Het neutrino's soveel inligting as fotone?

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As neutrino-detektors aanhou verbeter sodat 'n redelike aantal neutrino's waargeneem kan word, sou dit net so informatief wees vir sterrekunde soos fotone?

Dit is natuurlik 'n baie waardevolle aanvulling op fotone, maar ek dink aan neutrino's op sigself. Fotone het golflengte, spektrale lyne, rooi verskuiwing, diffraksie, polarisasie wat hul oorsprong en interaksies op pad na ons openbaar. Sê neutrino's meer as net uit watter rigting hulle kom?

Dit is 'n baie breë vraag en alhoewel 'n uitgebreide antwoord buite die bestek van hierdie eenvoudige Q & A-formaat val, gee ek u 'n paar voorbeelde waar 'neutrino-teleskope' revolusionêr sou wees.

Daar is 'n voorspelde kosmiese neutrino-agtergrond, analoog aan die kosmiese mikrogolfagtergrond. Neutrino's het op 1 sekonde na die oerknal ontkoppel en die heelal gevul met neutrino's wat nou tot ongeveer 1,9 K moes afgekoel het. Dit sou 'n (nog) skouspelagtige bevestiging van die oerknal-model wees. Daar is egter 'n rimpel; noudat ons weet dat neutrino's massa het, blyk dit dat hierdie neutrino's in die huidige tydvak nie-relativisties is nie. Dit beteken dat hulle deur gravitasiestrukture herlei en gekonsentreer kan word, wat dit moontlik fantastiese sondes van sulke strukture maak en miskien meer sensitief is as fotone. Neutrino-teleskope sou dus 'n fantastiese bydrae lewer tot ons begrip van kosmologie.
Neutrino's ontstaan uit energieke prosesse in die kern van sterre. Hierdie prosesse is andersins onsigbaar, ons kan dit beslis nie direk met die lig sien nie en kan dit slegs indirek ondersoek met behulp van tegnieke soos asteroseismologie of kyk na die vermenging van chemiese elemente van die kern tot die oppervlak. Neutrino's vertel ons moontlik baie meer; gee byvoorbeeld 'n direkte skatting van die aantal kernreaksies wat per sekonde plaasvind. Neutrino-emissie is ook die oorheersende manier waarop verkoeling in supernovas en in warm neutronsterre en wit dwerge plaasvind. 'N Neutrino-teleskoop kan dus fundamenteel wees vir 'n diep (letterlike) begrip van sterrewolusie en veral die laat stadiums van sterrewolusie.

In terme van wat gemeet kan word; u kan die vloed van neutrino's en die neutrino-energiespektrum meet. Daar is natuurlik ook die tydsberekening wat u van kortstondige gebeure soos supernovas kan kry. Daar is ook die vraag na geurswisselings en die mengsel van neutrino-soorte wat waargeneem word. Ek verstaan nie genoeg hieroor om voorspellings te maak oor hoe astronomies nuttig hierdie eiendom is nie, maar ek wed dat dit sal wees.

& # 8220Sien & # 8221 Dubbel: neutrino's en fotone waargeneem uit dieselfde kosmiese bron

Daar was al lank 'n vraag oor watter soorte gebeure en prosesse verantwoordelik is vir die neutrino's met die hoogste energie wat uit die ruimte kom en waargeneem word deur wetenskaplikes. 'N Ander vraag, waarskynlik verwant, is wat die meerderheid kosmiese strale met hoë energie en die deeltjies skep, meestal protone, wat voortdurend op die aarde reën.

As wetenskaplikes en die vermoë om hoë-energie neutrino's op te spoor (deeltjies wat baie volop is, elektries neutraal, baie lig en baie moeilik waarneem) en hoë-energie fotone (deeltjies lig, hoewel nie noodwendig van sigbare lig nie) meer kragtig en presies geword het, was daar 'n groot hoop om 'n antwoord op hierdie vraag te kry. Een van die dinge waarop ons gewag het (en al 'n paar keer teleurgesteld is) is 'n hewige ontploffing in die heelal wat gelyktydig hoë energie-fotone en neutrino's produseer, teen 'n hoë tempo dat albei soorte deeltjies kan op dieselfde tyd waargeneem word wat uit dieselfde rigting kom.

In onlangse jare was daar indirekte bewyse dat blazars en # 8212 smal deeltjiesstrale, soos in die loop van 'n geweer in ons algemene rigting gewys het, en geskep is terwyl materiaal naby en amper in reuse swart gate in die middelpunte van baie ver dwarrel. sterrestelsels & # 8212 kan verantwoordelik wees vir die hoë-energie neutrino's. Sterk regstreekse bewyse ten gunste van hierdie hipotese is pas aangebied. Verlede jaar het een van hierdie blazers helder opgevlam, en die fakkel het beide neutrino's met 'n hoë energie en hoë energie-fotone geskep wat binne dieselfde tydperk waargeneem is en van dieselfde plek in die lug gekom het.

Ek het al oor die IceCube neutrino-sterrewag geskryf voordat dit 'n kubieke kilometer ys onder die Suidpool is, met ligdetektors, en dit is ideaal vir die waarneming van neutrino's waarvan die bewegingsenergie die protone in die Large Hadron Collider ver oorskry. , waar die Higgs-deeltjie ontdek is. Hierdie neutrino's gaan meestal onopgemerk deur Ice Cube, maar een uit 100 000 tref iets, en puin van die botsing lewer sigbare lig wat Ice Cube se detektors kan opneem. IceCube het reeds belangrike ontdekkings gedoen deur 'n nuwe klas hoë-energie neutrino's op te spoor.

Op 22 September verlede jaar is een van hierdie baie energie-neutrino's by IceCube waargeneem. Meer presies, 'n muon wat ondergronds geskep is deur die botsing van hierdie neutrino met 'n atoomkern, is in IceCube waargeneem. Om die waargenome muon te skep, moes die neutrino 'n bewegingsenergie gehad het wat tienduisend keer groter was as die bewegingsenergie van elke proton by die Large Hadron Collider (LHC). En die rigting van die neutrino-beweging is ook bekend; dit is in wese dieselfde as dié van die waargenome muon. IceCube se wetenskaplikes het dus geweet waar hierdie neutrino vandaan gekom het.

(Dit werk nie vir tipiese kosmiese strale-protone nie, byvoorbeeld, beweeg in geboë paaie omdat hulle deur kosmiese magnetiese velde afgewyk word, dus selfs as u hul reisrigting meet by hul aankoms na die aarde, weet u nie waar Neutrino's, elektries neutraal, word nie deur magnetiese velde geraak nie en beweeg reguit, net soos fotone doen.)

Baie naby aan die rigting is 'n bekende baadjie (TXS-0506), vier miljard ligjare weg ('n goeie fraksie van die afstand oor die sigbare heelal).

Die IceCube-wetenskaplikes het onmiddellik hul neutrino-waarneming gerapporteer aan wetenskaplikes met hoë-energie fotondetektors. (Ek het ook geskryf oor sommige detektors wat gebruik word om die baie energieke fotone te bestudeer wat ons in die lug vind: veral die Fermi / LAT-satelliet het 'n rol gespeel in hierdie nuutste ontdekking.) Fermi / LAT, wat voortdurend hou die lug dop, het reeds hoë-energie-fotone opgespoor wat uit dieselfde rigting kom. Die Fermi-wetenskaplikes het binne 'n paar dae bevestig dat TXS-0506 op daardie stadium inderdaad opgevlam het en # 8212 reeds in April 2017 begin het, ses keer so helder as normaal. Met hierdie nuus van IceCube en Fermi / LAT, volg baie ander teleskope (onder andere die MAGIC kosmiese stralingsdetektor-teleskope) die voorbeeld en bestudeer die baadjie en leer meer oor die eienskappe van die fakkel.

Nou, net 'n enkele neutrino op sy eie is nie oortuigend nie, is dit moontlik dat dit net toevallig was? Die IceCube-mense het dus na hul ouer data gegaan om rond te snuffel. Daar ontdek hulle in hul data vir 2014-2015 'n dramatiese fakkel in neutrino's & # 8212 Meer as 'n dosyn neutrino's, wat meer as 150 dae gesien is, het uit dieselfde rigting gekom in die lug waar TXS-0506 sit. (Meer presies, byna 20 uit hierdie rigting is gesien, in 'n tydperk waar daar normaalweg net 6 of 7 toevallig is.) Dit bevestig dat hierdie baadjie inderdaad 'n bron van neutrino's is. En uit die energieë van die neutrino's in hierdie fakkel, kan nog meer geleer word oor hierdie baadjie en hoe dit terselfdertyd hoë-energie fotone en neutrino's maak. Interessant genoeg is daar tot dusver ten minste geen sterk bewyse vir hierdie fakkel van 2014 in fotone nie, behalwe miskien 'n toename in die aantal energiebesparende fotone & # 8230, maar nie die totale helderheid van die bron nie.

Die volledige prentjie, wat steeds opduik, is geneig om die idee te ondersteun dat die baadjie ontstaan uit 'n supermassiewe swart gat, wat optree as 'n natuurlike deeltjieversneller, wat 'n smal deeltjiesproei maak. protone ingesluit, teen uiters hoë energie. Hierdie protone, miljoene keer meer energiek as dié van die Large Hadron Collider, bots dan met meer gewone deeltjies wat net ronddwaal, soos sigbare ligfotone uit sterlig of infrarooi fotone uit die omringende hitte van die heelal. Die botsings lewer deeltjies op wat pione genoem word, gemaak van kwarks en anti-kwarks en gluone (net soos protone is), wat weer tot fotone of tot (onder andere) neutrino's verval. En dit is die resulterende fotone en neutrino's wat nou gesamentlik waargeneem is.

Aangesien kosmiese strale die geheimsinnige hoë-energie-deeltjies uit die buitenste ruim wat voortdurend op ons planeet reën, meestal protone is, dit is 'n bewys dat baie, miskien die meeste kosmiese strale met die hoogste energie, geskep word in die natuurlike deeltjieversnellers wat verband hou met baadjies. Baie wetenskaplikes het vermoed dat die mees ekstreme kosmiese strale geassosieer word met die mees aktiewe swart gate in die sentrums van sterrestelsels, en nou het ons bewyse en meer besonderhede ten gunste van hierdie idee. Dit blyk nou waarskynlik dat hierdie vraag met verloop van tyd beantwoord sal kan word, aangesien meer blazar fakkels waargeneem en bestudeer word.

Die aankondiging van hierdie belangrike ontdekking is by die National Science Foundation gedoen deur Francis Halzen, die hoofnavorser van IceCube, Olga Botner, voormalige IceCube-woordvoerder, Regina Caputo, die Fermi-LAT-analisekoördineerder, en Razmik Mirzoyan, MAGIC-woordvoerder.

Die feit dat sowel fotone as neutrino's uit dieselfde bron waargeneem is, is 'n voorbeeld van wat mense nou noem & # 8220multi-messenger astronomy & # 8221; 'n vorige voorbeeld was die waarneming in swaartekraggolwe, en in fotone van baie verskillende energieë, van twee samesmeltende neutronsterre. Natuurlik het so iets reeds in 1987 gebeur toe 'n supernova deur die oog gesien is en ook in neutrino's waargeneem is. Maar in hierdie geval het die neutrino's en fotone miljoene en miljarde kere energie!

Het neutrino's soveel inligting as fotone? - Sterrekunde

Vrae en antwoorde oor neutrino's
en die KamLAND-aankondiging van Desember 2002

voorberei deur
John geleer,
Universiteit van Hawaii, professor in fisika, KamLAND-medewerker

WAT IS NEUTRINOS?
Neutrino's is die minste massiewe elementêre deeltjie in die versameling boustene van die natuur, wat ses kwarks bevat (af, op, vreemd, bekoor, onder en bo) en leptone. Neutrino's is gratis en is in die familie van neutrale leptone. Hulle voel nie die sterk krag wat kwarke in protone en neutrone bind nie, en protone en neutrone in kerne nie.

Daar is drie soorte, of & quotflavors & quot, soos dit genoem word, van neutrino's elektron, muon en tau. Daar is ook drie anti-neutrino's van dieselfde geure. Die neutrino's kry hul name van hul gelaaide leptonbroers in volgorde van toenemende massa, die elektron, muon en tauon. Baie teoretici het gedink dat die massa neutrino's in die verlede jare nul was. Die bevindinge van sulke klein waardes is 'n raaisel en ongetwyfeld 'n leidraad.

WAAR KOM NEUTRINO'S?
Neutrino's word in baie omstandighede op die aarde, in die son, sterre en sterrestelsels geproduseer. Die dinge waaroor ons hier besorg is, is die gevolg van radioaktiewe verval in kernkragreaktors in Japan. Dit is elektron-anti-neutrino's. Dit is in teenstelling met neutrino-eksperimente op die son wat neutrino's uit die middel van die son meet.

Die KamLAND-eksperiment sien ook neutrino's van radioaktiewe vervalle dwarsoor die aarde, meestal uit Uraan en Thorium en hul vervalprodukte. Hierdie neutrino's het energie aan die lae kant van die KamLAND-waarnemingsgebied en vorm nie 'n problematiese agtergrond vir die opsporing van die kernkragreaktore nie. Uit die huidige resultate is dit inderdaad duidelik dat KamLAND binnekort genoeg statistieke sal hê om 'n positiewe sein van die totale radioaktiwiteit van die aarde te meet, tot dusver kan KamLAND slegs 'n boonste perk eis. Hierdie nooit ongekende waarneming het gevolge vir die verhitting van die aarde deur radioaktiwiteit en is van belang vir geoloë.

HOE WORD ELECTRON ANTI-NEUTRINOS GESPORTEER?
Neutrino's gaan oor die algemeen ongestrooid deur die aarde, maar soms kom 'n mens in wisselwerking, soms in die KamLAND-detector. Gewoonlik tref 'n elektron-antineutrino 'n kwark in die kern van 'n waterstofatoom, wat 'n proton is, in die paraffienolie. Die neutrino ruk 'n pluslading om 'n positron te word en verander die proton in 'n neutron. Die positron vernietig byna onmiddellik met 'n elektron en produseer twee gammastrale. Die gammastrale beweeg tien sentimeter in die vloeistof en wissel verder met mekaar, wat lei tot meer deeltjies. Die energie word uiteindelik in hitte neergesit, maar ook in opwindende molekules in toestande waaruit hulle kan verval met die vrystelling van lig. Skitterende materiaal wat by die olie gevoeg is, is baie doeltreffend. In die net kry jy ongeveer 3000 optiese fotone vir elke MeV energie wat neergelê word. met ander woorde ongeveer 5 000-10 000 fotone vir elke neutrino-interaksie van hierdie tipe. Dit alles gebeur binne 'n paar nanosekondes (miljardste van 'n sekonde).

Intussen begin die neutron wat deur die neutrino-interaksie geskep word, doelloos rondrammel, in atome stamp en uiteindelik gevang word deur 'n ander waterstofkern. Deuterium te word. Dit vind plaas met die vrystelling van die bindingsenergie van Deuterium van 2.2 MeV, en gebeur gewoonlik na ongeveer 200 mikrosekondes ('n lang tyd op hierdie skaal van gebeure). Daarom is die handtekening vir anti-neutrino interaksies 'n aanvanklike uitbarsting van fotone in die regte energiebereik vanaf die positron gevolg in 'n paar honderd mikrosekondes deur 'n tweede uitbarsting van die lig wat deur die neutron veroorsaak word.

Hierdie twee uitbarstings van lig moet dan in die regte intensiteitsgebied wees en betyds naby wees. Bovendien, met behulp van die tyd van ontvangs van die lig soos waargeneem deur die groot fotomultiplineerders (ligverklikkers), kan 'n mens die ligging van die oorsprong van die ligpulse rekonstrueer en eis dat dit binne 'n meter of twee van mekaar is. Hierdie stel vereistes dien dan om 'n baie skoon filter vir agtergrondgebeurtenisse te maak. Data word opgeteken vanaf elke klein ligstraaltjie in die detector. Die filterproses om antineutrino-gebeure op te spoor, word off-line gedoen, wat die agtergrond, kalibrasies en dies meer deeglik kan bestudeer. Uiteindelik is die anti-neutrino gebeure in wese sonder agtergrond (ongeveer 1 agtergrond tot 80 werklike anti-neutrino gebeure).

WAT IS DIE SOLAR NEUTRINO-PROBLEEM?
Die 'sonneutrino-probleem' duur nog baie jare, sedert 1968 toe die eerste sonneutrino-eksperiment, waarvoor Ray Davis vanjaar die Nobelprys bekroon is, waarnemings gedoen het van elektronneutrino's uit die son. Hierdie eksperiment het die transmissietempo van chloor in Argon in 'n tenk skoonmaakvloeistof in 'n diep myn in Homestake, Noord-Dakota, gemeet. Later het soortgelyke radiochemiese eksperimente in Rusland en Italië met Gallium na Germanium oorgedra, 'n soortgelyke verskil (alhoewel van verskillende omvang). Toe het die Kamiokande-detektor in Japan die elastiese verspreiding van neutrino's vanaf elektrone in water opgespoor en die eerste aanlynmetings van neutrino's uit die son gedoen. Hiervoor word Mashiboshi in 2002 met die Nobelprys bekroon. Die laaste legkaartstuk is in 2000 vervaardig en met meer oortuigende statistieke in 2002 deur die SNO-samewerking. Laasgenoemde het nie net 'n tekort in die elektronneutrino's gemeet nie, maar ook die voorspelde koers van sonneutrino's gevind in 'n nuwe meting wat die totale aantal sonneutrino's waarneem.

Dit lyk asof dit die mense regverdig wat al jare lank die neutrino-vloeistowwe van die son bereken, en wat dikwels daarvan beskuldig word dat hulle dit verkeerd begaan het en die probleem met die sonneutrino's veroorsaak het (en dus baie tyd en geld van eksperimente vermors het). Nou weet ons dat hulle dit inderdaad reggekry het, en dit blyk dat die neutrino's uit die son kom, maar dat daar iets met hulle gebeur om hul smaakmengsel te verander. Eenvoudige ossillasies tussen die drie bekende tipes neutrino's lyk die beste, maar daar is nog onduidelikhede. Natuurkundiges het dus gewag op die resultate van die KamLAND-eksperiment wat dit moontlik maak om hierdie gevolgtrekking te toets op 'n manier wat heeltemal onafhanklik is van die sonprobleme.

WAT IS ANDERS OOR KAMLAND-RESULTATE?
Die KamLAND-eksperiment is die nuutste in 'n lang reeks eksperimente wat neutrino's uit reaktore meet. In 1955 het Fred Reines en Clyde Cowan die heel eerste waarnemings van neutrino's gedoen deur presies dieselfde meganisme te gebruik as in KamLAND. Dit was 'n ongelooflike stap vorentoe, aangesien baie mense neutrino's onopspoorbaar gedink het.

In die tussentyd was daar bykans 'n dosyn eksperimente in die omgewing van reaktore wat groter en verder weg is. Die jongste twee eksperimente in die 1990's by CHOOZ in Frankryk en Palo Verde in die VSA is met verskeie tonverklikkers op 'n afstand van ongeveer 'n kilometer bedryf. Dus, KamLAND het 'n groot sprong van ongeveer 'n faktor van honderd, waarnemende reaktore op 'n tipiese afstand van 180 km verteenwoordig. Hierdie vorige eksperimente het baie goed gedien om die spektrum van neutrino's vanaf reaktore, neutrino-dwarsdeursnee, ens. Te bestudeer. Dit alles laat toe dat die tempo van die verwagte gebeure (indien geen ossillasies plaasvind nie) in KamLAND vir 'n paar persent bekend is. Vorige eksperimente het glad nie 'n teken gesien van neutrino-verdwyning nie, soos ons nou weet, omdat dit te naby was. (Daar was egter verskeie vals alarms van vroeëre eksperimente wat elk ter ruste gelê is deur verdere werk).

Die grootste verskil tussen sonneutrino-eksperimente en KamLAND is dat die KamLAND-eksperiment heeltemal afhang van plaaslike, sterk bestudeerde, mensgemaakte bronne van anti-neutrino's. En natuurlik is die sonbron neutrino's, nie anti-neutrino's nie. Dat ons nou vind dat neutrino's en anti-neutrino's op dieselfde manier optree met betrekking tot ossillasies, is 'n belangrike toets vir iets wat die fisici CPT-invariansie noem. In die praktyk beteken dit in hierdie geval dat neutrino's en anti-neutrino's op dieselfde manier optree (dieselfde massa en ossillasies). Hierdie simmetrie, hoewel dit baie vertrou word deur teoretici, word in hierdie geval bevraagteken as 'n ontvlugting vir nog 'n stel eksperimente wat oënskynlik onversoenbare resultate met alle ander (LSND) het. Dit is ook waar dat CPT tot op hede nie sterk beperk was vir neutrino's nie. Dit lyk asof die spekulatiewe teorieë deur die KamLAND-resultate opreg ontken word, en CPT is 'n veilige simmetrie tot 'n redelike mate van presisie.

WAT IS OSSILLASIES?
Neutrino-ossillasies is 'n eienaardige kwantummeganiese effek. Dit is moeilik om 'n goeie makroskopiese analogie te vind, aangesien dit te make het met die deeltjie-golfdualiteit van fundamentele materie.

Ons weet net wat 'n deeltjie is deur die manier waarop dit geproduseer word of interaksie het; dit is hoe ons dit noem. Wanneer 'n pion verval, lei dit tot 'n muon en 'n muon (anti) neutrino wanneer 'n neutron verval, lei dit tot 'n proton, 'n elektron en 'n elektron (anti) neutrino. Wanneer 'n muon deur 'n neutrino geproduseer word, weet ons dat dit deur 'n muonneutrino vervaardig is. En so aan.
'N Ander manier om 'n deeltjie te ken, is volgens gewig, uitgedruk deur spoed gegewe 'n sekere hoeveelheid energie en ook omdat dit deur swaartekrag aangetrek word. Gewoonlik is hierdie identifikasies vir elke deeltjie dieselfde, maar dit lyk asof muonneutrino's baie deurmekaar is.

As ons 'n muonneutrino-straal by 'n versneller skep en dit deur 'n kilometer aarde en ysterbeskerming lei om al die gelaaide deeltjies te elimineer, sien ons muons wat soms in 'n detector geproduseer word, in die regte rigting en net nadat die deeltjiebundelpuls die produksieteiken. Neutrino's is bekende deeltjies in hierdie sin, werklike en stabiele deeltjies, en hul interaksies word al langer as 30 jaar by die deeltjieversnellers, ondergronds en by reaktore bestudeer.

Die vreemde situasie vir neutrino's, anders as al die ander elementêre deeltjies, is dat die toestand van die deeltjie wat ons elektron of muon of tauon neutrino noem, miskien nie dieselfde is as die deeltjie massa toestand nie. Neutrino's is 'n soort verhouding tussen Dr. Jekyll en Mr. Hyde. Nog vreemder, die neutrino's bestaan blykbaar uit minstens drie verskillende massas,

Die muonneutrino kan effektief bestaan uit die helfte van twee toestande met 'n effens verskillende massa wat in en uit fase met mekaar beweeg, terwyl hulle saam beweeg, afwisselend as 'n muonneutrino wissel en dan 'n tau neutrino maak. Wat waargeneem word, hang af van waar die detektor die balk onderskep.

Die elektronneutrino (of antineutrino) blyk daarenteen een of ander mengsel van al drie die neutrino-massas te wees. Ons moet die totale mengsel nog nie regkry nie, maar dit blyk dat die mengsel van elektronneutrino's met 'n kombinasie van die tau- en muonneutrino's so groot is as wat dit mag wees. Hierdie eienaardige resultaat is nie deur teorie verwag nie en ons verstaan nie waarom dit so is nie.

WAT WYS ANDER STUDIES OOR NEUTRINO MASSA EN OSSILLASIES?
Die massa neutrino's en die moontlikheid van hul ossillasies ontwyk navorsers al jare. Versneller-gebaseerde eksperimente en ander wat reaktore en radioaktiewe bronne gebruik, het tot dusver slegs boonste perke vir neutrino-massas opgelewer. Geen oscillasies is oortuigend waargeneem voordat die Super-Kamiokande-uitslae in 1998 aangekondig is nie, wat 'die rookgeweer' getoon het dat muonneutrino's verdwyn het terwyl hulle die aarde deurkruis. Daaropvolgende studies (deur Super-Kamiokande, K2K, MACRO en Soudan II) het die inligting opgelewer dat muonneutrino's inderdaad ossilleer en dat hul vennote die tau-neutrino's is.

Die sonneutrino-situasie word hierbo beskryf, maar die kern is dat ons uit die SNO- en Super-Kamiokande-resultate op sonneutrino-metings, geneem met vroeëre resultate, eerder met vertroue kan aflei dat die tekort aan neutrino's in vergelyking met die berekeninge van die son dui aan dat elektronneutrino's ossilleer. As gevolg van die afstand tot die son, maak die feit dat die sonmodel baie ingewikkeld is en dat daar sommige modelle (hoewel nie veel in die mode nie) behalwe ossillasies bestaan, mense effens ongemaklik met die sonresultate. Die nuwe KamLAND-resultate, wat slegs met 'n aardse eksperiment in relatief beheerde toestande te make het, vee baie onsekerhede en ontsnapklousules uit: neutrino's het wel massa en almal ossilleer!

Baie eksperimente het probeer om absolute neutrino-massa direk te meet, wat baie moeilik is. Ons weet dat neutrino's lig is, baie minder as die massa van die elektron. Baie teoretici het inderdaad gedink dat neutrino-massa nul sou wees. Die ossillasie-eksperimente meet slegs massa-kwadraatverskille, en kan dus nie die absolute neutrino-massa vir ons vertel nie. Inderdaad, neutrino-massas kan amper so klein wees as die verskille, of hulle kan almal naby 1 eV wees, met klein verskille. Dit is moontlik dat die totale neutrino-massa gemeet sal word aan die gang van astrofisiese eksperimente, indien die totale massa ten minste ongeveer 0,1 eV is. Omdat neutrino's uit die oerknal baie meer as protone in die heelal is (met twee miljard tot een, of so), het selfs 'n klein neutrino-massa kosmologiese betekenis. Die saamgestelde massa van al die neutrino's in die heelal is naby die massa van al die sterre wat 'n mens in die lug sien.

WAT TOON DIE KAMLAND-GEGEVENS?
Die KamLAND-data dui aan dat die elektronneutrino's en anti-neutrino's waarskynlik ossilleer met 'n massa-kwadraatverskil van ongeveer 0,00007 eV ^ 2 en met 'n so groot vermenging as wat dit kan wees, of daarby. Die massaverskil is baie klein. ongeveer 0,008 eV. Die massaverskil tussen die twee swaarder neutrino-massatoestande blyk ongeveer 0,05 eV te wees, ongeveer ses keer meer. Dit moet vergelyk word met die massa van die elektron, wat 511,000 eV is. Ons het geen teoretiese model van die verspreiding van hierdie massas of hul totale totaal nie.

Soos hierbo gesê, toon die KamLAND-data ook volledige konsekwentheid tussen sonkragdata en 'n aardse eksperiment, en tussen neutrino's en anti-neutrino's.

WAAROM BEVIND DIE ELEKTRON NEUTRINOS?
Ons data plus die sonkragdata vertel ons ondubbelsinnig dat elektronneutrino's ossilleer, alhoewel ons nie seker kan wees met watter ander neutrino-toestand nie. Verskeie toetse dui nou aan dat die ossillerende maat van die elektronneutrino NIE 'n nuwe steriele neutrino is nie, maar dat dit heeltemal ooreenstem met 'n kombinasie van muon en tau neutrino.

STEM DIE NUWE BEVINDINGS in ooreenstemming met ander gegewens?
Soos hierbo gesê, stem KamLAND se resultate heeltemal ooreen met vroeëre eksperimente op kleiner afstande vanaf reaktore en met die ensemble van neutrino-eksperimente. Die KamLAND-uitslae is nie in stryd met enige ander resultate nie.

Waar kom die energiekste neutrino's vandaan?

Waar kom die neutrino's met die hoogste energie in die heelal vandaan? Wetenskaplikes het 'n paar idees, maar hulle weet dit nie regtig nie. Die neutrino's met die hoogste energie wat wetenskaplikes gesien het, word nie op die aarde vervaardig nie, dit kom van buite ons eie sonnestelsel en het energie wat baie groter is as enigiets wat ons in deeltjiesversnellers kan produseer. Dit is een van daardie tergende fisiese raaisels: Wat kook hierdie ongelooflike energieke deeltjies op?

Dieselfde ding wat neutrino's so moeilik maak om te vang en hul onwilligheid om met materie te kommunikeer, maak hulle ook wonderlike boodskappers van verafgeleë plekke in die kosmos. Anders as lig, kan kosmiese neutrino's na ons toe reis vanaf plekke wat ondeursigtig is en die geheime van baie digte plekke in ons heelal kan openbaar. En in teenstelling met kosmiese strale, gelaaide kerne wat ons vermoed dieselfde oorsprong het as kosmiese neutrino's, is neutrino's onverstoord op hul reis. Waar kosmiese strale deur magnetiese velde afgebuig kan word, kan kosmiese neutrino's na hul bronne terugwys en leidrade gee vir die uiters energieke prosesse wat hulle voortgebring het.

Sommige moontlike verklarings sluit in die oorblyfsels van supernovas, swart gate, pulse, ontploffings wat gammastraalbarstings genoem word, of reaksies in die digbevolkte sentrums van sterrestelsels (aktiewe galaktiese kerne genoem). Maar dit kan ook iets heeltemal nuuts wees wat ons nog nie voorheen teëgekom het nie.

Met behulp van die IceCube-eksperiment op Antarktika, het wetenskaplikes neutrino's so energiek vasgevang dat dit buite ons sonnestelsel moes ontstaan. Wetenskaplikes het van hulle grillerige name gegee wat gebaseer is op Sesamstraat-karakters: Big Bird, Bert en Ernie. Ander groot eksperimente (soos KM3NeT, wat instrumente sal gebruik wat versprei is oor 'n deel van die Middellandse See) is ook geïnteresseerd in die vaslegging van hierdie buitegewone deeltjies. Hierdie kosmiese boodskappers het die aanbreek van neutrino-sterrekunde ingelui: 'n ander manier om inligting oor ons groot, vreemde heelal te vind.

KM3NeT & # 8217 s skikkings van duisende optiese sensors sal die dowwe lig in die diepsee opspoor van gelaaide deeltjies afkomstig van botsings tussen die neutrino's en die aarde. Krediet: KM3NeT-samewerking / Edward Berbee / Nikhef

Uiters hoë-energie kosmiese strale is waargeneem in verskeie grondgebaseerde kosmiese straal-eksperimente regoor die wêreld, maar die oorsprong daarvan bly 'n raaisel. Dit is besonder moeilik om hul bronne op te spoor omdat kosmiese strale in die magnetiese veld van ons sterrestelsel afgebuig kan word sodat hulle nie meer na hul oorsprong terugwys nie. Wetenskaplikes hoop om kosmiese neutrino's te gebruik om meer te leer oor kosmiese strale met 'n hoë energie.

Daar is verskeie kosmiese bronne van hierdie kosmiese strale en neutrino's in ons eie sterrestelsel en daarbuite: galaktiese bronne, soos supernova-oorblyfsels, of ekstragalaktiese bronne, soos aktiewe galaktiese kerne en gammastraalbarstings. Neutrino-sterrekunde is 'n groeiende veld waarin wetenskaplikes die lug met neutrino's in plaas van fotone karteer. Hierdie uitdagende eksperimente, wat IceCube, ANTARES en KM3NeT insluit, hoop om lig te werp op die oorsprong van hierdie ultra-hoë-neutrino's.

Hierdie 2-PeV (petaelectronvolt) neutrino-gebeurtenis is op Dinsdag 4 Desember 2012 deur IceCube opgespoor. Dit is 'Big Bird' genoem. Krediet: IceCube-samewerking

Alhoewel die IceCube-eksperiment verskeie neutrino's met ongelooflike energieë waargeneem het, het niemand tot dusver 'n spesifieke kosmiese bron geïdentifiseer nie. Alhoewel wetenskaplikes 'n bewys van hul bestaan het, weet hulle eintlik nie watter meganismes hulle produseer of waar hulle vandaan kom nie. Maar uitdagings soos hierdie is waarvoor wetenskaplikes hou.

Hoë-energie neutrino's word besonder interessant namate ons die sogenaamde Greisen – Zatsepin – Kuzmin (GZK) limiet bereik. Die GZK-limiet is 'n teoretiese boonste grens van die energieë van kosmiese strale uit verre bronne, wat beskou word as iets meer as 150 miljoen ligjare weg van ons ligblou punt. Kosmiese strale moet hierdie limiet hê, ten minste in teorie, vanweë die manier waarop dit kosmiese mikrogolf-agtergrondfotone versprei, die lae-energie-deeltjies van die lig wat oorbly van die Oerknal.

Hierdie boonste grens is ongeveer 10 20 elektronvolts, ongeveer 70 miljoen keer groter as die hoogste energie wat ons in laboratoriums kon bereik. Alhoewel kosmiese strale hierdie limiet moet nakom, het neutrino's nie noodwendig so 'n beperking nie & # 8212 en diegene wat met hierdie ongelooflike energieë geproduseer word, kan ons steeds op aarde bereik. Die ontdekking van hierdie ongelooflik energieke, maar ongelukkig seldsame neutrino's, kan 'n heeltemal nuwe venster oopmaak waarmee u die heelal kan sien en verken.

Wetenskaplikes kom met opwindende idees om na die neutrino's te soek. Een voorbeeld? Gebruik radiotegnologie in 'n hoë ballon om die hele Antarktiese ysskild te bestudeer.

Neutrino-energieë

Asof dit nie verwarrend genoeg is om neutrino's van verskillende geure, verskillende massas en verskillende materie (antimaterie en gewone materie) te hê nie, kom neutrino's ook in 'n wye verskeidenheid energieë.

Die energie van 'n neutrino hang af van die proses wat dit gevorm het. Omdat neutrino's geen lading het nie, is daar geen manier om elektriese velde te gebruik om hulle te versnel en meer energie te gee nie, soos wetenskaplikes met deeltjies soos protone kan doen. Meer energieke reaksies sal meer energieke neutrino's veroorsaak. Dit is ideaal vir wetenskaplikes, omdat die meer geneigde deeltjies interaksie het en spore agterlaat. Dit is meer waarskynlik dat hulle deur gereelde materiaal gestuit word en die energie oordra na iets anders (ander deeltjies) wat detektore kan optel.

Hierdie spoor van die NOvA neutrino detector by Fermilab toon deeltjies wat geproduseer word deur 'n neutrino interaksie. Krediet: NOvA-samewerking

Low-energy neutrinos, such as those left over from the Big Bang, are very difficult to find because not only are they weakly interacting (like all neutrinos), but they also don’t have much energy to pass on to other particles we can see. Even if they do, that signal is likely to be weak and hard to pick out from all the other interactions shouting over it.

Neutrino energy is typically measured in electronvolts. But there is a big range of neutrino energies. Some have one-millionth of an electronvolt, and some have a quintillion electronvolts (that’s a 1 followed by 18 zeros). That means plenty of neutrinos to explore, and interesting information about the processes that formed those neutrinos.

Neutrinos come in a wide variety of energies. Some of the lowest-energy ones come from the Big Bang, while the most energetic seen thus far have come from extragalactic sources. The neutrino cross section (on the y axis) is a measure of how likely the neutrino is to be stopped by regular matter. The higher energy a neutrino has, the more likely it is to interact. Credit: J.A. Formaggio and G.P. Zeller

Because neutrinos come in a very broad range of energies, an even broader range of techniques have to be used to see them.

The lowest-energy neutrinos come from just a few seconds after the Big Bang, and it is expected that these neutrinos have only a fraction of an electronvolt of energy. This is less energy than it takes to even knock an electron out of a hydrogen atom, making them incredibly hard to detect, because you need a detector with an even lower threshold. It turns out that materials at room temperature are vibrating with thermal energies that are much higher than these Big Bang neutrinos, so one way to see these lowest-energy neutrinos is to use stiller, colder materials (at cryogenic temperatures) and look for nuclei that receive a small amount of energy seemingly out of the blue. Another proposed method to see these neutrinos is using these low-energy neutrinos to stimulate a beta decay, then searching for an outgoing electron that has just a little more energy than one would expect. The trick, then, is building a detector that can measure tiny differences in electron energies.

This plot shows the sun in neutrinos. The bright yellow at the center means a high concentration of neutrinos from that direction. Credit: Super-Kamiokande Collaboration/Kamioka Observatory, ICRR, Univ. of Tokyo

Neutrinos from the sun come in energies from tens to millions of electronvolts, a result of the many different fusion processes that take place there simultaneously. Scientists now know that most neutrinos from the sun are in the tens to hundreds of electronvolts. Researchers were able to see these only by building an extremely large scintillator detector and making sure there were no radioactive contaminants anywhere nearby.

The first neutrinos from the sun that scientists were able to see were the ones energetic enough to change a chlorine atom into an excited argon atom (by changing a neutron inside a nucleus to a proton). The was Ray Davis’s experiment at the Homestake Gold Mine. By measuring the radioactive decay of that excited argon atom, scientists made the first measurements of neutrinos from the sun.

Once a neutrino is energetic enough to knock an electron out of its orbital, then detectors that are sensitive to electric charges can pick the little particles up. The striking thing about this reaction is that the electron is knocked out of its orbital at exactly the same angle as the incoming neutrino hit with. If a detector can measure that outgoing electron angle and take into account the detector’s relationship to the sun, then you can actually “see” the sun with neutrinos. The detector, whether it’s on Earth’s surface or underground, will see the sun all the time, day or night.

Neutrinos from nuclear reactors have a million times more energy than Big Bang neutrinos, so they can be seen by measuring their interactions with atoms. One key difference is that the neutrinos from reactors are actually antineutrinos, so instead of changing neutrons to protons, they change protons to neutrons—and the neutrons are much harder to detect. The neutrons can be captured by certain particles that then decay and produce photons, particles of light, which can signal that a neutrino was there.

The Deep Underground Neutrino Experiment hosted by Fermilab will use an intense beam of neutrinos with billions of electronvolts of energy. Credit: DUNE/Fermilab

As neutrinos go from a million electronvolts to a billion electronvolts, they can start to transfer more energy to the particles in a detector. At a billion electronvolts, that same process of a neutrino colliding with a nucleus can produce an electron that travels through dozens of centimeters of plastic or a muon that travels through meters of steel. At 10 billion electronvolts, the neutrinos have enough energy to completely break up a nucleus.

Finally, if you need a meter of steel to see a 1-GeV muon, then you need a kilometer of steel to see a 1-TeV muon. The detectors that have seen the highest-energy neutrinos are those that are made with a cubic kilometer of detector material. The question is, how on Earth can you afford a cubic kilometer of detector? You have to use some material that’s already available in large quantities and figure out how to pull a signal out of it. People have made detectors out of both ocean water and the ice in Antarctica to see these highest-energy neutrinos.

The IceCube experiment uses a cubic kilometer of ice in Antarctica as its detector medium. More than 5,000 sensors in the ice look for neutrinos from outer space. Credit: IceCube Collaboration/University of Wisconsin-Madison

Swift 10 Years of Discovery, a novel approach to Time Domain Astronomy

1. Inleiding

The detection of MeV neutrinos emitted from the Sun and from the supernovae allowed the understanding of the physics of these astrophysical objects (see Bahcall, 2004 Raffelt, 2004 for a review). MeV neutrino “telescopes” are capable of detecting neutrinos from sources close to our galaxy up to a distance of < 100 kp . The main goal of the construction of high energy, > 1 TeV , neutrino telescopes ( Gaisser et al., 1995 ) is the extension of the distance accessible to neutrino astronomy to cosmological scales ( Waxman, 2009 ).

The existence of extra-galactic high-energy neutrino sources is implied by cosmic-ray observations. The cosmic-ray spectrum extends to energies ∼ 10 20 eV , and is likely dominated beyond ∼ 10 19 eV by extra-galactic sources. The origin of Cosmic Rays (CRs) has been a tantalizing mystery ever since their discovery by Hess (1912) nearly a century ago. While “lower energy” CRs of up to 10 16 eV are believed to originate from supernova explosions in our galaxy ( Blasi, 2008 ), the source of the more energetic CRs whose energies can exceed 10 19 eV remains unknown ( Cronin, 2005 ). Although ultrahigh energy cosmic rays (UHECR) are produced throughout the Universe, those observed at the Earth must have been produced locally (within 50 Mpc) since they lose energy while propagating through the Cosmic Microwave Background (CMB). This process (discussed by Greisen (1966) and Zatsepin and Kuzmin (1966) – “GZK”) not only causes energy loss for the primary, but also creates secondary particles of extremely high energy ( Beresinsky and Zatsepin, 1969 Stecker, 1973 ), including neutrinos above 10 17 eV . The secondary particles, particularly Ultra High Energy Neutrinos (UHEN), can be used to explore the origins of UHECR ( Seckel and Stanev, 2005 ). With no electric charge, neutrinos experience no scattering or energy loss, and so provide a probe of the source distribution even to high redshift. Since UHECRs and the expected “GZK cutoff” have been observed, the expectation of a GZK neutrino flux is on very strong footing ( Mannheim et al., 2001 ). UHE neutrinos are also likely to be created at the acceleration sites of the UHECRs, pointing directly to the source from the Earth, and providing information on the role of hadrons in the acceleration.

Only active galactic nuclei ( Rachen and Biermann, 1993 ) and gamma ray bursts ( Milgrom and Usov, 1995 Vietri, 1995 Waxman, 1995 ) are believed to be capable of accelerating CRs to such enormous energies ( Wick et al., 2004 ). Gamma Ray Bursts (GRBs) are powerful explosions, and are among the highest-redshift point sources observed. The most common phenomenological interpretation of these cosmological sources is through the so-called fireball model ( Piran, 2000 Mészáros, 1999, 2006 ). In this model, part of the energy is carried out (e.g., from a collapsed star) by hadrons at highly-relativistic energies, some of which is dissipated internally and eventually reconverted into internal energy, which is then radiated as γ-rays by synchrotron and inverse-Compton emission by shock-accelerated electrons. As the fireball sweeps up ambient material, it energizes the surrounding medium through, e.g., forward shocks, which are believed to be responsible for the longer-wavelength afterglow emission ( Mészáros, 1999 ).

If the GRB jet comprises PeV protons, it should produce energetic neutrinos through photon–hadron interactions. The photons for this process can be supplied by the GRB gamma rays during its prompt phase, or during the afterglow phase ( Waxman and Bahcall, 1997 Dermer, 2002 ). These lead to the production of charged pions, which subsequently decay to produce neutrinos. Within this picture, GRBs should produce neutrinos with energies of ∼ 100 TeV from the same region in which the GRB photons are produced ( Guetta et al., 2004 ). These neutrinos, if present, could be readily detected. Hence, the detectability of TeV to PeV neutrinos depends on the presence of (>) PeV protons and on the efficiency at which their energy is converted into neutrinos, as compared to how much of the energy is in electrons, which is manifested primarily in the prompt GRB photon emission.

Neutrino astronomy has steadily progressed over the last half century, with successive generations of detectors achieving sensitivity to neutrinos with increasingly higher energies. With each increase in neutrino energy, the required detector increases in size to compensate for the dramatic decrease of the flux with energy. The high-energy neutrinos from GRBs and the GZK neutrinos discussed above should be detected by large neutrino telescopes, such as IceCube, the Askarian Radio Array (ARA) and in the future KM3NeT. 1 All these detectors look for the Cherenkov radiation initiated by the neutrino interactions in the ice using either optical detection (IceCube, KM3NeT) or Radio Frequency (RF) detection (ARA).

Are all sources of cosmic rays blazars?

Ons weet nog nie. This is the first time we have been able to identify a likely source of neutrinos and cosmic rays. Scientists think there are other cosmic objects that might also accelerate cosmic rays, many even think that most of them will only be revealed with a larger neutrino detector, i.e., with larger samples of neutrino data, since the highest energy gamma rays might be absorbed in the environments around these sources or on their way to Earth.

Learn more about this in a short video featuring IceCuber Chad Finley, former convenor of the IceCube point-source working group.

Do neutrinos have as much information as photons do? - Sterrekunde

Do neutrinos undergo redshift?
Are there such things as "neutrino spectra"?
How would they compare to photon redshifts?

Iantresman 1.Yes, neutrinos can undergo redshift.
2.Yes, there are neutrino spectra. They aren't easy to see.
3. Like photons, E=hv applies to neutrinos, E/c =hv/c describes the momentum of a photon or a neutrino. They differ in spin. photons have integral spin. 1,2.. neutrinos half-integral spin . 1/2, -1/2. and photons participate in the electromagnetic interaction, neutrinos the weak interaction. Pete.

We fully expect that neutrinos are less energetic if they are emitted by something moving away in our frame, or if they climb out of a deep gravity well. As far as I know this has never been tested.

Neutrinos do have energies, and the ones we observe coming out of the Sun can be identified with the specific reaction that created them. Neutrinos observed by the spray of particles coming out of the Antarctic ice are higher energy, and have observable energies with some modest sized error bars.

In either case this is a great question, and someday we may be able to detect kown energy neutrinos from a source moving at a high specific velocity so we can test this.

Could neutrino redshift, compared to photon redshifts, be able to determine the proportion of Doppler to Cosmological redshift, or would that also depend on their mass?

I do not know the answer. I can say that I think it is pretty likely that we will have accurate parallax measurements to galaxies outside our local group before we'll accurately measure this effect in neutrinos.

Neutrino redshift should act like photon redshift if and only if neutrinos are massless as trinitree88 thinks. If they are massive, they are still highly relativistic, and so their velocity will still be imperceptably close to c, but their energy should drop following SR.
Hasn't the question of whether neutrinos have rest-mass been answered? Since some of the neutrinos from the Sun change "flavor" in the few minutes they takes to reach us, they must be travelling at less than the speed of light, and therefore they have rest mass.

Hasn't the question of whether neutrinos have rest-mass been answered? Since some of the neutrinos from the Sun change "flavor" in the few minutes they takes to reach us, they must be travelling at less than the speed of light, and therefore they have rest mass.

I believe that they have mass, but Trinitree88 who wrote post #2 above does not believe this. I was writing allowing for his doubt, which includes the possibility that the flavor change only happens in the presence of other matter (i.e. inside the Earth and Sun), but not in the sparsely populated parts of space between us and sn1987a for example.

Eroica. Wikipedia is an excellent, but not infallible (neither am I) source for quick info: for example. from your link. onder:

They suggest that a fourth generation of fermions beyond the Standard Model would involve a "heavy "neutrino . easy to make in a particle accelerator. This ignores the experimental results from CERN. There, the production of Z0 bosons was at an energy "resonance", matching the mass/energy of the Z. around 90 Mev/c2.
The more ways there were for those Z's to decay, the more rapidly they would. The more rapidly they decayed, the shorter-lived they were. The shorter-lived they were, the narrower the width of the energy "resonance". This was known as the Z-pole. If it was spread out. one family of fermions. A little less spread. two families. Sharper than Bell-Shaped. three families. Sharper still..four families. The data clearly point to three (all known) familys. (The late great Richard Feymann capitulated on the data). There is no known evidence for either a fourth set of quarks(along with all the new mesons, baryons, hyperons, involved). or it's commensurate "heavy" neutrino. "easy" to generate.:naughty:
Their information on possible mass limits for neutrinos are equally shaky. Antoniseb has allowed the discrepancy until the results are incontrovertible. Every experiment claiming masses has a matter path where superposition of neutrinos with entrained matter occurs. allowing MSW flavor mixing. I believe some of ETA-C's posts concur. that the experiments to date define upper limits to possible masses. but are not inconsistent with massless neutrinos. May the wind be at your back. Pete

Eroica. Wikipedia is an excellent, but not infallible (neither am I) source for quick info:
Actually, Wikipedia was not my source. I just posted the link for convenient reference. I thought the matter had been settled, but obviously I was mistaken.

May the wind be at your back. Pete
And may the road rise before you. :)

Re the OP: As long as you are careful with your definitions (etc), you can get to the 'redshift' of neutrinos (or anything else astrophysical) from first (classical) principles.

Bob is moving away from Alex (let's say we're sitting in Alex' spaceship). Bob shoots bullets, neutrons, neutrinos, and photons directly at Alex. Bob also tells Alex, via the 2-way radio, just how fast he's shooting them (or, if you prefer, the energy he's sent them off with, plus their speed in the case of the bullets and neutrinos we're going to need something to do with their mass, just to be sure).

Alex receives the photons, neutrinos, neutrons, and bullets, and records the speed, energy (and mass) of each.

We none of us have any difficulty with the 'doppler' effect, for each type of thing, right?

For photons, it's straight-forward (just your normal redshift).

What about bullets? Could they be said to have a 'redshift'? Sure!

And neutrons? and neutrinos??

Now here's one tricky part - as long as the bullets don't move too fast, the 'redshift' can be related to the kinetic energy, which just reduces to the speed, right?

But what if they're going at relativistic speeds? How to factor in gamma (into the 'redshift')?

However you do that, you'll need to do the same thing with neutrons.

Which brings us to neutrinos . do they get the 'photon' treatment (for determining redshift)? or the 'bullet' treatment?

Anyway, to address another question (in post #4): think of a blazar - we're staring down the barrel of a particle accelerator that would make the LHC look like the dim glow of a firefly. Lots of photons, neutrons, and neutrinos (but maybe no bullets).
Blazars can be seen right across the universe (well, to several billion pc anyway), in light, radio, X-rays, gammas, . and maybe some of the UHECRs originate in them too.

The photon redshift in blazars is a mixture of doppler and cosmological, in origin . if we could measure it, would the 'UHECR redshift' be any different? The 'neutrino redshift'? A very good question!

Finally, we do have strong observational evidence that there is, indeed, some sort of 'cosmological redshift' for neutrinos.

Just as the CMB is the 'surface of last scattering' of photons, when photons and (ordinary) matter 'decoupled', so there is a 'surface of last scattering' for neutrinos, when they decoupled from (ordinary) matter. It happened much earlier . when the universe was merely

1 second old (IIRC). These 'relict' neutrinos would be easily detectable if they were not 'redshifted' down to frigidity (IIRC, the neutrino equivalent of the CMB is expected to be, just like the CMB, an almost perfect blackbody, of

1K whatever a 'blackbody' neutrino spectrum turns out to be). However, such cold neutrinos are essentially undetectable (using today's technology).

But then, neutrino oscillation makes the story more complicated .

These 'relict' neutrinos would be easily detectable if they were not 'redshifted' down to frigidity (IIRC, the neutrino equivalent of the CMB is expected to be, just like the CMB, an almost perfect blackbody, of

1K whatever a 'blackbody' neutrino spectrum turns out to be). However, such cold neutrinos are essentially undetectable (using today's technology).

Has the potential for such "cold neutrinos" been considered as a candidate for the "cold dark matter"?

What’s a neutrino?

A neutrino is a particle! It’s one of the so-called fundamental particles, which means it isn’t made of any smaller pieces, at least that we know of. Neutrinos are members of the same group as the most famous fundamental particle, the electron (which is powering the device you’re reading this on right now). But while electrons have a negative charge, neutrinos have no charge at all.

Neutrinos are also incredibly small and light. They have some mass, but not much. They are the lightest of all the subatomic particles that have mass. They’re also extremely common—in fact, they’re the most abundant massive particle in the universe. Neutrinos come from all kinds of different sources and are often the product of heavy particles turning into lighter ones, a process called “decay.”

These little particles have an interesting history. First predicted in 1930, they weren’t discovered in experiments until 1956, and scientists thought they were massless until even later. While we keep learning more about neutrinos, with new answers come new mysteries.

Neutrinos are also tricky to study. The only ways they interact is through gravity and the weak force, which is, well, weak. This weak force is important only at very short distances, which means tiny neutrinos can skirt through the atoms of massive objects without interacting. Most neutrinos will pass through Earth without interacting at all. To increase the odds of seeing them, scientists build huge detectors and create intense sources of neutrinos.

Physicist Enrico Fermi popularized the name “neutrino”, which is Italian for “little neutral one.” Neutrinos are denoted by the Greek symbol ν, or nu (pronounced “new”). But not all neutrinos are the same. They come in different types and can be thought of in terms of flavors, masses, and energies. Some are antimatter versions. There may even be some yet undiscovered kinds of neutrinos!

Ask Ethan: Do Neutrinos Always Travel At Nearly The Speed Of Light?

Neutrino detectors, like the one used in the BOREXINO collaboration here, generally have an enormous . [+] tank that serves as the target for the experiment, where a neutrino interaction will produce fast-moving charged particles that can then be detected by the surrounding photomultiplier tubes at the ends. However, slow-moving neutrinos cannot produce a detectable signal in this fashion.

INFN / Borexino Collaboration

For decades, the neutrino was among the most puzzling and elusive of cosmic particles. It took more than two decades from when it was first predicted to when it was finally detected, and they came along with a bunch of surprises that make them unique among all the particles that we know of. They can “change flavor” from one type (electron, mu, tau) into another. All neutrinos always have a left-handed spin all anti-neutrinos always have a right-handed spin. And every neutrino we’ve ever observed moves at speeds indistinguishable from the speed of light. But must that be so? That’s what Patreon supporter Laird Whitehill wants to know, asking:

“I know neutrinos travel almost at the speed of light. But since they have mass, there is no reason that they couldn’t travel at any speed. But [you’ve implied] their mass dictates that they must travel almost at the speed of light.

But light travels at a constant speed. But anything with mass can travel at any speed.”

So why, then, do we only see neutrinos traveling at velocities consistent with the speed of light? It’s a fascinating question. Let’s dive on in.

According to the Standard Model, the leptons and antileptons should all be separate, independent . [+] particles from one another. But the three types of neutrino all mix together, indicating they must be massive and, furthermore, that neutrinos and antineutrinos may in fact be the same particle as one another: Majorana fermions.

E. Siegel / Beyond The Galaxy

The neutrino was first proposed in 1930, when a special type of decay — beta decay — seemed to violate two of the most important conservation laws of all: the conservation of energy and the conservation of momentum. When an atomic nucleus decayed in this fashion, it:

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increased in atomic number by 1,
emitted an electron,
and lost a little bit of rest mass.

When you added up the energy of the electron and the energy of the post-decay nucleus, including all the rest mass energy, it was always slightly less than the rest mass of the initial nucleus. In addition, when you measured the momentum of electron and the post-decay nucleus, it didn’t match the initial momentum of the pre-decay nucleus. Either energy and momentum were being lost, and these supposedly fundamental conservation laws were no good, or there was a hitherto undetected additional particle being created that carried that excess energy and momentum away.

Schematic illustration of nuclear beta decay in a massive atomic nucleus. Beta decay is a decay that . [+] proceeds through the weak interactions, converting a neutron into a proton, electron, and an anti-electron neutrino. Before the neutrino was known or detected, it appeared that both energy and momentum were not conserved in beta decays.

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It would take approximately 26 years for that particle to be detected: the elusive neutrino. Although we couldn’t quite see these neutrinos directly — and still can’t — we can detect the particles they collide or react with, providing evidence of the neutrino’s existence and teaching us about its properties and interactions. There are a myriad of ways the neutrino has shown itself to us, and each one provides us with an independent measurement and constraint on its properties.

We’ve measured neutrinos and antineutrinos produced in nuclear reactors.

We’ve measured neutrinos produced by the Sun.

We’ve measured neutrinos and antineutrinos produced by cosmic rays that interact with our atmosphere.

We’ve measured neutrinos and antineutrinos produced by particle accelerator experiments.

We’ve measured neutrinos produced by the closest supernova to occur in the past century: SN 1987A.

And, in recent years, we’ve even measured a neutrino coming from the center of an active galaxy — a blazar — from under the ice in Antarctica.

The remnant of supernova 1987a, located in the Large Magellanic Cloud some 165,000 light years away. . [+] It was the closest observed supernova to Earth in more than three centuries, and the neutrinos that arrived from it came in a burst lasting about

10 seconds: equivalent to the time that neutrinos are expected to be produced.

Noel Carboni & the ESA/ESO/NASA Photoshop FITS Liberator

With all of this information combined, we’ve learned an incredible amount of information about these ghostly neutrinos. Some particularly relevant facts are as follows:

Every neutrino and antineutrino we’ve ever observed moves at speeds so fast they’re indistinguishable from the speed of light.
Neutrinos and antineutrinos both come in three different flavors: electron, mu, and tau.
Every neutrino we’ve ever observed is left-handed (if you point your thumb in its direction of motion, your left hand’s fingers “curl” in the direction of its spin, or intrinsic angular momentum), and every anti-neutrino is right-handed.
Neutrinos and antineutrinos can oscillate, or change flavor, from one type into another when they pass through matter.
And yet neutrinos and antineutrinos, despite appearing to move at the speed of light, must have a non-zero rest mass, otherwise this “neutrino oscillation” phenomenon would not be possible.

If you begin with an electron neutrino (black) and allow it to travel through either empty space or . [+] matter, it will have a certain probability of oscillating, something that can only happen if neutrinos have very small but non-zero masses. The solar and atmospheric neutrino experiment results are consistent with one another, but not with the full suite of neutrino data including beamline neutrinos.

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Neutrinos and antineutrinos come in a wide variety of energies, and the odds of having a neutrino interact with you increase with a neutrino’s energy. In other words, the more energy your neutrino has, the more likely it is to interact with you. For the majority of neutrinos produced in the modern Universe, through stars, supernovae, and other natural nuclear reactions, it would take about a light-year worth of lead to stop approximately half of the neutrinos fired upon it.

All of our observations, combined, have enabled us to draw some conclusions about the rest mass of neutrinos and antineutrinos. First off, they cannot be zero. The three types of neutrino almost certainly have different masses from one another, where the heaviest a neutrino is allowed to be is about 1/4,000,000th the mass of an electron, the next-lightest particle. And through two independent sets of measurements — from the large-scale structure of the Universe and the remnant light left over from the Big Bang — we can conclude that approximately one billion neutrinos and antineutrinos were produced in the Big Bang for every proton in the Universe today.

If there were no oscillations due to matter interacting with radiation in the Universe, there would . [+] be no scale-dependent wiggles seen in galaxy clustering. The wiggles themselves, shown with the non-wiggly part subtracted out (bottom), is dependent on the impact of the cosmic neutrinos theorized to be present by the Big Bang. Standard Big Bang cosmology corresponds to β=1. Note that if there is a dark matter/neutrino interaction present, the acoustic scale could be altered.

D. Baumann et al. (2019), Nature Physics

Here’s where the disconnect between theory and experiment lies. In theory, because neutrinos have a non-zero rest mass, it should be possible for them to slow down to non-relativistic speeds. In theory, the neutrinos left over from the Big Bang should have already slowed down to these speeds, where they’ll only be moving at a few hundred km/s today: slow enough that they should have fallen into galaxies and galaxy clusters by now, making up approximately

1% of all the dark matter in the Universe.

But experimentally, we simply don’t have the capabilities to detect these slow-moving neutrinos directly. Their cross-section is literally millions of times too small to have a chance at seeing them, as these tiny energies wouldn’t produce recoils noticeable by our current equipment. Unless we could accelerate a modern neutrino detector to speeds extremely close to the speed of light, these low-energy neutrinos, the only ones that should exist at non-relativistic speeds, will remain undetectable.

A neutrino event, identifiable by the rings of Cherenkov radiation that show up along the . [+] photomultiplier tubes lining the detector walls, showcase the successful methodology of neutrino astronomy. This image shows multiple events, and is part of the suite of experiments paving our way to a greater understanding of neutrinos.

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And that’s unfortunate, because detecting these low-energy neutrinos — the ones that move slow compared to the speed of light — would enable us to perform an important test that we’ve never performed before. Imagine that you’ve got a neutrino, and you’re traveling behind it. If you look at this neutrino, you’ll measure it moving straight ahead: forwards, in front of you. If you go to measure the neutrino’s angular momentum, it will behave as though it’s spinning counterclockwise: the same as if you pointed your left hand’s thumb forward and watched your fingers curl around it.

If the neutrino always moved at the speed of light, it would be impossible to move faster than the neutrino. You’d never, no matter how much energy you put into yourself, be able to overtake it. But if the neutrino has a non-zero rest mass, you should be able to boost yourself to move faster than the neutrino is moving. Instead of seeing it move away from you, you’d see it move towards you. And yet, its angular momentum would have to be the same, in the counterclockwise direction, meaning you’d have to use your reg hand to represent it, rather than your left.

If you catch a neutrino or antineutrino moving in a particular direction, you'll find that its . [+] intrinsic angular momentum exhibits either clockwise or counterclockwise spin, corresponding to whether the particle in question is a neutrino or antineutrino. Whether right-handed neutrinos (and left-handed antineutrinos) are real or not is an unanswered question that could unlock many mysteries about the cosmos.

HYPERPHYSICS / R NAVE / GEORGIA STATE UNIVERSITY

This is a fascinating paradox. It seems to indicate that you could transform a matter particle (a neutrino) into an antimatter particle (an antineutrino) simply by changing your motion relative to the neutrino. Alternatively, it’s possible that there really could be right-handed neutrinos and left-handed antineutrinos, and that we’ve just never seen them for some reason. It’s one of the biggest open questions about neutrinos, and the capability to detect low-energy neutrinos — the ones moving slow compared to the speed of light — would answer that question.

But we can’t really do that in practice. The lowest-energy neutrinos we’ve ever detected have so much energy that their speed must be, at minimum, 99.99999999995% the speed of light, which means that they can move no slower than 299,792,457.99985 meters-per-second. Even over cosmic distances, when we’ve observed neutrinos arriving from galaxies other than the Milky Way, we’ve detected absolutely no difference between a neutrino’s speed and the speed of light.

When a nucleus experiences a double neutron decay, two electrons and two neutrinos get emitted . [+] conventionally. If neutrinos obey this see-saw mechanism and are Majorana particles, neutrinoless double beta decay should be possible. Experiments are actively looking for this.

LUDWIG NIEDERMEIER, UNIVERSITAT TUBINGEN / GERDA

Nevertheless, there’s a tantalizing chance we have to resolve this paradox, despite the difficulty inherent to it. It’s possible to have an unstable atomic nucleus that doesn’t just undergo beta decay, but double beta decay: where two neutrons in the nucleus simultaneously both undergo beta decay. We’ve observed this process: where a nucleus changes its atomic number by 2, emits 2 electrons, and energy and momentum are both lost, corresponding to the emission of 2 (anti)neutrinos.

But if you could transform a neutrino into an antineutrino simply by changing your frame-of-reference, that would mean that neutrinos are a special, new type of particle that exists only in theory thus far: a Majorana fermion. It would mean that the antineutrino emitted by one nucleus could, hypothetically, be absorbed (as a neutrino) by the other nucleus, and you’d be able to get a decay where:

the atomic number of the nucleus changed by 2,
2 electrons are emitted,
but 0 neutrinos or antineutrinos are emitted.

There are currently multiple experiments, including the MAJORANA experiment, looking specifically for this neutrinoless double beta decay. If we observe it, it will fundamentally change our perspective on the elusive neutrino.

The GERDA experiment, a decade ago, placed the strongest constraints on neutrinoless double beta . [+] decay at the time. The MAJORANA experiment, shown here, has the potential to finally detect this rare decay. It will likely take years for their experiment to yield robust results, but any events at all in excess above the expected background would be groundbreaking.

THE MAJORANA NEUTRINOLESS DOUBLE-BETA DECAY EXPERIMENT / UNIVERSITY OF WASHINGTON

But for right now, with current technology, the only neutrinos (and antineutrinos) we can detect via their interactions move at speeds indistinguishable from the speed of light. Neutrinos might have mass, but their mass is so small that of all the ways the Universe has to create them, only the neutrinos made in the Big Bang itself should be moving slow compared to the speed of light today. Those neutrinos might be all around us, as an inevitable part of the galaxy, but we cannot directly detect them.

In theory, however, neutrinos can absolutely travel at any speed at all, so long as it’s slower than the cosmic speed limit: the speed of light in a vacuum. The issue we have is twofold:

slow moving neutrinos have very low probabilities of interactions,
and those interactions that do occur are so low in energy that we cannot presently detect them.

The only neutrino interactions we see are the ones coming from neutrinos moving indistinguishably close to the speed of light. Until there’s a revolutionary new technology or experimental technique, this will, however unfortunate it is, continue to be the case.