Неутрино

Неутрино (се означува со грчката буква ''ν'') — лептон, елементарна честичка со половичен спин, која е ова интеракција само со слаби субатомски сили и гравитација.
Масата на неутриното е мала во споредба со другите субатомски честички. Неутриното е единствениот идентефикуван кандидат за темна материја, поточно врела темна материја.
Постојат три видови неутрина: електронски неутрина, мионски неутрина, и тау-неутрино. Секој вид има и сопствена античестичка, наречена „антинеутрино“, која исто така нема електричен полнеж. Неутрината се создадени на начин на кој го зачувуваат лептонскиот број, т.е.,за секое создадено електронско неутрино, се создава позитрон (антиелектрон), исто така и за секое создадено електронско неутрино, се создава електронско антинеутрино.
Неутрината, своето име го добиле според нивниот полнеж кој всушност е неутрален и затоа тие не се под влијание на силното меѓумолекулско заемодејство. Слабото заемодејство е дејство на куси растојанија а пак гравитацијата е премногу слаба на субатомската скала.
Неутрината можат да бидат создадени на повеќе начини, вклучувајќи и одредени видови на радиоактивни распади, при јадрените реакции како кај Сонцето. Поголемиот број на неутрина во близина на Земјата се од јадрените реакции во Сонцето. Околу 65 милијарди сончеви неутрина во секунда минуваат низ секој кубен сентиметар во правец Сонце-Земја.
Неутрината се колебаат помеѓу различните видови на Неутрина додека се во движење. Односно, електронското неутрино при бета-распад може да пристигне до детекторот како мионско или тау-неутрино. Овие колебања бараат различните неутрина да имаат различни маси, иако овие маси се многу мали. Од космолошките мерења,е пресметано дека сумата на трите неутрина заедно мора да е помала од еден милионит дел од оној на електронот.
1 Историја
1.1 Паулиев предлог
Неутриното било откриено и претставено за првпат од страна на Wolfgang Pauli во 1930 со цел да објасни како бета распаѓањето може да заштеди енергија, моментум, и аголен моментум (обрт). Во контраст на Niels Bohr,кој предложил статистичка верзија на законите за конзервација за да го објасни настанот, Pauli поставил хипотеза во врска со сè уште не одреден елемент кој тој го нарекол “неутрон”, и бил дел од конвенцијата во 1930 која ги дала имињата на протонот и електронот, кои во 1930 биле познати производи за алфа и бета распаѓањето !—што знеле за алфа и бета распаѓањето? Производот не е протон,туку е хелиум јадро--> Toj сметал дека новата честичка била емитирана од јадрото заедно со електронот или бета-честичката во процесот на бета распаѓањето.
James Chadwick открил многу помасивна јадрена честичка во 1932 и ја нарекол неутрон, оставајќи два типа на честички со исто име.Pauli го користел терминот "неутрон" и за честичка која зачувува енергија во бета распаѓање,и за неутрален елемент во јадрото на атомот. Зборот неутрино влегол во меѓународниот вокабулат преку Енрико Ферми, кој го искористил зборот за време на конференција во Париз во јули 1932.
Во Фермијовата теорија за бета распаѓањето,Чадвиковиот голем неутрален елемент може да распадне во протон,електрон,и во помалите неутрални елементи(наречени како електрон антинеутрино):
: → + +
Фермиовите лист,напишан во 1934,го унифицирал Паулиовиот неутрино со Paul Dirac позитрон и Werner Heisenberg неутрон-протон модел и дал солдина база за понатамошната експериментална работа. Сепак,весникот,the journal Природа го одбил Фермовиот лист,кажувајќи дека теоријата е премногу далеку од реалноста.Тој го принел листот до италијански весник,кој го прифатил,но општата незаинтересиранот околу неговата теорија во почетокот резултирала со неговата промена во експерименталнта физика.
Како и да е,дури и во 1934 постоеле сомнежи за Бохрсовата идеја-дека законите за зачувување на енергија не се прикладни-не се точни.На Солвејновата конференција во 1934, првите знаци на енергија во бета распаѓањето биле забележани,и овие спектри наметнале строгиот лимит врз електроните во секој тип на бета распаѓање.Таков лимит не бив очекуван ако законот за зачувување на енергија не бил наметнат за да биде прифатен. in 1934 was that only a limited (and conserved) amount of energy was available, and a new particle was sometimes taking a varying fraction of this limited energy, leaving the rest for the beta particle. Pauli made use of the occasion to publicly emphasize that the still-undetected "neutrino" must be an actual particle.
1.2 Директна детекција
Во 1942, Wang Ganchang прв предложил да се користи бета факање со цел експериментално да се откријат неутрината to. Во20 July 1956 проблемот на ''Наука'', Clyde Cowan, Frederick Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire издале потврда дека пронашле неутрино, a result that was rewarded almost forty years later with the 1995 Nobel Prize.
In this experiment, now known as the Cowan–Reines neutrino experiment, antineutrinos created in a nuclear reactor by beta decay reacted with protons to produce neutrons and positrons:
: + → +
Позитронот брзо наоѓа елекрон, и тие меѓусебно се поништуваат. Неутронот може да биде забележан ако се фати на соодветно јадро, ослободувајќи гама-зраци. Коинциденцијата на овие два евенти-позитрон анхилација и факањето на неутрон-дава уникатна значка на антинеутрино интеракција.
1.3 Неутрински вкус
Антинеутриното e откриено од страна на Кован and Рејнес и е античестичка на електронското неутрино. Во 1962, Лион Ледерман, Мелвин Шварц и Џек Стајнбергер покажале дека постојат повеќе видови на неутрина,но првично ги откриле заемодејствата на мионските неутрина (чије хипотетичко име било''неутретто''), за што ја добиле Нобеловата награда за Физика. Кога бил откриен третиот вид на лептон,тау-неутрино,откриена честичка во 1975 со Стенфордскиот линиски забрзувач на честичка,за која исто така се очекувало да биде поврзан со неутрино (тау-неутрино).Првите докази за овој трет вид на неутрино се појавиле при набљудување на исчезнатата енергија и импулс кај тау-распаѓањата кои наликуваат на бета распаѓањето,што довело до откривањето на електронското неутрино.Првите заменодејства на тау-неутриното биле јавно објавени во 2000 од страна на DONUT здружение во Фермилаб,а постоењето на оваа честичка било докажано теоретски и експериментално со податоците од Големиот електронско позитронски судирач.
1.4 Сончев неутрински проблем
При крајот на 60-тите години од минатиот век,биле изведени неколку последователни експерименти од кои се забележало дека бројот на неутрина кои пристугнуваат од Сонцето е помеѓу една третина и една половина од бројот кој бил предвиден за електронското неутрино од Стандардниот сончев модел.Овој спор,кој стана познат како сончевиот неутрински проблем,остана нерешен околу 30 години.Проблемот е разрешен со откривањето на неутринските колебања и маса
1.5 Колебања
Практичен метод за испитување на неутринските колебања првично бил предложен од Бруно Понтекорво во 1957 користејќи ѓи сличните каонски колебања,во последователен период од 10 години тој го развил математичкиот формализам и современиот запис на вакуумските колебања. Во 1985 г. Станислав Михеев и Алексеј Смирнов (надоврзувајќи се на работата на Линколн Волфенштајн) забележале дека вкусовите колебања можат да бидат изменети кога неутрината се движат низ материјат.Овој ефект е наречен Михеев-Смирнов-Волфштајнов ефект (МСВ-ефект) и е од важност да се разбере дека многу неутрина оддадени од многуте соединувања во Сонцето минуваат низ густата материја во сончевото јадро на нивниот пат кон детекторите на Земјата.
Кон почетокот на 1998 година,експериментите започнале да покажуваат дека сончевите и атмосферските неутрина ги менуваат своите вкусови (погледајте Суперкамиоканде и Сабдурска неутринска опсерваторија).Ова го разлешило проблемот на сончевото неутрино:електронските неутрина кои се создадени во Сонцето делумно се менуваат во друг вкус кој не може да биде забележан на инструментите на експереминтите.
Иако поединечните експерименти,како што е збирот на сончево неутрински експерименти се во согласност со не колебните механизми на запазувањето на неутринскиот вкус,земајќи ги предвид сите заедно,неутринските експериментите го покажуваат постоењето на неутринските колебања. Особено важни во оваа насока се експериментите во реактарот КамЛАНД и забржувачките експерименти,како на пример МИНОС.Експериментот КамЛАНД навистина ги забележал колебањата при механизмот на промената на неутринскиот вкус кај сончевото електронско неутирно.Слично и МИНОС потврдува дека колебањето на атмосферските неутрина обезбедува подобро определување на квадратното разделување. Јапонскиот научник Такааки Каџита и Канадскиот научник Артур Мекдоналд ја доибваат Нобеловата награда за физика во 2015 година токму за ова значајно откритие како теориско и експериментално неутрината го менуваат својот вкус.
1.6 Супернова-неутрина
Рејмонд Девис и Масатоши Кошиба биле наградени со Нобеловата награда во физика.Давис за неговата работа на Космичкото неутрино и Кошиба за неговата прва вистинска обзервација на супернова неутрината.Откривањето на сончевото неутрино,и на неутрината откриени на СН 1987A супернова го дале почетокот на неутринската астрономија.
2 Својства и реакции
Неутрината имаат полуброен спинот и од таму следи дека е фермион. Неутрината заемно дејствуваат примарно преку слаба сила.Откривањето на неутринското колебање покажува дека неутрината имаат маса.ПОстоењето на неутринската маса покажува за постоењето на ситно неутрино магнетниот момент од ред на ,дозволувајќи за електромагнетни заемодејства меѓу неутрината.Експеримент изведен од Ц. С. ВУ во Колумбијскиот универзитет покажал дека неутрината секогаш покажуваат лева хиралност. Многу е тешко да се идентификуваат поединечните заемодејства помеѓу неутрината поради природната радиација.Поради ова,во раните експерименти се користел специјален канал за оваа идентификација:интеракцијата на антинеутриното со водородното јадро кај молекулите на вода.Јадрото на водород е само еден протон и поради тоа јадрени реакции кои би се одвивале кај потешки елементи не мора да се земат предвид при ваквите реакции.Во еден кубен метар вода се случуваат само неколку реакции кога водата е поставена надвор од јадрен реактор,но со тоа се мери рекацијата на плутониумот во реакторот.
2.1 Михеев–Смирнов–Волфенштајнов ефект
Неутрина кои минуваат низ материја,поминуваат низ процес сличен на движењето на светлината низ проѕирен материјал.Овој процес не може да се набљудува директно,затоа што нема јонизирачко зрачење,но се јавува МСВ ефектот.Само мал дел од енергијата од неутриното се пренесува на материјалот.
2.2 Јадрени заемодејства
Неутрината може да дејстуваат на атомско јадро и да го променат во друго јадро.Овој процес се користи кај радиохемиски детектори за неутрина.Во овој случај енергетските нивоа и спинските состојби на целното јадро мора да се земат предвид за да се пресмета веројатноста на интеракцијата. Претежно, веројатноста на реакцијата се зголемува со зголемувањето на бројот на неутрони и протони во јадро.
2.3 Поттикнато цепење
Very much like neutrons do in nuclear reactors, neutrinos can induce fission reactions within heavy nuclei. So far, this reaction has not been measured in a laboratory, but is predicted to happen within stars and supernovae. The process affects the abundance of isotopes seen in the universe. Neutrino fission of deuterium nuclei has been observed in the Sudbury Neutrino Observatory, which uses a heavy water detector.
2.4 Без самозаемодејство
Observations of the cosmic microwave background suggest that neutrinos do not interact with themselves.
2.5 Видови
+Неутрината во стандардниот модел
Фермион, Симбол
Семејство 1
Електронско неутрино
Електронско антинеутрино
Семејство 2
Мионско неутрино
Мионско антинеутрино
Семејство 3
Тау-неутрино
Тау-антинеутрино
There are three known types (''flavors'') of neutrinos: electron neutrino , muon neutrino and tau neutrino , named after their partner leptons in the Standard Model (see table at right). The current best measurement of the number of neutrino types comes from observing the decay of the Z boson. This particle can decay into any light neutrino and its antineutrino, and the more types of light neutrinos available, the shorter the lifetime of the Z boson. Measurements of the Z lifetime have shown that the number of light neutrino types is 3. The correspondence between the six quarks in the Standard Model and the six leptons, among them the three neutrinos, suggests to physicists' intuition that there should be exactly three types of neutrino. However, actual proof that there are only three kinds of neutrinos remains an elusive goal of particle physics.
The possibility of ''sterile'' neutrinos—relatively light neutrinos which do not participate in the weak interaction but which could be created through flavor oscillation (see below)—is unaffected by these Z-boson-based measurements, and the existence of such particles is in fact hinted by experimental data from the LSND experiment. However, the currently running MiniBooNE experiment suggested, until recently, that sterile neutrinos are not required to explain the experimental data, although the latest research into this area is on-going and anomalies in the MiniBooNE data may allow for exotic neutrino types, including sterile neutrinos.
A recent re-analysis of reference electron spectra data from the Institut Laue-Langevin has also hinted at a fourth, sterile neutrino.
Recently analyzed data from the Wilkinson Microwave Anisotropy Probe of the cosmic background radiation is compatible with either three or four types of neutrinos. It is hoped that the addition of two more years of data from the probe will resolve this uncertainty.
2.6 Антинеутрина
Antineutrinos, the antiparticles of neutrinos, are neutral particles produced in nuclear beta decay. These are emitted during beta particle emissions, in which a neutron decays into a proton, electron, and antineutrino. They have a spin of ½, and are part of the lepton family of particles. All antineutrinos observed thus far possess right-handed helicity (i.e. only one of the two possible spin states has ever been seen), while neutrinos are left-handed. Antineutrinos, like neutrinos, interact with other matter only through the gravitational and weak forces, making them very difficult to detect experimentally. Neutrino oscillation experiments indicate that antineutrinos have mass, but beta decay experiments constrain that mass to be very small. A neutrino–antineutrino interaction has been suggested in attempts to form a composite photon with the neutrino theory of light.
Because antineutrinos and neutrinos are neutral particles, it is possible that they are the same particle. Particles that have this property are known as Majorana particles. Majorana neutrinos have the property that the neutrino and antineutrino could be distinguished only by chirality; what experiments observe as a difference between the neutrino and antineutrino could simply be due to one particle with two possible chiralities. If neutrinos are indeed Majorana particles, neutrinoless double beta decay, as well as a range of other lepton number violating phenomena, would be allowed. Several experiments have been and are being conducted to search for this process.
Researchers around the world have begun to investigate the possibility of using antineutrinos for reactor monitoring in the context of preventing the proliferation of nuclear weapons.
Antineutrinos were first detected as a result of their interaction with protons in a large tank of water. This was installed next to a nuclear reactor as a controllable source of the antineutrinos. (See: Cowan–Reines neutrino experiment)
Only antineutrinos, not neutrinos, take part in the Glashow resonance.
2.7 Вкусови колебања
Neutrinos are most often created or detected with a well defined flavor (electron, muon, tau). However, in a phenomenon known as neutrino flavor oscillation, neutrinos are able to oscillate among the three available flavors while they propagate through space. Specifically, this occurs because the neutrino flavor eigenstates are not the same as the neutrino mass eigenstates (simply called 1, 2, 3). This allows for a neutrino that was produced as an electron neutrino at a given location to have a calculable probability to be detected as either a muon or tau neutrino after it has traveled to another location. This quantum mechanical effect was first hinted by the discrepancy between the number of electron neutrinos detected from the Sun's core failing to match the expected numbers, dubbed as the "solar neutrino problem". In the Standard Model the existence of flavor oscillations implies nonzero differences between the neutrino masses, because the amount of mixing between neutrino flavors at a given time depends on the differences between their squared masses. There are other possibilities in which neutrino can oscillate even if they are massless. If Lorentz symmetry is not an exact symmetry, neutrinos can experience Lorentz-violating oscillations.
It is possible that the neutrino and antineutrino are in fact the same particle, a hypothesis first proposed by the Italian physicist Ettore Majorana. The neutrino could transform into an antineutrino (and vice versa) by flipping the orientation of its spin state.
This change in spin would require the neutrino and antineutrino to have nonzero mass, and therefore travel slower than light, because such a spin flip, caused only by a change in point of view, can take place only if inertial frames of reference exist that move faster than the particle: such a particle has a spin of one orientation when seen from a frame which moves slower than the particle, but the opposite spin when observed from a frame that moves faster than the particle.
On July 19, 2013 the results from the T2K experiment presented at the European Physical Society Conference on High Energy Physics in Stockholm, Sweden, confirmed neutrino oscillation theory.
2.8 Брзина
Before neutrinos were found to oscillate, they were generally assumed to be massless, propagating at the speed of light. According to the theory of special relativity, the question of neutrino velocity is closely related to their mass. If neutrinos are massless, they must travel at the speed of light. However, if they have mass, they cannot reach the speed of light.
Also some Lorentz-violating variants of quantum gravity might allow faster-than-light neutrinos. A comprehensive framework for Lorentz violations is the Standard-Model Extension (SME).
In the early 1980s, first measurements of neutrino speed were done using pulsed pion beams (produced by pulsed proton beams hitting a target). The pions decayed producing neutrinos, and the neutrino interactions observed within a time window in a detector at a distance were consistent with the speed of light. This measurement was repeated in 2007 using the MINOS detectors, which found the speed of neutrinos to be, at the 99% confidence level, in the range between and . The central value of 1.000051''c'' is higher than the speed of light but is also consistent with a velocity of exactly ''c'' or even slightly less. This measurement set an upper bound on the mass of the muon neutrino of at 99% confidence. After the detectors for the project were upgraded in 2012, MINOS refined their initial result and found agreement with the speed of light, with the difference in the arrival time of neutrinos and light of -0.0006% (±0.0012%).
A similar observation was made, on a much larger scale, with supernova 1987A (SN 1987A). 10-MeV antineutrinos from the supernova were detected within a time window that was consistent with the speed of light for the neutrinos. Currently, the question of whether or not neutrinos have mass cannot be decided; their speed is (as yet) indistinguishable from the speed of light.
In September 2011, the OPERA collaboration released calculations showing velocities of 17-GeV and 28-GeV neutrinos exceeding the speed of light in their experiments (see Faster-than-light neutrino anomaly). In November 2011, OPERA repeated its experiment with changes so that the speed could be determined individually for each detected neutrino. The results showed the same faster-than-light speed. However, in February 2012, reports came out that the results may have been caused by a loose fiber optic cable attached to one of the atomic clocks which measured the departure and arrival times of the neutrinos. An independent recreation of the experiment in the same laboratory by ICARUS found no discernible difference between the speed of a neutrino and the speed of light.
In June 2012, CERN announced that new measurements conducted by all four Gran Sasso experiments (OPERA, ICARUS, Borexino and LVD) found agreement between the speed of light and the speed of neutrinos, finally refuting the initial OPERA claim.
2.9 Маса
The Standard Model of particle physics assumed that neutrinos are massless. However the experimentally established phenomenon of neutrino oscillation, which mixes neutrino flavour states with neutrino mass states (analogously to CKM mixing), requires neutrinos to have nonzero masses. Massive neutrinos were originally conceived by Bruno Pontecorvo in the 1950s. Enhancing the basic framework to accommodate their mass is straightforward by adding a right-handed Lagrangian. This can be done in two ways. If, like other fundamental Standard Model particles, mass is generated by the Dirac mechanism, then the framework would require an SU(2) singlet. This particle would have no other Standard Model interactions (apart from the Yukawa interactions with the neutral component of the Higgs doublet), so is called a sterile neutrino. Or, mass can be generated by the Majorana mechanism, which would require the neutrino and antineutrino to be the same particle.
The strongest upper limit on the masses of neutrinos comes from cosmology: the Big Bang model predicts that there is a fixed ratio between the number of neutrinos and the number of photons in the cosmic microwave background. If the total energy of all three types of neutrinos exceeded an average of per neutrino, there would be so much mass in the universe that it would collapse. This limit can be circumvented by assuming that the neutrino is unstable; however, there are limits within the Standard Model that make this difficult. A much more stringent constraint comes from a careful analysis of cosmological data, such as the cosmic microwave background radiation, galaxy surveys, and the Lyman-alpha forest. These indicate that the summed masses of the three neutrinos must be less than .
In 1998, research results at the Super-Kamiokande neutrino detector determined that neutrinos can oscillate from one flavor to another, which requires that they must have a nonzero mass. While this shows that neutrinos have mass, the absolute neutrino mass scale is still not known. This is because neutrino oscillations are sensitive only to the difference in the squares of the masses. The best estimate of the difference in the squares of the masses of mass eigenstates 1 and 2 was published by KamLAND in 2005: Δ''m'' = . In 2006, the MINOS experiment measured oscillations from an intense muon neutrino beam, determining the difference in the squares of the masses between neutrino mass eigenstates 2 and 3. The initial results indicate |Δ''m''| = , consistent with previous results from Super-Kamiokande. Since |Δ''m''| is the difference of two squared masses, at least one of them has to have a value which is at least the square root of this value. Thus, there exists at least one neutrino mass eigenstate with a mass of at least .
In 2009, lensing data of a galaxy cluster were analyzed to predict a neutrino mass of about . This surprisingly high value requires that the three neutrino masses be nearly equal, with neutrino oscillations on the order of meV. The masses lie below the Mainz-Troitsk upper bound of for the electron antineutrino. The latter will be tested in 2015 in the KATRIN experiment, that searches for a mass between and .
A number of efforts are under way to directly determine the absolute neutrino mass scale in laboratory experiments. The methods applied involve nuclear beta decay (KATRIN and MARE).
On 31 May 2010, OPERA researchers observed the first tau neutrino candidate event in a muon neutrino beam, the first time this transformation in neutrinos had been observed, providing further evidence that they have mass.
In July 2010 the 3-D MegaZ DR7 galaxy survey reported that they had measured a limit of the combined mass of the three neutrino varieties to be less than . A tighter upper bound yet for this sum of masses, , was reported in March 2013 by the Planck collaboration, whereas a February 2014 result estimates the sum as 0.320 ± 0.081 eV based on discrepancies between the cosmological consequences implied by Planck's detailed measurements of the Cosmic Microwave Background and predictions arising from observing other phenomena, combined with the assumption that neutrinos are responsible for the observed weaker gravitational lensing than would be expected from massless neutrinos.
If the neutrino is a Majorana particle, the mass may be calculated by finding the half life of neutrinoless double-beta decay of certain nuclei. As of 2015, the lowest upper limit on the Majorana mass of the neutrino has been set by KamLAND-Zen: 0.12–0.25 eV.
The Nobel prize in Physics 2015 was awarded to both Takaaki Kajita and Arthur B. McDonald for their experimental discovery of neutrino oscillations, which demonstrates that neutrinos have mass.
2.10 Големина
Standard Model neutrinos are fundamental point-like particles. An effective size can be defined using their electroweak cross section (apparent size in electroweak interaction). The average electroweak characteristic size is ''r''<sup>2</sup> = ''n'' × 10<sup>−33</sup> см<sup>2</sup> (''n'' × 1 nanobarn), where ''n'' = 3.2 for electron neutrino, ''n'' = 1.7 for muon neutrino and ''n'' = 1.0 for tau neutrino; it depends on no other properties than mass. However, this is best understood as being relevant only to probability of scattering. Since the neutrino does not interact electromagnetically, and is defined quantum mechanically by a wavefunction, it does not have a size in the same sense as everyday objects. Furthermore, processes that produce neutrinos impart such high energies to them that they travel at almost the speed of light. Nevertheless, neutrinos are fermions, and thus obey the Pauli exclusion principle, i.e. that increasing their density forces them into progressively higher momentum states.
2.11 Хиралност
Experimental results show that (nearly) all produced and observed neutrinos have left-handed helicities (spins antiparallel to momenta), and all antineutrinos have right-handed helicities, within the margin of error. In the massless limit, it means that only one of two possible chiralities is observed for either particle. These are the only chiralities included in the Standard Model of particle interactions.
It is possible that their counterparts (right-handed neutrinos and left-handed antineutrinos) simply do not exist. If they do, their properties are substantially different from observable neutrinos and antineutrinos. It is theorized that they are either very heavy (on the order of GUT scale—see ''Seesaw mechanism''), do not participate in weak interaction (so-called sterile neutrinos), or both.
The existence of nonzero neutrino masses somewhat complicates the situation. Neutrinos are produced in weak interactions as chirality eigenstates. However, chirality of a massive particle is not a constant of motion; helicity is, but the chirality operator does not share eigenstates with the helicity operator. Free neutrinos propagate as mixtures of left- and right-handed helicity states, with mixing amplitudes on the order of ''m''<sub>ν</sub>/''E''. This does not significantly affect the experiments, because neutrinos involved are nearly always ultrarelativistic, and thus mixing amplitudes are vanishingly small. For example, most solar neutrinos have energies on the order of –, so the fraction of neutrinos with "wrong" helicity among them cannot exceed .
2.12 Вештачки
2.12.1 Реакторски неутрина
Nuclear reactors are the major source of human-generated neutrinos. Antineutrinos are made in the beta-decay of neutron-rich daughter fragments in the fission process. Generally, the four main isotopes contributing to the antineutrino flux are , , and (i.e. via the antineutrinos emitted during beta-minus decay of their respective fission fragments). The average nuclear fission releases about of energy, of which roughly 4.5% (or about ) is radiated away as antineutrinos. For a typical nuclear reactor with a thermal power of , meaning that the core produces this much heat, and an electrical power generation of , the total power production from fissioning atoms is actually , of which is radiated away as antineutrino radiation and never appears in the engineering. This is to say, of fission energy is ''lost'' from this reactor and does not appear as heat available to run turbines, since antineutrinos penetrate all building materials practically without interaction.
The antineutrino energy spectrum depends on the degree to which the fuel is burned (plutonium-239 fission antineutrinos on average have slightly more energy than those from uranium-235 fission), but in general, the ''detectable'' antineutrinos from fission have a peak energy between about 3.5 and , with a maximum energy of about . There is no established experimental method to measure the flux of low-energy antineutrinos. Only antineutrinos with an energy above threshold of can be uniquely identified (see ''neutrino detection'' below). An estimated 3% of all antineutrinos from a nuclear reactor carry an energy above this threshold. Thus, an average nuclear power plant may generate over antineutrinos per second above this threshold, but also a much larger number (97%/3% = ~30 times this number) below the energy threshold, which cannot be seen with present detector technology.
2.12.2 Забрзувачки неутрина
Some particle accelerators have been used to make neutrino beams. The technique is to collide protons with a fixed target, producing charged pions or kaons. These unstable particles are then magnetically focused into a long tunnel where they decay while in flight. Because of the relativistic boost of the decaying particle, the neutrinos are produced as a beam rather than isotropically. Efforts to construct an accelerator facility where neutrinos are produced through muon decays are ongoing. Such a setup is generally known as a neutrino factory.
2.12.3 Јадрени бомби
Nuclear bombs also produce very large quantities of neutrinos. Fred Reines and Clyde Cowan considered the detection of neutrinos from a bomb prior to their search for reactor neutrinos; a fission reactor was recommended as a better alternative by Los Alamos physics division leader J.M.B. Kellogg. Fission bombs produce antineutrinos (from the fission process), and fusion bombs produce both neutrinos (from the fusion process) and antineutrinos (from the initiating fission explosion).
2.13 Геолошки
Neutrinos are part of the natural background radiation. In particular, the decay chains of and isotopes, as well as, include beta decays which emit antineutrinos. These so-called geoneutrinos can provide valuable information on the Earth's interior. A first indication for geoneutrinos was found by the KamLAND experiment in 2005. KamLAND's main background in the geoneutrino measurement are the antineutrinos coming from reactors. Several future experiments aim at improving the geoneutrino measurement and these will necessarily have to be far away from reactors.
2.14 Атмосферски
Atmospheric neutrinos result from the interaction of cosmic rays with atomic nuclei in the Earth's atmosphere, creating showers of particles, many of which are unstable and produce neutrinos when they decay. A collaboration of particle physicists from Tata Institute of Fundamental Research (India), Osaka City University (Japan) and Durham University (UK) recorded the first cosmic ray neutrino interaction in an underground laboratory in Kolar Gold Fields in India in 1965.
2.15 Сончеви
Solar neutrinos originate from the nuclear fusion powering the Sun and other stars.
The details of the operation of the Sun are explained by the Standard Solar Model. In short: when four protons fuse to become one helium nucleus, two of them have to convert into neutrons, and each such conversion releases one electron neutrino.
The Sun sends enormous numbers of neutrinos in all directions. Each second, about 65 billion solar neutrinos pass through every square centimeter on the part of the Earth that faces the Sun. Since neutrinos are insignificantly absorbed by the mass of the Earth, the surface area on the side of the Earth opposite the Sun receives about the same number of neutrinos as the side facing the Sun.
2.16 Супернови
In 1966 Colgate and White
calculated that neutrinos carry away most of the gravitational energy released by the collapse of massive stars, events now categorized as Type Ib and Ic and Type II supernovae. When such stars collapse, matter densities at the core become so high that the degeneracy of electrons is not enough to prevent protons and electrons from combining to form a neutron and an electron neutrino. A second and more important neutrino source is the thermal energy (100 billion kelvins) of the newly formed neutron core, which is dissipated via the formation of neutrino–antineutrino pairs of all flavors.
Colgate and White's theory of supernova neutrino production was confirmed in 1987, when neutrinos from supernova 1987A were detected. The water-based detectors Kamiokande II and IMB detected 11 and 8 antineutrinos of thermal origin, respectively, while the scintillator-based Baksan detector found 5 neutrinos (lepton number = 1) of either thermal or electron-capture origin, in a burst lasting less than 13 seconds. The neutrino signal from the supernova arrived at earth several hours before the arrival of the first electromagnetic radiation, as expected from the evident fact that the latter emerges along with the shock wave. The exceptionally feeble interaction with normal matter allowed the neutrinos to pass through the churning mass of the exploding star, while the electromagnetic photons were slowed.
Because neutrinos interact so little with matter, it is thought that a supernova's neutrino emissions carry information about the innermost regions of the explosion. Much of the ''visible'' light comes from the decay of radioactive elements produced by the supernova shock wave, and even light from the explosion itself is scattered by dense and turbulent gases, and thus delayed. The neutrino burst is expected to reach Earth before any electromagnetic waves, including visible light, gamma rays or radio waves. The exact time delay depends on the velocity of the shock wave and on the thickness of the outer layer of the star. For a Type II supernova, astronomers expect the neutrino flood to be released seconds after the stellar core collapse, while the first electromagnetic signal may emerge hours later, after the explosion shock wave has had time to reach the surface of the star. The SNEWS project uses a network of neutrino detectors to monitor the sky for candidate supernova events; the neutrino signal will provide a useful advance warning of a star exploding in the Milky Way.
Although neutrinos pass through the outer gases of a supernova without scattering, they provide information about the deeper supernova core with evidence that here, even neutrinos scatter to a significant extent. In a supernova core the densities are those of a neutron star (which is expected to be formed in this type of supernova), becoming large enough to influence the duration of the neutrino signal by delaying some neutrinos. The length of the neutrino signal from SN 1987A, some 13 seconds, was far longer than it would take in theory for neutrinos to pass directly through the neutrino-generating core of a supernova, expected to be only 32 kilometers in diameter SN 1987A. The number of neutrinos counted was also consistent with a total neutrino energy of 2.2 x 10<sup>46</sup> joules, which was estimated to be nearly all of the total energy of the supernova.
2.17 Остатоци од супернови
The energy of supernova neutrinos ranges from a few to several tens of MeV. However, the sites where cosmic rays are accelerated are expected to produce neutrinos that are at least one million times more energetic, produced from turbulent gaseous environments left over by supernova explosions: the supernova remnants. The origin of the cosmic rays was attributed to supernovas by Walter Baade and Fritz Zwicky; this hypothesis was refined by Vitaly L. Ginzburg and Sergei I. Syrovatsky who attributed the origin to supernova remnants, and supported their claim by the crucial remark, that the cosmic ray losses of the Milky Way is compensated, if the efficiency of acceleration in supernova remnants is about 10 percent. Ginzburg and Syrovatskii's hypothesis is supported by the specific mechanism of "shock wave acceleration" happening in supernova remnants, which is consistent with the original theoretical picture drawn by Enrico Fermi, and is receiving support from observational data. The very-high-energy neutrinos are still to be seen, but this branch of neutrino astronomy is just in its infancy. The main existing or forthcoming experiments that aim at observing very-high-energy neutrinos from our galaxy are Baikal, AMANDA, IceCube, ANTARES, NEMO and Nestor. Related information is provided by very-high-energy gamma ray observatories, such as VERITAS, HESS and MAGIC. Indeed, the collisions of cosmic rays are supposed to produce charged pions, whose decay give the neutrinos, and also neutral pions, whose decay give gamma rays: the environment of a supernova remnant is transparent to both types of radiation.
Still-higher-energy neutrinos, resulting from the interactions of extragalactic cosmic rays, could be observed with the Pierre Auger Observatory or with the dedicated experiment named ANITA.
2.18 Големата експлозија
It is thought that, just like the cosmic microwave background radiation left over from the Big Bang, there is a background of low-energy neutrinos in our Universe. In the 1980s it was proposed that these may be the explanation for the dark matter thought to exist in the universe. Neutrinos have one important advantage over most other dark matter candidates: it is known that they exist. However, this idea also has serious problems.
From particle experiments, it is known that neutrinos are very light. This means that they easily move at speeds close to the speed of light. For this reason, dark matter made from neutrinos is termed "hot dark matter". The problem is that being fast moving, the neutrinos would tend to have spread out evenly in the universe before cosmological expansion made them cold enough to congregate in clumps. This would cause the part of dark matter made of neutrinos to be smeared out and unable to cause the large galactic structures that we see.
Further, these same galaxies and groups of galaxies appear to be surrounded by dark matter that is not fast enough to escape from those galaxies. Presumably this matter provided the gravitational nucleus for formation. This implies that neutrinos cannot make up a significant part of the total amount of dark matter.
From cosmological arguments, relic background neutrinos are estimated to have density of 56 of each type per cubic centimeter and temperature if they are massless, much colder if their mass exceeds . Although their density is quite high, they have not yet been observed in the laboratory, as their energy is below thresholds of most detection methods, and due to extremely low neutrino interaction cross-sections at sub-eV energies. In contrast, boron-8 solar neutrinos—which are emitted with a higher energy—have been detected definitively despite having a space density that is lower than that of relic neutrinos by some 6 orders of magnitude.
3 Откривање
Neutrinos cannot be detected directly, because they do not ionize the materials they are passing through (they do not carry electric charge and other proposed effects, like the MSW effect, do not produce traceable radiation). A unique reaction to identify antineutrinos, sometimes referred to as inverse beta decay, as applied by Reines and Cowan (see below), requires a very large detector in order to detect a significant number of neutrinos. All detection methods require the neutrinos to carry a minimum threshold energy. So far, there is no detection method for low-energy neutrinos, in the sense that potential neutrino interactions (for example by the MSW effect) cannot be uniquely distinguished from other causes. Neutrino detectors are often built underground in order to isolate the detector from cosmic rays and other background radiation.
Antineutrinos were first detected in the 1950s near a nuclear reactor. Reines and Cowan used two targets containing a solution of cadmium chloride in water. Two scintillation detectors were placed next to the cadmium targets. Antineutrinos with an energy above the threshold of caused charged current interactions with the protons in the water, producing positrons and neutrons. This is very much like decay, where energy is used to convert a proton into a neutron, a positron and an electron neutrino is emitted:
From known decay:
: Energy + → + +
In the Cowan and Reines experiment, instead of an outgoing neutrino, you have an incoming antineutrino from a nuclear reactor:
: Energy (>) + + → +
The resulting positron annihilation with electrons in the detector material created photons with an energy of about . Pairs of photons in coincidence could be detected by the two scintillation detectors above and below the target. The neutrons were captured by cadmium nuclei resulting in gamma rays of about that were detected a few microseconds after the photons from a positron annihilation event.
Since then, various detection methods have been used. Super Kamiokande is a large volume of water surrounded by photomultiplier tubes that watch for the Cherenkov radiation emitted when an incoming neutrino creates an electron or muon in the water. The Sudbury Neutrino Observatory is similar, but uses heavy water as the detecting medium, which uses the same effects, but also allows the additional reaction any-flavor neutrino photo-dissociation of deuterium, resulting in a free neutron which is then detected from gamma radiation after chlorine-capture. Other detectors have consisted of large volumes of chlorine or gallium which are periodically checked for excesses of argon or germanium, respectively, which are created by electron-neutrinos interacting with the original substance. MINOS uses a solid plastic scintillator coupled to photomultiplier tubes, while Borexino uses a liquid pseudocumene scintillator also watched by photomultiplier tubes and the proposed NOνA detector will use liquid scintillator watched by avalanche photodiodes. The IceCube Neutrino Observatory uses of the Antarctic ice sheet near the south pole with photomultiplier tubes distributed throughout the volume.
4 Мотивација за научен интерес
Neutrinos' low mass and neutral charge mean they interact exceedingly weakly with other particles and fields. This feature of weak interaction interests scientists because it means neutrinos can be used to probe environments that other radiation (such as light or radio waves) cannot penetrate.
Using neutrinos as a probe was first proposed in the mid-20th century as a way to detect conditions at the core of the Sun. The solar core cannot be imaged directly because electromagnetic radiation (such as light) is diffused by the great amount and density of matter surrounding the core. On the other hand, neutrinos pass through the Sun with few interactions. Whereas photons emitted from the solar core may require 40,000 years to diffuse to the outer layers of the Sun, neutrinos generated in stellar fusion reactions at the core cross this distance practically unimpeded at nearly the speed of light.
Neutrinos are also useful for probing astrophysical sources beyond the Solar System because they are the only known particles that are not significantly attenuated by their travel through the interstellar medium. Optical photons can be obscured or diffused by dust, gas, and background radiation. High-energy cosmic rays, in the form of swift protons and atomic nuclei, are unable to travel more than about 100 megaparsecs due to the Greisen–Zatsepin–Kuzmin limit (GZK cutoff). Neutrinos, in contrast, can travel even greater distances barely attenuated.
The galactic core of the Milky Way is fully obscured by dense gas and numerous bright objects. Neutrinos produced in the galactic core might be measurable by Earth-based neutrino telescopes.
Another important use of the neutrino is in the observation of supernovae, the explosions that end the lives of highly massive stars. The core collapse phase of a supernova is an extremely dense and energetic event. It is so dense that no known particles are able to escape the advancing core front except for neutrinos. Consequently, supernovae are known to release approximately 99% of their radiant energy in a short (10-second) burst of neutrinos. These neutrinos are a very useful probe for core collapse studies.
The rest mass of the neutrino is an important test of cosmological and astrophysical theories (see ''Dark matter''). The neutrino's significance in probing cosmological phenomena is as great as any other method, and is thus a major focus of study in astrophysical communities.
The study of neutrinos is important in particle physics because neutrinos typically have the lowest mass, and hence are examples of the lowest-energy particles theorized in extensions of the Standard Model of particle physics.
In November 2012 American scientists used a particle accelerator to send a coherent neutrino message through 780 feet of rock. This marks the first use of neutrinos for communication, and future research may permit binary neutrino messages to be sent immense distances through even the densest materials, such as the Earth's core.
5 Поврзано
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