A brief history of neutrino. From neutrinos to cosmic sources, DK&ER

Transcription

1 A brief history of neutrino

2 Two body decay m 1 M m 2 Energy-momentum conservation => Energy of the decay products always the same

3 : Puzzle of decay Continuous spectrum of particles Energy is not conserved?? Momentum is not conserved??

4 Dec 1930: A Desperate Remedy A A e I have done something very bad today by proposing a particle that cannot be detected; it is something no theorist should ever do. W.Pauli

5 Sir Arthur Eddington: In an ordinary way I might say that I do not believe in neutrinos. Dare I say that experimental physicists will not have sufficient ingenuity to make neutrinos.

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7 How to catch a neutrino? H. Bethe: If one observes: n > p e - ν then how about an inverse beta decay: ν p > n e +? Probability of a reaction (for one neutrino) = cross section x number of targets/ area Bethe calculated: cross section= cm 2 One needs either cm of water to absorb a neutrino, or a lot of neutrinos.

8 Reines and Cowan: a Proposal (1953) Liquid scintillator Water, Cadmium chloride Liquid scintillator γ-rays produced Compton electrons, which led to scintillation light detected by photomultipliers. A signal was selected by a coincidence of prompt light from positrons and delayed light (by 15 µsec) from the neutron absorption by a cadmium nucleus.

9 Reines i Cowan: Discovery of neutrino Reactor in Savannah River as a source of neutrinos from neutron-rich nuclei. Detector: 12 m underground: scyntil scyntil water water scyntil In 1956 a telegram to Pauli: We are happy to inform you that we have definitely detected neutrinos Nobel Prize for Reines

10 But weak interactions bring a new mystery: Left- right asymmetry T. D. Lee and C. N. Yang, Phys. Rev. 104, C. S. Wu et al., Phys. Rev. 105, Richard L. Garwin et al., Phys. Rev. 105,

11 Parity conservation Let s consider: and assume that cobalt nuclei are polarized (by magnetic field at low temperatures) If all laws of physics are symmetrical under parity transformation then: parity transformation reverses vectors but not axial vectors So IF parity is conserved then probab to emit electrons forward and backward should be equal

12 Left-right asymmetry in beta decay 1957: Parity is NOT conserved in weak interactions Mrs Wu et al. measured electrons from beta decays of Co 60 nuclei whose spins were oriented (for a few minutes) in a magnetic field. It appeared that there were more electrons in the direction opposite to Co 60 spins. Electrons are not symmetrically ejected over and under the plane perpendicular to the nuclear spins!

13 Left-right asymmetry in beta decay (cont) Starting with the experiment by Wu et al. the measurements showed that the angular distribution of electrons: positrons: where θ is the angle between the electron direction and its spin while v is the electron velocity Thus: electrons prefer backward positrons prefer forward emission with respect to their spins: direction of motion direction of spin Electrons are mostly left-handed (LH) and positrons right-handed (RH)

14 Left-right asymmetry in beta decay (cont.) We can define Longitudinal polarization : For massless neutrinos one can expect: or i.e. neutrino polarization P is: Left-handed or right-handed?

15 Measurement of neutrino polarization (or helicity) From Pauli hypothesis: neutrino spin=1/2 but what is its polarization? An experiment by Goldhaber et al. (1958) see a very good description by Grzegorz Brona (MSc). Conclusion: Neutrinos accompanying positrons are left-handed, while those accompanying electrons are right-handed Hence by convention: leptons are left-handed anti-leptons are right-handed electrons positrons neutrinos anti-neutrinos

16 Goldhaber s experiment all figures thanks to Mr Grzegorz Brona (MSc) K orbit electron Total angular momentum of the initial state is spin of a captured electron. Final states: i.e. spins are opposite spin spin velocity velocity i.e. the recoiling nucleus has the same polarization sense (or handedness) as the neutrino - along or against velocity vector. i.e. RH or LH

17 Goldhaber s experiment (cont.) RH LH Next: gamma has to carry away the angular momentum of the excited nucleus! Let s consider the LH case spins against velocities): if photon emitted forward i.e. forward γ has to be LH if photon emitted backward velocity spins In the same way one can show that: In RH case forward γ has to be RH Hence: polarization of forward γ is the same as that of neutrinos!!

18 Goldhaber s experiment (cont.) Hence we need to: select forward gammas measure their polarization Another great idea: use resonant scattering: possible only with a forward gamma because it has slightly higher energy than the excitation energy (thus allowing for some recoil energy of the nucleus)

19 Schematic view of Goldhaber experiment Experiment steps: Electron capture by 152 Eu Decay of 152 Sm* with emission of gammas Measurement of gamma polarization by scattering on polarized electrons in iron (by mgt field) Absorption and reemition of γ in 152 Sm selects only photons emitted forward Reemitted gammas measured by NaI

20 Result of the experiment + or is for magnetic field direction which polarizes spins of iron electrons which act as polarimeter for gamma polarization Compton scattering probability is bigger for opposite spin orientation of electron and photon measured photons had preferentially the same spin orientation as electrons (because scattered photons are not resonant and do not get to NaI) As a result of this experiment the neutrino polarization was found to be:

21 Neutrinos are Left-handed i.e its spin projection on a direction of motion (helicity) is negative

22 Neutrino polarization Massless neutrinos only rotate in one direction! Neutrino Anti Neutrino

23 Mass versus polarization All neutrinos left-handed massless If they have mass, can t go at speed of light. Because if an observer moves faster than neutrino: Now neutrino right-handed?? contradiction can t be massive

24 One neutrino or two? Muons were detected in cosmic rays.. µ > e +... muon decays were measured and the electron spectrum was again 3-body? Another puzzle: why µ > e + γ is not observed? Reines: In 1956 Cowan and I proposed to go to an accelerator and test the identity of the two neutrinos. The reaction we got from Los Alamos was difficult to understand: You two fellows have had enough fun. Why don t you go back to work. Fred Reines, 1982

25 Detection of ν µ Experiment by Schwartz, Lederman and Steinberger in 1962: Protons from an accelerator in Brookhaven (Long Island) interacted with target producing pions. Pions decayed producing muons and neutrinos. The experiment s goal was to study the nature of the neutrinos. Detector: iron plates interspersed with spark chambers target sparks along a muon track Conclusion: the neutrinos accompanying µ from π decays produce in the detector muons and not electrons. They are different from neutrinos discovered by Reines and Cowan.

26 How Many Neutrinos? Total width: Γ~ decay probability (~1/lifetime) Partial widths: Γ i ~ branching rate (channel i)

27 Detection of ν τ a challenge! 2000 ν τ has to produce a τ lepton one has to track a τ τ lifetime is 3x10-13 sec (c =90 µm) use emulsion Experiment: DONUT (Direct Observation of the NU Tau) at FermiLab accelerator. Out of neutrinos, only 1000 ν interactions recorded, out of which 4 were identified as ν τ DONUT searched for decays into 1 charged particle (86% of taon decays)

28 Detection of ν τ DONUT 800 GeV protons produced mesons containing c and s quarks, which decay into τ and ν τ

29 Detection of ν τ 2000

30 Who needs 3 generations? Neutrinos may help to solve the mystery

31 Standard Model with colors Generation I Generation II Generation III Leptons Quarks Gauge Bosons gluons >>>