a brief history of neutrino - warszawska grupa...

From neutrinos to cosmic sources, DK&ER

A brief history of neutrino


Two body decay

m1 m2

Energy-momentum conservation =>

Energy of the decay products always the same

M


1913-1930: Puzzle of b decay

  Continuous spectrum of b particles

  Energy is not conserved??

  Momentum is not conserved??


Dec 1930: A Desperate Remedy

“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

A

A’

e

n


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.”


How to catch a neutrino?

H. Bethe:

ν p —> n e + then how about an inverse beta decay:

n—> p e- ν If one observes:

Probability of a reaction (for one neutrino) = cross section x number of targets/ area

Bethe calculated: cross section= 10-44 cm2

One needs either 1021 cm of water to absorb a neutrino, or a lot of neutrinos.

?


Reines and Cowan: a Proposal (1953)

Water, Cadmium chloride

Liquid scintillator

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.


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

scyntil

water

water

In 1956 a telegram to Pauli: „We are happy to inform you that we have definitely detected neutrinos...”

1995 Nobel Prize for Reines


But weak interactions bring a new mystery:

Left- right asymmetry

T. D. Lee and C. N. Yang, Phys. Rev. 104, 254 http://link.aps.org/abstract/PR/v104/p254 C. S. Wu et al., Phys. Rev. 105, 1413 http://link.aps.org/abstract/PR/v105/p1413 Richard L. Garwin et al., Phys. Rev. 105, 1415 http://link.aps.org/abstract/PR/v105/p1415


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


Left-right asymmetry in beta decay

Mrs Wu et al. measured electrons from beta decays of Co60 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 Co60 spins. Electrons are not symmetrically ejected over and under the plane perpendicular to the nuclear spins!

1957:

Parity is NOT conserved in weak interactions


Left-right asymmetry in beta decay (cont)

where θ is the angle between the electron direction and its spin while v is the electron velocity

Starting with the experiment by Wu et al. the measurements showed that the angular distribution of

positrons: electrons:

Electrons are mostly left-handed (LH) and positrons right-handed (RH)

Thus: electrons prefer backward positrons prefer forward

emission with respect to their spins: direction of motion direction of spin


Left-right asymmetry in beta decay (cont.)

For massless neutrinos one can expect:

or

We can define „Longitudinal polarization”:

i.e. neutrino polarization P is:

Left-handed or right-handed?


Measurement of neutrino polarization (or helicity)

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

From Pauli hypothesis: neutrino spin=1/2 but what is its polarization ?


Goldhaber’s experiment all figures thanks to Mr Grzegorz Brona (MSc)

Total angular momentum of the initial state is spin of a captured electron.

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

Final states:

velocity

velocity

spin

spin

K orbit electron

i.e. spins are opposite


Goldhaber’s experiment (cont.)

Let’s consider the LH case – spins against velocities):

i.e. forward γ has to be LH

velocity

spins

if photon emitted forward

if photon emitted backward

Next:

RH

LH

gamma has to carry away the angular momentum of the excited nucleus!

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!!


Goldhaber’s experiment (cont.)

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)

Hence we need to: •  select forward gammas •  measure their polarization


Schematic view of Goldhaber experiment

 Electron capture by 152Eu

  Decay of 152Sm* with emission of gammas

 Measurement of gamma polarization by scattering on polarized electrons in iron (by mgt field)

 Absorption and reemition of γ in 152Sm selects only photons emitted forward

  Reemitted gammas measured by NaI

Experiment steps:


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:


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


Anti Neutrino

Neutrino polarization

Neutrino

Massless neutrinos only rotate in one direction !


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


One neutrino or two?

µ—> e + ...... muon decays were measured and the electron

spectrum was again 3-body?

Another puzzle: why µ—> e + γ is not observed?

Muons were detected in cosmic rays..

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


Detection of νµ

target

Experiment by Schwartz, Lederman and Steinberger in 1962:

Detector: iron plates interspersed

with spark chambers

----- sparks along a muon track

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.

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.


How Many Neutrinos?

Total width: Γ~ decay probability (~1/lifetime) Partial widths: Γi ~ branching rate (channel i)


Detection of ντ – a challenge!

Experiment: DONUT (Direct Observation of the NU Tau) at FermiLab accelerator.

Out of 1013 neutrinos , only 1000 ν interactions recorded, out of which 4 were identified as ντ

2000

DONUT searched for decays into 1 charged particle (86% of taon decays)

•  ντ has to produce a τ lepton •  one has to track a τ •  τ lifetime is 3x10-13 sec (ct=90 µm)

use emulsion


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


Detection of ντ 2000


Who needs 3 generations?

Neutrinos may help to solve the mystery


Standard Model with colors Generation I Generation II

Gauge Bosons gluons

Generation III

Leptons

Quarks

>>>

a brief history of neutrino - warszawska grupa...

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