How to catch a neutrino

1
How to catch a neutrino?
H. Bethe
ngt p e- ?
If one observes
? p gt n e
then how about an inverse beta decay
?
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.
From neutrinos to cosmic sources, lecture 1,
2003
2
Reines and Cowan a Proposal (1953)
Reactor in Savannah River as a source of
neutrinos from decays of neutron-rich nuclei.
Detector 12 m underground
?-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.
Liquid scintillator
Water, Cadmium chloride
In 1956 a telegram to Pauli We are happy to
inform you that we have definitely detected
neutrinos... 1995 Nobel Prize for Reines
Liquid scintillator
3
But weak interactions bringa new mystery
Left- right asymmetry
From neutrinos to cosmic sources, lecture 2,
2004
4
Left-right symmetry in beta decay
1957
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!
From neutrinos to cosmic sources, lecture 2,
2004
5
Left-right symmetry in beta decay (cont.)
Starting with the experiment by Wu et al. the
measurements showed that the angular distribution
of
positrons
electrons
where ? is the angle between the electron
direction and its spin and 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)
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Left-right symmetry 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?
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Measurement of neutrino polarization(or helicity)
From Pauli hypothesis neutrino spin1/2 but what
is its polarization ?
An experiment by Goldhaber et al. (1958) see a
very good description by Grzegorz Brona.
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
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Goldhabers experiment
all figures thanks to Mr Grzegorz Brona
K orbit electron
Total angular momentum of the initial state is
spin of a captured electron.
final states
Hence the recoiling nucleus has the same
polarization sense (or handedness) as the
neutrino - along or against velocity vector
spin
velocity
spin
velocity
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Goldhabers experiment
all figures thanks to Mr Grzegorz Brona
gamma carries away the angular momentum of the
excited nucleus (the LH case spins against
velocities)
if photon emitted forward
if photon emitted backward
i.e. forward g has the same polarization as the
neutrino
spins
How to check whether g is forward or backward?
velocity
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Goldhabers experiment (cont.)
Resonant scattering
possible only with a forward g because it has
slightly higher energy than the
excitation energy (thus allowing for some recoil
energy of the nucleus)
From neutrinos to cosmic sources, lecture 2,
2004
11
Schematic view of Goldhaber experiment
Experiments steps

• 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 ? in 152Sm
  selects only photons emitted forward

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Result of the experiment
or is for magnetic field direction (up
or down) which polarizes spins of iron electrons
which act as polarimeter for gamma polarization
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Neutrinos are Left-handed
i.e its spin projection on a direction of
motion (helicity) is negative
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Anti-Neutrinos are Right-handed

• CPT theorem in quantum field theory
• C interchange particles anti-particles
• P parity
• T time-reversal
• State obtained by CPT from nL must exist nR

_
From neutrinos to cosmic sources, lecture 2,
2004
15
Neutrino polarization
Neutrinos only rotate in one direction !
Anti Neutrino
Neutrino
From neutrinos to cosmic sources, lecture 2,
2004
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Weinberg Model
Weinbergs lepton model
Anti Leptons
Leptons
Gauge bosons
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Mass versus polarization

• All neutrinos left-handed ? massless
• If they have mass, cant go at speed of light.

• Now neutrino right-handed??
• ? contradiction ? cant be massive

From neutrinos to cosmic sources, lecture 2,
2004
18
One neutrino or two?
Muons were detected in cosmic rays..
µgt e ...... muon decays were measured and
the electron spectrum was again
3-body? Another puzzle why µgt e ? is not
observed?
In 1956 Cowan and I (Reines) 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 dont you go
back to work. Fred Reines, 1982
From neutrinos to cosmic sources, lecture 2,
2004
19
Detection of nµ
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
experiments goal was to study the nature of the
neutrinos.
Detector iron plates interspersed with spark
chambers
----- sparks along a muon track
target
Conclusion the neutrinos accompanying m from p
decays produce in the detector muons and not
electrons. They are different from neutrinos
discovered by Reines and Cowan.
From neutrinos to cosmic sources, lecture 2,
2004
20
How Many Neutrinos?
GZ measured Ghad ,Gl ,Gn calculated
total width G decay probability
(1/lifetime) partial Gi branching rate
(channel i)
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Detection of nt a challenge!
2000

• nt has to produce a t lepton
• one has to track a t
• t lifetime is 3x10-13 sec (ct90 µm)

use emulsion
Experiment DONUT (Direct Observation of the ?
Tau) at FermiLab accelerator. Out of 1013
neutrinos , only 1000 n interactions
recorded, out of which 4 were identified as nt
DONUT searched for decays into 1 charged
particle (86 of taon decays)
From neutrinos to cosmic sources, lecture 2,
2004
22
Detection of nt DONUT
800 GeV protons produced mesons containing c and
s quarks, which decay into t and nt
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Detection of ?t
2000
From neutrinos to cosmic sources, lecture 2,
2004
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Who needs 3 generations?
Neutrinos may help to solve the mystery
From neutrinos to cosmic sources, lecture 2,
2004
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Natural sources of neutrinos