Lecture 1: March 27, 2006

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Lecture 1 March 27, 2006 History,
Overview, Motivation
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1) Prehistory birth of neutrino In nuclear a
and g decay discrete lines were observed,however,
the b spectrum of 210Bi looked continuous
(Chadwick 1914). The Q-value is 1161 keV, and the
average b energy was measured to be 337 keV
(Meitner1930). This violated energy conservation
law. Also, the idea then was that a nucleus, e.g.
14N, was made of 14 protons and 7 electrons. Yet
its statistics looked like BE, not FD. The way
out was suggested by Pauli (famous letter,
12/4/1930), when he proposed the existence of
neutrinos. Fermi (1934) formulated the theory
of b decay. Neutrinos and electrons were created
in the decay, not present beforehand. Bethe and
Peierls (1934) estimated neutrino cross section
as 10-44 cm2, hence unobservable. Discovery of
the neutron (Chadwick 32) solved the issue of
statistics. Allen (1942) measured the recoil
momentum in the electron capture of 7Be (Q0.86
MeV). It was compatible with the emission of
(almost) massless neutrino.
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Estimate of the cross section Take n -gt p e-
n and consider n p -gt n e At low (MeV)
energies the cross section can depend only on
E (the energy of the neutrino or positron). Hence
s GF2 E2 (hc)2. GF 1.17 x 10-11 MeV-2, hc
2x10-11MeV cm Thus s 10-44 cm2 (as in Bethe and
Peierls)
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Allens experiment on 7Be electron capture
7Be
T1/2 53.3 days
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Recoil kinetic energy T En2/2 MLi 53 eV for
En 0.86 MeV
0.48 MeV
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QEC 0.86 MeV
7Li
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E photomultiplier to detect the recoil 7Li ions
G1 and G2 electrodes to accelerate the recoil
ions (G1) and retard them (G2)
S heated 7Be source
Geiger counter to check that the recoils are not
caused by g emission
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2) Early history In 1953-56 Reines and Cowan
detected electron antineutrinos from the nuclear
reactor using the reaction n p -gt n e and
observing the annihilation radiation in delayed
coincidence with the neutron capture. The cross
section agreed with expectations. Thus
neutrinos became real particles. (Reines got the
Nobel Prize in 1995). In 1956 Lee and Yang
proposed that parity is not conserved in weak
interactions. They were motivated by the study of
K decays, that decayed both into two and three
pion states, of opposite parity. The suggestion
that parity is not conserved was quickly verified
experimentally, by studies of b decay and m
decay. The two-component neutrino theory (Lee
Yang, Salam, Landau 1957) The observed maximum
parity violation in leptonic weak processes could
be accommodated if neutrinos are massless (and
hence helicity and chirality eigenstates). Only
lefthanded neutrinos (and righthanded antineutrino
s) would be needed. Lepton number would be
conserved.
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Schematic diagram of the Reines-Cowan experiment
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Determination of neutrino helicity (Goldhaber,
Grodzins,Sunyar 1958)
Consider the electron capture process 152Eu(0-)
e- -gt 152Sm(1-) ne followed by 152Sm(1-)
-gt 152Sm(0) g Here the ne energy g energy.
The experiment was arranged such that pg pSm
-pn If the g is circularly polarized (Jz 1 or
-1), it follows that the spin of the neutrino
must be opposite to the g polarization (since
the captured electron had s 1/2) and therefore
the helicities must be the same. The g circular
polarization was measured to be -0.67 - 0.10,
implying after corrections that the neutrino is
lefthanded.
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63Eu152 (0-), T1/2 9.3 h
EC, Q 0.962 MeV
62Sm152(1-)
E1 g, Eg 0.963 MeV
62Sm152(0)
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V-A theory A priori it was not clear which of
the possible Lorentz structures govern weak
interactions, in particular S-T or V-A. The
similarity of couplings in b and m decays lead to
the formulation of the V-A model (Feynman
Gell-Mann, 1958). Of particular interest was the
observation of the p -gt e ne decay. That
decay is forbidden in the limit me -gt 0. Hence
G(p -gt e n)/G(p -gt m n) me2/mm2mp2 -
me2/mp2 - mm2 1.284x10-4 in perfect
agreement with the experiment. Finally, the
finding that the vector coupling constants (but
not the axial ones) are the same (almost) in b
and m decay lead to the conserved vector current
hypothesis. All that set the stage for the
Standard Electroweak Model of Glashow, Weinberg
and Salam.
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Proof that is nm different from ne Even earlier
(Lokanathan Steinberger 1955) the attempts to
Observe m -gt e g gave a small upper limit
2x10-5 for the branch. That suggested that even
though the weak coupling appeared to be
universal, the nm may be different from ne. The
way to prove it is to show that the reaction nm
n -gt m p goes and nm n -gt e p does not
go. The proof was given by Danby et al. (1962,
Nobel prize 1988) who found that the beam of
dominantly nm from pion decay makes muons, but
essentially no electrons (some were made from
contaminants in the beam). Note Muons produce
long penetrating tracks but electrons produce
short intense electromagnetic showers.
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Discovery of neutral currents The Standard
Electroweak Model (Glashow 1961, Salam
1968, Weinberg 1967, Nobel Prize 1979) predicted
(among other things) the existence of neutral
currents. Their existence was established by
observing nm e -gt nm e (observing
recoiling electrons) at CERN (1973) using the
Gargamelle bubble chamber. The third lepton
family (t lepton) was discovered in 1975 at
SLAC (Nobel Prize for Perl in 1995). An
experimental proof of the existence of nt was
made only in 1999 (DONUT experiment).
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Particle content of the Standard Model.
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Determination of the number of active
neutrinos from the invisible width of Z at
LEP (decay channel Z -gt ni ni ). Nn 2.9841
-0.0083
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• Standard Model postulates that all neutrinos are
  exactly massless.
• Individual lepton numbers are conserved, i.e.
  processes not only
• like m-gt e g but also nm n -gt e p etc.
  are strictly forbidden.
• More recent discoveries challenge this postulate
  and show that
• neutrinos are massive (albeit much lighter than
  other fermions)
• and that the individual lepton numbers are not
  conserved.
• Description of these phenomena will be the main
  topic of these
• lectures. It is hoped that the pattern of
  neutrino masses and
• mixing that is emerging will offer a glimpse
  into the fundamental
• source of particle masses and the role of flavor.

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• Overview of the existing evidence for neutrino
  mass and mixing
• 1)Solar neutrinos Solar energy production is
  based on the pp cycle, the
• transformation of 4 protons into 4He 2e
  2ne. Comparing the flux of ne
• as well as the total neutrino flux on Earth with
  the predicted one, one can
• check whether the neutrino flavor is conserved.
  Experimentally, the total
• flux measured by the neutral current reactions
  agrees with expectations,
• but the ne flux is reduced by a factor 2-3
  depending on energy. This
• finding clearly shows that neutrino flavor is
  not conserved.
• 2) Atmospheric neutrinos Cosmic rays (protons
  and heavier nuclei) interact
• with the upper atmosphere and produce (primarily)
  pions, that decay into
• muons and nm. Muons in turn decay into electrons,
  nm and ne. By determining
• the ratio of nm to ne fluxes as well as the flux
  of nm coming from different
• directions (and hence traveling different
  distances), experiments have
• established that the nm flux is not conserved
  also, and that the flavor
• violation depends on distance and energy.
• 3) Reactor neutrinos Reactors produce low energy
  (lt10 MeV) ne. By
• measuring their flux as a function of distance
  and energy, one can test
• the flavor conservation. Again, ne flavor is
  found to be not conserved.

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Description and analysis of these experimental
findings, and the phenomenology of the involved
physics will be the main topic of this
course. What else should and could be done to
arrive at a coherent picture? Do these phenomena
point to a unique generalization of the Standard
Model? If not (and they do not) what are the
possibilities, and how to distinguish between
them?
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A unique feature of neutrino physics is its
relation to astrophysics and cosmology. Here is a
quick overview Unlike radiation and charged
particles, neutrinos are not absorbed or
scattered by dust, magnetic field, stellar matter
etc. Hence they open up a new probe of distant
objects, neutrino astronomy. So far neutrinos
coming from only two extraterrestrial objects
were observed the Sun and Supernova 1987A. In
the case of the Sun, observations confirm that
the present ideas about the solar energy
production are correct. In the case Of core
collapse SN (SNII), the one limited observation
confirmed That the basic ideas about these
objects are correct. At present we are eagerly
waiting for the next SNII in our galaxy.
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Supernova Neutrino Detection
IMB
KamII
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Neutrinos and cosmology Most of our
understanding of the early Universe comes
from observation and description of the abundance
of light primordial nuclei (4He,2H,3He,7Li) , so
called Big Bang Nucleosynthesis (BBN), and more
recently from the detailed study of the Cosmic
Microwave Background Radiation (CMB). In the
early Universe all particles, including
neutrinos, were in thermal equilibrium. At T 1
MeV neutrinos decouple , i.e. their interaction
rate becomes less than the expansion rate. One
can convincingly show that the density of these
cosmic background neutrinos is 3/11 of the CMB
number density for each flavor. Observing them is
one of the great challenges of the field.
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Neutrinos and structure formation Given the
present upper and lower limits on neutrino
masses, we know that neutrinos cannot account for
the observed Dark Matter in the Universe, which
should be cold, i.e. nonrelativistic at the
epoch of structure formation, while neutrinos
were hot. Neutrinos are relativistic until T
mn during that time they travel a distance dn
MPl/mn2 (MPl (hc/GN)1/2 1019 GeV). In the
early Universe these relativistic neutrinos wash
out inhomogeneities on the scale that depends on
their mass. Thus the distribution of structures
in the present day Universe depends on mn
structures smaller than some limiting size
should be less abundant. Thus, there is a
connection between the distribution of structures
and neutrino mass that can be used to constrain
the mn from above (or fit for it).