16'451 Lecture 16: Weak Interaction, Parity

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Nov.4, 2004
16.451 Lecture 16 Weak Interaction, Parity
Neutrinos
Famous experiment carried out by C.S. Wu (1957)
at the suggestion of Lee Yang (1956,
Nobel Prize 1957) demonstrated that the weak
interaction violates parity
Key observation when cobalt nuclei were
polarized in a magnetic field at low
temperature, electrons were emitted
preferentially in a direction opposite to the
nuclear spin...
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60 Co Decay Scheme

• two famous gamma rays, 1173 and 1332 keV
  (cobalt radiation therapy!)
• high spin of 60Co plus magnetic property means
  it can be polarized in a B field
• angular distribution of gamma rays reveals
  polarization of the 60Co parent nucleus

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Figures from C.S. Wu et al., Phys. Rev. 105, p.
1413 (1957)
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To PMT
(warm unpolarized)
light guide
B field (up or down)
? scintillator
? anisotropy measures nuclear polarization
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A pseudoscalar observable
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electron emission angle
Under a parity transformation
Angular momentum
Observer using a parity-reversed coordinate
system deduces the opposite correlation of e- and
J... but this is crazy.... ????
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Consider what a parity transformation does to a
coordinate system
Normal RIGHT-handed Cartesian system
x -x
y -y
z -z
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Principle of parity conservation Laws of
physics should be independent of coordinate
system! In particular, a right-handed and
left-handed choice of Cartesian coordinates
should be completely arbitrary. (We should
get the same answer both ways.) (True for
gravity, strong, and electromagnetic
interactions) This is not true for the weak
interaction has the opposite sign in LH
and RH systems ? by demonstrating a preferred
correlation , beta-decay prefers a LH
coordinate system ? symmetry is broken!
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In fact, the electron and antineutrino themselves
show a similar correlation define helicity h
for a particle with spin s, and
momentum p Electrons emitted in ?-decay have h
-v/c left handed (positrons
h v/c right handed) Neutrinos have
h -1 (LH) and antineutrinos have h 1 (RH)
-- this is the only perceptible difference
between them!!!!!
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Parity Nonconservation and the Standard Model
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Weak force carriers, W, Z, W- have spin 1
(bosons) and are left-handed, i.e. they have
h -1 always (spin opposite to direction of
motion) If this is the case, then parity
violation in the weak interaction is a built-in
feature. But nobody knows why.... Extensive
searches for physics Beyond the Standard Model
probe the existence of a symmetric set of
right-handed force carriers. None detected yet,
but if they exist, they are required to be
extremely heavy!
Ref Williams 9.13 see also chapter 12
Many precise experimental tests are in agreement
with this picture see the particle data group
web page for a current summary. High energy
collider experiments have played a major role in
discovering the heavier quarks and members of the
lepton family...
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Note on colliding beams and the production of new
particles
consider first a fixed-target experiment extra
kinetic energy in the beam is wasted rather
than used for new particle production, since
momentum has to be conserved (forward direction)
before
after
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Now work out how much beam energy is needed in
the lab
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TRICK length2 of a 4 vector is invariant!!!
Available energy to make new particles goes up as
the square root of the beam energy
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CERN Accelerator Complex 27 km long, under the
city of Geneva, Switzerland!
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several international collaborations built
sensitive calorimeter and tracking detectors
to record final state particles produced in
high energy e e- collsions to search for new
particles at LEP (now decommissioned to be
replaced by the LHC or Large Hadron Collider)
event from OPAL - Canadian group
ee- ? WW- ? many particles!
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• Particle Data Group cross section for ee-
  annihilation at high energy
• resonance peaks for production of neutral
  particles meson resonances seen
• at lower energy. All of these have J
  1, negative parity (recall pion has J 0)

Z boson, discovered 1983, CERN
Total center of mass energy (GeV)
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Close up of Z resonance peak gives the mass of
the Z and the number of neutrinos!
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Fit gives N 2.994 /- 0.012 neutrino types!
(N 3 looks pretty good!)
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And FINALLY, a quick word about SNO
Sudbury Neutrino Observatory http//www.sno.phy.
queensu.ca/
Prior to SNO, several other solar neutrino
experiments were constructed and in operation
world wide, e.g. the Kamiokande detector in
Japan, SAGE and GALLEX detectors in Europe ...
all had slightly different energy sensitivities
and operated using different reactions to detect
the neutrinos, but all found a discrepancy in
the solar flux!
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Calculated neutrino flux from fusion reactions in
the sun, J. Bahcall et. al
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Energy thresholds of various detectors are shown
Ref Williams, section 14.7
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All detectors, including SNO, show a deficit of
electron neutrinos from the sun
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SNO a unique D2O Cerenkov detector that can
see all neutrino types
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4700 underground in the Creighton nickel mine in
Sudbury, Canada, to suppress background from
cosmic ray muons, etc
20 diameter photo- multiplier tubes
looking inward detect Cerenkov light when a
neutrino interacts in the water
acrylic vessel holds 1000 tonnes of heavy water,
D2O that makes an ideal detector for neutrinos.
Neutrino candidate event Cerenkov ring on one
side of the detector with nothing entering from
the other side.
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Neutrino detection mechanisms in heavy water
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SNO published results http//www.sno.phy.queensu
.ca/sno/publications.html
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electron-neutrinos only
all neutrino types
Significance of the SNO result first
experiment to see what happened by measuring
all neutrino types
Ratio

• Interpretation
• the total number of neutrinos is consistent
  with expectations from the solar model.
• only electron-type neutrinos are produced in
  solar fusion reactions
• 2/3 of these must be turning into other
  neutrino types (?, ?) before reaching earth!

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Neutrino masses and mixing (see, e.g.
http//www.sns.ias.edu/jnb/ )
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The theory of neutrino mixing gets complicated
very quickly, but in a nutshell, the
observation of neutrino oscillations sets
limits on the mass-difference ?m2 and the
mixing angle ?, e.g. for only two neutrino
types, one could write
Then as time evolves, with the masses of 1 and 2
being different, the observed neutrino state
will be a different linear combination of 1 and 2
that depends on the parameters ?m2 and sin2? .
Combined data from all experiments can be used to
place limits on the mixing parameters.... so
far, the favoured situation looks like this
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The SNO result is an incredible achievement for
physics and Canada
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http//www.nserc.ca/news/2003/p031124.htm
It wasnt easy! For a few years, the subatomic
research community almost went broke trying to
pay for SNO .... but it was worth it!