Lecture 11: Weak Interactions • Cross-Section and the W Coupling • The Cabibbo Angle and the CKM Matrix • Parity Violation • Kaons and Mixing • CP Violation Sections 4.51, 8.1, Chapter 10 Useful Sections in Martin & Shaw:
Dec 22, 2015
Lecture 11: Weak Interactions
• Cross-Section and the W Coupling
• The Cabibbo Angle and the CKM Matrix
• Parity Violation
• Kaons and Mixing
• CP Violation
Sections 4.51, 8.1, Chapter 10
Useful Sections in Martin & Shaw:
(from ''Telephone Poles and Other Poems," 1963)
Neutrinos, they are very small. They have no charge, they have no mass And do not interact at all. The earth is just a silly ball To them, through which they simply pass, Like dustmaids down a drafty hall Or photons through a sheet of glass...
John Updyke
in fact, point-like in the Standard Model
and little(< 2eV)
hardly
true
should not be takento indicate a sensitive detection technique
interaction cross-sectionmuch higher than for typical neutrino energies
obvious foreshadowingof electroweak theory
Cosmic Gall
~ 2 /c
''cross-sectional area" of wave packet
time spent by wave packet in presence of the proton
typical timescale for weak interaction to occur
e + p n + e+Inverse -decay:
(Pontecorvo)
From standard -decay, the lifetime of the free neutron is ~ 1000 s and the energies of the e and
e are ~ 1 MeV
= h/p ≃ 1200fm = 1.2x1010cm
thus, ~ (1.2x10-10cm)3/[(3x1010 cm/s)(1000s)] ~ 1043cm2
Note E3t1 and, from previous discussion, t1 E5
~ 1043 (EMeV
)2 cm2 Almost exactly right!(and very, very small!!!)
Interaction Length for a 1 MeV Neutrino in Lead
~ 10-43 cm2 (per proton)
= (11.4 g/cm3) x [1/(207 g/mole)]x (6.02x1023 atoms/mole)
x (82 protons/atom)
= 2.7x1024 protons/cm3
= 1/(2.7x10-19) cm
= 3.7 x 1018 cm = 4 light-years !!
Parity Violation in Weak InteractionsFirst suggested in 1956 by Lee & Yang based on review of kaon decay modes
60Co
e
60Co
e
Pnuclear spins aligned by cooling to 0.01 oKin a magnetic field
Should be the same under parity transformation, but fewer electrons are actually seen going forward !
Directly observed by Wu et al. in 1957 from the decay 60Co 60 Ni* + e +
e
(1.173 MeV) + (1.332 MeV)
(degree of polarisation determined from the anisotropy of -rays)
Also, in 1958, Goldhaber et al. measured the helicity of the neutrino:
e + 152Eu(J=0) 152Sm*(J=1) + e
152Sm(J=0) +
events were chosen with the final states collinear and
e travel in opposite directions, so helicity
of the neutrino is found from that of the gamma
all neutrinos are left-handed !
Leon Lederman, Melvin Schwartz and Jack Steinberger, 1962
Neutrinos of the ''Second Kind"(not as popular as the Spielberg sequel)
Assume some Yukawa-like exchange process is at work.
Weak interactions obey a simple symmetry :
So, for example, for the process + (pion decay):
but, unfortunately, it is foundexperimentally that the couplingsare not the same!
W
ud ≃ 0.95 W
d
u
W
W
It can change ud (like -decay) sc tb
and, for leptons, ee
-decay (np+e+e) tells us the exchange particle must
be charged
s
u
W
Another hitch:
shouldn’t occur, but does ! (albeit infrequently)
We can explain all this (or, at least, parameterize our ignorance)by adopting the somewhat bizarre notion that the weak interactionactually couples to mixtures of quarks.
So, initially just considering the first twogenerations, the relevant quark doublets are:
ud
cs( ) and ( )
where d d cosC + s sin
C
s d sin
C + s cos
C
C ''Cabibbo angle"
or, alternatively d s sinC + d cos
C
s s cos
C + d sin
C
W
ud = W
cos2C
W
us = W
sin2C
~ 1/20 C = 12.7 + 0.1 degrees )
= tan2
C
(The factor of 1/20 delineates ''Cabibbo-suppressed" and ''Cabibbo-allowed" processes)
Generalizing to 3 generations and all possible mixings between quarks:
dsb
Vud
Vus
Vub
Vud
Vus
Vub
Vud
Vus
Vub
dsb( ) [ ] ( )=
(Cabibbo, Kobayashi and Maskawa)
CKM matrix
W
us
W
ud
=
Kaons: Ko = ds Ko = sd (S = +1) (S = 1)
But S is not conserved in weak interactions so Ko-Ko mixing can occur:
u
u
d
s
s
d
W+ WKo Ko
We can thus define two orthogonal mixtures:
K1
o = 1/2 ( Ko + Ko )
K2
o = 1/2 ( Ko Ko )
Note: C P K1
oK1
oand C P K2
oK2
o
K1
o + ; o o
K2
o + o ; o o o
Allowed
K1
o + o ; o o o
K2
o + ; o o
Forbidden
Experimentally, 2 kaon states are observed with different lifetimes:
KS
o ; o o ≃ 9x1011s
So we associate KS
o K1
o and KL
o K2
o
However, in 1964, Christenson, Cronin, Fitch & Turlay discovered
KL
o +
(branching ratio ~ 2x103)
KL
o ; o o o ; lepton () ≃ 5x10s
30 GeVprotons
steeltarget
beamcollimator
magnets sweeps out charged particles
lead-glass cutsout photons
KS+K
L KL
18 m
KL beam
direction
CM of + pair
KS
o = 1/ ( K1
o K2
o )
KL
o = 1/ ( K1
o + K2
o )
where small complex number parameterizing the size of the CP violation
(experimentally, ≃ 2.3x103 )
What does this mean??
Reason for antimatter assymmetry ??
Perhaps we can learn more from studyingCP violation in other particle systems...
dsb
Vud
Vus
Vub
Vud
Vus
Vub
Vud
Vus
Vub
dsb( ) [ ] ( )= ??
CP violation could be parameterized as partof the mixing angles in the CKM matrix
Unitarity of the matrix is needed to allow for local gauge symmetry
Which imposes constraints on the angles:
''Unitarity Triangle"
Matter-Antimatter Asymmetry Revisited:
Sakarov Conditions (1967)
1) Baryon Number Violationallows baryons and anti-baryons to appearand disappear independently of each other
2) CP Violationso the rate of appearance/disappearance of baryons is different from anti-baryons
EstablishesAsymmetry
3) Non-Equilibrium Conditionssince equilibrium would then tend to''average-out" any asymmetry
Locks InAsymmetry
!!!(GUTs)