lecture 11: weak interactions - university of oxfordbiller/particle_course/lectures/lecture_1… ·...
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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 taken to indicate a sensitive detection technique
interaction cross-section much higher than for typical neutrino energies
obvious foreshadowing of electroweak theory
Cosmic Gall
+ ν e (Pauli)
Beta Decay
n → p + e-
(Pauli)
''Inverse" Beta Decay
νe + p → n + e+
σ ~ λ2 × λ/c τ
''cross-sectional area" of ν wave packet
time spent by wave packet in presence of the proton
typical timescale for weak interaction to occur
νe + p → n + e+ Inverse β-decay:
(Pontecorvo)
From standard β-decay, the lifetime of the free neutron is τ ~ 1000 s and the energies of the e- and νe are ~ 1 MeV
⇒ λ = h/p ≃ 1200fm = 1.2x10-10cm
thus, σ ~ (1.2x10-10cm)3/[(3x1010 cm/s)(1000s)] ~ 10-43cm2
Note σ ∝ E-3t-1 and, from previous discussion, t-1 ∝ E5
⇒ σ ~ 10-43 (EMeV)2 cm2 Almost exactly right! (and very, very small!!!)
Interaction Length for a 1 MeV Neutrino in Lead
λ = 1/(σρ)
σ ~ 10-43 cm2 (per proton)
ρ = (11.4 g/cm3) x [1/(207 g/mole)] x (6.02x1023 atoms/mole) x (82 protons/atom)
= 2.7x1024 protons/cm3
λ = 1/(2.7x10-19) cm
= 3.7 x 1018 cm = 4 light-years !!
n → p + e- + νe
νe + p → n + e+
Reines and Cowan, 1956 (Nobel Prize – 1995 !!)
Parity Violation in Weak Interactions First suggested in 1956 by Lee & Yang based on review of kaon decay modes
60Co
e-
60Co
e-
P nuclear spins aligned by cooling to 0.01 oK in 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- + νe
γ (1.173 MeV) + γ (1.332 MeV)
(degree of polarisation determined from the anisotropy of γ-rays)
Garwin, Lederman & Weinrich (1957)
e+ νµ
νe
νµ
µ+
π+
precess polarised muons (polarised)
Also, in 1958, Goldhaber et al. measured the helicity of the neutrino:
e- + 152Eu(J=0) → 152Sm*(J=1) + νe
152Sm(J=0) + γ
events were chosen with the final states collinear ⇒ γ and νe 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 π- → µ- + νµ (pion decay):
but, unfortunately, it is found experimentally that the couplings are not the same!
αWud ≃ 0.95 αW
d
u
W- νµ
µ- π-
⇒ W±
It can change u↔d (like β-decay) s↔c t↔b
and, for leptons, e↔νe µ↔νµ τ↔ντ
β-decay (n→p+e-+νe) tells us the exchange particle must be charged
s
u
W- νµ
µ- Κ-
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 interaction actually couples to mixtures of quarks.
So, initially just considering the first two generations, the relevant quark doublets are:
u dʹ′
c sʹ′ ( ) and ( )
where dʹ′ ≡ d cosθC + s sinθC
sʹ′ ≡ -d sinθC + s cosθC
θC ⇒ ''Cabibbo angle"
or, alternatively d ≡ -sʹ′ sinθC + dʹ′ cosθC
s ≡ sʹ′ cosθC + dʹ′ sinθC
αWud = αW cos2θC
αWus = αW sin2θC
~ 1/20 ( θC = 12.7 + 0.1 degrees )
σ ( Κ- → µ- νµ )
σ ( π- → µ- νµ ) = tan2θC
(The factor of 1/20 delineates ''Cabibbo-suppressed" and ''Cabibbo-allowed" processes)
Generalizing to 3 generations and all possible mixings between quarks:
dʹ′ sʹ′ bʹ′
Vud Vus Vub Vud Vus Vub Vud Vus Vub
dʹ′ sʹ′ bʹ′ ( ) [ ] ( ) =
(Cabibbo, Kobayashi and Maskawa)
CKM matrix
αWus
αWud =
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+ W- Ko Ko
We can thus define two orthogonal mixtures:
⎜K1o〉 = 1/√2 ( ⎜Ko〉 + ⎜Ko〉 )
⎜K2o〉 = 1/√2 ( ⎜Ko〉 - ⎜Ko〉 )
Note: C P ⎜K1o〉 = + ⎜K1
o〉 and C P ⎜K2o〉 = - ⎜K2
o〉
K1o → π+ π- ; πo πo
K2o → π+ π- πo ; πo πo πo
Allowed
K1o → π+ π- πo ; πo πo πo
K2o → π+ π- ; πo πo
Forbidden
Experimentally, 2 kaon states are observed with different lifetimes:
KSo → π+ π- ; πo πo τ ≃ 9x10-11s
So we associate KSo ⇔ K1
o and KLo ⇔ K2
o
However, in 1964, Christenson, Cronin, Fitch & Turlay discovered
KLo → π+ π-
(branching ratio ~ 2x10-3)
KLo → π+ π- πο ; πo πo πo ; π± lepton ν (ν) τ ≃ 5x10-8s
±
30 GeV protons
steel target
beam collimator
magnets sweeps out charged particles
lead-glass cuts out photons
KS+KL KL
18 m
KL beam direction
CM of π+π- pair
)θ
⎜KSo〉 = 1/√1+ ε 2 ( ⎜K1
o〉 - ε ⎜K2o〉 )
⎜KLo〉 = 1/√1+ ε 2 ( ε ⎜K1
o〉 + ⎜K2o〉 )
where ε ≡ small complex number parameterizing the size of the CP violation
(experimentally, ε ≃ 2.3x10-3 )
What does this mean??
Reason for antimatter assymmetry ??
Perhaps we can learn more from studying CP violation in other particle systems...
Basically compare the rates for
B0 = Ψ + KS0
(π+π- mode)
B0 = Ψ + KS0
versus
(π+π- mode)
dʹ′ sʹ′ bʹ′
Vud Vus Vub Vud Vus Vub Vud Vus Vub
dʹ′ sʹ′ bʹ′ ( ) [ ] ( ) = ??
CP violation could be parameterized as part of 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:
γ
α
β
η
ρ
''Unitarity Triangle"
Matter-Antimatter Asymmetry Revisited:
Sakarov Conditions (1967)
1) Baryon Number Violation allows baryons and anti-baryons to appear and disappear independently of each other
2) CP Violation so the rate of appearance/disappearance of baryons is different from anti-baryons
Establishes Asymmetry
3) Non-Equilibrium Conditions since equilibrium would then tend to ''average-out" any asymmetry
Locks In Asymmetry
!!! (GUTs)