Section VI Weak Interactions The Weak Interaction The

Section VI - Weak Interactions

The Weak Interaction Ø The WEAK interaction accounts for many decays in particle physics Examples: Ø Characterized by long lifetimes and small cross-sections 81

Ø Two types of WEAK interaction: CHARGED CURRENT (CC): W Bosons NEUTRAL CURRENT (NC): Z 0 Boson Ø The WEAK force is mediated by MASSIVE VECTOR BOSONS: MW ~ 80 Ge. V MZ ~ 91 Ge. V Examples: Weak interactions of electrons and neutrinos: 82

Boson Self-Interactions Ø In QCD the gluons carry “COLOUR” charge. Ø In the WEAK interaction the W and Z 0 bosons carry the WEAK CHARGE Ø W also carry EM charge BOSON SELF-INTERACTIONS 83

Fermi Theory Weak interaction taken to be a “ 4 -fermion contact interaction” – No propagator – Coupling strength given by the FERMI CONSTANT, GF – GF = 1. 166 x 10 -5 Ge. V-2 b Decay in Fermi Theory GF Use Fermi’s Golden Rule to get the transition rate where Mif is the matrix element and r(Ef) is the density of final states. 84

Density of Final States: 2 -body vs. 3 -body ØTWO BODY FINAL STATE: Relativistic (E ~ p) i. e. neglect mass of final state particles. Only consider one of the particles since the other fixed by (E, p) conservation. Ø THREE BODY FINAL STATE (e. g. b decay): Now necessary to consider two particles – the third is given by (E, p) conservation. 85

Neutrino Scattering in Fermi Theory (Inverse b Decay). GF where Ee is the energy of the e- in the centre-of-mass system and the energy in the centre-of-mass system. is In the laboratory frame: Ø n’s only interact WEAKLY have very small interaction cross-sections Ø Here WEAK implies that you need approximately 50 light-years of water to stop a 1 Me. V neutrino ! However, as En the cross-section can become very large. Violates maximum allowed value by conservation of probability at (UNITARITY LIMIT). Fermi theory breaks down at high energies. 86

Weak Charged Current: W Boson Ø Fermi theory breaks down at high energy Ø True interaction described by exchange of CHARGED W BOSONS Ø Fermi theory is the low energy EFFECTIVE theory of the WEAK interaction. b Decay: u n d d GF u d p u e n e- Scattering: e GF 87

Compare WEAK and QED interactions: e e WEAK QED e CHARGED CURRENT WEAK INTERACTION Ø At low energies, , propagator i. e. appears as POINT-LIKE interaction of Fermi theory. Ø Massive propagator short range Ø Exchanged boson carries electromagnetic charge. Ø FLAVOUR CHANGING – ONLY WEAK interaction changes flavour Ø PARITY VIOLATING – ONLY WEAK interaction can violate parity conservation. 88

Compare Fermi theory c. f. massive propagator GF e For compare matrix elements: GF is small because m. W is large. The precise relationship is: The numerical factors are partly of historical origin (see Perkins 4 th ed. , page 210). The intrinsic strength of the WEAK interaction is GREATER than that of the electromagnetic interaction. At low energies (low q 2), it appears weak due to the massive propagator. 89

Neutrino Scattering with a Massive W Boson Replace contact interaction by massive boson exchange diagram: e with where q is the scattering angle. Integrate to give: Total cross-section now well behaved at high energies. 90

The Weak CC Lepton Vertex All weak charged current lepton interactions can be described by the W boson propagator and the weak vertex: W- STANDARD MODEL WEAK CC LEPTON VERTEX +antiparticles Ø W Bosons only “couple” to the lepton and neutrino within the SAME generation e. g. no coupling Ø Universal coupling constant g. W 91

Examples: e u n d d u d p u e 92

Decay Ø Muons are fundamental leptons (m ~ 206 me). Ø Electromagnetic decay is NOT observed; the EM interaction does not change flavour. Ø Only the WEAK CC interaction changes flavour. Ø Muons decay weakly: GF e As can use FERMI theory to calculate decay width (analogous to b decay). 93

FERMI theory gives decay width proportional to (Sargent rule). (see Perkins) However, more complicated phase space integration (previously neglected kinetic energy of recoiling nucleus) gives Ø Muon mass and lifetime known with high precision. Ø Use muon decay to fix strength of WEAK interaction GF Ø GF is one of the best determined fundamental quantities in particle physics. 94

Universality of Weak Coupling Can compare GF measured from - decay with that from b decay. e u n d d u d p u e From muon decay measure: From b decay measure: Ratio Conclude that the strength of the weak interaction is ALMOST the same for leptons as for quarks. We will come back to the origin of this difference 95

t Decay The t mass is relatively large and as there a number of possible decay modes. Examples Tau branching fractions: 96

Lepton Universality Test whether all leptons have the same WEAK coupling from measurements of the decay rates and branching fractions. Compare e If universal strength of WEAK interaction, expect are all measured precisely Predict Measure SAME WEAK CC COUPLING FOR AND t 97

Also compare IF same couplings expect: (the small difference is due to the slight reduction in phase space due to the non-negligible muon mass). The observed ratio the prediction. is consistent with SAME WEAK CC COUPLING FOR e, AND t LEPTON UNIVERSALITY 98

Electroweak Unification Ø Weak Charged Current interactions explained by W exchange. Ø W bosons are charged, couple to photon. 2 diagrams (+interference) s. WW (pb) Consider e+ e+ + e e Ø Cross-section DIVERGES at high energy Ø Divergence cured by introducing Z 0 Ø Extra diagram e+ e Ø Only works if g, W , Z 0 couplings are related ELECTROWEAK UNIFICATION 99

The Weak NC Vertex All weak neutral current interactions can be described by the Z 0 boson propagator and the weak vertices: Z 0 Z 0 STANDARD MODEL WEAK NC LEPTON VERTEX STANDARD MODEL WEAK NC QUARK VERTEX +antiparticles Ø Z 0 NEVER changes type of particle Ø Z 0 NEVER changes quark flavour Ø Z 0 couplings are a MIXTURE of EM and WEAK couplings and therefore depend on (see later) 100

Examples: 101