Weak Interactions II

M. Cobal, PIF 2006/7 Cabibbo theory

M. Cobal, PIF 2006/7 Weak doublets

M. Cobal, PIF 2006/7 The Cabibbo angle

M. Cobal, PIF 2006/7 Weak Neutral Currents

Neutral Current Z0 Charged Current W

M. Cobal, PIF 2006/7 M. Cobal, PIF 2006/7 Weak Charged Currents

M. Cobal, PIF 2006/7 Weak Neutral Currents

M. Cobal, PIF 2006/7 The GIM Mechanism

M. Cobal, PIF 2006/7 M. Cobal, PIF 2006/7 M. Cobal, PIF 2006/7 The CKM Matrix

M. Cobal, PIF 2006/7 M. Cobal, PIF 2006/7

W and Z production From the quark point of view, processes are quark-antiquark annihilations: u + d →W + , d + u →W −

0 0 u + u → Z , d + d → Z To obtain sufficient cms energies, proton and antiproton beams at SPS had energy of 270 GeV each W boson, UA1 detector in 1982

M. Cobal, PIF 2006/7 -A lepton with large momentum (> 10 GeV/c) emitted at a wide angle to the beam (> 5O )

-Large missing transverse momentum carried out by neutrino

If pt(W)=0 ⇒ missing pT = pT(l)

- From 43 events observed by UA1, the mass of W was defined as: M = 80.33 ± 0.15 GeV/c2 W

And the decay width as: ΓW = 2.07 ± 0.06 GeV Which corresponds to a lifetime of 3.2x10-25 s

-Branching ratios of leptonic decay modes of W are about 11% for each lepton generation

M. Cobal, PIF 2006/7 W bosons can be pair-produced in e+e- annihilation, and the up-to-date world average for the W-mass is: 2 MW = 80.39 ±0.06GeV /c

M. Cobal, PIF 2006/7 Z signature -Pair of leptons (e+e-) with very large momenta

-Mass of the Z0 is then invariant mass of leptons. Knowing M , M was predicted to be ~90 GeV/c2 w z

-

UA1

pp → ZX with Z → e+e− or Z → µ+ µ− M. Cobal, PIF 2006/7 Dilepton mass spectra near the Z0 peak (CDF Collaboration)

More precise methods give world average values of

2 MZ = 91.187±0.007 GeV/c 2 ΓZ = 2.490 ±0.007 GeV/c corresponding to a lifetime of 2.6x10-25 s Branching ratios of leptonic decay modes of Z0 are around 3.4% for each lepton generation

M. Cobal, PIF 2006/7 Carlo Rubbia (1934) Simon van der Meer (1925)

• Nobel Prize 1984 for their decisive contributions to the large project, which led to the discovery of the field particles W and Z, communicators of weak interaction

M. Cobal, PIF 2006/7 Sheldon Lee Glashow (1932) Abdus Salam (1926 – 1996) Steven Weinberg (1933) • Nobel Prize 1979 for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including, the prediction of the weak neutral current

M. Cobal, PIF 2006/7 • Self-coupling W-Z

• Couplings with photons

M. Cobal, PIF 2006/7