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Weak Interactions & Neutral Currents

M&S CH 9.1. Charged current. strangeness changing neutral current. Until the the mid-1970 ’ s all known weak interaction processes could be described by the exchange of a charged , spin 1 boson, the W boson.

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Weak Interactions & Neutral Currents

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  1. M&S CH 9.1 Charged current strangeness changing neutral current Until the the mid-1970’s all known weak interaction processes could be described by the exchange of a charged, spin 1 boson, the W boson. The initial and final state quarks or leptons differed by one unit of electric charge. Weak interactions mediated by a W-boson are called “charged current” interactions. Weak Interactions & Neutral Currents A key prediction of the Glashow-Weinberg-Salam model was the existence of weak interactions mediated by the Z0, a neutral vector boson. In the GWS model the Z0 did not change the flavor of the lepton or quark. Recall: the GIM mechanism was invented to eliminate (first order) neutral current reactions where the flavor of the quark or lepton changed. The measured branching fractions of K0®m+m- and K+®m+ne provided evidence for the absence of strangeness changing neutral currents. nm m+ m- m+ W+ ??0 u d s s

  2. The GIM mechanism eliminated strangeness changing neutral currents by adding a fourth quark (charm): Weak Interactions & Neutral Currents If we add the two amplitudes together we get: s¢ s¢ d¢ d¢ ??0 ??0 The strangeness changing terms are gone but there are still neutral current terms! In 1973 the first experimental evidence for neutral currents was found using the reaction: This reaction cannot occur by W exchange!

  3. Where do the neutrinos come from ? Proton-nucleon collisions produce lots of p's and K's which decay into neutrinos.   Neutrino BeamAnti-Neutrino BeamBranching Ratio p+®m+nmp-®m-nm 0.999 p+®e+nep-®e-ne»10-4 K+®m+nm K-®m-nm 0.63 K+®e+ne K-®e-ne»10-5 K+®p0m+nm K-®p0m-nm»0.03 K+®p0e+ne K-®p0e-ne»0.05 Also get neutrinos from muon decay: Rough calculation of nm/ne ratio: a) neglect neutrinos from muon decay. b) assume we produce 10X as many pions as kaons in a proton-nucleus collision. How do we produce a neutrino beam? have branching ratios of 100%. Relatively easy to make a beam of high energy nm's, but hard to make a "pure" beam of high energy ne's. To make a pure beam of ne's use nuclear beta decay which is below the energy threshold to produce muons.

  4. Many other examples of neutral current interactions were discovered in the 1970’s W exchange (charged current): nm+N ®m-+X Z exchange (neutral current): nm+N ®nm+X Neutrino cross section for neutral currents predicted to be 1/3 of charged current cross section. Neutrino Induced Neutral Current Interactions Typical layout for a neutrino beam line. “Observation of Neutrino-Like Interactions Without Muon Or Electrons in the Gargamelle Neutrino Experiment” PL, V45, 1973. “CC”= charged current event (e or m in final state) “NC”= neutral current event (no e or m in final state) Gargamelle was the name of the bubble chamber (BC). Expect neutrino interactions to be uniformly distributed in BC. Background from neutrons and K0’s expected mostly in front of BC. Data in good agreement with expectation of Standard Model. Distributions along beam axis

  5. qW »28 degrees M&S 9.2.1, C.6.3 M&S use gEM=gZcosqW, eq. 9.8. However, this is NOT the factor that would be used to describe coupling of the Z boson to fermions described on the next slide. The correct factor is gEM=gZsinqWcosqW. Weak Interactions & Neutral Currents By 1983 CERN was able to produce “real” Z’s and W’s and measure their properties: mass, branching fractions, and decay distributions. By 1989 accelerators that could produce Z0’s via e+e-®Z were online: SLC @ SLAC and LEP @ CERN By 1999 CERN could produce W’s via e+e-® W+W-. The model of Glashow, Weinberg and Salam becomes the Standard Model. In the standard model the coupling of the Z to quarks and leptons is different than the coupling of the W to quarks and leptons. The W has a “V-A” coupling to quarks and leptons: gm(1-g5) The Z coupling to quarks and leptons is much more complicated than the W’s: gm(cvf-cafg5)= gm(I3f-I3f g5-2Qfsin2qW) I3 is the third component of “weak” isospin f=fermion type qW is the “Weinberg angle” The Weinberg angle is a fundamental parameter of the standard model. It relates the “strength” of the W and Z boson couplings to the EM coupling: gEM=gWsinqW gEM=gZsinqWcosqW

  6. gZgm(cvf-cafg5) First order calculation Weak Interactions & Neutral Currents The standard model gives us the following for the Z boson coupling to a fermion anti-fermion pair: gm(cvf-cafg5)= gm(I3f-I3f g5-2Qfsin2qW) fermion (f) cv ca I3 Q ne, nm, nt 1/2 1/2 1/2 0 e, m, t -1/2+2sin2qW -1/2 -1/2 -1 u, c, t 1/2-4/3sin2qW 1/2 1/2 2/3 d, s, b -1/2-2/3sin2qW -1/2 -1/2 -1/3 f f Z0 In the standard model the masses of the W and Z are related by: MW=MZcosqW Interesting new development (2001) with measurement of sin2qW. World average for sin2qW =0.2227±0.0003 (mainly from LEP @ CERN) NuTeV result for sin2qW = 0.2277±0.0013(stat) ±0.0009(syst) NuTeV is a Fermilab neutrino experiment. Their result for sin2qW disagrees with previous measurements, may point to physics beyond standard model.

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