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Unformatted text preview: – 1– WBOSON SEARCHES Revised September 2009 by M.C. Chen (UC Irvine) and B.A. Dobrescu (Fermilab). The W boson is a hypothetical massive particle of electric charge ± 1 and spin 1, which is predicted in various extensions of the Standard Model. W couplings to quarks and leptons. The Lagrangian terms describing couplings of a W boson to fermions are given by W μ h u i ³ C R q ij P R + C L q ij P L ´ γ μ d j + ν i ³ C R l ij P R + C L l ij P L ´ γ μ e j i +h . c . (1) Here u, d, ν and e are the Standard Model fermions in the mass eigenstate basis, i, j = 1 , 2 , 3 label the fermion generation, and P R,L = (1 ± γ 5 ) / 2. The coeﬃcients C L q ij , C R q ij , C L l ij , C R l ij are complex dimensionless parameters. If C R l ij = 0, then the i th generation includes a righthanded neutrino. It is often assumed that there are correlations between the left and righthanded couplings [1]. Although this is true in some of the original models that include a W [2], there exist theories where all the left and righthanded couplings are free parameters. Unitarity considerations imply that the W is a gauge boson associated with a spontaneously brokengauge symmetry. This is true even when it is a composite particle ( e.g. , the charged techni ρ in technicolor theories [3]) , or a KaluzaKlein mode in theories where the W boson propagates in extra dimensions [4]. The simplest extension of the electroweak gauge group that includes a W is SU (2) 1 × SU (2) 2 × U (1), but larger groups are also encountered in some theories. A generic property of all these gauge theories is that besides a W they contain at least a Z boson, whose mass is typically comparable or smaller than M W . Despite the severe limits on Z bosons [5], theories where the properties of the new gauge bosons would allow the W to be discovered before the Z are quite common (for example, a leptophobic W decaying to t ¯ b may be observed easier than a Z in the t ¯ t final state which has higher backgrounds). CITATION: K. Nakamura et al. (Particle Data Group), JPG 37 , 075021 (2010) (URL: http://pdg.lbl.gov) July 30, 2010 14:34 – 2– The renormalizable photon W coupling is completely fixed by electromagnetic gauge invariance. By contrast, the renor malizable W W Z and W W Z couplings are modeldependent, and the same is true for the W couplings to Z or Higgs bosons. Depending on the symmetrybreaking sector, a treelevel mass mixing may be induced between the electricallycharged gauge bosons. Upon diagonalization of their mass matrix, the W − Z mass ratio and the couplings of the observed W are shifted from the Standard Model values. Given that these are well measured, the mixing angle between the two gauge bosons must be smaller than about 10 − 2 . Similarly, a Z − Z mixing is induced in generic theories, leading to even tighter constraints. There are, however, theories in which these mixings are negligible even when the W and Z...
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This note was uploaded on 06/07/2011 for the course PHYS 4132 taught by Professor Kutter during the Spring '11 term at University of Florida.
 Spring '11
 Kutter
 Charge, Mass

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