rpp2010-rev-zprime-searches

rpp2010-rev-zprime-searches - 1 Z -BOSON SEARCHES Revised...

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–1– Z ± -BOSON SEARCHES Revised September 2009 by M.-C. Chen (UC Irvine) and B.A. Dobrescu (Fermilab). The Z ± boson is a hypothetical massive, electrically-neutral and color-singlet particle of spin 1. This particle is predicted in many extensions of the standard model, and has been the object of extensive phenomenological studies [1]. Z ± couplings to quarks and leptons. The couplings of a Z ± boson to the ±rst-generation fermions are given by Z ± μ ( g L u u L γ μ u L + g L d d L γ μ d L + g R u u R γ μ u R + g R d d R γ μ d R + g L ν ν L γ μ ν L + g L e e L γ μ e L + g R e e R γ μ e R ) , (1) where u, d, ν and e are the quark and lepton ±elds in the mass eigenstate basis, and the coefficients g L u , g L d , g R u , g R d , g L ν , g L e , g R e are real dimensionless parameters. If the Z ± couplings to quarks and leptons are generation-independent, then these seven parameters describe the couplings of the Z ± to all standard- model fermions. More generally, however, the Z ± couplings to fermions are generation-dependent, in which case Eq. (1) may be written with some generation indices i, j =1 , 2 , 3 labelling the quark and lepton ±elds, and with the seven coefficients promoted to 3 × 3 Hermitian matrices. These parameters describing the Z ± interactions with quarks and leptons are subject to some theoretical constraints. Quan- tum ±eld theories that include a heavy spin-1 particle are well behaved at high energies only if that particle is a gauge bo- son associated with a spontaneously broken gauge symmetry. Quantum e²ects preserve the gauge symmetry only if the cou- plings of the gauge boson to fermions satisfy a certain set of equations called anomaly cancellation conditions. Furthermore, the fermion charges under the new gauge symmetry are con- strained by the requirement that the quarks and leptons get masses from gauge-invariant interactions with Higgs doublets or whatever else breaks the electroweak symmetry. The relation between the couplings displayed in Eq. (1) and the gauge charges z L f i and z R f i of the fermions f = u, d, ν, e CITATION: K. Nakamura et al. (Particle Data Group), JPG 37 , 075021 (2010) (URL: http://pdg.lbl.gov) July 30, 2010 14:34
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–2– involves the unitary 3 × 3 matrices V L f and V R f that transform the gauge eigenstate fermions f Li and f Ri , respectively, into the mass eigenstate ones. In addition, the Z ± couplings are modiFed if the new gauge boson ˜ Z ± μ (in the gauge eigenstate basis) has a kinetic mixing ( χ/ 2) B μν ˜ Z ± μν with the hypercharge gauge boson B μ , or a mass mixing δM 2 ˜ Z μ ˜ Z ± μ with the linear combination ( ˜ Z μ ) of neutral bosons which has same couplings as the Z 0 in the standard model [2]. Both the kinetic and mass mixings shift the mass and couplings of the Z boson, such that the electroweak measurements impose upper limits on χ and 2 / ( M 2 Z ± M 2 Z ) of the order of 10 3 [3]. Keeping only linear terms in these two small quantities, the couplings of the mass-eigenstate Z ± boson are given by g L f = g z V L f z L f ( V L f ) + e c W Ã s W χM 2
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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.

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rpp2010-rev-zprime-searches - 1 Z -BOSON SEARCHES Revised...

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