rpp2010-rev-z-boson - – 1– THE Z BOSON Revised March...

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Unformatted text preview: – 1– THE Z BOSON Revised March 2009 by M. Gr¨unewald (U. College Dublin and U. Ghent), and A. Gurtu (Tata Inst.). Precision measurements at the Z-boson resonance using electron–positron colliding beams began in 1989 at the SLC and at LEP. During 1989–95, the four LEP experiments (ALEPH, DELPHI, L3, OPAL) made high-statistics studies of the pro- duction and decay properties of the Z . Although the SLD experiment at the SLC collected much lower statistics, it was able to match the precision of LEP experiments in determining the effective electroweak mixing angle sin 2 θ W and the rates of Z decay to b- and c-quarks, owing to availability of polarized electron beams, small beam size, and stable beam spot. The Z-boson properties reported in this section may broadly be categorized as: • The standard ‘lineshape’ parameters of the Z con- sisting of its mass, M Z , its total width, Γ Z , and its partial decay widths, Γ(hadrons), and Γ( ) where = e, μ, τ, ν ; • Z asymmetries in leptonic decays and extraction of Z couplings to charged and neutral leptons; • The b- and c-quark-related partial widths and charge asymmetries which require special techniques; • Determination of Z decay modes and the search for modes that violate known conservation laws; • Average particle multiplicities in hadronic Z decay; • Z anomalous couplings. The effective vector and axial-vector coupling constants describing the Z-to-fermion coupling are also measured in p ¯ p and ep collisions at the Tevatron and at HERA. The corresponding cross-section formulae are given in Section 39 (Cross-section formulae for specific processes) and Section 16 (Structure Functions) in this Review . In this minireview, we concentrate on the measurements in e + e − collisions at LEP and SLC. The standard ‘lineshape’ parameters of the Z are deter- mined from an analysis of the production cross sections of CITATION: K. Nakamura et al. (Particle Data Group), JPG 37 , 075021 (2010) (URL: http://pdg.lbl.gov) July 30, 2010 14:34 – 2– these final states in e + e − collisions. The Z → ν ν ( γ ) state is identified directly by detecting single photon production and indirectly by subtracting the visible partial widths from the total width. Inclusion in this analysis of the forward-backward asymmetry of charged leptons, A (0 , ) FB , of the τ polarization, P ( τ ), and its forward-backward asymmetry, P ( τ ) fb , enables the separate determination of the effective vector ( g V ) and ax- ial vector ( g A ) couplings of the Z to these leptons and the ratio ( g V / g A ), which is related to the effective electroweak mixing angle sin 2 θ W (see the “Electroweak Model and Constraints on New Physics” review)....
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rpp2010-rev-z-boson - – 1– THE Z BOSON Revised March...

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