rpp2010-rev-top-quark - 1 THE TOP QUARK Updated April 2010...

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–1– THE TOP QUARK Updated April 2010 by T.M. Liss (Univ. Illinois) and A. Quadt (Univ. G¨ ottingen). A. Introduction: The top quark is the Q =2 / 3, T 3 =+1 / 2 member of the weak-isospin doublet containing the bottom quark (see the review on the “Standard Model of Electroweak Interactions” for more information). This note summarizes the properties of the top quark (mass, production cross section, decay branching ratios, etc. ), and provides a discussion of the experimental and theoretical issues involved in their determina- tion B. Top quark production at the Tevatron: All direct measurements of production and decay of the top quark have been made by the CDF and DØ experiments in p p collisions at the Fermilab Tevatron collider. The ±rst studies were performed during Run I, at s = 1.8 TeV, which was completed in 1996. The most recent, and most precise, measurements are from Run II, which started in 2001 at s = 1.96 TeV. This note will discuss primarily results from Run II. In hadron collisions, top quarks are produced dominantly in pairs through the QCD processes q q t t and gg t t .A t s =1.96 TeV the most recent calculations are at NLO with next-to-leading-log soft gluon resummation [1]. Cacciari et al. gives a production cross section of 7.61 pb for m t = 171 GeV/ c 2 with CTEQ6.5 PDFs. Over the range 150 GeV/ c 2 m t 190 GeV/ c 2 the calculated cross section decreases (increases) by approximately 0 . 24 pb / GeV for m t greater (less) than 171 GeV/ c 2 . A similar calculation by Kidonakis and Vogt yields a production cross section of 7.62 pb for m t = 171 GeV/ c 2 using CTEQ6.6M, with nearly the same mass-dependence. The di²erence in the central value obtained using di²erent PDFs is typically a few tenths of a pb or less. A detailed comparison of the most recent calculations is ongoing between the authors of the calculations. Approximately 85% of the production cross section at the Tevatron is from q q annihilation, with the re- mainder from gluon-gluon fusion [2]. Somewhat smaller cross sections are expected from electroweak single-top production 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– mechanisms, namely from q q ± t b [3] and qb q ± t [4], medi- ated by virtual s -channel and t -channel W bosons, respectively. The cross sections are calculated for m top = 175 GeV / c 2 to be 0 . 88 ± 0 . 11 pb for the s -channel, and 1 . 98 ± 0 . 25 pb for the t - channel [5], a little less than half of the t t production rate. The identiFcation of top quarks in the electroweak single-top chan- nel is much more difficult than in the QCD t t channel, due to a less distinctive signature and signiFcantly larger backgrounds. In top decay, the Ws and Wd Fnal states are expected to be suppressed relative to Wb by the square of the CKM matrix el- ements V ts and V td . Assuming unitarity of the three-generation CKM matrix, these matrix element values are estimated to be less than 0.043 and 0.014, respectively, implying a value of V tb > 0 . 999 (see the review “The CKM Quark-Mixing Matrix” for more information). With a mass above the
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rpp2010-rev-top-quark - 1 THE TOP QUARK Updated April 2010...

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