The fingerprints of the Top Quark

1. Top Turns Ten Symposium, Fermilab, October 21st 2005 Jessica Levêque, University of Arizona Search for resonances Top Quark Mass Measurement Top Quark Branching Fraction W Helicity Measurement Ten years after its discovery, the top quark is now being studied with improved accelerator and detectors at Fermilab. A precise determination of its characteristics, through the measurement of its mass, production modes and decay properties allows us to test the Standard model very deeply, constraint the Higgs boson mass and look for signs of new physics. The fingerprints of the Top Quark Projected uncertainty on the top quark mass as a function of integrated luminosity The top quark mass is a fundamental parameter of the Standard Model, but its value can not be predicted. DØ has measured the top quark mass using different techniques : - In the lepton+jets channels, the Template method uses a kinematic fit to reconstruct the top quark’s invariant mass. The mass spectrum is then fitted using template distributions for signal and background. In a different approach used in the Ideogram and Matrix Element methods an analytical likelihood including a description of the differential cross-sections and full event kinematics is taken to calculate a probability per event to be signal or background. - In the dilepton channel, the Matrix Weighting method is used. The production of 2 neutrinos leads to an underconstrained event kinematics. As proposed by Dalitz, Goldstein and Kondo, a hypothesized value of the top quark mass is used to determine the top quark pair momenta. A weight distribution is derived for each event as a function of the top quark mass. Using its peak as an estimator of the mass for each event, and comparing the resulting distribution in data to signal and background templates provides an estimate of the top quark mass. The goal is to measure the top quark mass with 1% precision during Run II. Summary of the most recent top quark mass measurements In the lepton+jets final states, the top quark pairs are selected with 2 analyses, topological and b-tagged. The four vectors of all final state particles are reconstructed using a kinematic fit. f+ is then extracted from a likelihood fit to the angular distribution of the charged lepton cos q*, using template distributions for various right-handed fractions. In the dilepton channel, f+ is extracted from a fit of the charged lepton momentum distribution. In the Standard Model, the W boson coming from a top quark decay is either left-handed (f- = 70%) or longitudinal (f0 = 30%). A measured fraction of right-handed W bosons f+ 0 would be an unambiguous signature of new physics. The W boson helicity is measured through the angle q between the charged lepton and the top quark direction in the W boson rest frame. The figure on the left shows the Standard Model prediction. L=230 pb-1 Combined result : The very large mass of the top quark could be an indication that this particle plays a special role in the electroweak symmetry breaking mechanism. Topcolor-assisted-technicolor models predict the existence of a new gauge boson Z’ which is strongly coupled to the third generation of quarks. An evidence for this new boson would be a significantly higher top quark production cross-section than predicted by the Standard Model, or a peak in the top quark’s pair invariant mass distribution. A measurement of R was performed in the lepton+jets channel. The excess of events with 1 and 2 or more b-jets after background estimate is used to simultaneously measure the top quark cross-section and the branching ratio fraction. Assuming the CKM matrix unitarity with exactly 3 quark generations, the Standard Model predicts the following branching ratio fraction : No evidence for a resonance was found. Upper limits on a leptophobic Z’ boson mass can be derived : Any deviation would be a hint for new physics.