Top properties

The sensitivity which can be reached at the LHC in the measurement of many top proper- ties, like the top charge, the spin and spin correlations, the rare top decays associated to flavour changing neutral currents (FCNC: t→ qX, with X =γ,Z,g) and thett¯resonances,

Figure 6.73: Left plot: Number of jets for single top candidates in the t-channel and for the relevant backgrounds (ATLAS). Right plot: Reconstructed mass for single top candidates in the Wt channel and for the relevant backgrounds (CMS).

has been studied. ATLAS measured the precision of these measurements which can be obtained with 1 fb−1 _{of integrated luminosity citeatlasproperties. For the tests of physics}

beyond the SM associated with the production of top quarks, the 95% CL limit (in the absence of a signal) was also derived. Several sources of systematic errors were considered using an approach common to all studies, as mentioned at the beginning. Few examples follows: the sensitivity of the ATLAS experiment to the top quark charge measurement is such that already with 1 fb−1_{(using the semi-leptonic b-decay) it is possible to distinguish}

with a 5σ significance, between the SM scenario (q=2/3) and the alternative (q=4/3). A complete study of the precision reachable on the W polarisation fractions F0, FL and FR

(respectively 5%, 12% and 0.03) and the t¯t spin correlation parameters A and AD (50%

and 34%) has been performed in the semileptonict¯tchannel. Reconstructed and corrected angular distributions are used to extract polarisation measurements. Expected limits on the top quark rare decays through FCNC processes (t → qZ, qγ,qg) were set at 95% CL in the absence of signal . The discovery potential of the ATLAS experiment for the t¯t resonances decaying in the semileptonic channel, have been studied as a function of the resonance mass. Using this information, Kaluza-Klein gluon resonances with masses up to 1.5 TeV can be excluded with 1 fb−1 _{of data.}

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The Standard Model Higgs Boson

Authors: Sara Bolognesi, Chiara Mariotti and Daniele Trocino Revisors: Barbara Mele, Paolo Nason

7.29

Higgs Boson Mass

The Higgs boson mass is the only yet unknown free parameter of the SM. The Higgs in fact has never been observed experimentally and its mass cannot be predicted by the SM. It depends on the parametersv andλ, but while the former can be estimated by its relation with the constant GF of Fermi’s theory, the latter is characteristic of the field φ and

cannot be determined other than measuring the Higgs mass itself. Both theoretical and experimental constraints exist, including those from direct search at colliders, in particular LEP.