Search forZ′resonances decaying tott¯indilepton+

Abstract

A search for resonances decaying to top quark-antiquark pairs is performed using a dilepton þ jets data sample recorded by the CMS experiment at the LHC in pp collisions at ffiffi ffi s p ¼ 7 TeV corresponding to an integrated luminosity of 5:0 fb À1 . No significant deviations from the standard model background are observed. Upper limits are presented for the production cross section times branching fraction of top quark-antiquark resonances for masses from 750 to 3000 GeV. In particular, the existence of a leptophobic topcolor particle Z 0 is excluded at the 95% confidence level for resonance masses M Z 0 < 1:3 TeV for À Z 0 ¼ 0:012M Z 0 , and M < 1:9 TeV for À Z 0 ¼ 0:10M Z 0 .

Machine learning techniques in searches fortt̄hin theh→bb̄ decay channel

“…Such techniques include artificial neural networks (ANN) with simple network structures [6][7][8] and Boosted Decision Trees [8][9][10], and build on significant recent interest in particle physics in studying and adapting machine learning (ML) techniques to improve object reconstruction and identification [11][12][13][14][15][16][17], triggering on interesting physics [18,19], and to classify events as in tth analyses and other searches [20][21][22][23][24][25]. It should be noted that some tth analyses also make use of "matrix element methods," which attempt to estimate leading order probabilities for events to come from signal or background processes by integrating over their production probabilities in a large multi-dimensional space [6,26,27], and are complimentary to the approaches studied here.…”

The MSSM Higgs sector at a high MSUSY: reopening the low tan β regime and heavy Higgs searches

“…While there was no search for the SM Higgs in this channel, the ATLAS [148] and CMS [149] collaborations have looked for heavy resonances (such as new Z ′ gauge bosons in extended gauge models or Kaluza-Klein excitations in scenarios with extra space-time dimensions) that decay into tt pairs with the data collected at the 7 TeV run. The lepton+jets final state has been studied in the topology where the top quarks are highly boosted which allows a good discrimination from the continuum tt background [150] (the ATLAS and CMS collaborations searches assume resonance masses M tt > ∼ 700 GeV to benefit from this topology).…”

“…the SM Higgs boson almost exclusively decays into W W or ZZ states, we will assume the cross section times decay branching ratio upper limit that has been given in this CMS study, -H/A → tt: we make use of the ATLAS [148] and CMS [149] searches at √ s = 7 TeV for new Z ′ or Kaluza-Klein gauge bosons that decay into tt pairs in the lepton+jets final state topology. Considering a small total width for the resonance, limits on the cross sections times branching ratio of ≈ 6, 3 and 0.75 pb for a resonance mass of, respectively, 350, 500 and 1000 GeV are assumed.…”

Colour octet extension of 2HDM

“…48 However, this is a for a model with production cross-section a few thousand times larger. Similarly, bounds on Z resonances decaying to tt pairs 49 can be interpreted as posing no significant constraint for these scalars. It is known 8,50 that the cross sections for producing pairs of coloured scalars are larger than those for single scalar production for much of the parameter space.…”