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Register a new userA new family dependent interaction in Tevatron top dilepton candidate events ?
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New family dependent fermionic interactions have been conjectured in several extensions of the Standard Model that range from Supersymmetry to composite theory up to flavor interactions. Strong constraints on these theoretical scenarios can be derived from light fermion phenomenology and from B-mesons studies. Corresponding constrains on the top quark sector are, on the other hand, rather week. Tevatron data, on top quark pair production and decay in dilepton channel, may suggest some deviation from the Standard Model expectations. Such a deviation can be successfully re-interpreted in terms of an exotic top decay that can arise in several theories beyond the Standard Model. Further investigations at present and future colliders will provide crucial tests on the models under discussion.
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CDF has observed a top forward-backward asymmetry discrepant with the Standard Model prediction at 3.4 sigma. We analyze models that could generate the asymmetry, including flavor-violating Ws, horizontal Z_Hs, triplet and sextet diquarks, and axigluons. We consider the detailed predictions of these models for the invariant mass and rapidity distributions of the asymmetry at the parton level, comparing against the unfolded parton-level CDF results. While all models can reproduce the asymmetry with the appropriate choice of mass and couplings, it appears at first examination that the extracted parton-level invariant mass distribution for all models are in conflict with Tevatron observations. We show on closer examination, however, that t tbar events in Z_H and W models have considerably lower selection efficiencies in high invariant mass bins as compared to the Standard Model, so that W, Z_H, and axigluon models can generate the observed asymmetry while being consistent with the total cross-section and invariant mass spectrum. Triplet and sextet models have greater difficulty producing the observed asymmetry while remaining consistent with the total cross-section and invariant mass distribution. To more directly match the models and the CDF results, we proceed to decay and reconstruct the tops, comparing our results against the raw CDF asymmetry and invariant mass distributions. We find that the models that successfully generate the corrected CDF asymmetry at the parton level reproduce very well the more finely binned uncorrected asymmetry. Finally, we discuss the early LHC reach for discovery of these models, based on our previous analysis [arXiv:1102.0018].
We carry out a comprehensive analysis of models for top A_{FB} at CDF in light of new top data arriving from the LHC. We begin with a careful Tevatron analysis, considering in general which sets of effective vertices give rise to a large forward-backward asymmetry while suppressing the contribution to the total t tbar cross-section. We show on general grounds that scalar models struggle to produce sufficient asymmetries consistent with CDF observations, while vector models can produce a large asymmetry with a less significant tension in the total cross-section and $tbar{t}$ invariant mass distribution at the Tevatron. We examine the essential observables of these models for top physics at LHC7 with 1 fb^{-1} of data, including the total cross-section, invariant mass distribution and number of additional jets in t tbar events. In the case of t-channel mediators, the LHC total cross-section places a strong constraint on light mediators, while the Tevatron invariant mass distributions place strong constraints on heavy mediators that are able to produce the asymmetry. Heavy axigluons are becoming increasingly squeezed by LHC7 t tbar and dijet resonance searches. We conclude that LHC7 top analyses are rapidly closing the window for viable models of the CDF top A_{FB}.
In this report a new search for a narrow-width heavy resonance decaying into top quark pairs (X -> ttbar) in ppbar collisions at sqrt(s)=1.96 TeV has been performed using data collected by the D0 detector at the Fermilab Tevatron collider. The analysis considers ttbar candidate events in the lepton+jets channel using a lifetime tag to identify b-jets and the ttbar invariant mass distribution to search for evidence of resonant production. The analyzed dataset corresponds to an integrated luminosity of approximately 370 pb^-1. Since no evidence for a ttbar resonance X is found, upper limits on sigma(X) x B(X -> ttbar) for different hypothesized resonance masses using a Bayesian approach are set. Within a topcolor-assisted technicolor model, the existence of a leptophobic Z boson with M(Z) < 680 GeV and width Gamma(Z) = 0.012 M(Z) can be excluded at 95% C.L..
We perform a model-independent study for top-antitop and top-top resonances in the dilepton channel at the Large Hadtron Collider. In this channel, we can solve the kinematic system to obtain the momenta of all particles including the two neutrinos, and hence the resonance mass and spin. For discovering top-antitop resonances, the dilepton channel is competitive to the semileptonic channel because of the good resolution of lepton momentum measurement and small standard model backgrounds. Moreover, the charges of the two leptons can be identified, which makes the dilepton channel advantageous for discovering top-top resonances and for distinguishing resonance spins. We discuss and provide resolutions for difficulties associated with heavy resonances and highly boosted top quarks.
The present talk is based on the assumption that New Bound States (NBSs) of top-anti-top quarks (named T-balls) exist in the Standard Model (SM): a) there exists the scalar 1S - bound state of 6t+6bar t - the bound state of 6 top-quarks with their 6 anti-top-quarks; b) the forces which bind these top-quarks are very strong and almost completely compensate the mass of the 12 top-anti-top-quarks forming this bound state; c) such strong forces are produced by the interactions of top-quarks via the virtual exchange of the scalar Higgs bosons having the large value of the top-quark Yukawa coupling constant g_tsimeq 1. Theory also predicts the existence of the NBS 6t + 5bar t, which is a color triplet and a fermion similar to the t-quark of the fourth generation. We have also considered b-replaced NBSs: n_b b + (6t + 6bar t - n_b t) and n_b b + (6t + 5bar t - n_b t), etc. We have estimated the masses of the lightest b-replaced NBS: M_{NBS}simeq (300 - 400) GeV, and discussed the larger masses of the NBSs. We have developed a theory of the scalar T-balls condensate, and predicted the existence of the three SM phases, calculating the top-quark Yukawa coupling constant at the border of two phases (with T-balls condensate and without it) equal to: g_t approx 1. The searching for the Higgs boson H and T-balls at the Tevatron and LHC is discussed.
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