Search

We examine flavour-changing neutral-current decays of the top quark, t→qγ, t→qZ, t→qH, and t→qg (with q=u,c), in the Alternative Left-Right Model, a left right-symmetric model featuring exotic quarks and light bosons. These decays have a very small probability of occurring within the Standard Model, but they can be enhanced in this model through the presence of the exotic states. While associated signals may be detected directly at the LHC, rare decays have the advantage of offering means to probe new particles indirectly, through loop-contributions. We perform a comprehensive analysis of the model's parameter space to demonstrate the possible existence of enhancements in the corresponding branching ratios, of 106 for the branching ratios B(t→uZ) and B(t→uH), and in the range of 102−104 for the other decays, relative to the Standard Model. We subsequently determine the preferred parameter space regions of the model in terms of potential of being reached in the near future.

The rare top quark decays mediated by a new neutral massive gauge boson that is predicted in models with extended gauge symmetries are studied. We focus on the processes t → c V , u V induced at the one loop level, where V = γ , g , by considering different extended models. It is found that, within a broad range of mass of the new neutral gauge boson, the models predict branching ratios for the decays in study that are competitive with respect to the corresponding branching ratios in the Standard Model (SM). In order to establish bound on our branching ratios, we consider the recent experimental bounds as m Z ′ ≥ 3. 8 – 4. 5 TeV , depending on the model, which also impose restrictions on our calculation. Even in this case, the resulting branching ratios are of the same order of magnitude as that predicted by the SM. It should be noted that for the case of two models studied here, since no experimental bound exists to compare with, the results could be important, as they are, in the best of cases, two orders of magnitude larger than the predicted by the SM. [ABSTRACT FROM AUTHOR]

Abstract: Precise measurements of the top quark decay properties at hadron colliders offer interesting new possibilities of testing the standard model. At the same time, recent intriguing experimental results concerning CP violation in the and systems have stimulated many studies of physics beyond the standard model. We investigate anomalous interactions as a possible source of new effects in oscillations within a model independent approach based on the assumptions of Minimal Flavor Violation. After matching our effective operators onto the low-energy effective Lagrangian describing meson mixing and evolving it down to the B-mass scale, we extract the preferred ranges of the anomalous interactions at the weak scale. These values are then compared to previously considered constraints coming from the rare radiative decay. Finally, we reconsider the associated effects in the decays and find that the W helicity fractions can deviate by as much as , from their standard model values, respectively. The deviations in in particular, can reach the level of expected precision measurements at the LHC. [Copyright &y& Elsevier]

Abstract: We study the prospects of measuring the CKM matrix element at the LHC with the top quarks produced in the processes and , and the subsequent decays and . To reduce the jet activity in top quark decays, we insist on tagging the leptonically, (), and analyse the anticipated jet profiles in the signal process and the dominant background from the decay . To that end, we analyse the V0 ( and Λ) distributions in the s- and b-quark jets concentrating on the energy and transverse momentum distributions of these particles. The V0s emanating from the branch have displaced decay vertexes from the interaction point due to the weak decays and the b-quark jets are rich in charged leptons. Hence, the absence of secondary vertexes and of the energetic charged leptons in the jet provide additional (b-jet vs. s-jet) discrimination in top quark decays. These distributions are used to train a boosted decision tree (BDT), a technique used successfully in measuring the CKM matrix element in single top production at the Tevatron. Using the BDT classifier, and a variant of it called BDTD, which makes use of decorrelated variables, we calculate the BDT(D)-response functions corresponding to the signal () and background (). Detailed simulations undertaken by us with the Monte Carlo generator PYTHIA are used to estimate the background rejection versus signal efficiency for three representative LHC energies , 10 TeV and 14 TeV, of which only the analysis for the case is shown in detail. We argue that a benchmark with 10% accuracy for the signal () at a background () rejection by a factor 103 (required due to the anticipated value of the ratio ) can be achieved at the LHC@14 TeV with an integrated luminosity of 10 fb−1. [ABSTRACT FROM AUTHOR]

The Flavor changing process of the top quark t → cγ is considered in a model with a single universal extra dimension which is compactified and the Rξ gauge is used. The Branching fraction associated can be bigger that the one predicted in the Standard Model because the dynamic arising from the extra dimension. When the inverse of the compactification radius is 1/R = 0,5 TeV, the branching fraction can get Br(t → cγ) ⋍10-10. [ABSTRACT FROM AUTHOR]