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Register a new userHard Photons in Hadroproduction of Top Quarks with Realistic Final States
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We present a complete description of top quark pair production in association with a hard photon in the dilepton channel. Our calculation is accurate to NLO in QCD. It is based on matrix elements for $e^+ u_e mu^-bar{ u}_mu b bar{b}gamma$ production and includes all resonant and non-resonant diagrams, interferences, and off-shell effects of the top quarks and the $W$ gauge bosons. This calculation constitutes the first full computation for top quark pair production with a final state photon in hadronic collisions at NLO in QCD. Numerical results for total and differential cross sections are presented for the LHC at a centre-of-mass energy of $sqrt{s}=13$ TeV. For a few observables relevant for new physics searches, beyond some kinematic bounds, we observe shape distortions of more than $100%$. In addition, we confirm that the size of the top quark off-shell effects for the total cross section is consistent with the expected uncertainties of the narrow width approximation. Results presented here are not only relevant for beyond the Standard Model physics searches but also important for precise measurements of the top-quark fiducial cross sections and top-quark properties at the LHC.
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Qualitative model for hadroproduction in high energy collisions considering two components (thermal and hard) to hadroproduction is proposed. Inclusive pseudorapidity distributions, $dsigma/deta$, and transverse momentum spectra, $d^2sigma/(deta dp_T^2)$, measured by different collaborations are considered in terms of this model. The shapes of the pseudorapidity distributions agree with that one can expect from the qualitative picture introduced. Finally, the differences between charged particle spectra produced in inclusive and diffractive events are discussed and the absence of the thermal component in the latter is observed.
We develop a new method for tagging jets produced by hadronically decaying top quarks. The method is an application of shower deconstruction, a maximum information approach that was previously applied to identifying jets produced by Higgs bosons that decay to bottom quarks. We tag an observed jet as a top jet based on a cut on a calculated variable that is an approximation to the ratio of the likelihood that a top jet would have the structure of the observed jet to the likelihood that a non-top QCD jet would have this structure. We find that the shower deconstruction based tagger can perform better in discriminating boosted top quark jets from QCD jets than other publicly available tagging algorithms.
Models incorporating flavoured dark matter provide an elegant solution to the dark matter problem, evading the tight LHC and direct direction constraints on simple WIMP models. In Dark Minimal Flavour Violation, a simple framework of flavoured dark matter with new sources of flavour violation, the constraints from thermal freeze-out, direct detection experiments, and flavour physics create well-defined benchmark scenarios for these models. We study the LHC phenomenology of four such scenarios, focusing on final states where a single top quark is produced accompanied by no jets, one jet from the fragmentation of light quarks or a $b$-tagged jet. For each of these signatures we develop a realistic LHC analysis, and we show that the proposed analyses would increase the parameter space coverage for the four benchmarks, compared to existing flavour-conserving LHC analyses. Finally we show the projected discovery potential of the considered signatures for the full LHC statistics at 14 TeV, and for the High Luminosity LHC.
A very light scalar top (stop) superpartner is motivated by naturalness and electroweak baryogenesis. When the mass of the stop is less than the sum of the masses of the top quark and the lightest neutralino superpartner, as well as the of the masses of the lightest chargino and the bottom quark, the dominant decay channels of the stop will be three-body, four-body, or flavour violating. In this work, we investigate the direct and indirect constraints on a light stop, we compute the relative decay branching fractions to these channels, and we study the sensitivity of existing LHC searches to each of them.
We show how single top production at the LHC can be used to discover (and characterize the couplings of) B quarks, which are an essential part of many natural models of new physics beyond the Standard Model. We present the B effective model and concentrate on resonant production via a colored anomalous magnetic moment. Generally, Bs preferentially decay into a single top quark produced in association with a W boson; thus, this production process makes associated single top production essential to B searches at the LHC. We demonstrate the background processes are manageable and the signal cross section is sufficient to yield a large signal significance even during the 7 TeV LHC run. Specifically, we show that B masses of 700 GeV or more can be probed. Moreover, if a B is found, then the chirality of its coupling can be determined. Finally, we present signal cross sections for several different LHC energies.
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