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Register a new userFinding top quarks with shower deconstruction
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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.
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We introduce shower deconstruction, a method to look for new physics in a hadronic environment. The method aims to be a full information approach using small jets. It assigns to each event a number chi that is an estimate of the ratio of the probability for a signal process to produce that event to the probability for a background process to produce that event. The analytic functions we derive to calculate these probabilities mimic what full event generators like Pythia or Herwig do and can be depicted in a diagrammatic way. As an example, we apply this method to a boosted Higgs boson produced in association with a Z-boson and show that this method can be useful to discriminate this signal from the Z+jets background.
We develop a matrix element based reconstruction method called event deconstruction. The method uses information from the hard matrix element and a parton shower to assign probabilities to whether a final state was initiated by a signal or background process. We apply this method in the signal process of a Z decaying to boosted top quarks in an all hadronic final state and discuss envisioned improvements of the method. We find that event deconstruction can considerably improve on existing reconstruction techniques.
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.
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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