Viacheslav Bunichev

Method for simulating electroweak top-quark production events in the NLO approximation : Singletop event generator

A new method for simulating electroweak top-quark production processes is described along with its computer realization in the SingleTop event generator. Special attention is paid to the correct combination of events from two parts of the main t-channel production process: 2 → 2 with a b quark in the initial state and 2 → 3, where an additional b quark appears in the final state. Integration of these two contributions enables the generation of event samples including the first correction to the leading perturbation order, avoiding the double-counting problem and negative-weight events. The SingleTop generator is based on the complete set of the tree Feynman diagrams calculated by the CompHEP package.

Higgs-radion mixing in stabilized brane world models

We consider a quartic interaction of the Higgs and Goldberger-Wise fields, which connects the mechanism of the extra dimension size stabilization with spontaneous symmetry breaking on our brane and gives rise to a coupling of the Higgs field to the radion and its KK tower. We estimate a possible influence of this coupling on the Higgs-radion mixing and study restrictions on model parameters from the LHC data.

Prospects of mass measurements for neutral MSSM Higgs bosons in the intense-coupling regime at a linear collider

Abstract We analyze the prospects for detecting the three neutral Higgs bosons of the Minimal Supersymmetric extension of the Standard Model in the intense-coupling regime at e + e − colliders. Due to the small mass differences between the Higgs states in this regime and their relative large total decay widths, the discrimination between the particles is challenging at the LHC and in some cases even impossible. We propose to use the missing mass technique in the Higgs-strahlung process in e + e − collisions to distinguish between the two CP-even Higgs eigenstates h and H , relying on their b b ¯ decay in the b b ¯ l + l − event sample. Ah and AH associated production is then studied in the 4 b -jet event sample to probe the CP-odd A boson. At collider energies s ≃ 300 GeV and an integrated luminosity of 500 fb −1 , accuracies in the mass measurement of the CP-even Higgs bosons are expected to range from 100 to 300 MeV, while for the CP-odd A boson, accuracies of less than 500 MeV can be obtained.

Heavy scalar boson in view of the unconfirmed 750 GeV LHC diphoton excess

We discuss the impact of the constraints from the measurements of the parameters of the observed 125 GeV Higgs boson and from the unconfirmed 750 GeV diphoton excess in the LHC experiments on the properties of a possible extra scalar boson predicted in various Standard Model extensions. We consider an SM extension based on a stabilized brane-world model, in which the effective low-energy Lagrangian for the scalar degrees of freedom turns out to be very general and, for different values of the model parameters, reproduces the scalar field Lagrangians of various SM extensions by a singlet scalar. It is shown that in the simplest variant of the model, where only the gravitational degrees of freedom propagate in the bulk, the 125 GeV scalar state can be consistently interpreted as a Higgs-dominated state for a rather wide range of the model parameters, whereas the production cross section of a heavier radion-dominated state with mass 750 GeV or more turns out to be too small in the allowed region of the model parameter space for producing the wouldbe diphoton excess.

Optimization of the analysis of single top-quark production at the Large Hadron Collider (LHC)

Several steps of an investigation of single top-quark production at the Large Hadron Collider (LHC) are discussed. A number of general problems associated with basic steps of this analysis are discussed. The investigations are performed for the CMS (Compact Muon Solenoid detector) Collaboration, and detailed results will be presented in CMS publications. Three event generators used by the CMS Collaboration in simulating single top-quark production (SingleTop, MC@NLO, and a generator based on the MadGraph package) are compared in the present study. Schemes for optimizing the observation of a single top quark on the basis of the neural-network method are presented. The first steps in searches for possible anomalous vector couplings in the Wtb vertex that manifest themselves in processes of single top-quark production are described.

Prospects of a search for a new massless neutral gauge boson at the ILC

Prospects to search for a new massless neutral gauge boson, the paraphoton, in e{sup +}e{sup -} collisions at center-of-mass energies of 0.5 and 1 TeV are studied. The paraphoton naturally appears in models with Abelian kinetic mixing. A possible realistic model-independent lowest order effective Lagrangian contains magnetic interactions of the paraphoton with the standard model fermion fields. These interactions are proportional to the fermion mass and grow with energy, with however very weak paraphoton couplings to ordinary matter. At the ILC, a potentially interesting process to search for the paraphoton is its radiation from top quarks, so that the event topology of interest is a pair of acoplanar top quarks decaying to jets and missing energy. By combining many discriminating features of signal and background events, efficient paraphoton event selection is achieved, allowing us to set limits for the top-paraphoton coupling. Arguments in favor of the missing energy as the paraphoton with spin 1 are discussed.

Searches for anomalous coupling in top-quark interaction with the W boson and b quark, along with searches for quark-flavor-changing neutral currents, in an analysis of data from the CMS experiment

The results of searches for effects beyond the Standard Model in processes of single top-quark production in the CMS experiment are presented. Anomalous contributions of the vector and magnetic types in top-quark interaction with the W boson and b quark and quark-flavor-changing neutral currents in top-quark interaction with the c or u quark via gluon exchange were studied. The respective analysis was performed with the aid of Bayesian neural networks. No statistically significant deviations were found, and upper limits on anomalous couplings at a 95% confidence level were set.

Testing extra dimensions hypothesis in high energy physics

M. Smolyakov SINP MSU E-mail: smolyakov@theory.sinp.msu.ru The history of the hypothesis about extra dimensions is revi ewed and the physics and phenomenology of models with large extra dimensions are discussed with an emphasis on the RandallSundrum model with two branes. We argue that the original Ran dall-Sundrum model is phenomenologically unacceptable, because the massless radio n couples too strongly to matter on the negative tension brane, and should be stabilized. For the st abilized Randall-Sundrum model, the physical degrees of freedom are explicitly isolated. The pr oblem of constructing brane world models with more than one extra dimensions is touched upon. T he phenomenology of the stabilized Randall-Sundrum model is discussed in detail and spec ific changes of the distribution tails for the processes of SM particles scattering at the Tevatron and the LHC due to the contribution of the KK tower are found. We also consider the case of the univ ersal extra dimensions, where the SM gauge bosons can propagate in the bulk, and present the cross-sections of the single top and Drell-Yan production processes taking into account the contributions of the Standard Model gauge bosons, of their first KK modes and of the corresponding KK towers and discuss the impact of the interference between them. For pp-collisions at the L HC with the center of mass energy 14TeV we also found changes of the distribution tails due to t he interference effects. Such a modification of distribution tails is characteristic for th e processes mediated by particles coming from extra dimensions and should always be taken into accoun t when looking for them.

The specificity of searches for W′, Z′ and γ′ coming from extra dimensions

A bstractWe discuss the specificity of searches for hypothetical W′, Z′ and γ′ bosons at hadron colliders in single top quark and μ+νμ production and Drell-Yan processes assuming these particles to be the Kaluza-Klein excitations of the gauge bosons of the Standard Model. In this case any process mediated by W is also mediated by the whole KK tower of its excitations, whereas to the processes mediated by Z and γ there is not only a contribution from their KK towers, but also from that of the graviton. The contributions of the towers above W′, Z′ and γ′ and above the first excitation of the graviton are included with the help of effective four-fermion Lagrangians. We compute the cross-sections of these processes taking into account the contributions of the Standard Model gauge bosons, of their first KK modes and of the corresponding KK towers and discuss the impact of the interference between them. For pp-collisions at the LHC with the center of mass energy 14 TeV we found specific changes of the distribution tails due to the interference effects. Such a modification of distribution tails is characteristic for the processes mediated by particles coming from extra dimensions and should always be taken into account when looking for them.

Geometry, physics, and phenomenology of the Randall-Sundrum model

This review is devoted to a discussion of the geometry and physics of the Randall-Sundrum model with two branes. The main focus is on its phenomenologically acceptable stabilized version. For this model, the second variation Lagrangian is constructed, its gauge invariance is studied, corresponding equations of motion are derived and decoupled, and physical degrees of freedom are explicitly isolated. For the stabilized model, possible phenomenological manifestations in experiments at the Tevatron and LHC are discussed for cases in which the center-of-mass energy is below or above the production threshold of the first Kaluza-Klein tensor resonance.