Viktor Savrin

We present a new version of the CompHEP program (version 4.4). We describe shortly new issues implemented in this version, namely, simplification of quark flavor combinatorics for the evaluation of hadronic processes, Les Houches Accord-based CompHEP-PYTHIA interface, processing the color configurations of events, implementation of MSSM, symbolical and numerical batch modes, etc. We discuss how the CompHEP program is used for preparing event generators for various physical processes. We mention a few concrete physics examples for CompHEP-based generators prepared for the LHC and Tevatron.

CompHEP is a package for automatic calculations of elementary particle decay and collision properties in the lowest order of perturbation theory (the tree approximation). The main idea prescribed into the CompHEP is to make available passing on from the Lagrangian to the final distributions effectively with a high level of automation.

We discuss the SM Higgs discovery potential of LHC in the reaction pp→H+jet→γγ+jet when the jet is observed at sufficiently high Et to be reliably identified. We conclude that this channel gives promising discovery possibilities for the Higgs boson mass range 100–140 GeV, during LHC operation at a low luminosity. With 30 fb−1 of accumulated data and for MH=120 GeV about 100 signal events could be observed with the number of background events larger by a factor of 2 only, showing a signal significance S/B∼7. We use the difference of distributions in the partonic subprocess energy ŝ for the signal and background for a better separation of the signal.

A possible mechanism for the GSI e+e- narrow resonances as being due to non-perturbative effects of conventional QED is proposed. An application of the quasi-potential approach discloses a set of new resonances in a system of two charged particles. Numerical calculations agree with data on e+e- and pp narrow resonances. Additional new resonances are predicted for e+e-, pp, e-e- systems.

The recently reported effect of long-range near-side angular correlations at LHC occurs for large multiplicities of particles with $1\,GeV\,<p_T\,<\,3\,GeV$. To understand the effect several possibilities have been discussed. In the letter we propose a simple qualitative mechanism which corresponds to gluon bremstralung of quarks moving with acceleration appropriate to the quark--anti-quark string. The smallness of azimuthal angle difference $\Delta \phi$ along with large $\Delta \eta$ at large multiplicities in this interval of $p_T$ are natural in the mechanism. The mechanism predicts also bremstralung photons with mean values of $p_T \approx 2.9$ and $0.72\,GeV$.

This Report summarises the activities of the "SM and Higgs" working group for the Workshop "Physics at TeV Colliders", Les Houches, France, 2-20 May, 2005. On the one hand, we performed a variety of experimental and theoretical studies on standard candles (such as W, Z, and ttbar production), treating them either as proper signals of known physics, or as backgrounds to unknown physics; we also addressed issues relevant to those non-perturbative or semi-perturbative ingredients, such as Parton Density Functions and Underlying Events, whose understanding will be crucial for a proper simulation of the actual events taking place in the detectors. On the other hand, several channels for the production of the Higgs, or involving the Higgs, have been considered in some detail. The report is structured into four main parts. The first one deals with Standard Model physics, except the Higgs. A variety of arguments are treated here, from full simulation of processes constituting a background to Higgs production, to studies of uncertainties due to PDFs and to extrapolations of models for underlying events, from small-$x$ issues to electroweak corrections which may play a role in vector boson physics. The second part of the report treats Higgs physics from the point of view of the signal. In the third part, reviews are presented on the current status of multi-leg, next-to-leading order and of next-to-next-to-leading order QCD computations. Finally, the fourth part deals with the use of Monte Carlos for simulation of LHC physics.

This Report summarises the activities of the SM and working group for the Workshop Physics at TeV Colliders, Les Houches, France, 2-20 May, 2005. On the one hand, we performed a variety of experimental and theoretical studies on standard candles (such as W, Z, and ttbar production), treating them either as proper signals of known physics, or as backgrounds to unknown physics; we also addressed issues relevant to those non-perturbative or semi-perturbative ingredients, such as Parton Density Functions and Underlying Events, whose understanding will be crucial for a proper simulation of the actual events taking place in the detectors. On the other hand, several channels for the production of the Higgs, or involving the Higgs, have been considered in some detail. The report is structured into four main parts. The first one deals with Standard Model physics, except the Higgs. A variety of arguments are treated here, from full simulation of processes constituting a background to Higgs production, to studies of uncertainties due to PDFs and to extrapolations of models for underlying events, from small-$x$ issues to electroweak corrections which may play a role in vector boson physics. The second part of the report treats Higgs physics from the point of view of the signal. In the third part, reviews are presented on the current status of multi-leg, next-to-leading order and of next-to-next-to-leading order QCD computations. Finally, the fourth part deals with the use of Monte Carlos for simulation of LHC physics.

The report summarizes the results of the activities of the Working Group on Event Generators for WW Physics at CERN during 1995.

The top quark and electroweak bosons (W and Z) represent the most massive fundamental particles yet discovered, and as such refer directly to the Standard Model's greatest remaining mystery: the mechanism by which all particles gained mass. This report summarizes the work done within the top-ew group of the Tevatron-for-LHC workshop. It represents a collection of both Tevatron results, and LHC predictions. The hope is that by considering and comparing both machines, the LHC program can be improved and aided by knowledge from the Tevatron, and that particle physics as a whole can be enriched. The report includes measurements of the top quark mass, searches for single top quark production, and physics of the electroweak bosons at hadron colliders.

At present time when a new generation of TeV colliders are beginning to operate one needs to calculate cross-sections for a great number of various reactions. Such calculations are united in the framework of the collider physical program, providing definite predictions how to detect the signatures of the new physics and separate them from the background. The CompHEP package was created for calculation of decay and high energy collision processes of elementary particles in the lowest order (tree) approximation. The main idea put into the CompHEP was to make available passing from the lagrangian to the final distributions effectively with the high level of automatization what is extremely needed in collider physics.

The PMTs of the CMS Hadron Forward calorimeter were found to generate a large size signal when their windows were traversed by energetic charged particles. This signal, which is due to Čerenkov light production at the PMT window, could interfere with the calorimeter signal and mislead the measurements. In order to find a viable solution to this problem, the response of four different types of PMTs to muons traversing their windows at different orientations is measured at the H2 beam-line at CERN. Certain kinds of PMTs with thinner windows show significantly lower response to direct muon incidence. For the four anode PMT, a simple and powerful algorithm to identify such events and recover the PMT signal using the signals of the quadrants without window hits is also presented. For the measurement of PMT responses to Čerenkov light, the Hadron Forward calorimeter signal was mimicked by two different setups in electron beams and the PMT performances were compared with each other. Superior performance of particular PMTs was observed.

CompHEP, as a partonic event generator, and PYTHIA, as a generator of final states of detectable objects, are interfaced. Thus, integrated tool is proposed for simulation of (almost) arbitrary collision processes at the level of detectable particles. Exact (multiparticle) matrix elements, convolution with structure functions, decays, partons hadronization and (optionally) parton shower evolution are basic stages of calculations. The PEVLIB library of event generators for LHC processes is described.

The structure functions of hadrons in deep inelastic scattering are calculated in the framework of the single-time formulation of QFT. The initial hadron is considered as a bound state of the quark-antiquark pair and the structure functions are expressed through the relativistic wave function of the bound state which obeys the covariant quasi-potential equation. A new scaling variable is introduced and explicit expressions for the scaling and prescaling parts of the structure functions are derived. It is shown that the prescaling and exclusive threshold asymptotics of the structure functions in the leading approximation contain only logarithmic terms. The exact solution of the quasi-potential equation for the relativistic potential with QCD large-Q 2 behaviour is used in order to calculate the structure functions.

In the paper the existence of new quasi-stationary levels in the relativistic Coulomb problem is predicted and their positions are calculated on the basis of the numerical solution of the quasi-potential equation. The results obtained are used for interpretation of the narrow electron-positron resonances revealed in heavy ions collisions and of the diproton resonances observed in neutron-proton interactions. The close relationship of the observed states with the von Neumann-Wigner levels embedded in the continuum, is indicated.

A mechanism of production of narrow e+e− peaks in heavy ion collisions is discussed. It is based on results giving new levels of a relativistic Coulomb system according to the numerical solution of the single-time relativistic equation. The mechanism describes the totality of data and provides a number of definite predictions.

In the framework of the relativistic independent quark model the parameters of the QCD-motivated static potential and the quark masses are calculated on the basis of the 1−− meson mass spectra. The value of the confining potential coefficient is found to be (0.197±0.005) GeV2 for quark–antiquark interaction independently on their flavours. The dependence of the quasi-Coulombic potential strength on the interaction distance are consistent with the QCD-motivated behaviour. The qq̄-separations are evaluated and the e+e− decay widths are estimated with the help of relativistic modification of the Van Royen–Weisskopf formula.

The CompHEP package was developed for calculations of decay and high energy collision processes with, correspondingly, up to 5 and 4 final particles in the lowest order (tree) approximation. The main idea put into CompHEP was to make available passing from the Lagrangian to final distributions efficiently with high level automation what is extremely needed in collider physics. The present talk describes a general structure of the CompHEP facilities and reports some physical results obtained with its help. The main purpose of the talk is to attract the attention of high energy physicists to this user-friendly package which is completely aimed at making easier their routine and tedious calculations in the TeV region.

We present a new version of the CompHEP program package, version 4.5. We describe new options and techniques implemented in the version: interfaces to ROOT and HERWIG, parallel calculations, generation of the XML-based header in event files (HepML), full implementation of the Les Houches agreements (LHA I, SUSY LHA, LHA PDF, Les Houches Event format), cascade matching for intermediate scalar resonances, etc.

In this paper we consider single leptoquark resonance production associated with the emission of a hard photon. We have obtained analytical formulæ for differential and total cross sections for the cases of scalar and vector leptoquarks. We have found that in reactions with scalar leptoquarks there is no photon radiation in some directions depending on the leptoquark electric charge (the radiative amplitude zero — the RAZ effect). For vector leptoquarks the exact RAZ is present only in the case of Yang-Mills coupling. We propose to use the RAZ effect to determine the types of leptoquarks. We conclude also that this effect opens a possibility to measure the leptoquark anomalous magnetic moment.

The report summarizes the results of the activities of the Working Group on Event Generators for WW Physics at CERN during 1995.

New developments of the CompHEP package and its applications to the top quark and the Higgs boson physics at the LHC collider are reviewed. These developments were motivated mainly by the needs of experimental searches of DO (Tevatron) and CMS (LHC) collaborations where identification of the top quark and the Higgs boson in the framework of the Standard Model (SM) or possible extensions of the SM played an important role. New useful features of the CompHEP Graphics User Interface (GUI) are described.

Parameters of the QCD-motivated static potential are evaluated on the basis of 1−− meson mass spectra. Their values are found to be with high precision the same for different quark-antiquark interactions independently on the quark flavours. Using these universal parameters the renormalization group invariant masses of quarks are calculated. It is also shown that the masses of π-meson radial excitations in this model are compatible with experimental data.

Meson mass spectra, evaluated in the framework of the relativistic model of quasi-independent quarks, are presented. Mass values are obtained with the help of numerical calculations based on the Dirac equation and by phenomenological mass formulae. The Dirac equation involves the potential, which is sum of the vector quasi-Coulombic potential and the scalar linear rising confinement potential. The phenomenological mass formulae are applied to excited meson states consisting of u-, d-quarks and antiquarks with isotopical spin I=1. A comparison of the evaluated mass spectra with existing data is performed. Problems of identification of some meson states in vector and scalar channels are discussed.

In this paper we consider single leptoquark resonance production associated with the emission of a hard photon. We have obtained analytical formulae for differential and total cross sections for the cases of scalar and vector leptoquarks. We have found that in reactions with scalar leptoquarks there is no photon radiation in some directions depending on the leptoquark electric charge (the radiative amplitude zero - the RAZ effect). For vector leptoquarks the exact RAZ is present only in the case of Yang-Mills coupling. We propose to use the RAZ effect to determine the types of leptoquarks. We conclude also that this effect opens a possibility to measure the leptoquark anomalous magnetic moment.

We analyse numerically manifestations of the radiative amplitude zero (RAZ) effect in single leptoquark production associated with hard photon emission. We present some quantitative conclusions on the possibility to distinguish leptoquark charges produced in ep collisions taking account of three-body final state subprocesses and of proton structure functions. Applying this method to HERA and possible LEP+LHC experiments we show that the RAZ analysis can serve as a tool to determine the leptoquark electric charge up to large leptoquark masses.

We present a new version of the CompHEP program package, version 4.5. We describe new options and techniques implemented in the version: interfaces to ROOT and HERWIG, parallel calculations, generation of the XML-based header in event files (HepML), full implementation of the Les Houches agreements (LHA I, SUSY LHA, LHA PDF, Les Houches Event format), cascade matching for intermediate scalar resonances, etc.

We discuss the SM Higgs discovery potential of the LHC in the channels and when the jet or lepton is observed at sufficiently high and a small rapidity to be reliably identified. We calculate all the signal subp rocesses and the irreducible background with realistic kinematical cuts. The reducible QCD background is also estimated. We conclude that the channel can give about 120-200 signal events for Higgs mass 100140 GeV at the integrated luminosity of 30fb . This signal rate should be compared with only 330-600 events for the irreducible background per two-photon invariant mass interval of 2 GeV. We estimate the QCD reducible background at the level of % of the irreducible one. Thus, one may hope that the Higgs boson can be discovered already after 1-1.5 years of LHC operation at a low luminosity. At a high luminosity of 10 cm s the observation of high Higgs bosons in this channel will be possible with significance higher than 15 for 100fb .

in quantum field theory. The distinctive feature of quantum field theory is the functional nature of the equations, and therefore any problem, even a two-particle one, here becomes a problem with infinitely many degrees of freedom. Modern mathematical formalism does not enable one to solve these flmctional equations in the general case. Therefore, in quantum theories with nonweak coupling, in which perturbation theory does not apply, it is more expedient to attempt to construct simpler dynamical equations that can serve as a basis for model constructions and enable one to describe systems of interacting particles. In particular, the wave function of a system of two interacting particles satisfies a quasipotential equation [1], which is a relativistic analog of the SchrSdinger equation. The kernel of this equation is a quasipotential, which is a complex function that depends on the energies of the particles and is a generalization of the ordinary potential in nonrelativistic quantum mechanics. The imaginary part of the quasipotential contains the contribution from all the many-particle intermediate states and must have a definite sign because of the unitarity condition. The quasipotential approach takes its simplest and most lucid form in the framework of the one-time formulation of the two-body problem in quantum field theory [2]. It is here that the analogy with nonrelativistic quantum mechanics becomes obvious. Of course, the interaction quasipotential of two relativistic particles itself remains unknown, but the analogy with quantum mechanics enables one to make a number of model assumption.s in order to construct this quasipotential and describe the experimental data [3]. In studying processes of the multiple production of particles, we arrive at the many-body problem in quantum field theory. Therefore, the description of such processes entails further difficulties. The point is that, compared with elastic processes, the number of variables is here increased drastically and it is much harder to construct schemes for calculating the corresponding matrix elements. Nevertheless, it appears to be possible to generalize the quasipotential approach to the case of three [4, 5] and more interacting particles by constructing a relativistic analog of the SchrSdinger equation for the wave function of a system with a fixed number of particles. In this case, the many-particle quasipotential is not a sum of the interaction quasipotentials of individual pairs of particles. However, the description of inelastic processes at high energies is further complicated by the fact that the number of possible reaction channels is extremely large, and it is hard to separate processes with a fixed number of particles in the final state. In this case, one is interested in the momentum distribution of, say, one or two particles in the final state and one is not interested in obtaining information Institute of High-Energy Physics, Serpuldlov. Translated from Teoreticheskaya i Matematiches

An equation for the density matrix is used to calculate the inclusive cross section for producing soft mesons. The results are in agreement with the conclusions of the optical model of multiple production, thereby confirming the assumptions of this model.

The propagator in the instanton background in the (−λφ4) scalar model in four dimensions is studied. Leading and sub-leading terms of its asymptotics for large momenta and its on-shell double residue are calculated. These results are applied to the analysis of the initial state and initial-final state corrections and the calculation of the next-to-leading (propagator) correction to the exponent of the cross section of multiparticle scattering processes.