S. Shichanin

CompHEP-PYTHIA interface: Integrated package for the collision events generation based on exact matrix elements

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 (multiparti-cle) 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. In the widely used generators PYTHIA [1], ISAJET [2] and HERWIG [3] data bases of matrix elements of hard subprocesses are built in. It means that matrix elements are stored as formulas. Furthermore, the matrix element squared |M| 2 is represented by means of some function modelling the behaviour of the integrand to get effective Monte-Carlo integration and events generation. Thus, as one can see, mainly 2 → 2 subprocesses are included in these data bases. However, the generation of events with 3, 4 and more bodies in the final states of hard subprocesses is needed for the Tevatron, LHC and future linear collider physics. One can note, in particular, that for such states there is no possibility to construct simple analytical formulae to match singular behaviour of |M| 2. Multidimensional phase space (4 dimensions in the 3-body case plus 2 dimensions in case of hadron collisons for convilution with parton distributions, 7+2 dimensions in the 4-body case etc) with untrivial regions corresponding to the singularities of the martrix element leads to complicated symbolic structures. Thus, one needs a new approach to the generation of events at the partonic level. Partonic level final states with top quarks, Higgs bosons and intermediate vector bosons, like W tj, ttH, W bb, ttbb and tttt, give practical examples of multidimensional phase space 1CompHEP, 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.

Single top quark production at LEP200

A complete tree-level calculation of the reaction e+e - ~ e+~,eTb (e-f, etb) in the electroweak standard theory in the LEP200 energy range is presented. For top quark masses in the range 130 to 190 GeV the cross sections are found to be in the order of 10 -5 to 10 -6 pb. Therefore, the number of single top quark events is expected to be negligible even with an integrated luminosity of E = 500 pb -~. It is further demonstrated that the Weizsiicker-Williams approximation approaches the accurate cross section calculations reasonably well.

On the conventional QED interpretation of the GSI e+e- narrow resonances

Abstract 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.

Complete tree-level calculation of the reaction

AbstractA complete tree-level calculation of the reaction

and the Higgs boson signal at LEP200 and NLC energies

e^ + e^ - \to \mu ^ + \mu ^ - b\bar b

The

in the electroweak standard theory for the energy range of LEP200 and the next linear collider is presented. The matrix elements have been calculated by employing the computer program CompHEP, the phase space integrals by the Monte Carlo integrator and event generator BASES/SPRING. The dependence of the 4-fermion cross section on energy and the Higgs boson mass is studied in detail. Interference contributions between the various diagrams are found not to alter significantly the production and decay properties of the Higgs boson. It is shown that already the counting rate of the reaction

-pair production and the Higgs signal in the reaction

e^ + e^ - \to \mu ^ + \mu ^ - b\bar b