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G. Nanava

Publications and source records attributed to G. Nanava.

At least 19 recordsLinked to original sources
arXiv2010

Matching NLO parton shower matrix element with exact phase space: case of W -> l nu (gamma) and gamma^* -> pi^+pi^-(gamma)

The PHOTOS Monte Carlo is often used for simulation of QED effects in decay of intermediate particles and resonances. Momenta are generated in such a way that samples of events cover the whole bremsstrahlung phase space. With the help of selection cuts, experimental acceptance can be then taken into account. The program is based on an exact multiphoton phase space. Crude matrix element is obtained by iteration of a universal multidimensional kernel. It ensures exact distribution in the soft photon region. Algorithm is compatible with exclusive exponentiation. To evaluate the program's precision, it is necessary to control the kernel with the help of perturbative results. If available, kernel is constructed from the exact first order matrix element. This ensures that all terms necessary for non-leading logarithms are taken into account. In the present paper we will focus on the W -> l nu and gamma^* -> pi^+ pi^- decays. The Born level cross sections for both processes approach zero in some points of the phase space. A process dependent compensating weight is constructed to incorporate the exact matrix element, but is recommended for use in tests only. In the hard photon region, where scalar QED is not expected to be reliable, the compensating weight for gamma^* decay can be large. With respect to the total rate, the effect remains at the permille level. It is nonetheless of interest. The terms leading to the effect are analogous to some terms appearing in QCD. The present paper can be understood either as a contribution to discussion on how to match two collinear emission chains resulting from charged sources in a way compatible with the exact and complete phase space, exclusive exponentiation and the first order matrix element of QED (scalar QED), or as the practical study of predictions for accelerator experiments.

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arXiv2010

Universal Interface of TAUOLA Technical and Physics Documentation

Because of their narrow width, tau decays can be well separated from their production process. Only spin degrees of freedom connect these two parts of the physics process of interest for high energy collision experiments. In the following, we present a Monte Carlo algorithm which is based on that property. The interface supplements events generated by other programs, with tau decays. Effects of spin, genuine weak corrections or of new physics may be taken into account at the time when a tau decay is generated and written into an event record.

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arXiv2009

NLO QCD corrections to Drell-Yan processes in the SANC framework

NLO QCD corrections to charged and neutral current Drell-Yan processes and their implementation in the computer system SANC are considered. On the partonic level both quark-antiquark and quark-gluon scattering channels are taken into account. Subtractions of the collinear singularities in the massive case are compared with ones in the MS-bar scheme. Results of SANC on the hadronic level are presented. Comparison with results of the MCFM package is shown.

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arXiv2008

PHOTOS Monte Carlo and its theoretical accuracy

Because of properties of QED, the bremsstrahlung corrections to decays of particles or resonances can be calculated, with a good precision, separately from other effects. Thanks to the widespread use of event records such calculations can be embodied into a separate module of Monte Carlo simulation chains, as used in High Energy Experiments of today. The PHOTOS Monte Carlo program is used for this purpose since nearly 20 years now. In the following talk let us review the main ideas and constraints which shaped the program version of today and enabled it widespread use. Finally, we will underline importance of aspects related to reliability of program results: event record contents and implementation of channel specific matrix elements.

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arXiv2007

Electroweak radiative corrections to the three channels of the process f_1 bar-f_1 ZA --> 0

We have calculated the electroweak radiative corrections at the O(alpha) level to the three channels of the process f_1 bar-f_1 Z A --> 0 and implemented them into the SANC system. Here A stands for the photon and f_1 for a first generation fermion whose mass is neglected everywhere except in arguments of logarithmic functions. The symbol --> 0 means that 4-momenta of all the external particles flow inwards. We present the complete analytical results for the covariant and helicity amplitudes for three cross channels: f_1 + bar-f_1 --> Z + gamma, Z --> f_1 + bar-f_1 + gamma and f_1 + gamma --> f_1 + Z. The one-loop scalar form factors of these channels are simply related by an appropriate permutation of their arguments s,t,u. To check the correctness of our results we first of all observe the independence of the scalar form factors on the gauge parameters and the validity of the Ward identity, i.e. external photon transversality, and, secondly, compare our numerical results with the other independent calculations available to us.

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arXiv2007

PHOTOS Monte Carlo for precision simulation of QED in decays - History and properties of the project

Because of properties of QED, the bremsstrahlung corrections to decays of particles or resonances can be calculated, with a good precision, separately from other effects. Thanks to the widespread use of event records such calculations can be embodied into a separate module of Monte Carlo simulation chains, as used in High Energy Experiments of today. The PHOTOS Monte Carlo program is used for this purpose since nearly 20 years now. In the following talk let us review the main ideas and constraints which shaped the program version of today and enabled it widespread use. We will concentrate specially on conflicting requirements originating from the properties of QED matrix elements on one side and degrading (evolving) with time standards of event record(s). These issues, quite common in other modular software applications, become more and more difficult to handle as precision requirements become higher.

hep-ph↗
arXiv2007

SANCnews: Sector ffbb

In this paper we describe the implementation of processes f_1 bar-f_1 ZZ --> 0 and f_1 bar-f_1 HZ --> 0 into the framework of SANC system. The f_1 stands for a massless fermion f whose mass is kept non-zero only in arguments of ln functions and --> 0 means that all 4-momenta flow inwards. The derived scalar form factors can be used for any cross channel after an appropriate permutation of their arguments (s,t,u). We present the covariant and helicity amplitudes for these processes: for the former only in the annihilation channel f_1 bar-f_1 --> ZZ, while for the latter in annihilation f_1 bar-f_1 --> HZ and decay H --> Z f_1 bar-f_1 channels. We briefly describe additional precomputation modules which were not covered in the previous paper. For the processes f_1 bar-f_1 --> HZ(ZZ) and decay H --> Z f_1 bar-f_1 we present compact results of calculation of the accompanying bremsstrahlung and discuss exhaustive numerical results. As applications there are two types of the Monte Carlo generators for the process H --> 4μ. The first one is the generator based on a single resonance approximation for one of the Z bosons. The second one, exploiting the double resonance approximation, is not described in this article. For the generator in the single approximation we present a short description. Whenever possible, we compare our results with those existing in the literature. For example, we present a comparison of the results for H --> 4 μdecay with those obtained by MC generator Prophecy4f.

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arXiv2007

Scalar QED, NLO and PHOTOS Monte Carlo

Recently, QED bremsstrahlung in $B$-meson decays into pair of scalars (π's and/or K's) is of interest. If experimental acceptance must be taken into account, PHOTOS Monte Carlo is often used in experimental simulations. We will use scalar QED to benchmark PHOTOS, even though this theory is of limited use for complex objects. We present the analytical form of the kernel used in the older versions of PHOTOS, and the new, exact (scalar QED) one. Matrix element and phase-space Jacobians are separated in the final weight and future extensions based on measurable electromagnetic form-factors are thus possible. The massive phase-space is controlled in the the program with no approximations. Thanks to the iterative solution all leading and next to leading logarithmic terms are properly reproduced by the Monte Carlo simulation. Simultaneously, full differential distributions over complete multiple body phase-space are provided. An agreement of better than 0.01% with independent calculations of scalar QED is demonstrated.

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arXiv2007

SANCnews: Sector 4f, Charged Current

In this paper we describe the implementation of the charged current decays of the kind t --> b l^+ nu_l (gamma) into framework of SANC system. All calculations are done taking into account one-loop electroweak correction in the Standard Model. The emphasis of this paper is done on the presentation of numerical results. Various distributions are produced by means of a Monte Carlo integrator and event generator. Comparison with the results of CompHEP and PYTHIA packages are presented for the Born and hard photon contributions. The validity of the cascade approximation at one-loop level is also studied.

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arXiv2007

The three channels of the process f_1 bar-f_1 H A --> 0 in the SANC framework

In this paper we describe the implementation of the processes f_1 bar-f_1 H A --> 0 into the framework of SANC system. Here A stands for a photon and f_1 -- for a massless fermion whose mass is neglected everywhere besides arguments of logarithmic functions. The symbol --> 0 means that all 4-momenta flow inwards. The derived one-loop scalar form factors can be used for any cross channel after an appropriate permutation of their arguments s,t,u. We present the covariant and helicity amplitudes for all three possible cross channels. For checking of the correctness of the results first of all we observe the independence on the gauge parameters and the validity of Ward identity (external photon transversality), and, secondly, we make an extensive comparison with the other independent calculations.

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arXiv2005

SANCscope -- v.1.00

In this article we have summarized the status of the system SANC version 1.00. We have implemented theoretical predictions for many high energy interactions of fundamental particles at the one-loop precision level for up to 4-particle processes. In the present part of our SANC description we place emphasis on an extensive discussion of an important first step of calculations of the one-loop amplitudes of 3- and 4-particle processes in QED, QCD and EW theories. SANC version v1.00 is accessible from servers at Dubna http://sanc.jinr.ru/ (159.93.74.10) and CERN http://pcphsanc.cern.ch/ (137.138.180.42)

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arXiv2003

How to use SANC to improve the PHOTOS Monte Carlo simulation of bremsstrahlung in leptonic W-Boson decays

Using the SANC system we study the one-loop electroweak standard model predictions, including virtual and real photon emission, for the decays of the on-shell vector boson, W --> L ANTI-NU (GAMMA). The complete one-loop corrections and exact photon emission matrix element are taken into account. For the phase-space integration, the Monte Carlo technique is used. This provides a useful element, first for the evaluation of the theoretical uncertainty of PHOTOS. Later we analyse the source of the differences between SANC and PHOTOS and we calculate the additional weight, which once installed, improves predictions of PHOTOS simulations. We can conclude that, after the correction of the weight is implemented, the theoretical uncertainty of PHOTOS simulations due to an incomplete first-order matrix element is reduced to below alpha/pi, for observables not tagging the photon in a direct way, and to 10% otherwise. This is interesting for applications in the phenomenology of the ongoing LEP2 and future LC and LHC experimental studies.

hep-ph↗
arXiv2003

Update of one-loop corrections for E+ E- --> F anti-F, first run of SANC system

We present a description of calculations of the amplitude for E+ E- --> F anti-F process with account of electroweak and QED one-loop corrections. This study is performed within the framework of the project SANC. The calculations are done within the OMS (on-mass-shell) renormalization scheme in two gauges: in Rxi, which allows an explicit control of gauge invariance by examining cancellation of gauge parameters and search for gauge-invariant subsets of diagrams, and in the unitary gauge as a cross-check. The formulae we derived are realized in two independent FORTRAN codes, eeffLib, which was written in an old fashioned way, i.e. manually, and another one, created automatically with an aid of s2n.f (symbols to numbers) software -- a part of SANC system. We present a comprehensive comparison with the results of the well-known program ZFITTER for all the light fermion production channels, as well as with the results existing in the world literature for the process E+ E- --> F anti-F;

hep-ph↗
arXiv2002

Comparison of SANC with KORALZ and PHOTOS

Using the SANC system we study the one-loop electroweak standard model prediction, including virtual and real photon emissions, for the decays of on-shell vector and scalar bosons B --> f anti-f (gamma), where B is a vector boson, Z or W, or a Standard Model Higgs. The complete one-loop corrections and exact photon emission matrix element are taken into account. For the phase-space integration, the Monte Carlo technique is used. For Z decay the QED part of the calculation is first cross-checked with the exact one-loop QED prediction of KORALZ. For Higgs boson and W decays, a comparison is made with the approximate QED calculation of PHOTOS Monte Carlo. This provides a useful element for the evaluation of the theoretical uncertainty of PHOTOS, very interesting for its application in ongoing LEP2 and future LC and LHC phenomenology.

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arXiv2002

Project {\tt SANC} (former {\tt CalcPHEP}): Support of Analytic and Numeric calculations for experiments at Colliders

The project, aimed at the theoretical support of experiments at modern and future accelerators -- TEVATRON, LHC, electron Linear Colliders (TESLA, NLC, CLIC) and muon factories, is presented. Within this project a four-level computer system is being created, which must automatically calculate, at the one-loop precision level the pseudo- and realistic observables (decay rates and event distributions) for more and more complicated processes of elementary particle interaction, using the principle of knowledge storing. It was already used for a recalculation of the EW radiative corrections for Atomic Parity Violation [1] and complete one-loop corrections for the process $e^+ e^-\to t\bar{t}$ [2-4]; for the latter an, agreement up to 11 digits with FeynArts and the other results is found. The version of {\tt SANC} that we describe here is capable of automatically computing the decay rates and the distributions for the decays $Z(H,W)\to f\bar{f}$ in the one-loop approximation.

hep-ph↗
arXiv2002

Project "CalcPHEP: Calculus for Precision High Energy Physics"

In this paper we describe the present status and our plans for the realization of next phases of the CalcPHEP project aimed at the theoretical support of experiments at modern and future accelerators: TEVATRON, LHC, electron Linear Colliders (LC's) i.e. TESLA, NLC, CLIC, and muon factories. Within this project, we are creating a four-level computer system which eventually must automatically calculate pseudo- and realistic observables for more and more complicated processes of elementary particle interactions, using the principle of knowledge storing. The first phase of the CalcPHEP system was realized in the site http://brg.jinr.ru/ in 2000--2001.

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