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M. Melles

Publications and source records attributed to M. Melles.

10 recordsLinked to original sources
arXiv2003

Two-loop electroweak angular-dependent logarithms at high energies

We present results on the two-loop leading and angular-dependent next-to-leading logarithmic virtual corrections to arbitrary processes at energies above the electroweak scale. In the `t Hooft-Feynman gauge the relevant Feynman diagrams involving soft and collinear gauge bosons γ, Z, W^\pm coupling to external legs are evaluated in the eikonal approximation in the region where all kinematical invariants are much larger than the electroweak scale. The logarithmic mass singularities are extracted from massive multi-scale loop integrals using the Sudakov method and alternatively the sector-decomposition method in the Feynman-parameter representation. The derivations are performed within the spontaneously broken phase of the electroweak theory, and the two-loop results are in agreement with the exponentiation prescriptions that have been proposed in the literature based on a symmetric SU(2) x U(1) theory matched with QED at the electroweak scale.

hep-ph↗
arXiv2002

Electroweak Sudakov corrections in the MSSM

For superpartner masses not much heavier than the weak scale $M=M_{\rm W}$, large logarithmic corrections of the Sudakov type arise at TeV energies. In this paper we summarize recent results of supersymmetric (susy) electroweak radiative corrections for sfermion and charged Higgs production at $e^+e^-$ colliders in the MSSM. The results are given to subleading logarithmic (SL) accuracy to all orders in perturbation theory in the ``light'' susy-mass scenario. Prospects for the determination of $\tan β\geq 10$ are discussed which is independent of soft susy-breaking terms to SL accuracy.

hep-ph↗
arXiv2002

Exact Differential O(alpha**2) Results for Hard Bremsstrahlung in e+e- Annihilation to 2f At and Beyond LEP2 Energies

We present the exact O(alpha) correction to the process e+ e- -> f f-bar + gamma, f neq e, for ISR oplus FSR at and beyond LEP2 energies. We give explicit formulas for the completely differential cross section. As an important application, we compute the size of the respective sub-leading corrections of O(alpha L) to the f f-bar cross section, where L is the respective big logarithm in the renormalization group sense so that it is identifiable as L = ln |s|/m_e^2 when s is the squared e+e- cms energy. Comparisons are made with the available literature. We show explicitly that our results have the correct infrared limit, as a cross-check. Some comments are made about the implementation of our results in the framework of the Monte Carlo event generator KK MC.

hep-ph↗
arXiv2001

SUSY Scalar Production in the Electroweak Sudakov Regime of Lepton Colliders

We consider the production of SUSY scalar pairs at lepton colliders, for c.m. energies much larger than the mass of the heaviest SUSY (real or virtual) particle involved in the process. In that energy regime, we derive the leading and subleading terms of the electroweak Sudakov logarithms in the MSSM, first working at one loop with physical states and then resumming to all orders with asymptotic expansions. We show that the first order of the resummed expression reproduces the physical one loop approximation, and compute systematically the possible effects on various observables both at one loop and to all orders. We discuss the regimes and the processes where the one loop approximation can or cannot be trusted, working in an energy range between 1 TeV and 4 TeV under a "light" SUSY mass assumption. As a byproduct of our analysis, we propose a determination of the MSSM parameter tan(beta) showing how a relative accuracy of about 25 percent can be easily achieved in the region tan(beta)>14, under reasonable experimental assumptions.

hep-ph↗
arXiv1999

Resummation of double logarithms in electroweak high energy processes

At future linear $e^+e^-$ collider experiments in the TeV range, Sudakov double logarithms originating from massive boson exchange can lead to significant corrections to the cross sections of the observable processes. These effects are important for the high precision objectives of the Next Linear Collider. We use the infrared evolution equation, based on a gauge invariant dispersive method, to obtain double logarithmic asymptotics of scattering amplitudes and discuss how it can be applied, in the case of broken gauge symmetry, to the Standard Model of electroweak processes. We discuss the double logarithmic effects to both non-radiative processes and to processes accompanied by soft gauge boson emission. In all cases the Sudakov double logarithms are found to exponentiate. We also discuss double logarithmic effects of a non-Sudakov type which appear in Regge-like processes.

hep-ph↗
arXiv1999

Precision Calculation of Bhabha Scattering at LEP

For the small-angle Bhabha-scattering process, we consider the error budget for the calculation of the LEP/SLC luminosity in the Monte Carlo event generator BHLUMI 4.04, from the standpoint of new calculations of exact results for the respective O(alpha**2) photonic corrections in the context of the Yennie-Frautchi-Suura exponentiation. We find that an over-all precision tag for the currently available program BHLUMI 4.04 can be reduced from 0.11% to 0.061% at LEP1 and from 0.25% to 0.122% at LEP2. For the large-angle Bhabha process, we present the Monte Carlo program BHWIDE and compare its predictions with predictions of other Monte Carlo programs as well as semi-analytical calculations.

hep-ph↗
arXiv1999

New Results on the Theoretical Precision of the LEP/SLC Luminosity

We consider the error budget for the calculation of the LEP/SLC luminosity in the Monte Carlo event generator BHLUMI4.04 from the standpoint of new calculations of the exact result for the O(alpha) correction to the process e+ e- -> e+ e- + gamma in the low angle luminosity regime at SLC/LEP energies, for the double bremsstrahlung effect e+ e- -> e+ e- + gamma gamma in this regime, and for the size of the two-loop virtual correction to e+ e- -> e+ e- in this regime in context of Yennie-Frautschi-Suura exponentiation. We find that the error on the O(alpha**2) photonic correction can be reduced from the currently published value 0.1% to the value 0.027%. This leads to an over-all precision tag for the currently available program BHLUMI4.04 of 0.061%. This reduction of the precision of the calculation is important for the final LEP1 EW precision Z physics tests of the Standard Model.

hep-ph↗
arXiv1996

Bhabha Process at LEP - Theoretical Calculations

In this contribution we give a short overview of the situation in the precision calculation of the Bhabha process and we present a preliminary numerical result on the second-order sub-leading correction to the small angle Bhabha process.

hep-ph↗
arXiv1996

Exact O(alpha) Corrections to the Single Hard Bremsstrahlung Process in Low Angle Bhabha Scattering in the SLC/LEP Energy Regime

We present the exact O(alpha) correction to the process e+ e- -> e+ e- + gamma in the low angle luminosity regime at SLC/LEP energies. We give explicit formulas for the completely differential cross section. As an important application, we illustrate the size of the respective corrections of O(alpha^2) to the SLC/LEP luminosity cross section. We show explicitly that our results have the correct infrared limit, as a cross-check. Some comments are made about the implementation of our results in the framework of a Monte Carlo event generator. This latter implementation will appear elsewhere.

hep-ph↗