arXiv subjects

M. Spira

Publications and source records attributed to M. Spira.

82 records · Page 5Linked to original sources

arXiv1995

Large-$p_\perp$ Heavy-Quark Production in Two-Photon Collisions

The next-to-leading-order (NLO) cross section for the production of heavy quarks at large transverse momenta ($p_\perp$) in $γγ$ collisions is calculated with perturbative fragmentation functions (PFF's). This approach allows for a resummation of terms $\proptoα_s\ln(p_\perp^2/m^2)$ which arise in NLO from collinear emission of gluons by heavy quarks at large $p_\perp$ or from almost collinear branching of photons or gluons into heavy-quark pairs. We present single-inclusive distributions in $p_\perp$ and rapidity including direct and resolved photons for $γγ$ production of heavy quarks at $e^+e^-$ colliders and at high-energy $γγ$ colliders. The results are compared with the fixed-order calculation for $m$ finite including QCD radiative corrections. The two approaches differ in the definitions and relative contributions of the direct and resolved terms, but essentially agree in their sum. The resummation of the $α_s \ln(p_\perp^2/m^2)$ terms in the PFF approach leads to a softer $p_\perp$ distribution and to a reduced sensitivity to the choice of the renormalization and factorization scales.

hep-ph↗

arXiv1995

QCD Corrections to Hadronic Higgs Decays

We present an update of the branching ratios for Higgs decays in the Standard Model and the Minimal Supersymmetric extension of the Standard Model. In particular, the decays of the Higgs particles to quark and gluon jets are analyzed and the spread in the theoretical predictions due to uncertainties of the quark masses and the QCD coupling is discussed.

hep-ph↗

arXiv1995

HIGLU: A Program for the Calculation of the Total Higgs Production Cross Section at Hadron Colliders via Gluon Fusion including QCD Corrections

A program for the calculation of the total Higgs production cross section via gluon fusion at hadron colliders including next--to--leading order QCD corrections is presented. It is suitable especially for Standard Model Higgs bosons and the neutral Higgs particles of the minimal supersymmetric extension. The program provides a model--independent calculation for scalar [\CP--even] as well as pseudoscalar [\CP--odd] Higgs bosons including the contributions of virtual top and bottom quarks inside the loop coupled to the Higgs particles. The relevant input parameters can be chosen from an input file. As a special case the minimal supersymmetric extension of the Standard Model can be investigated. The corresponding couplings are implemented including the leading higher order corrections.

hep-ph↗

arXiv1995

Cross Sections for Charm Production in $ep$ Collisions: Massive versus Massless Scheme

The next--to--leading order inclusive cross section for large-$p_\perp$ photoproduction of charm quarks at HERA is calculated in two different approaches. In the first approach the charm quarks are treated as massive objects which are strictly external to the proton and the photon while in the second approach the charm mass is neglected and the $c$ quark is assumed to be one of the active flavours in the proton and photon structure functions. We present single-inclusive distributions in transverse momentum and rapidity including direct and resolved photons. The cross section in the massless approach is found to be significantly larger than in the massive scheme. The deviation originates from several contributions which are disentangled. We argue that large-$p_\perp$ photoproduction of charm quarks at HERA will be sensitive to the charm content of the photon structure function.

hep-ph↗

arXiv1995

Gluino-Pair Production at the Tevatron

The next-to-leading order QCD corrections to the production of gluino pairs at the Tevatron are presented in this paper. Similar to the production of squark-antisquark pairs, the dependence of the cross section on the renormalization/factorization scale is reduced considerably by including the higher-order corrections. The cross section increases with respect to the lowest-order calculation which, in previous experimental analyses, had been evaluated at the scale of the invariant energy of the partonic subprocesses.

hep-ph↗

arXiv1995

Low-Energy Theorems in Higgs Physics

We present low-energy theorems for the calculation of loop amplitudes with external scalar or pseudoscalar Higgs bosons which are light compared to the loop particles. Starting from existing lowest-order versions of these theorems, we show how their applicability may be extended to the two--loop level. To illustrate the usefulness of these theorems, we discuss a number of applications to Higgs production and decay at and beyond the one--loop order.

hep-ph↗

arXiv1995

HIGGS BOSON PRODUCTION AT THE LHC

Gluon fusion is the main production mechanism for Higgs particles at the LHC. We present the QCD corrections to the fusion cross sections for the Higgs boson in the Standard Model, and for the neutral Higgs bosons in the minimal supersymmetric extension of the Standard Model. The QCD corrections are in general large and they increase the cross sections significantly. In two steps preceding the calculation of the production processes, we determine the QCD radiative corrections to Higgs decays into two photons and gluons.

hep-ph↗

arXiv1994

Squark Production at the Tevatron

We have determined the QCD corrections to the production of squark-antisquark pairs in $p\bar p$ collisions at the Tevatron. If the next-to-leading order corrections are taken into account, the renormalization/factorization scale dependence of the theoretical prediction for the cross section is reduced considerably. The higher order corrections increase the production cross section at the Tevatron by about a factor two if we compare the next-to-leading order prediction at a scale near the squark mass with the lowest order prediction for which, in the experimental analyses, the scale was identified with the invariant energy of the parton subprocess. This results in a rise of the experimental lower bound on the squark mass from the Tevatron by about $20$ GeV.

hep-ph↗

arXiv1994

Bounds on Radii and Magnetic Dipole Moments of Quarks and Leptons from LEP, SLC and HERA

Leptons, quarks and gauge bosons are assumed to be pointlike particles in the Standard Model. Stringent bounds on the radii of quarks and leptons and their weak anomalous magnetic moments can be derived from the high-precision measurements at LEP and SLC. We find a model-independent bound of $R\lsim 10^{-17}cm$ for quark and lepton radii. HERA will provide complementary information on the electromagnetic static properties of the quarks and the parameters of the charged quark currents.

hep-ph↗

arXiv1993

Two-Photon Decay Widths of Higgs Particles

Two--photon decays of Higgs bosons are important channels for the search of these particles in the intermediate mass range at the $pp$ colliders LHC and SSC. Dynamical aspects of the Higgs coupling to two photons can also be studied by means of the $γγ$ fusion of Higgs particles at high--energy e$^+$e$^-$ linear colliders. Extending earlier analyses which had been restricted to the Standard Model, we present in this note the QCD radiative corrections to the $γγ$ decay widths of scalar and pseudoscalar Higgs particles in multi--doublet extensions of the Higgs sector, as realized for instance in supersymmetric theories.

hep-ph↗