M. Caffo

QCD corrections to P wave quarkonium decays

We give a detailed account of the calculation of the one-loop QCD corrections to the annihilation widths into hadrons and into two photons of P wave quarkonium states of JPC = 0++, 2++.

Event generators for Bhabha scattering

The results obtained by the Event Generators for Bhabha Scattering working group during the CERN Workshop Physics at LEP2 (1994/1995) are presented.

Strong QCD corrections to P-wave quarkonium decays

Abstract We have calculated one-loop QCD corrections to the annihilation widths of P-wave quarkonium states into hadrons and into two photons. Comparison with accurate experimental data may lead to a clear test of perturbative QCD if α S turns out to be small enough for reasonable convergence.

The present theoretical error on the Bhabha scattering cross section in the luminometry region at LEP

Abstract The results concerning the theoretical evaluation of the small-angle Bhabha Scattering cross section obtained during the Workshop on Physics at LEP2 (CERN, Geneva, Switzerland, 1995) by the Working Group “Event Generators for Bhabha Scattering” are summarized. The estimate of the theoretical error on the cross section in the luminometry region is updated.

Neutrino counting in e+e− collisions

We consider neutrino counting through e+e− → vvγ both for unpolarized beams and longitudinally polarized electron beam. We fully discuss the main background, from radiative Bhabha scattering, which we calculate for the relevant kinematical configurations. Possible measurements immediately beyond the Z0 peak or well above it are discussed and compared. The background would in principle be eliminated by taking cross section differences for opposite helicities leaving the statistical error only. Our formulae and program can be used to discuss the electromagnetic background to similar high energy reactions e+e− → Xγ, where X is the missing energy-momentum.

The threshold expansion of the 2-loop sunrise self-mass master amplitudes

Abstract The threshold behavior of the master amplitudes for two loop sunrise self-mass graph is studied by solving the system of differential equations, which they satisfy. The expansion at the threshold of the master amplitudes is obtained analytically for arbitrary masses.

On neutrino counting from electron-positron collisions☆☆☆

Abstract The background from e+e−→ e+e−γ to the neutrino counting reaction e + e − → v vγ is calculated and discussed for different energies and experimental cuts. It is found that in general the background is larger than previous estimates, especially for low photon energies. The regions where the signal dominates the background are indicated.

Numerical evaluation of master integrals from differential equations

Abstract The 4-th order Runge-Kutta method in the complex plane is proposed for numerically advancing the solutions of a system of first order differential equations in one external invariant satisfied by the master integrals related to a Feynman graph. The particular case of the general massive 2-loop sunrise self-mass diagram is analyzed. The method offers a reliable and robust approach to the direct and precise numerical evaluation of master integrals.

Bhabha scattering at high energy

SummaryWe show how to use the results of first-order (one-loop) perturbation theory to build a nonperturbative description of realistic high-energy Bhabha scattering, for a wide range of experimental cuts and both large and small scattering angles, with 1% estimated precision. Correspondingly, an event generator BHA-GEN is implemented. The resulting values of the cross-section are compared with those of the other programs existing in the literature, which are also mainly based on the same one-loop results. The agreement, after accounting the different kinematical cuts, is in general within 1%. We stress that, to improve the theoretical accuracy, a complete understanding of the different implementations of the nonperturbative effects based on one-loop corrections is necessary; we also emphasize that a correct definition of the events is an essential point of the accurate description of the process, accounting by itself for a few per cent difference. Only after settling those problems one can meaningfully consider the two-loop corrections, which are anyhow needed to push the theoretical precision below the 1% level.

Collinear radiation and lepton universality measurements in Z0 and W decay

Abstract We give analytical expressions for the collinear photon emission in Z 0 → l + l − γ and W→ lνγ and for the radiative corrections to Z 0 and W decays corresponding to possible experimental set-ups. We find that the correct definition of the “elastic” events increases remarkably the ratio of bremsstrahlung to “elastic”. It also reduces the amount of the apparent violation of lepton universality originated from radiative corrections.