Giovanni Ridolfi

Updated predictions for the total production cross sections of top and of heavier quark pairs at the Tevatron and at the LHC

We present updated predictions for the total production cross section of top-quark pairs at the Tevatron and at the LHC, and, at the LHC, of heavy-quark pairs with mass in the range 0.5 - 2 TeV. For t (t) over bar production at the LHC we also present results at root S = 10 TeV, in view of the expected accelerator conditions during the forthcoming 2008 run. Our results are accurate at the level of next-to-leading order in as, and of next-to-leading threshold logarithms (NLO+NLL). We adopt the most recent parametrizations of parton distribution functions, and compute the corresponding uncertainties. We study the dependence of the results on the top mass, and we assess the impact of missing higher-order corrections by independent variations of factorisation and renormalisation scales.

On the metastability of the Standard Model vacuum

If the Higgs mass mH is as low as suggested by present experimental information, the Standard Model ground state might not be absolutely stable. We present a detailed analysis of the lower bounds on mH imposed by the requirement that the electroweak vacuum be sufficiently long-lived. We perform a complete one-loop calculation of the tunnelling probability at zero temperature, and we improve it by means of two-loop renormalization-group equations. We find that, for mH = 115 GeV, the Higgs potential develops an instability below the Planck scale for mt > (166±2) GeV, but the electroweak vacuum is sufficiently long-lived formt < (175 ± 2) GeV.

Improving the Weizsäcker-Williams approximation in electron-proton collisions

Abstract We critically examine the validity of the Weizsacker-Williams approximation in electron-hadron collisions. We show that in its commonly used form it can lead to large errors, and we show how to improve it in order to get accurate results. In particular, we present an improved form that is valid beyond the leading logarithmic approximation in the case when a small-angle cut is applied to the scattered electron. Furthermore we include comparisons of the approximate expressions with the exact electroproduction calculation in the case of heavy-quark production.

Heavy-Quark Correlations in Photon-Hadron Collisions

We describe a next-to-leading-order calculation of the fully exclusive parton cross section at next-to-leading order for the photoproduction of heavy quarks. We use our result to compute quantities of interest for current fixed-target experiments. We discuss heavy-quark total cross sections, distributions, and correlations.

Total cross sections for heavy flavour production at HERA

Abstract We compute total cross sections for charm and bottom photoproduction at HERA energies, and discuss the relevant theoretical uncertainties. In particular we discuss the problems arising from the small- x region, the uncertainties in the gluon parton density, and the uncertainties in the hadronic component of the cross section. Total electroproduction cross sections, calculated in the Weizsacker-Williams approximation, are also given.

Charm and bottom production: theoretical results versus experimental data

Abstract We present recent theoretical results in heavy-quark production, and we compare them with available experimental data. In particular, we consider the total cross sections, the xf and pT2 single-inclusive distributions, and results on heavy-quark pair correlations in both hadroproduction and photoproduction of charm and bottom hadrons.

Indication for light sneutrinos and gauginos from precision electroweak data

The present standard model fit of precision data has a low confidence level, and is characterized by a few inconsistencies. We look for supersymmetric effects that could improve the agreement among the electroweak precision measurements and with the direct lower bound on the Higgs mass. We find that this is the case particularly if the 3.6 σ discrepancy between sin 2θeff from leptonic and hadronic asymmetries is finally settled more on the side of the leptonic ones. After the inclusion of all experimental constraints, our analysis selects light sneutrinos, with masses in the range 55-80 GeV, and charged sleptons with masses just above their experimental limit, possibly with additional effects from light gauginos. The phenomenological implications of this scenario are discussed.

Scale dependence and small-χ behaviour of polarized parton distributions

Abstract We discuss perturbative evolution of the polarized structure function g 1 in the ( χ , Q 2 ) plane, with special regard to the small- χ region. We determine g 1 in terms of polarized quark and gluon distributions using coefficient functions to order α s . At small χ g 1 then displays substantial scale dependence, which necessarily implies a corresponding scale dependence in the large- χ region. This scale dependence has significant consequences for the extraction of the first moment from the experimental data, reducing its value while increasing the error. Conversely, the scale dependence may be used to constrain the size of the polarized gluon distribution.

Top quark distributions in hadronic collisions

Abstract We present kinematical distributions for top quark pairs produced at the Tevatron p p Collider, as predicted within Next-to-Leading-Order QCD. We consider single and double inclusive distributions, and compare our results to those obtained with the shower Monte Carlo HERWIG. We discuss the implications of our findings for experimental issues such as the measurement of the top quark mass.

On the Q2 dependence of the measured polarized structure functions

Abstract We analyse the available data on the polarized asymmetries A 1 for proton, neutron and deutron targets. We use a homogeneous and updated set of unpolarized structure functions to derive g 1 from A 1 , and we accurately correct for the scaling violations in order to obtain g 1 ( x , Q 2 ) with the same Q 2 for all x values. The contribution to the Q 2 evolution of a possible large gluon polarized density is also considered. The implications for the Ellis-Jaffe and for the Bjorken sum rules are discussed.