Mario Greco

Vector boson production at colliders: A theoretical reappraisal

Abstract We study the production of vector bosons in hadron-hadron collisions via the Drell-Yan mechanism in QCD. Our treatment of the transverse momentum and rapidity distributions of the produced bosons takes all available theoretical principles and results into account in a systematic way. The resulting q T distribution reduces to the perturbative limit for large q T , includes the summation of soft gluons and reproduces the known results for the total cross section. A full numerical analysis of W and Z cross sections at collider and tevatron energies is made.

Deep-inelastic processes

Abstract Electron-positron annihilation into hadrons, photoproduction, total and singly inclusive electro-production are discussed in the framework of a scaling model of electromagnetic interactions in which the photon is coupled to a continuum of hadronic states whose properties are established consistently. The model makes use of the concept of the shrinkage of the size of the particle with its mass. The exhibition of scaling properties in e.m. interactions demands a similar scaling behaviour of strong interactions. Predictions are made which are in good agreement with experiments.

Large infra-red corrections in QCD processes

Abstract The effect of soft gluon emission in hard processes is examined to all orders in QCD. Large corrections have been found, which sensibly modify the parton model results, and show the relevance of the appropriate kinematical limits in leptoproduction and in the Drell-Yan process.

QCD jets from coherent states

A recently proposed approach to the problem of infrared and mass singularities in QCD, based on the formalism of coherent states, is extended to discuss massless quark and gluon jets. Our results include all leading (ln δ) terms as well as finite terms in the energy loss ϵ, in addition to the usual ln ϵ associated with ln δ. Our formulae agree with explicit perturbative calculations, whenever available. Explicit expressions for the total KT distributions are given which take into account transverse-momentum conservation. Predictions are also made for the Q2 dependence of the mean KT2 for quark and gluon jets. Our jet kT distributions are extrapolated for low kT and shown to describe with good accuracy the data for ee → qq → hadrons. Numerical predictions are also presented for the forthcoming PETRA, PEP and LEP machines.

Radiative corrections to e+e− → μ+μ− around the Z0

Abstract Analytical expressions are presented for e.m. radiative effects in the reaction e − e + → μ − μ + , in the vicinity of the Z 0 mass, in the Weinberg-Salam model. Our formulae contain all finite first-order corrections, as well as soft photon effects resummed to all orders, with no restriction on the relative magnitudes of the energy resolution Δω and the Z 0 width Γ. Hard photon effects, other than the tail effect, are not included. Weak interactions are considered only to lowest order. Radiative corrections are explicitly shown to change the lowest-order results drastically, and must therefore be taken in full account for a realistic description of experiments at LEP energies.

Large-p⊥ hadroproduction of heavy quarks

Abstract The production of heavy quarks at large p ⊥ ( p ⊥ ⪢ m ) in hadronic collisions is considered. The analysis is carried out in the framework of perturbative fragmentation functions, thereby allowing a resummation at the NLO level of final-state large-mass logarithms of the kind log( p ⊥ / m ). The case of b-quark production is considered in detail. The resulting theoretical uncertainty from renormalization/factorization scales at large p ⊥ is found to be much smaller than that shown by the full O( α s 3 ) perturbative calculation.

Intercepts of the non-singlet structure functions

Abstract Infrared evolution equations for small- x behaviour of the non-singlet structure functions f 1 NS and g 1 NS are obtained and solved in the next-to-leading approximation, to all orders in α s , and including running α s effects. The intercepts of these structure functions, i.e., the exponents of the power-like small- x behaviour, are calculated. A detailed comparison with the leading logarithmic approximation (LLA) and DGLAP is made. We explain why the LLA predictions for the small- x dependence of the structure functions may be more reliable than the prediction for the Q 2 dependence in the range ofxa0 Q 2 explored at HERA.

Coherent state approach to the infra-red behaviour of non-abelian gauge theories

Abstract The infrared behaviour of matrix elements between coherent states of definite color is studied in non-abelian gauge theories. The matrix elements are shown to be finite to the lowest non-trivial order, and to all orders if the soft meson formula for real gluons holds to all orders. Factorization occurs in the fixed angle regime.

Non-singlet structure functions: Combining the leading logarithms resummation at small-x with DGLAP

Abstract The explicit expressions for the non-singlet DIS structure functions F 1 and g 1 , obtained at small-x by resumming the leading logarithmic contributions to all orders, are discussed and compared in detail with the DGLAP evolution for different values of x and Q 2 . The role played by the DGLAP inputs for the initial parton densities on the small-x behavior of the non-singlet structure functions is discussed. It is shown that the singular factors included into the fits ensure the Regge asymptotics of the non-singlet structure functions and mimic the impact of the total resummation of the leading logarithms found explicitly in our approach. Explicit expressions are presented which implement the NLO DGLAP contributions with our small-x results.

Running coupling effects for the singlet structure function g1 at small x

Abstract The running of the QCD coupling is incorporated into the infrared evolution equations for the flavour structure function g1. The explicit expressions for g1 including the total resummation of the double-logarithmic contributions and accounting for the running coupling are obtained. We predict that asymptotically g1∼x−ΔS, with the intercept ΔS≈0.86, which is a factor of 2 larger than the non-singlet intercept ΔNS. The impact of the initial quark (δq) and gluon (δg) densities on the sign of g1 at x⪡1 is discussed and explicit expressions relating δq and δg are obtained.