Stefano Catani

A general algorithm for calculating jet cross sections in NLO QCD

We present a new general algorithm for calculating arbitrary jet cross sections in arbitrary scattering processes to next-to-leading accuracy in perturbative QCD. The algorithm is based on the subtraction method. The key ingredients are new factorization formulae, called dipole formulae, which implement in a Lorentz covariant way both the usual soft and collinear approximations, smoothly interpolating the two. The corresponding dipole phase space obeys exact factorization, so that the dipole contributions to the cross section can be exactly integrated analytically over the whole of phase space. We obtain explicit analytic results for any jet observable in any scattering or fragmentation process in lepton, lepton-hadron or hadron-hadron collisions. All the analytical formulae necessary to construct a numerical program for next-to-leading order QCD calculations are provided. The algorithm is straightforwardly implementable in general purpose Monte Carlo programs.

Longitudinally-invariant k⊥-clustering algorithms for hadron-hadron collisions☆

Abstract We propose a version of the QCD-motivated “k⊥” jet-clustering algorithm for hadron-hadron collisions which is invariant under boosts along the beam directions. This leads to improved factorization properties and closer correspondence to experimental practice at hadron colliders. We examine alternative definitions of the resolution variables and cluster recombination scheme, and show that the algorithm can be implemented efficiently on a computer to provide a full clustering history of each event. Using simulated data at √S = 1.8 TeV, we study the effects of calorimeter segmentation, hadronization and the soft underlying event, and compare the results with those obtained using a conventional cone-type algorithm.

Resummation of the QCD perturbative series for hard processes

Abstract We study the region of inhibited radiation in hard hadronic processes, as for jet cross sections and heavy flavour production near threshold. The cases of deep inelastic scattering and Drell-Yan annihilation are explicitly considered. A general method to exponentiate leading and next-to-leading logarithms to all orders in perturbation theory is developed. A complete formula for the large N -moments is given and shown to agree with previous two-loop calculations. The resummation procedure suggests how to connect the perturbative and nonperturbative regions. The natural limit within the perturbative phase is shown to be the intrinsic transverse momentum.

Vector Boson Production at Hadron Colliders: A Fully Exclusive QCD Calculation at Next-to-Next-to-Leading Order

We consider QCD radiative corrections to the production of W and Z bosons in hadron collisions. We present a fully exclusive calculation up to next-to-next-to-leading order (NNLO) in QCD perturbation theory. To perform this NNLO computation, we use a recently proposed version of the subtraction formalism. The calculation includes the gamma-Z interference, finite-width effects, the leptonic decay of the vector bosons, and the corresponding spin correlations. Our calculation is implemented in a parton level Monte Carlo program. The program allows the user to apply arbitrary kinematical cuts on the final-state leptons and the associated jet activity and to compute the corresponding distributions in the form of bin histograms. We show selected numerical results at the Fermilab Tevatron and the LHC.

QCD Matrix Elements + Parton Showers

We propose a method for combining QCD matrix elements and parton showers in Monte Carlo simulations of hadronic flnal states in e + e i annihilation. The matrix element and parton shower domains are separated at some value yini of the jet resolution, deflned according to the kT-clustering algorithm. The matrix elements are modifled by Sudakov form factors and the parton showers are subjected to a veto procedure to cancel dependence on yini to next-to-leading logarithmic accuracy. The method provides a leading-order description of hard multi-jet conflgurations together with jet fragmentation, while avoiding the most serious problems of double counting. We present flrst results of an approximate implementation using the event generator APACIC++.

Next-to-next-to-leading-order subtraction formalism in hadron collisions and its application to Higgs-boson production at the large hadron collider.

We consider higher-order QCD corrections to the production of colorless high-mass systems (lepton pairs, vector bosons, Higgs bosons, etc.) in hadron collisions. We propose a new formulation of the subtraction method to numerically compute arbitrary infrared-safe observables for this class of processes. To cancel the infrared divergences, we exploit the universal behavior of the associated transverse-momentum (qT) distributions in the small-qT region. The method is illustrated in general terms up to the next-to-next-to-leading order in QCD perturbation theory. As a first explicit application, we study Higgs-boson production through gluon fusion. Our calculation is implemented in a parton level Monte Carlo program that includes the decay of the Higgs boson into two photons. We present selected numerical results at the CERN Large Hadron Collider.

Soft-gluon resummation for Higgs boson production at hadron colliders

We consider QCD corrections to Higgs boson production through gluon-gluon fusion in hadron collisions. We compute the cross section, performing the all-order resummation of multiple soft-gluon emission at next-to-next-to-leading logarithmic level. Known fixed-order results (up to next-to-next-to-leading order) are consistently included in our calculation. We give phenomenological predictions for Higgs boson production at the Tevatron and at the LHC. We estimate the residual theoretical uncertainty from perturbative QCD contributions. We also quantify the differences obtained by using the presently available sets of parton distributions.

The singular behaviour of QCD amplitudes at two-loop order

Abstract We discuss the structure of infrared singularities in on-shell QCD amplitudes at two-loop order. We present a general factorization formula that controls all the ϵ-poles of the dimensionally regularized amplitudes. The dependence on the regularization scheme is considered and the coefficients of the 1/ϵ4,1/ϵ3 and 1/ϵ2 poles are explicitly given in the most general case. The remaining single-pole contributions are also explicitly evaluated in the case of amplitudes with a q q pair.

The Dipole Formalism for the Calculation of QCD Jet Cross Sections at Next-to-Leading Order

In order to make quantitative predictions for jet cross sections in perturbative QCD, it is essential to calculate them to next-to-leading accuracy. This has traditionally been an extremely laborious process. Using a new formalism, imaginatively called the dipole formalism, we are able to construct a completely general algorithm for next-to-leading order calculations of arbitrary jet quantities in arbitrary processes. In this paper we present the basic ideas behind the algorithm and illustrate them with a simple example.

NLL resummation of the heavy quark hadroproduction cross-section

Abstract We compute the effect of soft-gluon resummation, at the next-to-leading-logarithmic level, in the hadroproduction cross section for heavy flavours. Applications to top, bottom and charm total cross sections are discussed. We find in general that the corrections to the fixed next-to-leading order results are larger for larger renormalization scales, and small, or even negative, for smaller scales. This leads to a significant reduction of the scale dependence of the results, for most experimental configurations of interest.