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.

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.

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 ǫ-poles of the dimensionally regularized amplitudes. The dependence on the regularization scheme is considered and the coefficients of the 1/ǫ 4 , 1/ǫ 3 and 1/ǫ 2 poles are explicitly given in the most general case. The remaining singlepole contributions are also explicitly evaluated in the case of amplitudes with a qq pair. † The RS that are mostly used in one-loop computations are the 't Hooft and Veltman scheme [22], the dimensional-reduction scheme [25] and the four-dimensional helicity scheme [26].

We propose a version of the QCD-motivated`k ? ' jet-clustering algorithm for hadron-hadron collisions which i s i n v ariant under boosts along the beam directions. This leads to improved factorization properties and closer correspondence to experimental practice at hadron colliders. We examine alternative denitions of the resolution variables and cluster recombination scheme, and show that the algorithm can be implemented eciently on a computer to provide a full clustering history of each e v ent. Using simulated data at p s = 1 : 8 T eV, we study the eects of calorimeter segmentation, hadronization and the soft underlying event, and compare the results with those obtained using a conventional conetype algorithm.

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We have recently proposed a form of high-energy factorization in order to find the small-x behaviour of heavy mass production in QCD. This factorization is k-dependent and provides all leading In x corrections to the coefficient function in various kinds of single-k 1 and double-k1processes. Here we investigate in detail its application to heavy flavour production in lepton and hadron initiated processes. We find that the resummation procedure yields large correction factors with respect to the lowest-order result, and we compute their explicit form in various cases.

We consider higher-order QCD corrections to the production of colourless highmass systems (lepton pairs, vector bosons, Higgs bosons, . . . ) 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 behaviour of the associated transverse-momentum (q T ) distributions in the small-q T region. The method is illustrated in general terms up to the next-to-next-to-leading order (NNLO) 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 in two photons. We present selected numerical results at the LHC.

We consider the transverse-momentum (q T ) distribution of generic high-mass systems (lepton pairs, vector bosons, Higgs particles, ....) produced in hadron collisions. At small q T , we concentrate on the all-order resummation of the logarithmicallyenhanced contributions in QCD perturbation theory. We elaborate on the b-space resummation formalism and introduce some novel features: the large logarithmic contributions are systematically exponentiated in a process-independent form and, after integration over q T , they are constrained by perturbative unitarity to give a vanishing contribution to the total cross section. At intermediate and large q T , resummation is consistently combined with fixed-order perturbative results, to obtain predictions with uniform theoretical accuracy over the entire range of transverse momenta. The formalism is applied to Standard Model Higgs boson production at LHC energies. We combine the most advanced perturbative information available at present for this process: resummation up to next-to-next-to-leading logarithmic accuracy and fixed-order perturbation theory up to next-to-leading order. The results show a high stability with respect to perturbative QCD uncertainties.