Resummation of non-global QCD observables

We discuss issues related to the resummation of non-global observables in QCD, those that are sensitive to radiation in only a part of phase space. Examples of such observables are certain single-hemisphere event shapes in e+e- and DIS. Compared to global observables (those sensitive to all emissions, e.g. the e+e- thrust) a new class of single-logarithmic terms arises. These have been neglected in recent calculations in the literature. For a whole set of single hemisphere e+e- and DIS event shapes, we analytically evaluate the first such term, at order alpha_s^2, and give numerical results for the resummation of these terms in the large-Nc limit.Comment: 10 page

A novel subtraction scheme for double-real radiation at NNLO

A general subtraction scheme, STRIPPER (SecToR Improved Phase sPacE for real Radiation), is derived for the evaluation of next-to-next-to-leading order (NNLO) QCD contributions from double-real radiation to processes with at least two particles in the final state at leading order. The result is a Laurent expansion in the parameter of dimensional regularization, the coefficients of which should be evaluated by numerical Monte Carlo integration. The two main ideas are a two-level decomposition of the phase space, the second one factorizing the singular limits of amplitudes, and a suitable parameterization of the kinematics allowing for derivation of subtraction and integrated subtraction terms from eikonal factors and splitting functions without non-trivial analytic integration.Comment: 14 pages, 3 figures, matches published version, includes new name for the scheme, extended discussion of massless final states, and some new reference

Direct Higgs production and jet veto at hadron colliders

We consider Higgs boson production through gluon--gluon fusion in hadron collisions, when a veto is applied on the transverse momenta of the accompanying hard jets. We compute the QCD corrections to this process at NLO and NNLO, and present numerical results at the Tevatron and the LHC.Comment: 5 pages, 4 ps figures, presented at the XXXVIIth Rencontres de Moriond, QCD and Hadronic interactions, Les Arc1800, Franc

Higgs production in hadron collisions: soft and virtual QCD corrections at NNLO

We consider QCD corrections to Higgs boson production through gluon-gluon fusion in hadron collisions. Using the recently evaluated two-loop amplitude for this process and the corresponding factorization formulae for soft-gluon bremsstrahlung at O(alpha_s^2), we compute the soft and virtual contributions to the NNLO cross section. We also discuss soft-gluon resummation at next-to-next-to-leading logarithmic accuracy. Numerical results for Higgs boson production at the LHC are presented.Comment: 17 pages, 4 figures included. One note and one reference adde

Semi-leptonic Decays of Heavy Quarks in Dijet Photoproduction at HERA

The production of heavy quarks has been studied in photoproduction processes with the ZEUS detector at HERA using an integrated luminosity of $36.9 pb^{-1}$. Events with a photon virtuality, $Q^2<1 GeV^2$, were selected with two jets of high transverse energy and an electron in the final state. Consideration of the distribution in $p_T^{rel}$ - the momentum of the electron transverse to the axis of the jet to which the electron is closest - allows a measurement of the beauty cross-section in a restricted region of phase space.Comment: 3 pages, 3 figures. To appear in DIS99 conference proceeding

An NNLO subtraction formalism in hadron collisions and its application to Higgs boson production at the LHC

We consider higher-order QCD corrections to the production of colourless high-mass 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.Comment: References adde

Steps towards NNLO QCD calculations: collinear factorization at O(\alpha_S^2)

I consider the singular behaviour of tree-level QCD amplitudes when the momenta of three partons become simultaneously parallel and I discuss the universal factorization formula that controls the singularities of the multiparton matrix elements in this collinear limit.Comment: 3 pages, 1 eps figure, talk given at the DIS99 Workshop, Zeuthen, Germany, April 19-23, 199

Azimuthal asymmetries in QCD hard scattering: infrared safe but divergent

We consider high-mass systems of two or more particles that are produced by QCD hard scattering in hadronic collisions. We examine the azimuthal correlations between the system and one of its particles. We point out that the perturbative QCD computation of such azimuthal correlations and asymmetries can lead to divergent results at fixed perturbative orders. The fixed-order divergences affect basic (and infrared safe) quantities such as the total cross section at fixed (and arbitrary) values of the azimuthal-correlation angle $\varphi$. Examples of processes with fixed-order divergences are heavy-quark pair production, associated production of vector bosons and jets, dijet and diboson production. A noticeable exception is the production of high-mass lepton pairs through the Drell--Yan mechanism of quark-antiquark annihilation. However, even in the Drell--Yan process, fixed-order divergences arise in the computation of QED radiative corrections. We specify general conditions that produce the divergences by discussing their physical origin in fixed-order computations. We show lowest-order illustrative results for $\cos(n\varphi)$ asymmetries (with $n=1,2,4,6$) in top-quark pair production and associated production of a vector boson and a jet at the LHC. The divergences are removed by a proper all-order resummation procedure of the perturbative contributions. Resummation leads to azimuthal asymmetries that are finite and computable. We present first quantitative results of such a resummed computation for the $\cos(2\varphi)$ asymmetry in top-quark pair production at the LHC.Comment: 43 pages, 5 eps figure

The Resummation of Soft Gluon in Hadronic Collisions

We compute the effects of soft gluon resummation for the production of high mass systems in hadronic collisions. We carefully analyse the growth of the perturbative expansion coefficients of the resummation formula. We propose an expression consistent with the known leading and next-to-leading resummation results, in which the coefficients grow much less than factorially. We apply our formula to Drell--Yan pair production, heavy flavour production, and the production of high invariant mass jet pairs in hadronic collisions. We find that, with our formula, resummation effects become important only fairly close to the threshold region. In the case of heavy flavour production we find that resummation effects are small in the experimental configurations of practical interest.Comment: 45 pages, Latex, epsfig, 10 figures. Minor corrections to text, notation and references. The previously quoted HERAb NLO bottom production cross section was wrong, and it has been fixed. Accepted for Publication on Nucl. Phys.