We consider the production of a pair of heavy quarks ($Q{\bar Q}$) in hadronic collisions. When the transverse momentum $q_T$ of the heavy-quark pair is much smaller than its invariant mass, the QCD perturbative expansion is affected by large logarithmic terms that must be resummed to all-orders. This behavior is well known from the simpler case of hadroproduction of colourless high-mass systems, such as vector or Higgs boson(s). In the case of $Q{\bar Q}$ production, the final-state heavy quarks carry colour charge and are responsible for additional soft radiation (through direct emission and interferences with initial-state radiation) that complicates the evaluation of the logarithmically-enhanced terms in the small-$q_T$ region. We present the all-order resummation structure of the logarithmic contributions, which includes colour flow evolution factors due to soft wide-angle radiation. Resummation is performed at the completely differential level with respect to the kinematical variables of the produced heavy quarks. Soft-parton radiation produces azimuthal correlations that are fully taken into account by the resummation formalism. These azimuthal correlations are entangled with those that are produced by initial-state collinear radiation. We present explicit analytical results up to next-to-leading order and next-to-next-to-leading logarithmic accuracy.Comment: Some comments expanded and references added. Version published on NP

We consider the production of Zγ pairs at hadron colliders. We report on the first complete and fully differential computation of radiative corrections at next-to-next-toleading order in QCD perturbation theory. We present selected numerical results for pp collisions at 7 TeV and compare them to available LHC data. We find that the impact of the NNLO QCD corrections on the fiducial cross section ranges between 4 and 15%, depending on the applied cuts.

We consider QCD radiative corrections to topquark pair production at hadron colliders. We use the q T subtraction formalism to perform a fully differential computation for this process. Our calculation is accurate up to the next-to-leading order in QCD perturbation theory and it includes all the flavour off-diagonal partonic channels at the next-to-next-to-leading order. We present a comparison of our numerical results with those obtained with the publicly available numerical programs MCFM and Top++.The top quark (t) has a special role [1] in elementary particle physics. Being the heaviest known fundamental constituent, with a mass of about 173.3 GeV [2], it couples strongly to the Higgs boson and it is crucial to the hierarchy problem. Within the Standard Model (SM) the main source of top-quark events in collisions at hadron colliders is top-quark pair production. Many New Physics (NP) models predict the existence of top partners with masses close to the electroweak symmetry breaking scale, which exhibit similar properties as the top quark and can decay into it. Studying the production of tt pairs at hadron colliders can not only shed light on the nature of the electroweak symmetry breaking but it also provides information on the backgrounds of many NP models.The theoretical efforts for obtaining precision predictions for top-quark pair production at hadron colliders started almost three decades ago with the calculation of the nextto-leading order (NLO) QCD corrections to the total cross section [3-6] and kinematical distributions [7] for this production process. The NLO calculations of the total cross section of Refs. [3][4][5][6] were carried out numerically. The expresa e-mail: grazzini@physik.uzh.ch b On leave of absence from INFN, Sezione di Firenze, Sesto Fiorentino, Florence, Italy sions in analytic form of the total partonic cross section 1 at NLO were obtained in Ref. [11]. Recently the calculation of the next-to-next-to-leading order (NNLO) QCD corrections to the tt total cross section was completed [12][13][14][15]. Besides the total cross section, differential cross sections and more general kinematical distributions are of great importance for precision studies. For instance, the tt (forward-backward and charge) asymmetry has received much attention in recent years (see, e.g., Ref.[16]). The tt asymmetry, which is nonvanishing starting from the NLO level [17,18], has recently been computed up to the NNLO level [19]. Other NNLO results on differential distributions are starting to appear [20][21][22][23].This letter is devoted to the NNLO (and NLO) QCD calculation of tt production. In particular, we present the results of the first NNLO application of the q T subtraction formalism [24] to the process of tt production in hadron collisions.At the partonic level, the NNLO calculation of tt production requires the evaluation of tree-level contributions with two additional unresolved partons in the final state, of oneloop contributions with one unresolved parton and of purely virtual contribution...

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We consider the cross section for one-particle inclusive production at high transverse momentum in hadronic collisions. We present the all-order resummation formula that controls the logarithmically-enhanced perturbative QCD contributions to the partonic cross section in the threshold region, at fixed rapidity of the observed parton (hadron). The explicit resummation up to next-to-leading logarithmic accuracy is supplemented with the computation of the general structure of the near-threshold contributions to the next-to-leading order cross section. This next-to-leading order computation allows us to extract the one-loop hard-virtual amplitude that enters into the resummation formula. This is a necessary ingredient to explicitly extend the soft-gluon resummation beyond the next-to-leading logarithmic accuracy. These results equally apply to both spin-unpolarized and spin-polarized scattering processes.