Top quark pair production at complete NLO accuracy with NNLO+NNLL′ corrections in QCD

We describe predictions for top-quark pair differential distributions at hadron colliders, which combine state-of-the-art NNLO QCD calculations and NLO electroweak corrections together with double resummation at NNLL$'$ accuracy of threshold logarithms and small-mass logarithms. This is the first time that such a combination has appeared in the literature. Numerical results are presented for the invariant-mass distribution, the transverse-momentum distribution as well as rapidity distributions.

Calculation of the two-loop heavy-flavor contribution to Bhabha scattering

We describe in detail the calculation of the two-loop corrections to the QED Bhabha scattering cross section due to the vacuum polarization by heavy fermions. Our approach eliminates one mass scale from the most challenging part of the calculation and allows us to obtain the corrections in a closed analytical form. The result is valid for arbitrary values of the heavy fermion mass and the Mandelstam invariants, as long as s,t,u >> m_e^2.

NNLL momentum-space resummation for stop-pair production at the LHC

If supersymmetry near the TeV scale is realized in Nature, the pair production of scalar top squarks is expected to be observable at the Large Hadron Collider. Recently, effective field-theory methods were employed to obtain approximate predictions for the cross section for this process, which include soft-gluon emission effects up to next-to-next-to-leading order (NNLO) in perturbation theory. In this work we employ the same techniques to resum soft-gluon emission effects to all orders in perturbation theory and with next-to-next-to-logarithmic (NNLL) accuracy. We analyze the effects of NNLL resummation on the stop-pair production cross section by obtaining NLO+NNLL predictions in pair inv…

Precision predictions for the tt¯ production cross section at hadron colliders

Abstract We make use of recent results in effective theory and higher-order perturbative calculations to improve the theoretical predictions of the top-quark pair production cross section at hadron colliders. In particular, we supplement the fixed-order NLO calculation with higher-order corrections from soft gluon resummation at NNLL accuracy. Uncertainties due to power corrections to the soft limit are estimated by combining results from single-particle inclusive and pair invariant-mass kinematics. We present our predictions as functions of the top-quark mass in both the pole scheme and the M S ¯ scheme. We also discuss the merits of using threshold masses as an alternative, and calculate …

Heavy-Flavor Contribution to Bhabha Scattering

We evaluate the last missing piece of the two-loop QED corrections to the high-energy electron-positron scattering cross section originating from the vacuum polarization by heavy fermions. The calculation is performed within a new approach applicable to a wide class of perturbative problems with mass hierarchy. The result is crucial for the high-precision physics program at existing and future e(+) e(-) colliders.

Two-loop tensor integrals in quantum field theory

A comprehensive study is performed of general massive, tensor, two-loop Feynman diagrams with two and three external legs. Reduction to generalized scalar functions is discussed. Integral representations, supporting the same class of smoothness algorithms already employed for the numerical evaluation of ordinary scalar functions, are introduced for each family of diagrams.

Soft Gluon Resummation in t anti-t Production at Hadron Colliders

Two-Loop Vertices in Quantum Field Theory: Infrared Convergent Scalar Configurations

A comprehensive study is performed of general massive, scalar, two-loop Feynman diagrams with three external legs. Algorithms for their numerical evaluation are introduced and discussed, numerical results are shown for all different topologies, and comparisons with analytical results, whenever available, are performed. An internal cross-check, based on alternative procedures, is also applied. The analysis of infrared divergent configurations, as well as the treatment of tensor integrals, will be discussed in two forthcoming papers.

Threshold expansion at order αs4 for the tt¯ invariant mass distribution at hadron colliders

Abstract We calculate the leading O ( α s 4 ) contributions to the invariant mass distribution of top-quark pairs produced at the Tevatron and LHC, in the limit where the invariant mass of the t t ¯ pair approaches the partonic center-of-mass energy. Our results determine at NNLO in α s the coefficients of all singular plus distributions and scale-dependent logarithms in the differential partonic cross sections for q q ¯ , g g → t t ¯ + X . A numerical analysis showing the effects of the NNLO corrections on the central values and scale dependence of the invariant mass distribution is performed. The NNLO corrections are found to significantly enhance the cross section and reduce the perturba…

Forward-Backward and Charge Asymmetries in the Standard Model

This talk reviews the Standard Model predictions for the top-quark forward backward and charge asymmetries measured at the Tevatron and at the LHC.

Top-Quark Pair Production Beyond Next-to-Leading Order

We report on recent calculations of the differential cross section for top-quark pair production at hadron colliders. The results are differential with respect to the top-pair invariant mass and to the partonic scattering angle. In these calculations, which were carried out by employing soft-collinear effective theory techniques, we resummed threshold logarithms up to next-to-next-to-leading logarithmic order. Starting from the differential cross section, it is possible to obtain theoretical predictions for the invariant-mass distribution and the total cross section. We summarize here our results for these observables, and we compare them with the results obtained from different calculation…

Two-Loop Planar Corrections to Heavy-Quark Pair Production in the Quark-Antiquark Channel

We evaluate the planar two-loop QCD diagrams contributing to the leading color coefficient of the heavy-quark pair production cross section, in the quark-antiquark annihilation channel. We obtain the leading color coefficient in an analytic form, in terms of one- and two-dimensional harmonic polylogarithms of maximal weight 4. The result is valid for arbitrary values of the Mandelstam invariants s and t, and of the heavy-quark mass m. Our findings agree with previous analytic results in the small-mass limit and numerical results for the exact amplitude.

Top-pair forward-backward asymmetry beyond next-to-leading order

We make use of recent results in effective theory and higher-order perturbative calculations to improve the theoretical predictions of the QCD contribution to the top-quark pair production forward-backward asymmetry at the Tevatron. In particular, we supplement the fixed-order next-to-leading order calculation with higher-order corrections from soft-gluon resummation at next-to next-to-leading order accuracy performed in two different kinematic schemes, which allows us to make improved predictions for the asymmetry in the $p\overline{p}$ and $t\overline{t}$ rest frames as a function of the rapidity and invariant mass of the $t\overline{t}$ pair. Furthermore, we provide binned results which …

Approximate NNLO predictions for the stop-pair production cross section at the LHC

If the minimal supersymmetric standard model at scales of around 1 TeV is realized in nature, the total top-squark pair production cross section should be measurable at the CERN Large Hadron Collider. In this work we present precise predictions for this observable, which are based upon approximate NNLO formulas obtained using soft-collinear effective theory methods.

Chromomagnetic dipole-operator corrections inB¯→XsγatO(β0αs2)

We calculate the fermionic corrections to the photon-energy spectrum of $\overline{B}\ensuremath{\rightarrow}{X}_{s}\ensuremath{\gamma}$ which arise from the self-interference of the chromomagnetic dipole operator ${Q}_{8}$ at next-to-next-to-leading order by applying naive non-Abelianization. The resulting $\mathcal{O}({\ensuremath{\beta}}_{0}{\ensuremath{\alpha}}_{s}^{2})$ correction to the $\overline{B}\ensuremath{\rightarrow}{X}_{s}\ensuremath{\gamma}$ branching ratio amounts to a relative shift of $+0.12%$ ($+0.27%$) for a photon-energy cut of 1.6 GeV (1.0 GeV). We also comment on the potential size of resummation and nonperturbative effects related to ${Q}_{8}$.

Two-loop QED Corrections to Bhabha Scattering

Recent developments in the calculation of the NNLO corrections to the Bhabha scattering differential cross section in pure QED are briefly reviewed and discussed.

Infrared Singularities and Soft Gluon Resummation with Massive Partons

Infrared divergences of QCD scattering amplitudes can be derived from an anomalous dimension matrix, which is also an essential ingredient for the resummation of large logarithms due to soft gluon emissions. We report a recent analytical calculation of the anomalous dimension matrix with both massless and massive partons at two-loop level, which describes the two-loop infrared singularities of any scattering amplitudes with an arbitrary number of massless and massive partons, and also enables soft gluon resummation at next-to-next-to-leading-logarithmic order. As an application, we calculate the infrared poles in the q qbar -> t tbar and gg -> t tbar scattering amplitudes at two-loop …

Two-loop divergences of scattering amplitudes with massive partons

We complete the study of two-loop infrared singularities of scattering amplitudes with an arbitrary number of massive and massless partons in non-abelian gauge theories. To this end, we calculate the universal functions F_1 and f_2, which completely specify the structure of three-parton correlations in the soft anomalous-dimension matrix, at two-loop order in closed analytic form. Both functions are found to be suppressed like O(m^4/s^2) in the limit of small parton masses, in accordance with mass factorization theorems proposed in the literature. On the other hand, they are unsuppressed and diverge logarithmically near the threshold for pair production of two heavy particles. As an applica…

Top quark pair production beyond the next-to-leading order

We report on recent calculations of the total cross section and differential distributions of top quark pair production at hardon colliders, including the invariant mass distribution, the transverse momentum and rapidity distributions, as well as the forward-backward asymmetry. The calculations are based on soft gluon resummation at the next-to-next-to-leading logarithmic accuracy.

Quest for precision in hadronic cross sections at low energy: Monte Carlo tools vs. experimental data

We present the achievements of the last years of the experimental and theoretical groups working on hadronic cross section measurements at the low-energy e (+) e (-) colliders in Beijing, Frascati, Ithaca, Novosibirsk, Stanford and Tsukuba and on tau decays. We sketch the prospects in these fields for the years to come. We emphasise the status and the precision of the Monte Carlo generators used to analyse the hadronic cross section measurements obtained as well with energy scans as with radiative return, to determine luminosities and tau decays. The radiative corrections fully or approximately implemented in the various codes and the contribution of the vacuum polarisation are discussed.

Soft-gluon resummation for boosted top-quark production at hadron colliders

We investigate the production of highly energetic top-quark pairs at hadron colliders, focusing on the case where the invariant mass of the pair is much larger than the mass of the top quark. In particular, we set up a factorization formalism appropriate for describing the differential partonic cross section in the double soft and small-mass limit, and explain how to resum simultaneously logarithmic corrections arising from soft gluon emission and from the ratio of the pair-invariant mass to that of the top quark to next-to-next-to-leading logarithmic accuracy. We explore the implications of our results on approximate next-to-next-to-leading order formulas for the differential cross section…

Two-loop divergences of massive scattering amplitudes in non-abelian gauge theories

The infrared divergences of QCD scattering amplitudes can be derived from an anomalous dimension \Gamma, which is a matrix in color space and depends on the momenta and masses of the external partons. It has recently been shown that in cases where there are at least two massive partons involved in the scattering process, starting at two-loop order \Gamma receives contributions involving color and momentum correlations between three (and more) partons. The three-parton correlations can be described by two universal functions F_1 and f_2. In this paper these functions are calculated at two-loop order in closed analytic form and their properties are studied in detail. Both functions are found …