External leg corrections in the unitarity method

Unitarity cuts diverge in the channel of a single massive external fermion. We propose an off-shell continuation of the momentum that allows a finite evaluation of the unitarity cuts. If the cut is taken with complete amplitudes on each side, our continuation and expansion around the on-shell configuration produces the finite contribution to the bubble coefficient. Finite parts in the expansion of the external leg counterterms must be included explicitly as well.Comment: 28 pages, 9 figures. Published version. Eq. (B.17) corrected, minor clarifications, typos fixe

Integrand Reduction for Two-Loop Scattering Amplitudes through Multivariate Polynomial Division

We describe the application of a novel approach for the reduction of scattering amplitudes, based on multivariate polynomial division, which we have recently presented. This technique yields the complete integrand decomposition for arbitrary amplitudes, regardless of the number of loops. It allows for the determination of the residue at any multiparticle cut, whose knowledge is a mandatory prerequisite for applying the integrand-reduction procedure. By using the division modulo Groebner basis, we can derive a simple integrand recurrence relation that generates the multiparticle pole decomposition for integrands of arbitrary multiloop amplitudes. We apply the new reduction algorithm to the two-loop planar and nonplanar diagrams contributing to the five-point scattering amplitudes in N=4 super Yang-Mills and N=8 supergravity in four dimensions, whose numerator functions contain up to rank-two terms in the integration momenta. We determine all polynomial residues parametrizing the cuts of the corresponding topologies and subtopologies. We obtain the integral basis for the decomposition of each diagram from the polynomial form of the residues. Our approach is well suited for a seminumerical implementation, and its general mathematical properties provide an effective algorithm for the generalization of the integrand-reduction method to all orders in perturbation theory.Comment: 32 pages, 4 figures. v2: published version, text improved, new subsection 4.4 adde

NLO electroweak contributions to squark pair production at the LHC

We present the tree-level and next-to-leading order (NLO) electroweak (EW) contributions to squark-squark production at the Large Hadron Collider (LHC) within the framework of the Minimal Supersymmetric Standard Model (MSSM).Comment: To appear in the proceedings of 17th International Conference on Supersymmetry and the Unification of Fundamental Interactions (SUSY09), Boston, USA, 5-10 Jun 200

Electroweak corrections to squark-antisquark production at the LHC

We present the calculation of the electroweak corrections for squark-antisquark pair production at the LHC within the Minimal Supersymmetric Standard Model. Taking into account all possible chirality and light-flavor configurations, we evaluate the NLO EW corrections, which are of O(a_s^2 a), as well as the subleading tree-level contributions of O(a_s a) and O(a^2). Numerical results are presented for several scans in the SUSY parameter space and relevant differential distributions are investigated. The impact of the electroweak corrections is nonnegligible and strongly depends on the chirality configuration of the produced squarks. Our analysis includes a discussion of photon-gluon initiated processes with a focus on the impact of the corresponding large PDF uncertainties.Comment: 33 pages, 12 figures; V2: Original results unchanged. Various minor modifications. Matches version published in JHE

Top-squark pair production at the LHC: a complete analysis at next-to-leading order

We present a complete next-to-leading order study of top-squark pair production at the LHC, including QCD and EW corrections. The calculation is performed within the Minimal Supersymmetric Standard Model and numerical results are presented for parameter regions compatible with the observed Higgs boson. We employ the most recent parton distribution functions including QED corrections and we find NLO EW corrections to the inclusive stop-pair production cross section up to $25 - 30\%$ compared to the leading-order prediction. Besides corrections to inclusive cross sections also important kinematic distributions are investigated.Comment: 27 pages, 10 figures. Version published in JHEP. The numerical discussion in Section 3 has been extended. References have been adde

Multi-leg One-loop Massive Amplitudes from Integrand Reduction via Laurent Expansion

We present the application of a novel reduction technique for one-loop scattering amplitudes based on the combination of the integrand reduction and Laurent expansion. We describe the general features of its implementation in the computer code NINJA, and its interface to GoSam. We apply the new reduction to a series of selected processes involving massive particles, from six to eight legs.Comment: v3: 39 pages, minor typos and one benchmark point correcte

Hadronic Production of Colored SUSY Particles with Electroweak NLO Contributions

We consider the production of squarks and gluinos at hadronic colliders. An overview over the class of processes is given. We investigate in detail the tree-level and higher order EW contributions to the cross sections. Special care has to be taken to obtain infrared finite observables. We study numerically stop--anti-stop and squark--gluino production at the LHC.Comment: 3 pages, 2 figures; To appear in the proceedings of 16th International Conference on Supersymmetry and the Unification of Fundamental Interactions (SUSY08), Seoul, Korea, 16-21 Jun 200

Magnus and Dyson Series for Master Integrals

We elaborate on the method of differential equations for evaluating Feynman integrals. We focus on systems of equations for master integrals having a linear dependence on the dimensional parameter. For these systems we identify the criteria to bring them in a canonical form, recently identified by Henn, where the dependence of the dimensional parameter is disentangled from the kinematics. The determination of the transformation and the computation of the solution are obtained by using Magnus and Dyson series expansion. We apply the method to planar and non-planar two-loop QED vertex diagrams for massive fermions, and to non-planar two-loop integrals contributing to 2 -> 2 scattering of massless particles. The extension to systems which are polynomial in the dimensional parameter is discussed as well.Comment: 32 pages, 6 figures, 2 ancillary files. v2: references added, typos corrected in the text and in the ancillary file