We study the discovery potential of the Tevatron for a Z ′ gauge boson. We introduce a parametrization of the Z ′ signal which provides a convenient bridge between collider searches and specific Z ′ models. The cross section for pp → Z ′ X → ℓ + ℓ − X depends primarily on the Z ′ mass and the Z ′ decay branching fraction into leptons times the average square coupling to up and down quarks. If the quark and lepton masses are generated as in the standard model, then the Z ′ bosons accessible at the Tevatron must couple to fermions proportionally to a linear combination of baryon and lepton numbers in order to avoid the limits on Z − Z ′ mixing. More generally, we present several families of U (1) extensions of the standard model that include as special cases many of the Z ′ models discussed in the literature. Typically, the CDF and D0 experiments are expected to probe Z ′ -fermion couplings down to 0.1 for Z ′ masses in the 500-800 GeV range, which in various models would substantially improve the limits set by the LEP experiments.

The antenna subtraction method for the computation of higher order corrections to jet observables and exclusive cross sections at collider experiments is extended to include hadronic initial states. In addition to the already known antenna subtraction with both radiators in the final state (final-final antennae), we introduce antenna subtractions with one or two radiators in the initial state (initial-final or initial-initial antennae). For those, we derive the phase space factorization and discuss the allowed phase space mappings at NLO and NNLO. We present integrated forms for all antenna functions relevant to NLO calculations, and describe the construction of the full antenna subtraction terms at NLO on two examples. The extension of the formalism to NNLO is outlined.

Abstract:We compute all two-loop master integrals which are required for the evaluation of next-to-leading order QCD corrections in Higgs boson production via gluon fusion. Many two-loop amplitudes for 2 → 1 processes in the Standard Model and beyond can be expressed in terms of these integrals using automated reduction techniques. These integrals also form a subset of the master integrals for more complicated 2 → 2 amplitudes with massive propagators in the loops. As a first application, we evaluate the two-loop amplitude for Higgs boson production in gluon fusion via a massive quark. Our result is the first independent check of the calculation of Spira, Djouadi, Graudenz and Zerwas. We also present for the first time the two-loop amplitude for gg → h via a massive squark.

We extend the antenna subtraction method to include initial states containing one hadron at NNLO. We present results for all the necessary subtraction terms, antenna functions, for the master integrals required to integrate them over the relevant phase space and finally for the integrated antennae themselves. Where applicable, our results are cross-checked against the known NNLO coefficient functions for deep inelastic scattering processes.

We present a new method for the numerical evaluation of arbitrary loop integrals in dimensional regularization. We first derive Mellin-Barnes integral representations and apply an algorithmic technique, based on the Cauchy theorem, to extract the divergent parts in the ǫ → 0 limit. We then perform an ǫ-expansion and evaluate the integral coefficients of the expansion numerically. The method yields stable results in physical kinematic regions avoiding intricate analytic continuations. It can also be applied to evaluate both scalar and tensor integrals without employing reduction methods. We demonstrate our method with specific examples of infrared divergent integrals with many kinematic scales, such as two-loop and three-loop box integrals and tensor integrals of rank six for the one-loop hexagon topology.

We present a method to evaluate numerically Feynman diagrams directly from their Feynman parameters representation. We first disentangle overlapping singularities using sector decomposition. Threshold singularities are treated with an appropriate contour deformation. We have validated our technique comparing with recent analytic results for the gg → h two-loop amplitudes with heavy quarks and scalar quarks.

We compute the order α 2 s corrections to the one particle inclusive electroproduction cross section of hadrons with non vanishing transverse momentum. We perform the full calculation analytically, and obtain the expression of the factorized (finite) cross section at this order. We compare our results with H1 data on forward production of π 0 , and discuss the phenomenological implications of the rather large higher order contributions obtained in that case. Specifically, we analyze the cross section sensitivity to the factorization and renormalization scales, and to the input fragmentation functions, over the kinematical region covered by data. We conclude that the data is well described by the O(α 2 s ) predictions within the theoretical uncertainties and without the inclusion of any physics content beyond the DGLAP approach.

We present the two-loop QCD amplitude for the interaction of two gluons and a CP-even Higgs boson in the Minimal Supersymmetric Standard Model. We apply a novel numerical method for the evaluation of Feynman diagrams with infrared, ultraviolet and threshold singularities. We discuss subtleties in the ultraviolet renormalization of the amplitude with conventional dimensional regularization, dimensional reduction, and the four dimensional helicity scheme. Finally, we show numerical results for scenarios of supersymmetry breaking with a rather challenging phenomenology in which the Higgs signal in the MSSM is suppressed in comparison to the Standard Model.The loop-mediated interaction of a Higgs boson and gluons is the main production mechanism for a Higgs boson at hadron colliders. Depending on the decay channels in which a Standard Model Higgs boson may be discovered, the measurement of the signal cross-sections can be achieved with a precision of about ±10% or better [1]. Amid the discovery of a Higgs boson, the signal crosssection will be an independent precision test of the Standard Model and its extensions.The gluon-fusion Higgs boson production cross-section is sensitive to higher order QCD corrections [2,3,4]. At the LHC, the signal cross-sections are known to change up to a factor of two when perturbative corrections from O α 2 s through O α 4 s are included [5,6]. The accuracy of these theoretical predictions is about ±10%. The partonic decay width to gluons also increases by a factor of two when including the known higher order QCD corrections. This quantity is now computed through order O α 5 s with an accuracy better than 1% [7]. Theoretical uncertainties in the ggh interaction within the Standard Model are currently adequately small for a future comparison with LHC data at a 10% precision level.Extensions of the Standard Model (SM) postulate diverse mechanisms for breaking the electroweak symmetry. A different Higgs boson sector than the one in the Standard Model and additional new particles are often introduced. The effects of undiscovered particles on the gluon-fusion cross-section are rather unconstrained from experimental data. Novel colored particles can change the Higgs and gluon interaction dramatically. For example, an additional heavy-quark with SM-like Yukawa coupling to the Higgs boson would contribute almost as much as the top-quark to the ggh amplitude. Recent examples in well motivated models were shown in [8].The complexity of the two-loop SM computations at O α 3 s in the full theory and at O α 4 s in the limit of a heavy top-quark is serious. The methods that have been employed are powerful and may be employed in other models. In especially simple modifications of the SM Lagrangian, for example adding a fourth generation with heavy leptons and quarks, the existing calculations are already sufficient. However, it will be important to know the gluon-fusion cross-section through at least order O α 3 s in all models which aspire to explain LHC data. Many viable extensions of the SM contai...