N. Kauer

Gluon-induced W-boson pair production at the LHC

Pair production of W bosons constitutes an important background to Higgs boson and new physics searches at the Large Hadron Collider LHC. We have calculated the loop-induced gluon-fusion process gg → W ∗ W ∗ → leptons, including intermediate light and heavy quarks and allowing for arbitrary invariant masses of the W bosons. While formally of next-to-next-to-leading order, the gg → W ∗ W ∗ → leptons process is enhanced by the large gluon flux at the LHC and by experimental Higgs search cuts, and increases the next-to-leading order W W background estimate for Higgs searches by about 30%. We have extended our previous calculation to include the contribution from the intermediate top- bottom massive quark loop and the Higgs signal process. We provide updated results for cross sections and differential distributions and study the interference between the different gluon scattering contributions. We describe important analytical and numerical aspects of our calculation and present the public GG2WW event generator.

H → WW as the discovery mode for a light Higgs boson

The production cross section for a m_H=115 GeV, SM Higgs boson in weak boson fusion at the LHC is sizable. However, the branching fraction for H-->WW is expected to be relatively small. The signal, with its two forward jets, is sufficiently different from the main backgrounds that a signal to background ratio of better than 1:1 can nevertheless be obtained, with large enough rate to allow for a 5 sigma signal with 35 fb^{-1} of data. The H-->WW signal in weak boson fusion may thus prove to be the discovery mode for the Higgs boson at the LHC.

A Proposal for a standard interface between Monte Carlo tools and one-loop programs

Many highly developed Monte Carlo tools for the evaluation of cross sections based on tree matrix elements exist and are used by experimental collaborations in high energy physics. As the evaluation of one-loop matrix elements has recently been undergoing enormous progress, the combination of one-loop matrix elements with existing Monte Carlo tools is on the horizon. This would lead to phenomenological predictions at the next-to-leading order level. This note summarises the discussion of the next-to-leading order multi-leg (NLM) working group on this issue which has been taking place during the workshop on Physics at TeV Colliders at Les Houches, France, in June 2009. The result is a proposal for a standard interface between Monte Carlo tools and one-loop matrix element programs. Dedicated to the memory of, and in tribute to, Thomas Binoth, who led the effort to develop this proposal for Les Houches 2009. Thomas led the discussions, set up the subgroups, collected the contributions, and wrote and edited this paper. He made a promise that the paper would be on the arXiv the first week of January, and we are faithfully fulfilling his promise. In his honour, we would like to call this the Binoth Les Houches Accord.

Gluon-induced WW background to Higgs boson searches at the LHC

Vector-boson pair production is an important background for Higgs boson and new physics searches at the Large Hadron Collider LHC. We have calculated the loop-induced gluon-fusion process gg → W W → leptons, allowing for arbitrary invariant masses of the intermediate W bosons. This process contributes at O(α 2) relative to quark- antiquark annihilation, but its importance is enhanced by the large gluon flux at the LHC and by experimental cuts employed in Higgs boson searches. We find that gg → W W provides only a moderate correction (ca. 5%) to the inclusive W-pair production cross section at the LHC. However, after taking into account realistic experimental cuts, the gluon-fusion process becomes significant and increases the theoretical W W background estimate for Higgs searches in the pp → H → W W → leptons channel by approximately 30%.

Multi-higgs boson production in the standard model and beyond

We present a calculation of the loop-induced processes gg ! HH and gg ! HHH, and investigate the observability of multi-Higgs boson production at the CERN Large Hadron Collider (LHC) in the Standard Model (SM) and beyond. While the SM cross sections are too small to allow observation at the LHC, we demonstrate that physics beyond the SM can lead to amplified, observable cross sections. Furthermore, the applicability of the heavy top quark approximation in two- and three-Higgs boson production is investigated. We conclude that multi-Higgs boson production at the SuperLHC is an interesting probe of Higgs sectors beyond the SM and warrants further study.

A numerical evaluation of the scalar hexagon integral in the physical region

Abstract We derive an analytic expression for the scalar one-loop pentagon and hexagon functions which is convenient for subsequent numerical integration. These functions are of relevance in the computation of next-to-leading order radiative corrections to multi-particle cross sections. The hexagon integral is represented in terms of n -dimensional triangle functions and ( n +2)-dimensional box functions. If infrared poles are present this representation naturally splits into a finite and a pole part. For a fast numerical integration of the finite part we propose simple one- and two-dimensional integral representations. We set up an iterative numerical integration method to calculate these integrals directly in an efficient way. The method is illustrated by explicit results for pentagon and hexagon functions with some generic physical kinematics.

Breakdown of the Narrow Width Approximation for New Physics

The narrow-width approximation is used in high-energy physics to reduce the complexity of scattering calculations. It is a fortunate accident that it works so well for the standard model, but in general it will fail in the context of new physics. We find numerous examples of significant corrections when the calculation is performed fully off shell including a finite width, notably from effects from the decay matrix elements, not just phase space. If not taken into account, attempts to reconstruct the Lagrangian of a new physics discovery from data would result in considerable inaccuracies and likely inconsistencies.

Update of the Binoth Les Houches Accord for a standard interface between Monte Carlo tools and one-loop programs

We present an update of the Binoth Les Houches Accord (BLHA) to standardise the interface between Monte Carlo programs and codes providing one-loop matrix elements.

NLO QCD corrections to ZZ jet production at hadron colliders

A fully differential calculation of the next-to-leading order QCD corrections to the production of Z-boson pairs in association with a hard jet at the Tevatron and LHC is presented. This process is an important background for Higgs particle and new physics searches at hadron colliders. We find sizable corrections for cross sections and differential distributions, particularly at the LHC. Residual scale uncertainties are typically at the 10% level and can be further reduced by applying a veto against the emission of a second hard jet. Our results confirm that NLO corrections do not simply rescale LO predictions.

Narrow-width approximation accuracy

Abstract A study of general properties of the narrow-width approximation (NWA) with polarization/spin decorrelation is presented. We prove for sufficiently inclusive differential rates of arbitrary resonant decay or scattering processes with an on-shell intermediate state decaying via a cubic or quartic vertex that decorrelation effects vanish and the NWA is of order Γ . Its accuracy is then determined numerically for all resonant 3-body decays involving scalars, spin- 1 2 fermions or vector bosons. We specialize the general results to MSSM benchmark scenarios. Significant off-shell corrections can occur — similar in size to QCD corrections. We qualify the configurations in which a combined consideration is advisable. For this purpose, we also investigate process-independent methods to improve the NWA.