Raoul Röntsch

W + W − , WZ and ZZ production in the POWHEG BOX

A bstractWe present an implementation of the vector boson pair production processes ZZ, W+W− and WZ within the POWHEG framework, which is a method that allows the interfacing of NLO calculations to shower Monte Carlo programs. The implementation is built within the POWHEG BOX package. The Z/γ* interference, as well as singly resonant contributions, are properly included. We also considered interference terms arising from identical leptons in the final state. As a result, all contributions leading to the desired four-lepton system have been included in the calculation, with the sole exception of the interference between ZZ and W+W− in the production of a pair of same-flavour, oppositely charged fermions and a pair of neutrinos, which we show to be fully negligible. Anomalous trilinear couplings can be also set in the program, and we give some examples of their effect at the LHC. We have made the relevant code available at the POWHEG BOX web site.

Next-to-leading order QCD predictions for W+W+jj production at the LHC

Because the LHC is a proton-proton collider, sizable production of two positively charged W-bosons in association with two jets is possible. This process leads to a distinct signature of same sign high-p⊥ leptons, missing energy and jets. We compute the NLO QCD corrections to the QCD-mediated part of pp → W+W+jj. These corrections reduce the dependence of the production cross-section on the renormalization and factorization scale to about ±10 percent. We find that a large number of W+W+jj events contain a relatively hard third jet. The presence of this jet should help to either pick up the W+W+jj signal or to reject it as an unwanted background.

Next-to-leading order QCD corrections forW+W−pair production in association with two jets at hadron colliders

We compute the NLO QCD corrections to the pair production of W-bosons in association with two jets at the Tevatron and the LHC. This process is an important background to heavy Higgs-boson production in association with two jets, either in gluon or weak boson fusion. We consider leptonic decays of W-bosons and include all the spin correlations exactly. For natural choices of the renormalization scale, the NLO QCD corrections to pp(ppbar) -> W^+W^-jj are moderate but different for various values of the center-of-mass collision energy at the LHC and the Tevatron, emphasizing the need to compute them explicitly.

W+W+ plus dijet production in the POWHEG BOX

We present an implementation of the calculation of the production of W+W+ plus two jets at hadron colliders, at next-to-leading order (NLO) in QCD, in the POWHEG framework, which is a method that allows the interfacing of NLO calculations to shower Monte Carlo programs. This is the first 2→4 process to be described to NLO accuracy within a shower Monte Carlo framework. The implementation was built within the POWHEG BOX package. We discuss a few technical improvements that were needed in the POWHEG BOX to deal with the computer intensive nature of the NLO calculation, and argue that further improvements are possible, so that the method can match the complexity that is reached today in NLO calculations. We have interfaced our POWHEG implementation with PYTHIA and HERWIG, and present some phenomenological results, discussing similarities and differences between the pure NLO and the POWHEG+PYTHIA calculation both for inclusive and more exclusive distributions. We have made the relevant code available at the POWHEG BOX web site.

QCD corrections to W + W − production through gluon fusion.

We compute the next-to-leading order (NLO) QCD corrections to the gg→W+W−→l1+ν1l2−ν¯2 process, mediated by a massless quark loop, at the LHC. This process first contributes to the hadroproduction of W+W− at O(αs2), but, nevertheless, has a sizable impact on the total production rate. We find that the NLO QCD corrections to the gg→W+W− process amount to O(50)%, and increase the NNLO QCD cross sections of pp→W+W− by approximately two percent, at both the 8 TeV and 13 TeV LHC. We also compute the NLO corrections to gluonic W+W− production within a fiducial volume used by the ATLAS Collaboration in their 8 TeV measurement of the W+W− production rate and find that the QCD corrections are significantly smaller than in the inclusive case. While the current experimental uncertainties are still too large to make these differences relevant, the observed strong dependence of perturbative corrections on kinematic cuts underscores that extrapolation from a fiducial measurement to the total cross section is an extremely delicate matter, and calls for the direct comparison of fiducial volume measurements with corresponding theoretical computations.

QCD corrections to ZZ production in gluon fusion at the LHC

We compute the next-to-leading order QCD corrections to the production of two Z-bosons in the annihilation of two gluons at the LHC. Being enhanced by a large gluon flux, these corrections provide distinct and, potentially, the dominant part of the N 3 LO QCD contributions to Z-pair production in proton collisions. The gg! ZZ annihilation is a loop-induced process that receives the dominant contribution from loops of five light quarks, that are included in our computation in the massless approximation. We find that QCD corrections increase the gg! ZZ production cross section byO(50% 100%) depending on the values of the renormalization and factorization scales used in the leading order computation, and the collider energy. The large corrections to gg! ZZ channel increase the pp! ZZ cross section by about six to eight percent, exceeding the estimated theoretical uncertainty of the recent NNLO QCD calculation.

Probing top-Z dipole moments at the LHC and ILC

A bstractWe investigate the weak electric and magnetic dipole moments of top quark-Z boson interactions at the Large Hadron Collider (LHC) and the International Linear Collider (ILC). Their vanishingly small magnitude in the Standard Model makes these couplings ideal for probing New Physics interactions and for exploring the role of top quarks in electroweak symmetry breaking. In our analysis, we consider the production of two top quarks in association with a Z boson at the LHC, and top quark pairs mediated by neutral gauge bosons at the ILC. These processes yield direct sensitivity to top quark-Z boson interactions and complement indirect constraints from electroweak precision data. Our computation is accurate to next-to-leading order in QCD, we include the full decay chain of top quarks and the Z boson, and account for theoretical uncertainties in our constraints. We find that LHC experiments will soon be able to probe weak dipole moments for the first time.

Gluon fusion contribution to W+W- + jet production

A bstractWe describe the computation of the gg → W+W−g process that contributes to the production of two W -bosons and a jet at the CERN Large Hadron Collider (LHC). While formally of next-to-next-to-leading order (NNLO) in QCD, this process can be evaluated separately from the bulk of NNLO QCD corrections because it is finite and gauge-invariant. It is also enhanced by the large gluon flux and by selection cuts employed in the Higgs boson searches in the decay channel H → W+W−, as was first pointed out by Binoth et al. in the context of gg → W+W− production. For cuts employed by the ATLAS collaboration, we find that the gluon fusion contribution to pp → W+W−j enhances the background by about ten percent and can lead to moderate distortions of kinematic distributions which are instrumental for the ongoing Higgs boson searches at the LHC. We also release a public code to compute the NLO QCD corrections to this process, in the form of an add-on to the package MCFM.

Constraining anomalous Higgs boson couplings to the heavy flavor fermions using matrix element techniques

In this paper we investigate anomalous interactions of the Higgs boson with heavy fermions, employing shapes of kinematic distributions. We study the processes

Interference effects for Higgs boson mediated

pp \to t\bar{t} + H