Publications (53)121.18 Total impact
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ABSTRACT: Electroweak vectorboson production, accompanied by multiple jets, is an important background to searches for physics beyond the Standard Model. A precise and quantitative understanding of this process is helpful in constraining deviations from known physics. We study four key ratios in $W + n$jet production at the LHC. We compute the ratio of cross sections for $W + n$ to $W + (n1)$jet production as a function of the minimum jet transverse momentum. We also study the ratio differentially, as a function of the $W$boson transverse momentum; as a function of the scalar sum of the jet transverse energy, $H_T^{\rm jets}$; and as a function of certain jet transverse momenta. We show how to use such ratios to extrapolate differential cross sections to $W+6$jet production at nexttoleading order, and we crosscheck the method against a direct calculation at leading order. We predict the differential distribution in $H_T^{\rm jets}$ for $W+6$ jets at nexttoleading order using such an extrapolation. We use the BlackHat software library together with SHERPA to perform the computations.12/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We study $W$boson production accompanied by multiple jets at 7 TeV at the LHC. We study the jetproduction ratio, of total cross sections for $W$+$n$ to $W$+($n1$)jet production, and the ratio of distributions in the total transverse hadronic jet energy $H_{\rm T}^{\rm jets}$. We use the ratios to extrapolate the total cross section, and the differential distribution in $H_{\rm T}^{\rm jets}$, to $W$+6jet production. We use the BlackHat software library in conjunction with SHERPA to perform the computations.07/2014;  [Show abstract] [Hide abstract]
ABSTRACT: In this contribution, we present an intermediate storage format for nexttoleading order (NLO) events and explain the advantages of presenting a NLO calculation in this format. We also present some recent applications, including the calculation of PDF uncertainties and the combination of different multiplicity samples for the prediction of gap fractions in inclusive dijet events.07/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We present nexttoleading order QCD predictions for cross sections and for a comprehensive set of distributions in diphoton + 2jet production at the Large Hadron Collider. We consider the contributions from loop amplitudes for two photons and four gluons, but we neglect top quarks. We use BlackHat together with SHERPA to carry out the computation. We use a Frixione cone isolation for the photons. We study standard sets of cuts on the jets and the photons, and also sets of cuts appropriate for studying backgrounds to Higgsboson production via vectorboson fusion.02/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We present results from a recent calculation of prompt photonpair production in association with two jets to nexttoleading order (NLO) at the LHC. The virtual contribution is evaluated using the BlackHat library, a numerical implementation of onshell methods for oneloop amplitudes, in conjunction with SHERPA. We study four sets of cuts: standard jet cuts, a set of Higgsrelated cuts suggested by ATLAS, and corresponding sets which isolate the kinematic region where the process becomes the largest background to Higgs production via vectorboson fusion.12/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We present an eventfile format for the dissemination of nexttoleadingorder (NLO) predictions for QCD processes at hadron colliders. The files contain all information required to compute generic jetbased infraredsafe observables at fixed order (without showering or hadronization), and to recompute observables with different factorization and renormalization scales. The files also make it possible to evaluate cross sections and distributions with different parton distribution functions. This in turn makes it possible to estimate uncertainties in NLO predictions of a wide variety of observables without recomputing the shortdistance matrix elements. The event files allow a user to choose among a wide range of commonlyused jet algorithms and jetsize parameters. We provide event files for a $W$ or $Z$ boson accompanied by up to four jets, and for purejet events with up to four jets. The files are for the Large Hadron Collider with a center of mass energy of 7 or 8 TeV. A C++ library along with a Python interface for handling these files are also provided and described in this article. The library allows a user to read the event files and recompute observables transparently for different pdf sets and factorization and renormalization scales.Computer Physics Communications 10/2013; 185(5). · 2.41 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present recent nexttoleading order (NLO) results in perturbative QCD obtained using the BlackHat software library. We discuss the use of ntuples to separate the lengthy matrixelement computations from the analysis process. The use of ntuples allows many analyses to be carried out on the same phasespace samples, and also allows experimenters to conduct their own analyses using the original NLO computation.Journal of Physics Conference Series 10/2013; 523.  [Show abstract] [Hide abstract]
ABSTRACT: In these proceedings we present results from a recent calculation for the production of a W boson in conjunction with five jets at nexttoleading order in perturbative QCD. We also use results at lower multiplicities to extrapolate the cross section to the same process with six jets.08/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We present an update of the Binoth Les Houches Accord (BLHA) to standardise the interface between Monte Carlo programs and codes providing oneloop matrix elements.Computer Physics Communications 08/2013; 185(2). · 2.41 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present nexttoleading order QCD predictions for the total cross section and for a comprehensive set of transversemomentum distributions in W + 5jet production at the Large Hadron Collider. We neglect the small contributions from subleadingcolor virtual terms, top quarks and some terms containing four quark pairs. We also present ratios of total cross sections, and use them to obtain an extrapolation formula to an even larger number of jets. We include the decay of the $W$ boson into leptons. This is the first such computation with six finalstate vector bosons or jets. We use BlackHat together with SHERPA to carry out the computation.04/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We extend our investigation of backgrounds to new physics signals, following CMS's datadriven search for supersymmetry at the LHC. The aim is to use different sets of cuts in gamma + 3jet production to predict the irreducible Z + 3jet background (with the Z boson decaying to neutrinos) to searches with missing transverse energy + 3jet signal topologies. We compute ratios of Z + 3jet to gamma + 3jet production cross sections and kinematic distributions at nexttoleading order (NLO) in alpha_s. We compare these ratios with those obtained using a parton shower matched to leadingorder matrix elements (ME+PS). This study extends our previous work [arXiv:1106.1423 [hepph]] on the Z + 2jet to gamma + 2jet ratio. We find excellent agreement with the ratio determined from the earlier NLO results involving two instead of three jets, and agreement to within 10% between the NLO and ME+PS results for the ratios. We also examine the possibility of large QCD logarithms in these processes. Ratios of Z + njet to gamma + njet cross sections are plausibly less sensitive to such corrections than the cross sections themselves. Their effect on estimates of Z + 3jet to gamma + 3jet ratios can be assessed experimentally by measuring the gamma + 3jet to gamma + 2jet production ratio in search regions. We partially address the question of potentially large electroweak logarithms by computing the realemission part of the electroweak corrections to the ratio using ME+PS, and find that it is 1% or less. Our estimate of the remaining theoretical uncertainties in the Z to gamma ratio is in agreement with our earlier study.Physical Review D 06/2012; · 4.69 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In this contribution we present recent progress in the computation of nexttoleading order (NLO) QCD corrections for the production of an electroweak vector boson in association with jets at hadron colliders. We focus on results obtained using the virtual matrix element library BLACKHAT in conjunction with SHERPA, focusing on results relevant to understanding the background to top production.01/2012;  [Show abstract] [Hide abstract]
ABSTRACT: We present the cross sections for production of up to four jets at the Large Hadron Collider, at nexttoleading order in the QCD coupling. We use the BlackHat library in conjunction with SHERPA and a recently developed algorithm for assembling primitive amplitudes into colordressed amplitudes. We adopt the cuts used by ATLAS in their study of multijet events in pp collisions at \sqrt{s} = 7 TeV. We include estimates of nonperturbative corrections and compare to ATLAS data. We store intermediate results in a framework that allows the inexpensive computation of additional results for different choices of scale or parton distributions.Physical Review Letters 12/2011; 109(4). · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We describe the decomposition of oneloop QCD amplitudes in terms of colourordered building blocks. We give new expressions for the coefficients of QCD colour structures in terms of ordered objects called primitive amplitudes, for processes with up to seven partons. These results are needed in computations of highmultiplicity scattering cross sections in nexttoleadingorder (NLO) QCD. We explain the origin of new relations between multiquark primitive amplitudes which can be used to optimise efficiency of NLO computations. As a first application we compute the fullcolour virtual contribution to the cross section for W+4jet production at the Large Hadron Collider, and verify that it is very well approximated by keeping only the leading terms in an expansion around the formal limit of a large number of colours.Journal of High Energy Physics 11/2011; 2012(2). · 5.62 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We review onshell and unitarity methods and discuss their application to precision predictions for LHC physics. Being universal and numerically robust, these methods are straightforward to automate for nexttoleadingorder computations within Standard Model and beyond. Several stateoftheart results including studies of W/Z+3jet and W+4jet production have explicitly demonstrated the effectiveness of the unitarity method for describing multiparton scattering. Here we review central ideas needed to obtain efficient numerical implementations. This includes onshell looplevel recursions, the unitarity method, color management and further refined tricks.Journal of Physics A Mathematical and Theoretical 09/2011; 44(45). · 1.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present the cross section for production of a Z boson in association with four jets at the Large Hadron Collider, at nexttoleading order in the QCD coupling. When the Z decays to neutrinos, this process is a key irreducible background to many searches for new physics. Its computation has been made feasible through the development of the onshell approach to perturbative quantum field theory. We present the total cross section for pp collisions at sqrt{s} = 7 TeV, after folding in the decay of the Z boson, or virtual photon, to a chargedlepton pair. We also provide distributions of the transverse momenta of the four jets, and we compare cross sections and distributions to the corresponding ones for the production of a W boson with accompanying jets.08/2011;  [Show abstract] [Hide abstract]
ABSTRACT: The prediction of backgrounds to new physics signals in topologies with large missing transverse energy and jets is important to new physics searches at the LHC. Following a CMS study, we investigate theoretical issues in using measurements of gamma + 2jet production to predict the irreducible background to searches for missing energy plus two jets that originates from Z + 2jet production where the Z boson decays to neutrinos. We compute ratios of gamma + 2jet to Z + 2jet production cross sections and kinematic distributions at nexttoleading order in alpha_s, as well as using a parton shower matched to leadingorder matrix elements. We find that the ratios obtained in the two approximations are quite similar, making gamma + 2jet production a theoretically reliable estimator for the missing energy plus two jets background. We employ a Frixionestyle photon isolation, but we also show that for isolated prompt photon production at high transverse momentum the difference between this criterion and the standard cone isolation used by CMS is small.Physical Review D 06/2011; 84(11). · 4.69 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We combine the unitarity method with the sixdimensional helicity formalism of Cheung and O’Connell to construct looplevel scattering amplitudes. As a first example, we construct dimensionally regularized QCD oneloop fourpoint amplitudes. As a nontrivial multiloop example, we confirm that the recently constructed fourloop fourpoint amplitude of N=4 superYangMills theory, including nonplanar contributions, is valid for dimensions D≤6. We comment on the connection of our approach to the recently discussed Higgs infrared regulator and on dual conformal properties in six dimensions.Physical Review D 04/2011; 83(8):085022. · 4.69 Impact Factor 
Article: LeftHanded W Bosons at the LHC
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ABSTRACT: The production of W bosons in association with jets is an important background to new physics at the LHC. Events in which the W carries large transverse momentum and decays leptonically lead to large missing energy and are of particular importance. We show that the lefthanded nature of the W coupling, combined with valence quark domination at a pp machine, leads to a large lefthanded polarization for both W^+ and W^ bosons at large transverse momenta. The polarization fractions are very stable with respect to QCD corrections. The leptonic decay of the W bosons translates the common lefthanded polarization into a strong asymmetry in transverse momentum distributions between positrons and electrons, and between neutrinos and antineutrinos (missing transverse energy). Such asymmetries may provide an effective experimental handle on separating W + jets from top quark production, which exhibits very little asymmetry due to C invariance, and from various types of new physics.03/2011;  [Show abstract] [Hide abstract]
ABSTRACT: We present the nexttoleading order (NLO) QCD results for W+4jet production at hadron colliders. This is the first hadroncollider process with five finalstate objects to be computed at NLO. It represents an important background to many searches for new physics at the energy frontier. Total cross sections, as well as distributions in the jet transverse momenta, are provided for the initial LHC energy of √s = 7 TeV. We use a leadingcolor approximation, known to be accurate to 3% for W production with fewer jets. The calculation uses the BLACKHAT library along with the SHERPA package.Physical Review Letters 03/2011; 106(9):092001. · 7.73 Impact Factor
Publication Stats
925  Citations  
121.18  Total Impact Points  
Top Journals
Institutions

2009–2011

Massachusetts Institute of Technology
 Center for Theoretical Physics
Cambridge, MA, United States


2008–2011

University of California, Los Angeles
 Department of Physics and Astronomy
Los Angeles, California, United States


2005–2007

Swansea University
 Department of Physics
Swansea, WLS, United Kingdom


2001

CERN
Genève, Geneva, Switzerland
