Publications (132)473.15 Total impact
 [Show abstract] [Hide abstract]
ABSTRACT: We use the hexagon function bootstrap to compute the ratio function which characterizes the nexttomaximallyhelicityviolating (NMHV) sixpoint amplitude in planar $\mathcal{N} = 4$ superYangMills theory at four loops. A powerful constraint comes from dual superconformal invariance, in the form of a $\bar{Q}$ differential equation, which heavily constrains the first derivatives of the transcendental functions entering the ratio function. At four loops, it leaves only a 34parameter space of functions. Constraints from the collinear limits, and from the multiRegge limit at the leadinglogarithmic (LL) and nexttoleadinglogarithmic (NLL) order, suffice to fix these parameters and obtain a unique result. We test the result against multiRegge predictions at NNLL and N$^3$LL, and against predictions from the operator product expansion involving one and two fluxtube excitations; all crosschecks are satisfied. We study the analytical and numerical behavior of the parityeven and parityodd parts on various lines and surfaces traversing the threedimensional space of cross ratios. As part of this program, we characterize all irreducible hexagon functions through weight eight in terms of their coproduct. We also provide representations of the ratio function in particular kinematic regions in terms of multiple polylogarithms.  [Show abstract] [Hide abstract]
ABSTRACT: Evanescent operators such as the GaussBonnet term have vanishing perturbative matrix elements in exactly D=4 dimensions. Similarly, evanescent fields do not propagate in D=4; a threeform field is in this class, since it is dual to a cosmologicalconstant contribution. In this Letter, we show that evanescent operators and fields modify the leading ultraviolet divergence in pure gravity. To analyze the divergence, we compute the twoloop identicalhelicity fourgraviton amplitude and determine the coefficient of the associated (nonevanescent) R^3 counterterm studied long ago by Goroff and Sagnotti. We compare two pairs of theories that are dual in D=4: gravity coupled to nothing or to threeform matter, and gravity coupled to zeroform or to twoform matter. Duff and van Nieuwenhuizen showed that, curiously, the oneloop conformal anomaly  the coefficient of the GaussBonnet operator  changes under pform duality transformations. We concur, and also find that the leading R^3 divergence changes under duality transformations. Nevertheless, in both cases the physical renormalized twoloop identicalhelicity fourgraviton amplitude can be chosen to respect duality. In particular, its renormalizationscale dependence is unaltered.  [Show abstract] [Hide abstract]
ABSTRACT: We compute the interference between the resonant process pp→H(→γγ)+2 jets and the corresponding continuum background at leading order in QCD. For the Higgs signal, we include gluon fusion (GF) and vector boson fusion (VBF) production channels, while for the background we consider all treelevel contributions, including pure EW effects (O(α^4_(QED))) and QCD contributions (O(α^2_(QED) α^2_s)), plus the loopinduced gluoninitiated process. After convolution with the experimental mass resolution, the main effect of the interference is to shift the position of the mass peak, as in the inclusive GF case studied previously. The apparent mass shift is small in magnitude but strongly dependent on the Higgs width, potentially allowing for a measurement of, or bound on, the width itself. In the H(→γγ)+2 jets channel, the VBF and GF contributions generate shifts of opposite signs which largely cancel, depending on the sets of cuts used, to as little as 5 MeV (toward a lower Higgs mass). The small magnitude of the shift makes this channel a good reference mass for measuring the inclusive mass shift of around 60 MeV in the Standard Model.  [Show abstract] [Hide abstract]
ABSTRACT: We compute the interference between the resonant process $pp\to H(\rightarrow \gamma\gamma)+2 \text{ jets}$ and the corresponding continuum background at leading order in QCD. For the Higgs signal, we include gluon fusion (GF) and vector boson fusion (VBF) production channels, while for the background we consider all treelevel contributions, including pure EW effects (${\cal O}(\alpha_{QED}^4)$) and QCD contributions (${\cal O}(\alpha_{QED}^2 \alpha_{s}^2)$), plus the loopinduced gluoninitiated process. After convolution with the experimental mass resolution, the main effect of the interference is to shift the position of the mass peak, as in the inclusive GF case studied previously. The apparent mass shift is small in magnitude but strongly dependent on the Higgs width, potentially allowing for a measurement of, or bound on, the width itself. In the $H(\rightarrow \gamma\gamma)+2 \text{ jets}$ channel, the VBF and GF contributions generate shifts of opposite signs which largely cancel, depending on the sets of cuts used, to as little as 5 MeV (toward a lower Higgs mass). The small magnitude of the shift makes this channel a good reference mass for measuring the inclusive mass shift of around 60 MeV in the Standard Model.  [Show abstract] [Hide abstract]
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.  [Show abstract] [Hide abstract]
ABSTRACT: We extend the hexagon function bootstrap to the nexttomaximallyhelicityviolating (NMHV) configuration for sixpoint scattering in planar \( \mathcal{N} \) = 4 superYangMills theory at three loops. Constraints from the \( \overline{Q} \) differential equation, from the operator product expansion (OPE) for Wilson loops with operator insertions, and from multiRegge factorization, lead to a unique answer for the threeloop ratio function. The threeloop result also predicts additional terms in the OPE expansion, as well as the behavior of NMHV amplitudes in the multiRegge limit at one higher logarithmic accuracy (NNLL) than was used as input. Both predictions are in agreement with recent results from the fluxtube approach. We also study the multiparticle factorization of multiloop amplitudes for the first time. We find that the function controlling this factorization is purely logarithmic through three loops. We show that a function U , which is closely related to the parityeven part of the ratio function V , is remarkably simple; only five of the nine possible final entries in its symbol are nonvanishing. We study the analytic and numerical behavior of both the parityeven and parityodd parts of the ratio function on simple lines traversing the space of cross ratios (u, v, w), as well as on a few twodimensional planes. Finally, we present an empirical formula for V in terms of elements of the coproduct of the sixgluon MHV remainder function R 6 at one higher loop, which works through three loops for V (four loops for R 6).  [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.  [Show abstract] [Hide abstract]
ABSTRACT: We describe the hexagon function bootstrap for solving for sixgluon scattering amplitudes in the large $N_c$ limit of ${\cal N}=4$ superYangMills theory. In this method, an ansatz for the finite part of these amplitudes is constrained at the level of amplitudes, not integrands, using boundary information. In the nearcollinear limit, the dual picture of the amplitudes as Wilson loops leads to an operator product expansion which has been solved using integrability by Basso, Sever and Vieira. Factorization of the amplitudes in the multiRegge limit provides additional boundary data. This bootstrap has been applied successfully through four loops for the maximally helicity violating (MHV) configuration of gluon helicities, and through three loops for the nonMHV case.  [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.  [Show abstract] [Hide abstract]
ABSTRACT: We present the fourloop remainder function for sixgluon scattering with maximal helicity violation in planar N=4 superYangMills theory, as an analytic function of three dualconformal cross ratios. The function is constructed entirely from its analytic properties, without ever inspecting any multiloop integrand. We employ the same approach used at three loops, writing an ansatz in terms of hexagon functions, and fixing coefficients in the ansatz using the multiRegge limit and the operator product expansion in the nearcollinear limit. We express the result in terms of multiple polylogarithms, and in terms of the coproduct for the associated Hopf algebra. From the remainder function, we extract the BFKL eigenvalue at nexttonexttoleading logarithmic accuracy (NNLLA), and the impact factor at NNNLLA. We plot the remainder function along various lines and on one surface, studying ratios of successive loop orders. As seen previously through three loops, these ratios are surprisingly constant over large regions in the space of cross ratios, and they are not far from the value expected at asymptotically large orders of perturbation theory.  [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.  [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.  [Show abstract] [Hide abstract]
ABSTRACT: I provide a basic introduction to modern helicity amplitude methods, including color organization, the spinor helicity formalism, and factorization properties. I also describe the BCFW (onshell) recursion relation at tree level, and explain how similar ideas  unitarity and onshell methods  work at the loop level. These notes are based on lectures delivered at the 2012 CERN Summer School and at TASI 2013.  [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.  [Show abstract] [Hide abstract]
ABSTRACT: We introduce a generating function for the coefficients of the leading logarithmic BFKL Green's function in transversemomentum space, order by order in alpha_s, in terms of singlevalued harmonic polylogarithms. As an application, we exhibit fully analytic azimuthalangle and transversemomentum distributions for MuellerNavelet jet cross sections at each order in alpha_s. We also provide a generating function for the total cross section valid to any number of loops.  [Show abstract] [Hide abstract]
ABSTRACT: We present a study on highperformance computing and largescale distributed computing for perturbative QCD calculations.  [Show abstract] [Hide abstract]
ABSTRACT: We study the change in the diphotoninvariantmass distribution for Higgs boson decays to two photons, due to interference between the Higgs resonance in gluon fusion and the continuum background amplitude for gg→γγ. Previously, the apparent Higgs mass was found to shift by around 100 MeV in the standard model in the leadingorder approximation, which may potentially be experimentally observable. We compute the nexttoleadingorder QCD corrections to the apparent mass shift, which reduce it by about 40%. The apparent mass shift may provide a way to measure, or at least bound, the Higgs boson width at the Large Hadron Collider through "interferometry." We investigate how the shift depends on the Higgs width, in a model that maintains constant Higgs boson signal yields. At Higgs widths above 30 MeV, the mass shift is over 200 MeV and increases with the square root of the width. The apparent mass shift could be measured by comparing with the ZZ^{*} channel, where the shift is much smaller. It might be possible to measure the shift more accurately by exploiting its strong dependence on the Higgs transverse momentum.  [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.  [Show abstract] [Hide abstract]
ABSTRACT: We present the threeloop remainder function, which describes the scattering of six gluons in the maximallyhelicityviolating configuration in planar N=4 superYangMills theory, as a function of the three dual conformal cross ratios. The result can be expressed in terms of multiple Goncharov polylogarithms. We also employ a more restricted class of "hexagon functions" which have the correct branch cuts and certain other restrictions on their symbols. We classify all the hexagon functions through transcendental weight five, using the coproduct for their Hopf algebra iteratively, which amounts to a set of firstorder differential equations. The threeloop remainder function is a particular weightsix hexagon function, whose symbol was determined previously. The differential equations can be integrated numerically for generic values of the cross ratios, or analytically in certain kinematics limits, including the nearcollinear and multiRegge limits. These limits allow us to impose constraints from the operator product expansion and multiRegge factorization directly at the function level, and thereby to fix uniquely a set of Riemannzetavalued constants that could not be fixed at the level of the symbol. The nearcollinear limits agree precisely with recent predictions by Basso, Sever and Vieira based on integrability. The multiRegge limits agree with the factorization formula of Fadin and Lipatov, and determine three constants entering the impact factor at this order. We plot the threeloop remainder function for various slices of the Euclidean region of positive cross ratios, and compare it to the twoloop one. For large ranges of the cross ratios, the ratio of the threeloop to the twoloop remainder function is relatively constant, and close to 7.  [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.
Publication Stats
9k  Citations  
473.15  Total Impact Points  
Top Journals
Institutions

19942015

Stanford University
 • SLAC National Accelerator Laboratory
 • Department of Physics
Stanford, California, United States


2011

CERN
 Physics Department (PH)
Genève, Geneva, Switzerland


2004

Durham University
 Department of Physics
Durham, England, United Kingdom
