Showing papers on "Parton published in 2011"
TL;DR: In this article, the authors presented the resummation of collinear and soft-gluon corrections to single top quark production in the $t$ channel at next-to-next to-leading logarithm accuracy using two-loop soft anomalous dimensions.
Abstract: I present the resummation of collinear and soft-gluon corrections to single top quark production in the $t$ channel at next-to-next-to-leading logarithm accuracy using two-loop soft anomalous dimensions. The expansion of the resummed cross section yields approximate next-to-next-to-leading-order cross sections. Numerical results for $t$-channel single top quark (or single antitop) production at the Tevatron and the LHC are presented, including the dependence of the cross sections on the top quark mass and the uncertainties from scale variation and parton distributions. Combined results for all single top quark production channels are also given.
617 citations
TL;DR: In this article, the authors review the physics basis, main features and use of general-purpose Monte Carlo event generators for the simulation of proton-proton collisions at the Large Hadron Collider.
608 citations
11 Jul 2011
TL;DR: The first measurement of the triangular v3, quadrangular v4, and pentagonal v5 charged particle flow in Pb-Pb collisions is reported, and a double peaked structure in the two-particle azimuthal correlations is observed, which can be naturally explained from the measured anisotropic flow Fourier coefficients.
Abstract: We report on the first measurement of the triangular nu(3), quadrangular nu(4), and pentagonal nu(5) charged particle flow in Pb-Pb collisions at root s(NN) = 2.76 TeV measured with the ALICE detector at the CERN Large Hadron Collider. We show that the triangular flow can be described in terms of the initial spatial anisotropy and its fluctuations, which provides strong constraints on its origin. In the most central events, where the elliptic flow nu(2) and nu(3) have similar magnitude, a double peaked structure in the two-particle azimuthal correlations is observed, which is often interpreted as a Mach cone response to fast partons. We show that this structure can be naturally explained from the measured anisotropic flow Fourier coefficients.
515 citations
TL;DR: In this paper, an exact all-order expression for the Drell-Yan cross section at small transverse momentum is derived directly in q676 T672 space, in which all large logarithms are resummed.
Abstract: Using methods from effective field theory, an exact all-order expression for the Drell–Yan cross section at small transverse momentum is derived directly in q
T
space, in which all large logarithms are resummed. The anomalous dimensions and matching coefficients necessary for resummation at NNLL order are given explicitly. The precise relation between our result and the Collins–Soper–Sterman formula is discussed, and as a by-product the previously unknown three-loop coefficient A
(3) is obtained. The naive factorization of the cross section at small transverse momentum is broken by a collinear anomaly, which prevents a process-independent definition of x
T
-dependent parton distribution functions. A factorization theorem is derived for the product of two such functions, in which the dependence on the hard momentum transfer is separated out. The remainder factors into a product of two functions of longitudinal momentum variables and $x_{T}^{2}$
, whose renormalization-group evolution is derived and solved in closed form. The matching of these functions at small x
T
onto standard parton distributions is calculated at $\mathcal{O}(\alpha_{s})$
, while their anomalous dimensions are known to three loops.
447 citations
TL;DR: In this paper, a determination of the parton distributions of the nucleon from a global set of hard scattering data using the NNPDF methodology including heavy quark mass effects is presented, and the authors assess the impact of these uncertainties on LHC observables by providing parton sets with different values of the charm and bottom quark masses.
432 citations
TL;DR: In this paper, a factorization theorem for Drell-Yan process at low q-T using effective field theory methods is derived and the obtained quantities are gauge invariant and the special role of the soft function and its subtraction thereof is emphasized.
Abstract: We derive a factorization theorem for Drell-Yan process at low q_T using effective field theory methods. In this theorem all the obtained quantities are gauge invariant and the special role of the soft function--and its subtraction thereof--is emphasized. We define transverse-momentum dependent parton distribution functions (TMDPDFs) which are free from light-cone singularities while all the Wilson lines are defined on-the-light-cone. We show explicitly to first order in \alpha_s that the partonic Feynman PDF can be obtained from the newly defined partonic TMDPDF by integrating over the transverse momentum of the parton inside the hadron. We obtain a resummed expression for the TMDPDF, and hence for the cross section, in impact parameter space. The universality of the newly defined matrix elements is established perturbatively to first order in \alpha_s. The factorization theorem is validated to first order in \alpha_s and also the gauge invariance between Feynman and light-cone gauges.
354 citations
TL;DR: The qcdnum program numerically solves the evolution equations for parton densities and fragmentation functions in perturbative QCD, a versatile convolution engine that provides tools to compute parton luminosities, cross-sections in hadron–hadron scattering, and deep inelastic structure functions in the zero-mass scheme or in generalised mass schemes.
312 citations
TL;DR: In this paper, the basic underlying theory of medium induced gluon radiation based on perturbative Quantum Chromo Dynamics (pQCD) and current experimental results from Deep Inelastic Scattering on large nuclei and high energy heavy-ion collisions is reviewed.
307 citations
TL;DR: In this article, the authors considered direct diphoton production in hadron collisions, and they computed the next-to-next-toleading order (NNLO) QCD radiative corrections at the fully-differential level.
Abstract: We consider direct diphoton production in hadron collisions, and we compute the next-to-next-to-leading order (NNLO) QCD radiative corrections at the fully-differential level. Our calculation uses the $q_T$ subtraction formalism and it is implemented in a parton level Monte Carlo program. The program allows the user to apply arbitrary kinematical cuts on the final-state photons and the associated jet activity, and to compute the corresponding distributions in the form of bin histograms. We present selected numerical results related to Higgs boson searches at the LHC and corresponding results at the Tevatron.
225 citations
TL;DR: In this article, a method for incorporating the information contained in new datasets into an existing set of parton distribution functions without the need for refitting is presented, which involves reweighting the ensemble of the parton densities through the computation of the χ 2 to the new dataset.
199 citations
TL;DR: In this paper, the Wigner functions of the nucleon were obtained through a Fourier transform in the transverse space of the generalized transverse-momentum dependent parton distributions.
Abstract: We study the Wigner functions of the nucleon which provide multidimensional images of the quark distributions in phase space. These functions can be obtained through a Fourier transform in the transverse space of the generalized transverse-momentum dependent parton distributions. They depend on both the transverse position and the three-momentum of the quark relative to the nucleon, and therefore combine in a single picture all the information contained in the generalized parton distributions and the transverse-momentum dependent parton distributions. We focus the discussion on the distributions of unpolarized/longitudinally polarized quark in an unpolarized/longitudinally polarized nucleon. In this way, we can study the role of the orbital angular momentum of the quark in shaping the nucleon and its correlations with the quark and nucleon polarizations. The quark orbital angular momentum is also calculated from its phase-space average weighted with the Wigner distribution of unpolarized quarks in a longitudinally polarized nucleon. The corresponding results obtained within different light-cone quark models are compared with alternative definitions of the quark orbital angular momentum, as given in terms of generalized parton distributions and transverse-momentum dependent parton distributions.
TL;DR: In this article, the PHENIX 2007 data set of J/psi yields at forward rapidity (1.2 < vertical bar y vertical bar < 2.2) in Au + Au collisions at root s(NN) = 200 GeV.
Abstract: Heavy quarkonia are observed to be suppressed in relativistic heavy-ion collisions relative to their production in p + p collisions scaled by the number of binary collisions. In order to determine if this suppression is related to color screening of these states in the produced medium, one needs to account for other nuclear modifications including those in cold nuclear matter. In this paper, we present new measurements from the PHENIX 2007 data set of J/psi yields at forward rapidity (1.2 < vertical bar y vertical bar < 2.2) in Au + Au collisions at root s(NN) = 200 GeV. The data confirm the earlier finding that the suppression of J/. at forward rapidity is stronger than at midrapidity, while also extending the measurement to finer bins in collision centrality and higher transverse momentum (p(T)). We compare the experimental data to the most recent theoretical calculations that incorporate a variety of physics mechanisms including gluon saturation, gluon shadowing, initial-state parton energy loss, cold nuclear matter breakup, color screening, and charm recombination. We find J/psi suppression beyond cold-nuclear-matter effects. However, the current level of disagreement between models and d + Au data precludes using these models to quantify the hot-nuclear-matter suppression.
TL;DR: The Parton-Hadron-String Dynamics (PHSD) transport approach is applied to nucleus-nucleus collisions at RHIC energies with respect to differential hadronic spectra in comparison to available data as mentioned in this paper.
TL;DR: In this paper, the authors use Soft Collinear Effective Theory (SCET) to analyze the transverse momentum broadening of an energetic parton propagating through quark-gluon plasma.
Abstract: We use Soft Collinear Effective Theory (SCET) to analyze the transverse momentum broadening, or diffusion in transverse momentum space, of an energetic parton propagating through quark-gluon plasma. Since we neglect the radiation of gluons from the energetic parton, we can only discuss momentum broadening, not parton energy loss. The interaction responsible for momentum broadening in the absence of radiation is that between the energetic (collinear) parton and the Glauber modes of the gluon fields in the medium. We derive the effective Lagrangian for this interaction, and we show that the probability for picking up transverse momentum k_\perp is given by the Fourier transform of the expectation value of two transversely separated light-like path-ordered Wilson lines. This yields a field theoretical definition of the jet quenching parameter \hat q, and shows that this can be interpreted as a diffusion constant. We close by revisiting the calculation of \hat q for the strongly coupled plasma of N=4 SYM theory, showing that previous calculations need some modifications that make them more straightforward and do not change the result.
TL;DR: In this article, the authors investigated several ingredients for a theory of multiple hard scattering in hadron-hadron collisions, including the space-time structure of multiple interactions, their power behavior, spin and color correlations, interference terms, scale evolution and Sudakov logarithms.
TL;DR: In this article, the antenna subtraction method was extended to NLO calculations at hadron colliders and the full set of initial-initial double real radiation antenna functions were derived in integrated form.
Abstract: The antenna subtraction method handles real radiation contributions in higher order corrections to jet observables. The method is based on antenna functions, which encapsulate all unresolved radiation between a pair of hard radiator partons. To apply this method to compute hadron collider observables, initial-initial antenna functions with both radiators in the initial state are required in unintegrated and integrated forms. In view of extending the antenna subtraction method to next-to-next-to-leading order (NNLO) calculations at hadron colliders, we derive the full set of initial-initial double real radiation antenna functions in integrated form.
TL;DR: In this article, a new formalism for dipole-type parton showers which maintains exact energy-momentum conservation at each step of the evolution is presented, with particular emphasis on the coherence properties, the level at which recoil effects do enter and the role of transverse momentum generation from initial state radiation.
Abstract: We outline a new formalism for dipole-type parton showers which maintain exact energy-momentum conservation at each step of the evolution. Particular emphasis is put on the coherence properties, the level at which recoil effects do enter and the role of transverse momentum generation from initial state radiation. The formulated algorithm is shown to correctly incorporate coherence for soft gluon radiation.
TL;DR: In this article, a bilocal quark-quark operator connected by a straight Wilson line is proposed to study transverse momentum dependent parton distribution functions (TMDs) using lattice QCD.
Abstract: We discuss in detail a method to study transverse momentum dependent parton distribution functions (TMDs) using lattice QCD. To develop the formalism and to obtain first numerical results, we directly implement a bilocal quark-quark operator connected by a straight Wilson line, allowing us to study T-even, ''process-independent'' TMDs. Beyond results for x-integrated TMDs and quark densities, we present a study of correlations in x and k{sub perpendicular}. Our calculations are based on domain wall valence quark propagators by the LHP Collaboration calculated on top of gauge configurations provided by the MILC Collaboration with N{sub f}=2+1 asqtad-improved staggered sea quarks.
TL;DR: In this article, the authors derived the general formulae for the two dijet production in pp collisions by exploring the difference in scales between soft and hard QCD processes and space-time struc- ture of Feynman diagrams.
Abstract: By exploring the difference in scales between soft and hard QCD processes and space-time struc- ture of Feynman diagrams we derive within the parton model the general formulae for the two dijet production in pp collisions. Cross section resulting from collisions of four partons is calculated in terms of new generalized two-particle parton distributions (GPDs) in proton. We explain why the derived formulae hold modulo simple modifications even when the pQCD corrections are taken into account. Our derivation justifies the intuitive geometrical approach in the impact parameter space used before to describe the four-jet production. Comparison with CDF and D0 data shows that the independent parton approximation to light cone wave function of proton is insufficient to explain the data. We argue that the nonperturbative multi-parton correlations in the light cone wave functions of colliding protons play an important role. We explain kinematics where the discussed process is enhanced as compared to the four jet production in the hard collisions of two partons.
TL;DR: In this paper, the effects of jet broadening and gluon bremsstrahlung induced by the propagation of a highly energetic quark in dense QCD matter are reconsidered from effective theory point of view.
Abstract: Two effects, jet broadening and gluon bremsstrahlung induced by the propagation of a highly energetic quark in dense QCD matter, are reconsidered from effective theory point of view. We modify the standard Soft Collinear Effective Theory (SCET) Lagrangian to include Glauber modes, which are needed to implement the interactions between the medium and the collinear fields. We derive the Feynman rules for this Lagrangian and show that it is invariant under soft and collinear gauge transformations. We find that the newly constructed theory SCETG recovers exactly the general result for the transverse momentum broadening of jets. In the limit where the radiated gluons are significantly less energetic than the parent quark, we obtain a jet energy-loss kernel identical to the one discussed in the reaction operator approach to parton propagation in matter. In the framework of SCETG we present results for the fully-differential bremsstrahlung spectrum for both the incoherent and the Landau-Pomeranchunk-Migdal suppressed regimes beyond the soft-gluon approximation. Gauge invariance of the physics results is demonstrated explicitly by performing the calculations in both the light-cone and covariant R
ξ
gauges. We also show how the process-dependent medium-induced radiative corrections factorize from the jet production cross section on the example of the quark jets considered here.
TL;DR: In this paper, a simple formalism for the evolution of timelike jets is presented, in which tree-level matrix element corrections can be systematically incorporated, up to arbitrary parton multiplicities and over all of phase space, in a way that exponentiates the matching corrections.
Abstract: We present a simple formalism for the evolution of timelike jets in which tree-level matrix element corrections can be systematically incorporated, up to arbitrary parton multiplicities and over all of phase space, in a way that exponentiates the matching corrections. The scheme is cast as a shower Markov chain which generates one single unweighted event sample, that can be passed to standard hadronization models. Remaining perturbative uncertainties are estimated by providing several alternative weight sets for the same events, at a relatively modest additional overhead. As an explicit example, we consider Z {yields} q{bar q} evolution with unpolarized, massless quarks and include several formally subleading improvements as well as matching to tree-level matrix elements through {alpha}{sub s}{sup 4}. The resulting algorithm is implemented in the publicly available VINCIA plugin to the PYTHIA8 event generator.
TL;DR: In this article, the two-loop QCD corrections to the helicity amplitudes for the processes $H \to ggg$ and $H\to q \bar q g$ in an effective theory with infinite top quark mass are derived from the coefficients appearing in the general tensorial structure for each process.
Abstract: Many search strategies for the Standard Model Higgs boson apply specific selection criteria on hadronic jets observed in association with the Higgs boson decay products, either in the form of a jet veto, or by defining event samples according to jet multiplicity. To improve the theoretical description of Higgs-boson-plus-jet production (and the closely related Higgs boson transverse momentum distribution), we derive the two-loop QCD corrections to the helicity amplitudes for the processes $H \to ggg$ and $H\to q \bar q g$ in an effective theory with infinite top quark mass. The helicity amplitudes are extracted from the coefficients appearing in the general tensorial structure for each process. The coefficients are derived from the Feynman graph amplitudes by means of projectors within the conventional dimensional regularization scheme. The infrared pole structure of our result agrees with the expectation from infrared factorization and the finite parts of the amplitudes are expressed in terms of one- and two-dimensional harmonic polylogarithms.
TL;DR: In this paper, the role of two-photon exchange (TPE) in electron-hadron scattering, focusing in particular on hadronic frameworks suitable for describing the low and moderate Q 2 region relevant to most experimental studies, is discussed.
TL;DR: In this paper, the authors study production of two pairs of jets in %hard hadron-hadron collisions in view of extracting contribution of double hard interactions of three and four partons.
Abstract: We study production of two pairs of jets in %hard hadron--hadron collisions in view of extracting contribution of {\em double hard interactions} of three and four partons ($3\to4$, $4\to4$). Such interactions, in spite of being power suppressed at the level of the total cross section, become comparable with the standard hard collisions of two partons, $2\to4$, in the {\em back-to-back kinematics} when the transverse momentum imbalances of two pairing jets are relatively small.
We express differential and total cross sections for two-dijet production in double parton collisions through the generalized two-parton distributions, $_2$GPDs \cite{BDFS1}, that contain large-distance two-parton correlations of non-perturbative origin as well as small-distance correlations due to parton evolution. We find that these large- and small-distance correlations participate in different manner in 4-jet production, and treat them in the leading logarithmic approximation of pQCD that resums collinear logarithms in all orders.
A special emphasis is given to $3\to4$ double hard interaction processes that occur as an interplay between large- and short-distance parton correlations and were not taken into consideration by approaches inspired by the parton model picture. We demonstrate that the $3\to4$ mechanism, being of the same order in $\as$ as the $4\to4$ process, turns out to be {\em geometrically enhanced} compared to the latter and should contribute significantly to 4-jet production.
The framework developed here takes into systematic consideration perturbative $Q^2$ evolution of $_2$GPDs. It can be used as a basis for future analysis of NLO corrections to multi-parton interactions (MPI) at LHC and Tevatron colliders, in particular for improving evaluation of QCD backgrounds to new physics searches.
TL;DR: In this article, the authors presented predictions for the production cross-section of a Standard Model Higgs boson in association with a pair at next-to-leading order accuracy using matrix elements obtained from the HELAC-Oneloop package.
Abstract: We present predictions for the production cross-section of a Standard Model Higgs boson in association with a pair at next-to-leading order accuracy using matrix elements obtained from the HELAC-Oneloop package. The NLO prediction was interfaced to the PYTHIA and HERWIG shower Monte Carlo programs with the help of POWHEG-Box, allowing for decays of massive particles, showering and hadronization, thus leading to final results at the hadron level.
TL;DR: This work proposes a method to measure the double parton scattering at LHCb using leptonic final states from the decay of two prompt J/ψ mesons.
Abstract: We argue that the recent LHCb observation of J/ψ-pair production indicates a significant contribution from double parton scattering, in addition to the standard single parton scattering component. We propose a method to measure the double parton scattering at LHCb using leptonic final states from the decay of two prompt J/ψ mesons.
TL;DR: The medium modification of a partonic jet shower traversing in a hot quark-gluon plasma is investigated and a differential equation that governs the evolution of the radiated gluon distribution as the jet propagates through the medium is derived.
Abstract: We investigate the medium modification of a partonic jet shower traversing in a hot quark-gluon plasma. We derive and solve a differential equation that governs the evolution of the radiated gluon distribution as the jet propagates through the medium. Energy contained inside the jet cone is lost by dissipation through elastic collisions with the medium and by scattering of shower partons to larger angles. We find that the jet energy loss at early times is dominated by medium effects on the vacuum radiation, and by medium-induced radiation effects at late times. We compare our numerical results for the nuclear modification of the dijet asymmetry with that recently reported by the ATLAS Collaboration.
TL;DR: In this paper, the antenna subtraction method was extended to NLO calculations at hadron colliders and the full set of initial-initial double real radiation antenna functions were derived in integrated form.
Abstract: The antenna subtraction method handles real radiation contributions in higher order corrections to jet observables. The method is based on antenna functions, which encapsulate all unresolved radiation between a pair of hard radiator partons. To apply this method to compute hadron collider observables, initial-initial antenna functions with both radiators in the initial state are required in unintegrated and integrated forms. In view of extending the antenna subtraction method to next-to-next-to-leading order (NNLO) calculations at hadron colliders, we derive the full set of initial-initial double real radiation antenna functions in integrated form.
TL;DR: In this article, generalized transverse-momentum dependent parton distributions (GTMDs) are used to simultaneously model different observables. But the authors consider a light-cone wave function overlap representation of the GTMDs, and construct the different quark-quark correlation functions from the 3 quark Fock components.
Abstract: We present a systematic study of generalized transverse-momentum dependent parton distributions (GTMDs). By taking specific limits or projections, these GTMDs yield various transverse-momentum dependent and generalized parton distributions, thus providing a unified framework to simultaneously model different observables. We present such simultaneous modeling by considering a light-cone wave function overlap representation of the GTMDs. We construct the different quark-quark correlation functions from the 3-quark Fock components within both the light-front constituent quark model and the chiral quark-soliton model. We provide a comparison with available data and make predictions for different observables.
TL;DR: The leading twist theory of nuclear shadowing was proposed in this paper, which is based on the combination of the Gribov-Glauber theory, QCD factorization theorems, and the HERA QCD analysis of diffraction in lepton-proton deep inelastic scattering (DIS).
Abstract: We present and discuss the theory and phenomenology of the leading twist theory of nuclear shadowing which is based on the combination of the generalization of the Gribov-Glauber theory, QCD factorization theorems, and the HERA QCD analysis of diffraction in lepton-proton deep inelastic scattering (DIS). We apply this technique for the analysis of a wide range of hard processes with nuclei---inclusive DIS on deuterons, medium-range and heavy nuclei, coherent and incoherent diffractive DIS with nuclei, and hard diffraction in proton-nucleus scattering---and make predictions for the effect of nuclear shadowing in the corresponding sea quark and gluon parton distributions. We also analyze the role of the leading twist nuclear shadowing in generalized parton distributions in nuclei and in certain characteristics of final states in nuclear DIS. We discuss the limits of applicability of the leading twist approximation for small x scattering off nuclei and the onset of the black disk regime and methods of detecting it. It will be possible to check many of our predictions in the near future in the studies of the ultraperipheral collisions at the Large Hadron Collider (LHC). Further checks will be possible in pA collisions at the LHC and forward hadron production at the Relativistic Heavy Ion Collider (RHIC). Detailed tests will be possible at an Electron-Ion Collider (EIC) in the USA and at the Large Hadron-Electron Collider (LHeC) at CERN.