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Showing papers on "Parton published in 2007"


Proceedings ArticleDOI
01 Jan 2007
TL;DR: In this paper, a preliminary set of updated NLO parton distributions and their uncertainties determined from CCFR and NuTeV dimuon cross sections are presented, along with additional jet data from HERA and the Tevatron.
Abstract: We present a preliminary set of updated NLO parton distributions. For the first time we have a quantitative extraction of the strange quark and antiquark distributions and their uncertainties determined from CCFR and NuTeV dimuon cross sections. Additional jet data from HERA and the Tevatron improve our gluon extraction. Lepton asymmetry data and neutrino structure functions improve the flavour separation, particularly constraining the down quark valence distribution.

1,288 citations


Journal ArticleDOI
TL;DR: MadGraph/MadEvent Monte Carlo as mentioned in this paper is a Monte Carlo event generator for hadron collider physics that can be used to generate events at the parton, hadron and detector level from a web interface.
Abstract: We present the latest developments of the MadGraph/MadEvent Monte Carlo event generator and several applications to hadron collider physics. In the current version events at the parton, hadron and detector level can be generated directly from a web interface, for arbitrary processes in the Standard Model and in several physics scenarios beyond it (HEFT, MSSM, 2HDM). The most important additions are: a new framework for implementing user-defined new physics models; a standalone running mode for creating and testing matrix elements; generation of events corresponding to different processes, such as signal(s) and backgrounds, in the same run; two platforms for data analysis, where events are accessible at the parton, hadron and detector level; and the generation of inclusive multi-jet samples by combining parton-level events with parton showers. To illustrate the new capabilities of the package some applications to hadron collider physics are presented: 1) Higgs search in pp \to H \to W^+W^-: signal and backgrounds. 2) Higgs CP properties: pp \to H jj$in the HEFT. 3) Spin of a new resonance from lepton angular distributions. 4) Single-top and Higgs associated production in a generic 2HDM. 5) Comparison of strong SUSY pair production at the SPS points. 6) Inclusive W+jets matched samples: comparison with the Tevatron data. Comment: 38 pages, 15 figures

1,183 citations


Journal ArticleDOI
TL;DR: A new formulation of the subtraction method is proposed to numerically compute arbitrary infrared-safe observables for this class of processes and exploits the universal behavior of the associated transverse-momentum distributions in the small-qT region to cancel the infrared divergences.
Abstract: We consider higher-order QCD corrections to the production of colorless high-mass systems (lepton pairs, vector bosons, Higgs bosons, etc.) in hadron collisions. We propose a new formulation of the subtraction method to numerically compute arbitrary infrared-safe observables for this class of processes. To cancel the infrared divergences, we exploit the universal behavior of the associated transverse-momentum (qT) distributions in the small-qT region. The method is illustrated in general terms up to the next-to-next-to-leading order in QCD perturbation theory. As a first explicit application, we study Higgs-boson production through gluon fusion. Our calculation is implemented in a parton level Monte Carlo program that includes the decay of the Higgs boson into two photons. We present selected numerical results at the CERN Large Hadron Collider.

820 citations


Journal ArticleDOI
TL;DR: In this article, the authors studied the matching of multijet matrix elements and shower evolution in the case of top production in hadronic collisions at the Tevatron and at the LHC.
Abstract: We study the matching of multijet matrix elements and shower evolution in the case of top production in hadronic collisions at the Tevatron and at the LHC. We present the results of the matching algorithm implemented in the ALPGEN Monte Carlo generator, and compare them with results obtained at the parton level, and with the predictions of the MC@NLO approach. We highlight the consistency of the matching algorithm when applied to these final states, and the excellent agreement obtained with MC@NLO for most inclusive quantities. We nevertheless identify also a remarkable difference in the rapidity spectrum of the leading jet accompanying the top quark pair, and comment on the likely origin of this discrepancy.

725 citations


Journal ArticleDOI
TL;DR: In this paper, the authors compare different procedures for combining fixed-order tree-level matrix-element generators with parton showers and find that although similar results are obtained in all cases, there are important differences.
Abstract: We compare different procedures for combining fixed-order tree-level matrix-element generators with parton showers. We use the case of W-production at the Tevatron and the LHC to compare different implementations of the so-called CKKW and MLM schemes using different matrix-element generators and different parton cascades. We find that although similar results are obtained in all cases, there are important differences.

619 citations


Journal ArticleDOI
TL;DR: In this article, the cross section for one-particle inclusive deep inelastic scattering off the nucleon for low transverse momentum of the detected hadron was studied and the results were complete in the one-photon exchange approximation at leading and first sub-leading twist accuracy, with both beam and target polarization.
Abstract: We study the cross section for one-particle inclusive deep inelastic scattering off the nucleon for low transverse momentum of the detected hadron. We decompose the cross section in terms of structure functions and calculate them at tree level in terms of transverse-momentum-dependent parton distribution and fragmentation functions. Our results are complete in the one-photon exchange approximation at leading and first subleading twist accuracy, with both beam and target polarization.

509 citations


Journal ArticleDOI
David D'Enterria1, David D'Enterria2, M. Ballintijn3, M. Bedjidian4  +2185 moreInstitutions (141)
TL;DR: In this paper, the capabilities of the CERN Large Hadron Collider (LHC) experiment to explore the rich heavy-ion physics program offered by the LHC are presented, and the potential of the CMS experiment to carry out a series of representative Pb-Pb measurements.
Abstract: This report presents the capabilities of the CMS experiment to explore the rich heavy-ion physics programme offered by the CERN Large Hadron Collider (LHC). The collisions of lead nuclei at energies , will probe quark and gluon matter at unprecedented values of energy density. The prime goal of this research is to study the fundamental theory of the strong interaction ? Quantum Chromodynamics (QCD) ? in extreme conditions of temperature, density and parton momentum fraction (low-x).This report covers in detail the potential of CMS to carry out a series of representative Pb-Pb measurements. These include bulk observables, (charged hadron multiplicity, low pT inclusive hadron identified spectra and elliptic flow) which provide information on the collective properties of the system, as well as perturbative probes such as quarkonia, heavy-quarks, jets and high pT hadrons which yield tomographic information of the hottest and densest phases of the reaction.

361 citations


Journal ArticleDOI
TL;DR: In this article, the perturbative QCD was extended to include elastic as well as inelastic parton energy losses and jet path length fluctuations, which significantly reduced the discrepancy between theory and current data without violating the global entropy bounds.

356 citations


Journal ArticleDOI
TL;DR: In this paper, the physics of large impact parameter interactions at the LHC were discussed, and the dominant processes in UPCs are photon-nucleon (nucleus) interactions.
Abstract: We discuss the physics of large impact parameter interactions at the LHC: ultraperipheral collisions (UPCs). The dominant processes in UPCs are photon-nucleon (nucleus) interactions. The current LHC detector configurations can explore small $x$ hard phenomena with nuclei and nucleons at photon-nucleon center-of-mass energies above 1 TeV, extending the $x$ range of HERA by a factor of ten. In particular, it will be possible to probe diffractive and inclusive parton densities in nuclei using several processes. The interaction of small dipoles with protons and nuclei can be investigated in elastic and quasi-elastic $J/\psi$ and $\Upsilon$ production as well as in high $t$ $\rho^0$ production accompanied by a rapidity gap. Several of these phenomena provide clean signatures of the onset of the new high gluon density QCD regime. The LHC is in the kinematic range where nonlinear effects are several times larger than at HERA. Two-photon processes in UPCs are also studied. In addition, while UPCs play a role in limiting the maximum beam luminosity, they can also be used a luminosity monitor by measuring mutual electromagnetic dissociation of the beam nuclei. We also review similar studies at HERA and RHIC as well as describe the potential use of the LHC detectors for UPC measurements.

335 citations


Journal ArticleDOI
TL;DR: In this article, the expectation values of Wilson loops in strongly coupled plasma of N = 4 super Yang-Mills (SYM) theory were derived for the case of heavy ion collisions.
Abstract: Expectation values of Wilson loops define the nonperturbative properties of the hot medium produced in heavy ion collisions that arise in the analysis of both radiative parton energy loss and quarkonium suppression. We use the AdS/CFT correspondence to calculate the expectation values of such Wilson loops in the strongly coupled plasma of N = 4 super Yang-Mills (SYM) theory, allowing for the possibility that the plasma may be moving with some collective flow velocity as is the case in heavy ion collisions. We obtain the N = 4 SYM values of the jet quenching parameter ˆ q, which describes the energy loss of a hard parton in QCD, and of the velocity-dependence of the quark-antiquark screening length for a moving dipole as a function of the angle between its velocity and its orientation. We show that if the quark-gluon plasma is flowing with velocity vf at an angle θ with respect to the trajectory of a hard parton, the jet quenching parameteris modified by a factor γf (1 − vf cos θ), and show that this result applies in QCD as in N = 4 SYM. We discuss the relevance of the lessons we are learning from all these calculations to heavy ion collisions at RHIC and at the LHC. Furthermore, we discuss the relation between our results and those obtained in other theories with gravity duals, showing in particular that the ratio betweenin any two conformal theories with gravity duals is the square root of the ratio of their central charges. This leads us to conjecture that in nonconformal theories ˆ q defines a quantity that always decreases

330 citations


Journal ArticleDOI
A. Aktas, Calin Alexa, V. P. Andreev, T. Anthonis1  +283 moreInstitutions (35)
TL;DR: In this article, a new set of diffractive parton distribution functions is obtained through a simultaneous fit to the diffractive inclusive and dijet cross sections, which allows for a precise determination of both diffractive quark and gluon distributions in the range 0.05 < zIP < 0.9.
Abstract: Differential dijet cross sections in diffractive deep-inelastic scattering are measured with the H1 detector at HERA using an integrated luminosity of 51.5 pb−1. The selected events are of the type ep → eXY , where the system X contains at least two jets and is well separated in rapidity from the low mass proton dissociation system Y . The dijet data are compared with QCD predictions at next-to-leading order based on diffractive parton distribution functions previously extracted from measurements of inclusive diffractive deepinelastic scattering. The prediction describes the dijet data well at low and intermediate zIP (the fraction of the momentum of the diffractive exchange carried by the parton entering the hard interaction) where the gluon density is well determined from the inclusive diffractive data, supporting QCD factorisation. A new set of diffractive parton distribution functions is obtained through a simultaneous fit to the diffractive inclusive and dijet cross sections. This allows for a precise determination of both the diffractive quark and gluon distributions in the range 0.05 < zIP < 0.9. In particular, the precision on the gluon density at high momentum fractions is improved compared to previous extractions.

Journal ArticleDOI
TL;DR: In this article, the authors review the experimental data base in view of the new results for the proton, and neutron, obtained at JLab, MAMI, and MIT-Bates.

Journal ArticleDOI
TL;DR: In this paper, a review of the present knowledge on nontrivial relations between generalized parton distributions and transverse momentum dependent distributions is presented, and as far as spectator models are concerned, the existing results are considerably extended.
Abstract: Recent work suggests nontrivial relations between generalized parton distributions on the one hand and (naive time-reversal odd) transverse momentum dependent distributions on the other. Here we review the present knowledge on such type of relations. Moreover, as far as spectator model calculations are concerned, the existing results are considerably extended. While various relations between the two types of parton distributions can be found in the framework of spectator models, so far no nontrivial model-independent relations have been established.

Journal ArticleDOI
TL;DR: MadGraph/MadEvent Monte Carlo as discussed by the authors is a Monte Carlo event generator and several applications to hadron collider physics, such as Higgs CP properties and the spin of a new resonance from lepton angular distributions.
Abstract: We present the latest developments of the MadGraph/MadEvent Monte Carlo event generator and several applications to hadron collider physics. In the current version events at the parton, hadron and detector level can be generated directly from a web interface, for arbitrary processes in the Standard Model and in several physics scenarios beyond it (HEFT, MSSM, 2HDM). The most important additions are: a new framework for implementing user-defined new physics models; a standalone running mode for creating and testing matrix elements; generation of events corresponding to different processes, such as signal(s) and backgrounds, in the same run; two platforms for data analysis, where events are accessible at the parton, hadron and detector level; and the generation of inclusive multi-jet samples by combining parton-level events with parton showers. To illustrate the new capabilities of the package some applications to hadron collider physics are presented: 1) Higgs search in pp \to H \to W^+W^-: signal and backgrounds. 2) Higgs CP properties: pp \to H jj$in the HEFT. 3) Spin of a new resonance from lepton angular distributions. 4) Single-top and Higgs associated production in a generic 2HDM. 5) Comparison of strong SUSY pair production at the SPS points. 6) Inclusive W+jets matched samples: comparison with the Tevatron data.

Journal ArticleDOI
TL;DR: In this paper, a detailed study of the production of a high transverse-momentum lepton pair at hadron colliders, which includes the exact O(�) electroweak corrections properly matched with leading logarithmic effects due to multiple photon emission, is presented.
Abstract: We present a detailed study of the production of a high transverse-momentum lepton pair at hadron colliders, which includes the exact O(�) electroweak corrections properly matched with leading logarithmic effects due to multiple photon emission, as required by the experiments at the Fermilab Tevatron and the CERN LHC. Numerical results for the relevant observables of single Z-boson production at hadron colliders are presented. The impact of the radiative corrections is discussed in detail. The presence in the proton of a photon density is considered and the effects of the photon-induced partonic subprocesses are analyzed. The calculation has been implemented in the new version of the event generator HORACE, which is available for precision simulations of the neutral and charged current Drell-Yan processes.

Journal ArticleDOI
TL;DR: In this paper, the angular correlations in leptonic decays of vector bosons and top quarks can be included in Monte Carlo parton showers, in particular those matched to NLO QCD computations.
Abstract: We explain how angular correlations in leptonic decays of vector bosons and top quarks can be included in Monte Carlo parton showers, in particular those matched to NLO QCD computations. We consider the production of n pairs of leptons, originating from the decays of n electroweak vector bosons or of n top quarks, in the narrow-width approximation. In the latter case, the information on the n b quarks emerging from the decays is also retained. We give results of implementing this procedure in MC@NLO.

Journal ArticleDOI
TL;DR: In this paper, a new set of parton distributions obtained at NNLO were presented, including a full treatment of heavy flavors in the region near the quark mass, and the improved treatment leads to a significant change in the gluon and heavy quark distributions, and a larger value of the QCD coupling at N NLO, α S (M Z 2 ) = 0.1191 ± 0.002 ( expt. ) ± 0.003 ( theory ).

Journal ArticleDOI
TL;DR: In this article, the authors derived forward inclusive dijet production in the scattering of a dilute hadron off an arbitrary dense target, whose partons with small fraction of momentum x are described by a color glass condensate.

Journal ArticleDOI
TL;DR: In this article, it was shown that hard-scattering factorization is violated in the production of high-p}-T$ hadrons in hadron-hadron collisions, in the case that the hadrons are back-to-back.
Abstract: We show that hard-scattering factorization is violated in the production of high-${p}_{T}$ hadrons in hadron-hadron collisions, in the case that the hadrons are back-to-back, so that ${k}_{T}$ factorization is to be used. The explicit counterexample that we construct is for the single-spin asymmetry with one beam transversely polarized. The Sivers function needed here has particular sensitivity to the Wilson lines in the parton densities. We use a greatly simplified model theory to make the breakdown of factorization easy to check explicitly. But the counterexample implies that standard arguments for factorization fail not just for the single-spin asymmetry but for the unpolarized cross section for back-to-back hadron production in QCD in hadron-hadron collisions. This is unlike corresponding cases in ${e}^{+}{e}^{\ensuremath{-}}$ annihilation, Drell-Yan, and deeply inelastic scattering. Moreover, the result endangers factorization for more general hadroproduction processes.

Journal ArticleDOI
TL;DR: In this article, the authors proposed recursive equations that could be used to generate a lowest order parton shower for hard scattering in hadron-hadron collisions, based on factorization soft and collinear interactions from relatively harder interactions in QCD amplitudes.
Abstract: We specify recursive equations that could be used to generate a lowest order parton shower for hard scattering in hadron-hadron collisions. The formalism is based on the factorization soft and collinear interactions from relatively harder interactions in QCD amplitudes. It incorporates quantum interference between different amplitudes in those cases in which the interference diagrams have leading soft or collinear singularities. It incorporates the color and spin information carried by partons emerging from a hard interaction. One motivation for this work is to have a method that can naturally cooperate with next-to-leading order calculations.

Journal ArticleDOI
TL;DR: In this article, a new implementation of the general PQCD formalism of Collins, including heavy quark mass effects, is described, and important features that contribute to the accuracy and efficiency of the calculation of both neutral current (NC) and charged current (CC) processess are explicitly discussed.
Abstract: A new implementation of the general PQCD formalism of Collins, including heavy quark mass effects, is described. Important features that contribute to the accuracy and efficiency of the calculation of both neutral current (NC) and charged current (CC) processess are explicitly discussed. This new implementation is applied to the global analysis of the full HERA I data sets on NC and CC cross sections, with correlated systematic errors, in conjunction with the usual fixed-target and hadron collider data sets. By using a variety of parametrizations to explore the parton parameter space, robust new parton distribution function (PDF) sets (CTEQ6.5) are obtained. The new quark distributions are consistently higher in the region x ~ 10−3 than previous ones, with important implications on hadron collider phenomenology, especially at the LHC. The uncertainties of the parton distributions are reassessed and are compared to the previous ones. A new set of CTEQ6.5 eigenvector PDFs that encapsulates these uncertainties is also presented.

Posted ContentDOI
TL;DR: Generalized parton distributions have been introduced as a suitable theoretical tool to study the structure of the nucleon as discussed by the authors, and they provide a comprehensive framework for describing the quark and gluon structure.
Abstract: Generalized parton distributions have been introduced in recent years as a suitable theoretical tool to study the structure of the nucleon. Unifying the concepts of parton distributions and hadronic form factors, they provide a comprehensive framework for describing the quark and gluon structure of the nucleon. In this review their formal properties and modeling are discussed, summarizing the most recent developments in the phenomenological description of these functions. The status of available data is also presented.

Journal ArticleDOI
TL;DR: In this article, the dynamical parton distributions of the nucleon generated radiatively from valence-like positive input distributions at optimally chosen low resolution scales are compared with the standard distributions generated from positive input distribution at some fixed and higher resolution scale.
Abstract: Utilizing recent DIS measurements (F_{2,L}) and data on dilepton and high-E_{T} jet production we determine the dynamical parton distributions of the nucleon generated radiatively from valence-like positive input distributions at optimally chosen low resolution scales. These are compared with `standard' distributions generated from positive input distributions at some fixed and higher resolution scale. It is shown that up to the next to leading order NLO(\bar{MS}, DIS) of perturbative QCD considered in this paper, the uncertainties of the dynamical distributions are, as expected, smaller than those of their standard counterparts. This holds true in particular in the presently unexplored extremely small-x region relevant for evaluating ultrahigh energy cross sections in astrophysical applications. It is noted that our new dynamical distributions are compatible, within the presently determined uncertainties, with previously determined dynamical parton distributions.

Journal ArticleDOI
TL;DR: In this paper, the authors explore possible degrees of freedom in the parton parameter space associated with nonperturbative (intrinsic) charm in the nucleon and find that the range of IC is constrained to be from zero (no charm) to a level 2-3 times larger than previous model estimates.
Abstract: We investigate the charm sector of the nucleon structure phenomenologically, using the most up-to-date global QCD analysis. Going beyond the common assumption of purely radiatively generated charm, we explore possible degrees of freedom in the parton parameter space associated with nonperturbative (intrinsic) charm in the nucleon. Specifically, we explore the limits that can be placed on the intrinsic charm (IC) component, using all relevant hard-scattering data, according to scenarios in which the IC has a form predicted by light-cone wave function models; or a form similar to the light sea-quark distributions. We find that the range of IC is constrained to be from zero (no IC) to a level 2-3 times larger than previous model estimates. The behaviors of typical charm distributions within this range are described, and their implications for hadron collider phenomenology are briefly discussed.

Journal ArticleDOI
TL;DR: Generalized parton distributions have been introduced as a suitable theoretical tool to study the structure of the nucleon as discussed by the authors, and they provide a comprehensive framework for describing the quark and gluon structure.
Abstract: Generalized parton distributions have been introduced in recent years as a suitable theoretical tool to study the structure of the nucleon. Unifying the concepts of parton distributions and hadronic form factors, they provide a comprehensive framework for describing the quark and gluon structure of the nucleon. In this review their formal properties and modeling are discussed, summarizing the most recent developments in the phenomenological description of these functions. The status of available data is also presented.

Journal ArticleDOI
TL;DR: In this article, a formal recurrence relation approach to multiple parton scattering was used to find the complete solution to the problem of medium-induced gluon emission from partons propagating in cold nuclear matter.
Abstract: We use a formal recurrence relation approach to multiple parton scattering to find the complete solution to the problem of medium-induced gluon emission from partons propagating in cold nuclear matter. The differential bremsstrahlung spectrum, where Landau-Pomeranchuk-Migdal destructive interference effects are fully accounted for, is calculated for three different cases: (i) a generalization of the incoherent Bertsch-Gunion solution for asymptotic on-shell jets (ii) initial-state energy loss of incoming jets that undergo hard scattering, and (iii) final-state energy loss of jets that emerge out of a hard scatter. Our analytic solutions are given as an infinite opacity series, which represents a cluster expansion of the sequential multiple scattering. These new solutions allow, for the first time, direct comparison between initial- and final-state energy loss in cold nuclei. We demonstrate that, contrary to the naive assumption, energy loss in cold nuclear matter can be large. Numerical results to first order in opacity show that, in the limit of large jet energies, initial- and final-state energy losses exhibit different path length dependences, linear versus quadratic, in contrast to earlier findings. In addition, in this asymptotic limit, initial-state energy loss is considerably larger than final-state energy loss. These new results have significant implications for heavy-ionmore » phenomenology in both p+A and A+A reactions.« less

Journal ArticleDOI
M. Benali1, C. Desnault2, M. Mazouz, Z. Ahmed3, H. Albataineh4, Kalyan Allada5, K. A. Aniol6, V. Bellini, W. U. Boeglin7, P. Bertin8, P. Bertin1, M. Brossard1, A. Camsonne8, Mustafa Canan9, S. Chandavar10, Chunhui Chen11, J. P. Chen8, Maxime Defurne12, C. W. de Jager8, R. De Leo13, A. Deur8, L. El Fassi14, L. El Fassi15, Rolf Ent8, D. Flay16, M. Friend17, E. Fuchey1, S. Frullani, F. Garibaldi, David Gaskell8, A. Giusa, O. Glamazdin18, S. Golge19, J. Gomez8, Olfred Hansen8, Douglas Higinbotham8, T. Holmstrom20, Tanja Horn21, J. Huang5, M. Huang22, G. M. Huber23, Charles Hyde9, Charles Hyde1, S. Iqbal6, F. Itard1, Ho. Kang24, H. Kang24, A. Kelleher25, Cynthia Keppel8, S. Koirala9, I. Korover26, J. LeRose8, R. A. Lindgren27, E. Long28, M. Magne1, Juliette Mammei29, D. J. Margaziotis6, Pete Markowitz7, A. Martí Jiménez-Argüello30, A. Martí Jiménez-Argüello2, Franco Meddi, D. G. Meekins8, R. Michaels8, M. Mihovilovic31, N. Muangma5, C. Munoz Camacho2, C. Munoz Camacho1, P. Nadel-Turonski8, N. Nuruzzaman11, R. Paremuzyan2, R. I. Pomatsalyuk18, Andrew Puckett32, V. A. Punjabi33, Yujie Qiang8, Abdurahim Rakhman3, M. N. H. Rashad9, S. Riordan34, J. Roche10, G.V. Russo, F. Sabatié12, Kiadtisak Saenboonruang35, Kiadtisak Saenboonruang27, Arijit Saha8, B. Sawatzky8, B. Sawatzky16, L. Selvy28, A. Shahinyan36, Simon Širca31, P. Solvignon8, M. L. Sperduto, R. Subedi37, Vincent Sulkosky5, C. M. Sutera, William A. Tobias27, G. M. Urciuoli, D. Wang27, Bogdan Wojtsekhowski8, H. Yao16, Z. Ye27, L. Zana3, X. Zhan34, Jie Zhang8, B. Zhao25, Z. W. Zhao27, X. Zheng27, P. Zhu27 
TL;DR: The present experiment exploits the interference between the deeply virtual Compton scattering (DVCS) and the Bethe-Heitler processes to extract a linear combination of generalized parton distributions (GPDs) particularly sensitive to E_{q}, the least constrained GPD.
Abstract: The three-dimensional structure of nucleons (protons and neutrons) is embedded in so-called generalized parton distributions, which are accessible from deeply virtual Compton scattering. In this process, a high-energy electron is scattered off a nucleon by exchanging a virtual photon. Then, a highly energetic real photon is emitted from one of the quarks inside the nucleon, which carries information on the quark’s transverse position and longitudinal momentum. By measuring the cross-section of deeply virtual Compton scattering, Compton form factors related to the generalized parton distributions can be extracted. Here, we report the observation of unpolarized deeply virtual Compton scattering off a deuterium target. From the measured photon-electroproduction cross-sections, we have extracted the cross-section of a quasifree neutron and a coherent deuteron. Due to the approximate isospin symmetry of quantum chromodynamics, we can determine the contributions from the different quark flavours to the helicity-conserved Compton form factors by combining our measurements with previous ones probing the proton’s internal structure. These results advance our understanding of the description of the nucleon structure, which is important to solve the proton spin puzzle. The internal structure of the neutron has now been probed by highly energetic photons scattering off it. Combined with previous results for protons, these measurements reveal the contributions of quark flavours to the nucleon structure.

Journal ArticleDOI
TL;DR: In this paper, the authors systematically calculate various flavor-changing neutral-current top-quark processes induced by supersymmetry at the Large Hadron Collider, which include five decay modes and six production channels.
Abstract: We systematically calculate various flavor-changing neutral-current top-quark processes induced by supersymmetry at the Large Hadron Collider, which include five decay modes and six production channels. To reveal the characteristics of these processes, we first compare the dependence of the rates for these channels on the relevant supersymmetric parameters, then we scan the whole parameter space to find their maximal rates, including all the direct and indirect current experimental constraints on the scharm-stop flavor mixings. We find that, under all these constraints, only a few channels, through cg -> t at parton level and t -> ch, may be observable at the Large Hadron Collider.

Journal ArticleDOI
TL;DR: In this article, the authors present analytic results for the complete weak corrections to gluon-induced top-quark pair production at the Tevatron and the Large Hadron Collider (LHC) at CERN.
Abstract: Top-quark physics plays an important role at hadron colliders such as the Tevatron at Fermilab or the upcoming Large Hadron Collider (LHC) at CERN. Given the planned experimental precision, detailed theoretical predictions are mandatory. In this article we present analytic results for the complete weak corrections to gluon-induced top-quark pair production – neglecting purely photonic corrections, completing our earlier results for the quark-induced reaction. As an application we discuss top-quark pair production at the Tevatron and LHC. In particular we show that, although they are small for inclusive quantities, weak corrections can be sizeable for differential distributions.

Journal ArticleDOI
TL;DR: In this paper, the results obtained combining LO partonic matrix elements with either LO or NLO partons distributions are compared to the best prediction using NLO for both matrix elements and parton distributions.
Abstract: We present a study of the results obtained combining LO partonic matrix elements with either LO or NLO partons distributions. These are compared to the best prediction using NLO for both matrix elements and parton distributions. The aim is to determine which parton distributions are most appropriate to use in those cases where only LO matrix elements are available, e.g. as in many Monte Carlo generators. Both LO and NLO parton distributions have flaws, sometimes serious, for some processes, so a modified optimal LO set is suggested. We investigate a wide variety of process, and the LO* pdf works at least as well as, and often better than, both LO and NLO pdfs in nearly all cases.The LO* pdf set is now available in the LHAPDF package.