Showing papers on "Parton published in 2017"
TL;DR: A new set of parton distributions, NNPDF3.1, is presented, which updates NN PDF3.0, the first global set of PDFs determined using a methodology validated by a closure test, and investigates the impact of parametrizing charm and evidence that the accuracy and stability of the PDFs are improved.
Abstract: We present a new set of parton distributions, NNPDF3.1, which updates NNPDF3.0, the first global set of PDFs determined using a methodology validated by a closure test. The update is motivated by recent progress in methodology and available data, and involves both. On the methodological side, we now parametrize and determine the charm PDF alongside the light quarks and gluon ones, thereby increasing from seven to eight the number of independent PDFs. On the data side, we now include the D0 electron and muon W asymmetries from the final Tevatron dataset, the complete LHCb measurements of W and Z production in the forward region at 7 and 8 TeV, and new ATLAS and CMS measurements of inclusive jet and electroweak boson production. We also include for the first time top-quark pair differential distributions and the transverse momentum of the Z bosons from ATLAS and CMS. We investigate the impact of parametrizing charm and provide evidence that the accuracy and stability of the PDFs are thereby improved. We study the impact of the new data by producing a variety of determinations based on reduced datasets. We find that both improvements have a significant impact on the PDFs, with some substantial reductions in uncertainties, but with the new PDFs generally in agreement with the previous set at the one sigma level. The most significant changes are seen in the light-quark flavor separation, and in increased precision in the determination of the gluon. We explore the implications of NNPDF3.1 for LHC phenomenology at Run II, compare with recent LHC measurements at 13 TeV, provide updated predictions for Higgs production cross-sections and discuss the strangeness and charm content of the proton in light of our improved dataset and methodology. The NNPDF3.1 PDFs are delivered for the first time both as Hessian sets, and as optimized Monte Carlo sets with a compressed number of replicas.
921 citations
TL;DR: A global analysis of collinearly factorized nuclear parton distribution functions (PDFs) including, for the first time, data constraints from LHC proton–lead collisions is introduced including a new set of next-to-leading order nuclear PDFs called EPPS16.
Abstract: We introduce a global analysis of collinearly factorized nuclear parton distribution functions (PDFs) including, for the first time, data constraints from LHC proton–lead collisions. In comparison to our previous analysis, EPS09, where data only from charged-lepton–nucleus deep inelastic scattering (DIS), Drell–Yan (DY) dilepton production in proton–nucleus collisions and inclusive pion production in deuteron–nucleus collisions were the input, we now increase the variety of data constraints to cover also neutrino–nucleus DIS and low-mass DY production in pion–nucleus collisions. The new LHC data significantly extend the kinematic reach of the data constraints. We now allow much more freedom for the flavor dependence of nuclear effects than in other currently available analyses. As a result, especially the uncertainty estimates are more objective flavor by flavor. The neutrino DIS plays a pivotal role in obtaining a mutually consistent behavior for both up and down valence quarks, and the LHC dijet data clearly constrain gluons at large momentum fraction. Mainly for insufficient statistics, the pion–nucleus DY and heavy-gauge-boson production in proton–lead collisions impose less visible constraints. The outcome – a new set of next-to-leading order nuclear PDFs called EPPS16 – is made available for applications in high-energy nuclear collisions.
408 citations
TL;DR: In this article, a new set of parton distribution functions (ABMP16), the strong coupling constant αs and the quark masses mc, mb and mt were determined in a global fit to next-to-next-toleading order (NNLO) in QCD.
Abstract: We determine a new set of parton distribution functions (ABMP16), the strong coupling constant αs and the quark masses mc, mb and mt in a global fit to next-to-next-to-leading order (NNLO) in QCD. The analysis uses the MS¯ scheme for αs and all quark masses and is performed in the fixed-flavor number scheme for nf=3, 4, 5. Essential new elements of the fit are the combined data from HERA for inclusive deep-inelastic scattering (DIS), data from the fixed-target experiments NOMAD and CHORUS for neutrino-induced DIS, data from Tevatron and the LHC for the Drell-Yan process and the hadro-production of single-top and top-quark pairs. The theory predictions include new improved approximations at NNLO for the production of heavy quarks in DIS and for the hadro-production of single-top quarks. The description of higher twist effects relevant beyond the leading twist collinear factorization approximation is refined. At NNLO, we obtain the value αs(nf=5)(MZ)=0.1147±0.0008.
357 citations
TL;DR: In this article, the authors presented the N3LO contributions to the non-singlet splitting functions for both parton distribution and fragmentation functions in perturbative QCD and derived exact expressions for the terms contributing in the limit of a large number of colours.
Abstract: We present the next-to-next-to-next-to-leading order (N3LO) contributions to the non-singlet splitting functions for both parton distribution and fragmentation functions in perturbative QCD. The exact expressions are derived for the terms contributing in the limit of a large number of colours. For the remaining contributions, approximations are provided that are sufficient for all collider-physics applications. From their threshold limits we derive analytical and high-accuracy numerical results, respectively, for all contributions to the four-loop cusp anomalous dimension for quarks, including the terms proportional to quartic Casimir operators. We briefly illustrate the numerical size of the four-loop corrections, and the remarkable renormalization-scale stability of the N3LO results, for the evolution of the non-singlet parton distribution and the fragmentation functions. Our results appear to provide a first point of contact of four-loop QCD calculations and the so-called wrapping corrections to anomalous dimensions in $$ \mathcal{N}=4 $$
super Yang-Mills theory.
181 citations
TL;DR: This article performed the first global QCD analysis of polarized inclusive and semi-inclusive deep-inelastic scattering and singleinclusive e^{+}e^{-} annihilation data, simultaneously fitting the parton distribution and fragmentation functions using the iterative Monte-Carlo method.
Abstract: We perform the first global QCD analysis of polarized inclusive and semi-inclusive deep-inelastic scattering and single-inclusive e^{+}e^{-} annihilation data, simultaneously fitting the parton distribution and fragmentation functions using the iterative Monte Carlo method Without imposing SU(3) symmetry relations, we find the strange polarization to be very small, consistent with zero for both inclusive and semi-inclusive data, which provides a resolution to the strange quark polarization puzzle The combined analysis also allows the direct extraction from data of the isovector and octet axial charges, and is consistent with a small SU(2) flavor asymmetry in the polarized sea
167 citations
TL;DR: In this paper, all major differential distributions with stable top-quarks at the LHC were calculated in the form of high-quality binned distributions and the results were presented in terms of the fastest perturbative convergence applied to the differential and inclusive crosssection.
Abstract: We calculate all major differential distributions with stable top-quarks at the LHC. The calculation covers the multi-TeV range that will be explored during LHC Run II and beyond. Our results are in the form of high-quality binned distributions. We offer predictions based on three different parton distribution function (pdf) sets. In the near future we will make our results available also in the more flexible fastNLO format that allows fast re-computation with any other pdf set. In order to be able to extend our calculation into the multi-TeV range we have had to derive a set of dynamic scales. Such scales are selected based on the principle of fastest perturbative convergence applied to the differential and inclusive cross-section. Many observations from our study are likely to be applicable and useful to other precision processes at the LHC. With scale uncertainty now under good control, pdfs arise as the leading source of uncertainty for TeV top production. Based on our findings, true precision in the boosted regime will likely only be possible after new and improved pdf sets appear. We expect that LHC top-quark data will play an important role in this process.
165 citations
Université Paris-Saclay1, SLAC National Accelerator Laboratory2, University of the Witwatersrand3, Reed College4, Lund University5, University of Manchester6, Durham University7, CERN8, Polish Academy of Sciences9, Monash University, Clayton campus10, Centre national de la recherche scientifique11, Massachusetts Institute of Technology12
TL;DR: In the eikonal (double-logarithmic) limit, quark/gluon discrimination is determined solely by the color factor of the initiating parton (CF versus CA) as discussed by the authors.
Abstract: By measuring the substructure of a jet, one can assign it a “quark” or “gluon” tag. In the eikonal (double-logarithmic) limit, quark/gluon discrimination is determined solely by the color factor of the initiating parton (CF versus CA). In this paper, we confront the challenges faced when going beyond this leading-order understanding, using both parton-shower generators and first-principles calculations to assess the impact of higher-order perturbative and nonperturbative physics. Working in the idealized context of electron-positron collisions, where one can define a proxy for quark and gluon jets based on the Lorentz structure of the production vertex, we find a fascinating interplay between perturbative shower effects and nonperturbative hadronization effects. Turning to proton-proton collisions, we highlight a core set of measurements that would constrain current uncertainties in quark/gluon tagging and improve the overall modeling of jets at the Large Hadron Collider.
162 citations
TL;DR: The first calculation of fully differential jet production at leading color in all partonic channels at next-to-next-to leading order in perturbative QCD is reported and the possible implications for parton distribution function fits are discussed.
Abstract: We report the first calculation of fully differential jet production at leading color in all partonic channels at next-to-next-to leading order in perturbative QCD and compare to the available ATLAS 7 TeV data. We discuss the size and shape of the perturbative corrections along with their associated scale variation across a wide range in jet transverse momentum, p_{T}, and rapidity, y. We find significant effects, especially at low p_{T}, and discuss the possible implications for parton distribution function fits.
139 citations
TL;DR: In this article, a connection between the transverse momentum dependent distributions (TMDs) F ( x, k ⊥ 2 ) and quasi-distributions (PQDs) Q ( y, p 3 ) was established.
137 citations
TL;DR: In this article, the von Neumann entropy of the system of partons resolved by deep inelastic scattering at a given Bjorken x and momentum transfer q2=-Q2 was computed.
Abstract: Using nonlinear evolution equations of QCD, we compute the von Neumann entropy of the system of partons resolved by deep inelastic scattering at a given Bjorken x and momentum transfer q2=-Q2. We interpret the result as the entropy of entanglement between the spatial region probed by deep inelastic scattering and the rest of the proton. At small x the relation between the entanglement entropy S(x) and the parton distribution xG(x) becomes very simple: S(x)=ln[xG(x)]. In this small x, large rapidity Y regime, all partonic microstates have equal probabilities—the proton is composed by an exponentially large number exp(ΔY) of microstates that occur with equal and exponentially small probabilities exp(-ΔY), where Δ is defined by xG(x)∼1/xΔ. For this equipartitioned state, the entanglement entropy is maximal—so at small x, deep inelastic scattering probes a maximally entangled state. We propose the entanglement entropy as an observable that can be studied in deep inelastic scattering. This will require event-by-event measurements of hadronic final states, and would allow to study the transformation of entanglement entropy into the Boltzmann one. We estimate that the proton is represented by the maximally entangled state at x≤10-3; this kinematic region will be amenable to studies at the Electron Ion Collider.
136 citations
TL;DR: In this article, the authors provided an analysis of the dependence of the bare unpolarized, helicity, and transversity isovector parton distribution functions (PDFs) from lattice calculations employing (maximally) twisted mass fermions.
Abstract: We provide an analysis of the $x$ dependence of the bare unpolarized, helicity, and transversity isovector parton distribution functions (PDFs) from lattice calculations employing (maximally) twisted mass fermions. The $x$ dependence of the calculated PDFs resembles the one of the phenomenological parameterizations, a feature that makes this approach very promising. Furthermore, we apply momentum smearing for the relevant matrix elements to compute the lattice PDFs and find a large improvement factor when compared to conventional Gaussian smearing. This allows us to extend the lattice computation of the distributions to higher values of the nucleon momentum, which is essential for the prospects of a reliable extraction of the PDFs in the future.
TL;DR: It is demonstrated that the inclusion of small-x resummation leads to a quantitative improvement in the perturbative description of the HERA inclusive and charm-production reduced cross-sections in the small x region.
Abstract: We present a determination of the parton distribution functions of the proton in which NLO and NNLO fixed-order calculations are supplemented by NLLx small-x resummation. Deep inelastic structure functions are computed consistently at NLO+NLLx or NNLO+NLLx, while for hadronic processes small-x resummation is included only in the PDF evolution, with kinematic cuts introduced to ensure the fitted data lie in a region where the fixed-order calculation of the hard cross-sections is reliable. In all other respects, the fits use the same methodology and are based on the same global dataset as the recent NNPDF3.1 analysis. We demonstrate that the inclusion of small-x resummation leads to a quantitative improvement in the perturbative description of the HERA inclusive and charm-production reduced cross-sections in the small x region. The impact of the resummation in our fits is greater at NNLO than at NLO, because fixed-order calculations have a perturbative instability at small x due to large logarithms that can be cured by resummation. We explore the phenomenological implications of PDF sets with small-x resummation for the longitudinal structure function $F_L$ at HERA, for parton luminosities and LHC benchmark cross-sections, for ultra-high energy neutrino-nucleus cross-sections, and for future high-energy lepton-proton colliders such as the LHeC.
TL;DR: In this article, the authors showed that the power divergence can be removed by a mass counterterm in the auxiliary z-field formalism, in the same way as the renormalization of power divergence for an open Wilson line.
TL;DR: In this paper, the authors present two independent procedures through which background subtraction can be performed and discuss the impact of the medium recoil on jet shape observables, which significantly improves the Jewel description of jet shape measurements.
Abstract: Realistic modeling of medium-jet interactions in heavy ion collisions is becoming increasingly important to successfully predict jet structure and shape observables. In Jewel, all partons belonging to the parton showers initiated by hard scattered partons undergo collisions with thermal partons from the medium, leading to both elastic and radiative energy loss. The recoiling medium partons carry away energy and momentum from the jet. Since the thermal component of these recoils’ momenta is part of the soft background activity, comparison with data requires the implementation of a subtraction procedure. We present two independent procedures through which background subtraction can be performed and discuss the impact of the medium recoil on jet shape observables. Keeping track of the medium response significantly improves the Jewel description of jet shape measurements.
TL;DR: In this paper, the authors present two independent procedures through which background subtraction can be performed and discuss the impact of the medium recoil on jet shape observables, which significantly improves the JEWEL description of jet shape measurements.
Abstract: Realistic modeling of medium-jet interactions in heavy ion collisions is becoming increasingly important to successfully predict jet structure and shape observables. In JEWEL, all partons belonging to the parton showers initiated by hard scattered partons undergo collisions with thermal partons from the medium, leading to both elastic and radiative energy loss. The recoiling medium partons carry away energy and momentum from the jet. Since the thermal component of these recoils' momenta is part of the soft background activity, comparison with data requires the implementation of a subtraction procedure. We present two independent procedures through which background subtraction can be performed and discuss the impact of the medium recoil on jet shape observables. Keeping track of the medium response significantly improves the JEWEL description of jet shape measurements.
TL;DR: In this paper, the soft anomalous dimension of scattering amplitudes is computed at three-loop order for massless partons by explicit evaluation of all relevant Feynman diagrams, up to an overall numerical factor using a bootstrap procedure.
Abstract: The soft anomalous dimension governs the infrared singularities of scattering amplitudes to all orders in perturbative quantum field theory, and is a crucial ingredient in both formal and phenomenological applications of non-abelian gauge theories. It has recently been computed at three-loop order for massless partons by explicit evaluation of all relevant Feynman diagrams. In this paper, we show how the same result can be obtained, up to an overall numerical factor, using a bootstrap procedure. We first give a geometrical argument for the fact that the result can be expressed in terms of single-valued harmonic polylogarithms. We then use symmetry considerations as well as known properties of scattering amplitudes in collinear and high-energy (Regge) limits to constrain an ansatz of basis functions. This is a highly non-trivial cross-check of the result, and our methods pave the way for greatly simplified higher-order calculations.
TL;DR: It is found that the leading-order medium-induced splitting functions, here obtained in the framework of soft-collinear effective theory with Glauber gluon interactions, capture the essential many-body physics, which is different from proton-proton reactions.
Abstract: Heavy ion collisions present exciting opportunities to study the effects of quantum coherence in the formation of subatomic particle showers. We report on the first calculation of the momentum sharing and angular separation distributions between the leading subjets inside a reconstructed jet in such collisions. These observables are directly sensitive to the hardest branching within jets and can probe the early stage of the jet formation. We find that the leading-order medium-induced splitting functions, here obtained in the framework of soft-collinear effective theory with Glauber gluon interactions, capture the essential many-body physics, which is different from proton-proton reactions. Qualitative and in most cases quantitative agreement between theory and preliminary CMS measurements suggests that hard parton branching in strongly interacting matter can be dramatically modified. We also propose a new measurement that will illuminate its angular structure.
TL;DR: In this article, it was shown that although the Ioffe-time distribution provides an alternative way to extract the parton distribution from the same lattice observables, it also requires the same large momentum (or short distance) limit as in LaMET to obtain a precision calculation.
TL;DR: The first combination of NLO QCD matrix elements for di-Higgs production, retaining the full top quark mass dependence, with a parton shower was presented in this paper.
Abstract: We present the first combination of NLO QCD matrix elements for di-Higgs production, retaining the full top quark mass dependence, with a parton shower. Results are provided within both the POWHEG-BOX and MadGraph5_aMC@NLO Monte Carlo frameworks. We assess in detail the theoretical uncertainties and provide differential results. We find that, as expected, the shower effects are relatively large for observables like the transverse momentum of the Higgs boson pair, which are sensitive to extra radiation. However, these shower effects are still much smaller than the differences between the Born-improved HEFT approximation and the full NLO calculation in the tails of the distributions.
TL;DR: In this paper, the authors recast the QCD evolution equations in terms of parton branching processes and presented a new numerical solution of the equations, which can be applied to analyze infrared contributions to evolution, order-by-order in the strong coupling α s, as a function of the soft-gluon resolution scale parameter.
TL;DR: KaTie as discussed by the authors is a parton-level event generator for hadron scattering processes that can deal with partonic initial-state momenta with an explicit transverse momentum dependence causing them to be space-like.
TL;DR: In this article, the small-x gluon was determined from the forward charm production data provided by LHCb for three different center-of-mass (c.m.) energies: 5, TeV, 7,TeV, and 13,TeV.
Abstract: The small-x gluon in global fits of parton distributions is affected by large uncertainties from the lack of direct experimental constraints. In this Letter, we provide a precision determination of the small-x gluon from the exploitation of forward charm production data provided by LHCb for three different center-of-mass (c.m.) energies: 5 TeV, 7 TeV, and 13 TeV. The LHCb measurements are included in the parton distribution function (PDF) fit by means of normalized distributions and cross-section ratios between data taken at different c.m. values, R_{13/7} and R_{13/5}. We demonstrate that forward charm production leads to a reduction of the PDF uncertainties of the gluon down to x≃10^{-6} by up to an order of magnitude, with implications for high-energy colliders, cosmic ray physics, and neutrino astronomy.
TL;DR: In this paper, a new type-II model with fractal logical operators is presented, in which quasiparticle excitations are restricted to move along planes, lines, or not to move at all.
Abstract: Fracton phases are three dimensional topological phases in which quasiparticle excitations are restricted to move along planes, lines, or not to move at all. In this work, we show how such phases can be understood in terms of parton constructions, and in the process we find a new type-II model with fractal logical operators.
TL;DR: In this article, the relativistic bound states of charmonium and bottomonium were studied in a light-front quantized Hamiltonian formalism, where the effective Hamiltonian was based on light front holography and the spin structure was generated from the onegluon exchange interaction with a running coupling.
Abstract: We study charmonium and bottomonium as relativistic bound states in a light-front quantized Hamiltonian formalism. The effective Hamiltonian is based on light-front holography. We use a recently proposed longitudinal confinement to complete the soft-wall holographic potential for the heavy flavors. The spin structure is generated from the one-gluon exchange interaction with a running coupling. The adoption of asymptotic freedom improves the spectroscopy compared with previous light-front results. Within this model, we compute the mass spectroscopy, decay constants and the r.m.s. radii. We also present a detailed study of the obtained light-front wave functions and use the wave functions to compute the light-cone distributions, specifically the distribution amplitudes and parton distribution functions. Overall, our model provides a reasonable description of the heavy quarkonia.
TL;DR: A reliable prescription to estimate the theoretical uncertainties, including those associated to the very definition of the process at NLO, is formulated and the sensitivity to a non-Standard-Model relative phase between the Higgs couplings to the top quark and to the W boson in tWH production is studied.
Abstract: We study Higgs boson production in association with a top quark and a $W$ boson at the LHC. At next-to-leading order (NLO) in QCD, this process interferes with $t\bar t H$ production and a procedure to meaningfully separate the two processes needs to be employed. In order to define $tWH$ production for both total rates and differential distributions, we consider the methods that have been previously proposed for treating intermediate resonances at NLO, in particular in the context of $tW$ production. To this aim, we first critically revisit the $tW$ process, for which an extensive literature exists and where an analogous (yet smaller) interference with $t \bar t$ production takes place. We analyse the currently employed techniques, including implementations of diagram removal and diagram subtraction, which feature approximations that need to be carefully taken into account when theoretical predictions are compared to experimental measurements. We then provide robust results for total and differential cross sections for $tW$ and $tWH$ at the LHC at 13 TeV, at the parton level as well as by matching short-distance events to a parton shower. We include a reliable prescription to estimate the theoretical uncertainties, including those associated to the very definition of the process at NLO. Finally, we study the sensitivity to a non-Standard-Model relative phase between the Higgs couplings to the top quark and to the $W$ boson in $tWH$ production.
TL;DR: The first combination of NLO QCD matrix elements for di-Higgs production, retaining the full top quark mass dependence, with a parton shower was presented in this article.
Abstract: We present the first combination of NLO QCD matrix elements for di-Higgs production, retaining the full top quark mass dependence, with a parton shower. Results are provided within both the POWHEG-BOX and MadGraph5_aMC@NLO Monte Carlo frameworks. We assess in detail the theoretical uncertainties and provide differential results. We find that, as expected, the shower effects are relatively large for observables like the transverse momentum of the Higgs boson pair, which are sensitive to extra radiation. However, these shower effects are still much smaller than the differences between the Born-improved HEFT approximation and the full NLO calculation in the tails of the distributions.
TL;DR: In this article, a framework to include triple collinear splitting functions into parton showers is presented, and the implementation of flavor-changing next-to-leading-order (NLO) splitting kernels is discussed as a first application.
Abstract: A framework to include triple collinear splitting functions into parton showers is presented, and the implementation of flavor-changing next-to-leading-order (NLO) splitting kernels is discussed as a first application. The correspondence between the Monte Carlo integration and the analytic computation of NLO DGLAP evolution kernels is made explicit for both timelike and spacelike parton evolution. Numerical simulation results are obtained with two independent implementations of the new algorithm, using the two independent event generation frameworks PYTHIA and SHERPA.
Abstract: We revisit the global QCD analysis of parton-to-kaon fragmentation functions at next-to-leading-order accuracy using the latest experimental information on single-inclusive kaon production in electron-positron annihilation, lepton-nucleon deep-inelastic scattering, and proton-proton collisions. An excellent description of all data sets is achieved, and the remaining uncertainties in parton-to-kaon fragmentation functions are estimated and discussed based on the Hessian method. Extensive comparisons to the results from our previous global analysis are made.
TL;DR: In this paper, a framework for including second-order perturbative corrections to the radiation patterns of parton showers is presented, which allows to combine O ( α s 2 ) -corrected iterated 2 → 3 kernels for ordered gluon emissions with tree-level 2 → 4 kernels for unordered ones.
Michigan State University1, University of Oxford2, Southern Methodist University3, College of William & Mary4, Thomas Jefferson National Accelerator Facility5, VU University Amsterdam6, Hampton University7, The Cyprus Institute8, University of Cyprus9, University of Pavia10, National Taiwan Ocean University11, University of Regensburg12, Temple University13, New Mexico State University14, Los Alamos National Laboratory15, University College London16, Shanghai Jiao Tong University17, University of Kentucky18, University of Virginia19, University of Washington20, University of Grenoble21, University of Tübingen22, University of Mainz23, University of Adelaide24
TL;DR: In this article, the authors present an overview of lattice-QCD and global-analysis techniques used to determine unpolarized and polarized proton PDFs and their moments.
Abstract: In the framework of quantum chromodynamics (QCD), parton distribution functions (PDFs) quantify how the momentum and spin of a hadron are divided among its quark and gluon constituents. Two main approaches exist to determine PDFs. The first approach, based on QCD factorization theorems, realizes a QCD analysis of a suitable set of hard-scattering measurements, often using a variety of hadronic observables. The second approach, based on first-principle operator definitions of PDFs, uses lattice QCD to compute directly some PDF-related quantities, such as their moments. Motivated by recent progress in both approaches, in this document we present an overview of lattice-QCD and global-analysis techniques used to determine unpolarized and polarized proton PDFs and their moments. We provide benchmark numbers to validate present and future lattice-QCD calculations and we illustrate how they could be used to reduce the PDF uncertainties in current unpolarized and polarized global analyses. This document represents a first step towards establishing a common language between the two communities, to foster dialogue and to further improve our knowledge of PDFs.