Showing papers on "Parton published in 2019"
TL;DR: In this article, the authors presented the analytic form of all leading-color two-loop five-parton helicity amplitudes in QCD, which were analytically reconstructed from exact numerical evaluations over finite fields.
Abstract: We present the analytic form of all leading-color two-loop five-parton helicity amplitudes in QCD. The results are analytically reconstructed from exact numerical evaluations over finite fields. Combining a judicious choice of variables with a new approach to the treatment of particle states in D dimensions for the numerical evaluation of amplitudes, we obtain the analytic expressions with a modest computational effort. Their systematic simplification using multivariate partial-fraction decomposition leads to a particularly compact form. Our results provide all two-loop amplitudes required for the calculation of next-to-next-to-leading order QCD corrections to the production of three jets at hadron colliders in the leading-color approximation.
128 citations
TL;DR: Recently, breakthrough ideas changed the landscape and several approaches were proposed to access the distributions themselves on the lattice as discussed by the authors, such as renormalization, matching to light-cone distributions, and lattice techniques.
Abstract: Within the theory of Quantum Chromodynamics (QCD), the rich structure of hadrons can be quantitatively characterized, among others, using a basis of universal nonperturbative functions: parton distribution functions (PDFs), generalized parton distributions (GPDs), transverse momentum dependent parton distributions (TMDs), and distribution amplitudes (DAs). For more than half a century, there has been a joint experimental and theoretical effort to obtain these partonic functions. However, the complexity of the strong interactions has placed severe limitations, and first-principle information on these distributions was extracted mostly from their moments computed in Lattice QCD. Recently, breakthrough ideas changed the landscape and several approaches were proposed to access the distributions themselves on the lattice. In this paper, we review in considerable detail approaches directly related to partonic distributions. We highlight a recent idea proposed by X. Ji on extracting quasidistributions that spawned renewed interest in the whole field and sparked the largest amount of numerical studies within Lattice QCD. We discuss theoretical and practical developments, including challenges that had to be overcome, with some yet to be handled. We also review numerical results, including a discussion based on evolving understanding of the underlying concepts and the theoretical and practical progress. Particular attention is given to important aspects that validated the quasidistribution approach, such as renormalization, matching to light-cone distributions, and lattice techniques. In addition to a thorough discussion of quasidistributions, we consider other approaches: hadronic tensor, auxiliary quark methods, pseudodistributions, OPE without OPE, and good lattice cross-sections. In the last part of the paper, we provide a summary and prospects of the field, with emphasis on the necessary conditions to obtain results with controlled uncertainties.
124 citations
TL;DR: In this article, the parton densities satisfying angular ordering conditions are obtained from fits to precision measurements of deep-inelastic scattering (DIS) cross sections at HERA, which are used to predict the transverse momentum spectrum of $Z$ bosons at the LHC.
Abstract: Collinear and transverse-momentum-dependent (TMD) parton densities are obtained from fits to precision measurements of deep-inelastic scattering (DIS) cross sections at HERA. The parton densities are evolved by Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution with next-to-leading-order (NLO) splitting functions using the parton branching method, allowing one to determine simultaneously collinear and TMD densities for all flavors over a wide range in $x$, ${\ensuremath{\mu}}^{2}$ and ${k}_{t}$, relevant for predictions at the LHC. The DIS cross section is computed from the parton densities using perturbative NLO coefficient functions. Parton densities satisfying angular ordering conditions are presented. Two sets of parton densities are obtained, differing in the renormalization scale choice for the argument in the strong coupling ${\ensuremath{\alpha}}_{\mathrm{s}}$. This is taken to be either the evolution scale $\ensuremath{\mu}$ or the transverse momentum ${q}_{t}$. While both choices yield similarly good ${\ensuremath{\chi}}^{2}$ values for the fit to DIS measurements, the gluon density especially turns out to differ between the two sets. The TMD densities are used to predict the transverse momentum spectrum of $Z$ bosons at the LHC.
97 citations
TL;DR: In this article, the authors presented the extraction of unpolarized quark transverse momentum dependent parton distribution functions (TMDPDFs) and the non-perturbative part of TMD evolution kernel from the global analysis of Drell-Yan and Z-boson production data.
Abstract: We present the extraction of unpolarized quark transverse momentum dependent parton distribution functions (TMDPDFs) and the non-perturbative part of TMD evolution kernel from the global analysis of Drell-Yan and Z-boson production data. The analysis is performed at the next-to-next-to-leading order (NNLO) in perturbative QCD, using the ζ-prescription. The estimation of the error-propagation from the experimental uncertainties to non-perturbative function is made by the replica method. The importance of the inclusion of the precise LHC data and its influence on the determination of non-perturbative functions is discussed.
97 citations
TL;DR: In this article, a lattice QCD study of the valence parton distribution inside the pion within the framework of Large Momentum Effective Theory was presented, where a mixed action approach with 1-HYP smeared valence Wilson clover quarks on $2+1$ flavor HISQ sea with the value of 300 MeV pion mass was used.
Abstract: We present a lattice QCD study of the valence parton distribution inside the pion within the framework of Large Momentum Effective Theory. We use a mixed action approach with 1-HYP smeared valence Wilson clover quarks on $2+1$ flavor HISQ sea with the valence quark mass tuned to 300 MeV pion mass. We use $4{8}^{3}\ifmmode\times\else\texttimes\fi{}64$ lattice at a fine lattice spacing $a=0.06\text{ }\text{ }\mathrm{fm}$ for this computation. We renormalize the quasi parton distribution function matrix element in the nonperturbative regularization independent momentum subtraction (RI-MOM) scheme. As a byproduct, we test the validity of a 1-loop matching procedure by comparing the RI-MOM renormalized quasi parton distribution function matrix element with off-shell quark external states as computed in the continuum 1-loop perturbation theory with the lattice results at $a=0.04$ and 0.06 fm. By applying the RI-MOM to $\overline{\mathrm{MS}}$ one-loop matching, implemented through a fit to phenomenologically motivated parton distribution functions, we obtain the valence parton distribution function of pion.
96 citations
TL;DR: In this paper, an unpolarised Transverse-Momentum-Dependent Parton distribution function is extracted from Drell-Yan production data from different experiments, including those at the LHC and spanning a wide kinematic range.
Abstract: We present an extraction of unpolarised Transverse-Momentum-Dependent Parton Distribution Functions based on Drell-Yan production data from different experiments, including those at the LHC, and spanning a wide kinematic range. We deal with experimental uncertainties by properly taking into account correlations. We include resummation of logarithms of the transverse momentum of the vector boson up to N$^3$LL order, and we include non-perturbative contributions. These ingredients allow us to obtain a remarkable agreement with the data.
89 citations
TL;DR: In this paper, the structure of power corrections to parton quasidistributions and pseudodistribution which appear in lattice calculations of parton distribution functions is studied.
Abstract: Perturbative expansions for short-distance quantities in QCD are factorially divergent and this deficiency can be turned into a useful tool to investigate nonperturbative corrections. In this work, we use this approach to study the structure of power corrections to parton quasidistributions and pseudodistributions which appear in lattice calculations of parton distribution functions. As the main result, we predict the functional dependence of the leading power corrections to quasi(pseudo)-distributions on the Bjorken $x$ variable. We also show that these corrections can be strongly affected by the normalization procedure.
85 citations
TL;DR: In this paper, the authors proposed a method to extract the evolution kernel of transverse-momentum dependent parton distribution functions (TMDPDFs) using lattice QCD and the large-momentsum effective theory.
Abstract: At small transverse momentum ${q}_{T}$, transverse-momentum dependent parton distribution functions (TMDPDFs) arise as genuinely nonperturbative objects that describe Drell-Yan like processes in hadron collisions as well as semi-inclusive deep-inelastic scattering. TMDPDFs naturally depend on the hadron momentum, and the associated evolution is determined by the Collins-Soper equation. For ${q}_{T}\ensuremath{\sim}{\mathrm{\ensuremath{\Lambda}}}_{\mathrm{QCD}}$ the corresponding evolution kernel (or anomalous dimension) is nonperturbative and must be determined as an independent ingredient in order to relate TMDPDFs at different scales. We propose a method to extract this kernel using lattice QCD and the large-momentum effective theory, where the physical TMD correlation involving light-like paths is approximated by a quasi-TMDPDF, defined using equal-time correlation functions with a large-momentum hadron state. The kernel is determined from a ratio of quasi-TMDPDFs extracted at different hadron momenta.
82 citations
TL;DR: This work identifies four independent quasi-PDF correlators that can be multiplicatively renormalized on the lattice and provides a LaMET factorization formula for these renormalization quasi- PDFs from which one can extract the gluon PDFs.
Abstract: Gluon parton distribution functions (PDFs) in the proton can be calculated directly on Euclidean lattices using large momentum effective theory (LaMET). To realize this goal, one has to find renormalized gluon quasi-PDFs in which power divergences and operator mixing are thoroughly understood. For the unpolarized distribution, we identify four independent quasi-PDF correlators that can be multiplicatively renormalized on the lattice. Similarly, the helicity distribution can be derived from three independent multiplicatively renormalizable quasi-PDFs. We provide a LaMET factorization formula for these renormalized quasi-PDFs from which one can extract the gluon PDFs.
81 citations
TL;DR: In this article, a detailed study of the helicity-dependent and helicity independent collinear parton distribution functions (PDFs) of the nucleon was performed using the quasi-PDF approach.
Abstract: We present a detailed study of the helicity-dependent and helicity-independent collinear parton distribution functions (PDFs) of the nucleon, using the quasi-PDF approach. The lattice QCD computation is performed employing twisted mass fermions with a physical value of the light quark mass. We give a systematic and in-depth account of the salient features entering in the evaluation of quasi-PDFs and their relation to the light-cone PDFs. In particular, we give details for the computation of the matrix elements, including the study of the various sources of systematic uncertainties, such as excited-states contamination. In addition, we discuss the nonperturbative renormalization scheme used here and its systematics, effects of truncating the Fourier transform and different matching prescriptions. Finally, we show improved results for the PDFs and discuss future directions, challenges and prospects for evaluating precisely PDFs from lattice QCD with fully quantified uncertainties.
81 citations
TL;DR: In this paper, the authors showed that the exponential regulator provides a consistent framework for the calculation of various ingredients in transverse momentum dependent factorization and obtained the two-loop quark jet function for the Energy-Energy Correlator in the backto-back limit.
Abstract: We revisit the calculation of perturbative quark transverse momentum de- pendent parton distribution functions and fragmentation functions using the exponential regulator for rapidity divergences. We show that the exponential regulator provides a consistent framework for the calculation of various ingredients in transverse momentum dependent factorization. Compared to existing regulators in the literature, the exponential regulator has a couple of advantages which we explain in detail. As a result, the calcula- tion is greatly simplified and we are able to obtain the next-to-next-to-leading order results up to O(E2) in dimensional regularization. These terms are necessary for a higher order calculation which is made possible with the simplification brought by the new regulator. As a by-product, we have obtained the two-loop quark jet function for the Energy-Energy Correlator in the back-to-back limit, which is the last missing ingredient for its N3LL resummation.
TL;DR: In this paper, a detailed study of the unpolarized nucleon parton distribution function (PDF) employing the approach of parton pseudo-distribution func- tions is presented.
Abstract: In this paper, we present a detailed study of the unpolarized nucleon parton distribution function (PDF) employing the approach of parton pseudo-distribution func- tions. We perform a systematic analysis using three lattice ensembles at two volumes, with lattice spacings a = 0.127 fm and a = 0.094 fm, for a pion mass of roughly 400 MeV. With two lattice spacings and two volumes, both continuum limit and infinite volume ex- trapolation systematic errors of the PDF are considered. In addition to the x dependence of the PDF, we compute their first two moments and compare them with the pertinent phenomenological determinations.
TL;DR: A review of the present understanding of the spin structure of protons and neutrons, the fundamental building blocks of nuclei collectively known as nucleons, can be found in this article.
Abstract: We review the present understanding of the spin structure of protons and neutrons, the fundamental building blocks of nuclei collectively known as nucleons. The field of nucleon spin provides a critical window for testing Quantum Chromodynamics (QCD), the gauge theory of the strong interactions, since it involves fundamental aspects of hadron structure which can be probed in detail in experiments, particularly deep inelastic lepton scattering on polarized targets. QCD was initially probed in high energy deep inelastic lepton scattering with unpolarized beams and targets. With time, interest shifted from testing perturbative QCD to illuminating the nucleon structure itself. In fact, the spin degrees of freedom of hadrons provide an essential and detailed verification of both perturbative and nonperturbative QCD dynamics. Nucleon spin was initially thought of coming mostly from the spin of its quark constituents, based on intuition from the parton model. However, the first experiments showed that this expectation was incorrect. It is now clear that nucleon physics is much more complex, involving quark orbital angular momenta as well as gluonic and sea quark contributions. Thus, the nucleon spin structure remains a most active aspect of QCD research, involving important advances such as the developments of generalized parton distributions (GPD) and transverse momentum distributions (TMD). Elastic and inelastic lepton-proton scattering, as well as photoabsorption experiments provide various ways to investigate non-perturbative QCD. Fundamental sum rules-such as the Bjorken sum rule for polarized photoabsorption on polarized nucleons-are also in the non-perturbative domain. This realization triggered a vigorous program to link the low energy effective hadronic description of the strong interactions to fundamental quarks and gluon degrees of freedom of QCD. This has also led to advances in lattice gauge theory simulations of QCD and to the development of holographic QCD ideas based on the AdS/CFT or gauge/gravity correspondence, a novel approach providing a well-founded semiclassical approximation to QCD. Any QCD-based model of the nucleon's spin and dynamics must also successfully account for the observed spectroscopy of hadrons. Analytic calculations of the hadron spectrum, a long sought goal of QCD research, have now being realized using light-front holography and superconformal quantum mechanics, a formalism consistent with the results from nucleon spin studies. We begin this review with a phenomenological description of nucleon structure in general and of its spin structure in particular, aimed to engage non-specialist readers. Next, we discuss the nucleon spin structure at high energy, including topics such as Dirac's front form and light-front quantization which provide a frame-independent, relativistic description of hadron structure and dynamics, the derivation of spin sum rules, and a direct connection to the QCD Lagrangian. We then discuss experimental and theoretical advances in the nonperturbative domain-in particular the development of light-front holographic QCD and superconformal quantum mechanics, their predictions for the spin content of nucleons, the computation of PDFs and of hadron masses.
14 Oct 2019
TL;DR: In this article, a review of photon-induced interactions reveal the density distribution of partons in nuclei, thereby probing quantum chromodynamics in high-density environments, and a future electron-ion collider with high-luminosity and centre-of-mass energy will make exquisitely detailed images of partsons in a nucleus.
Abstract: Understanding how quarks and gluons behave in the nuclear environment is an important focus of modern nuclear physics. Recent measurements have provided an improved understanding of how quark and gluon densities are altered in heavy nuclei. It has also become possible to make multi-dimensional pictures of the nucleus, exploring how these alterations are distributed within heavy nuclei. The modifications are naturally expected to be largest in the core of a nucleus and smaller near its periphery; this variation can change the effective shape of the nucleus. Experiments have also started to explore the transverse momentum distribution of the partons in the nuclei, and, by using incoherent photoproduction as a probe, measure event-by-event fluctuations in nucleon and nuclei parton densities. This Review explores recent progress in measurements of nuclear structure at high energy, emphasizing these multi-dimensional pictures. We also discuss how a future electron–ion collider with high luminosity and centre-of-mass energy will make exquisitely detailed images of partons in a nucleus. The partonic (quark and gluon) structure of protons and neutrons is modified in heavy nuclei. This Review surveys how studies of photon-induced interactions reveal the density distribution of partons in nuclei, thereby probing quantum chromodynamics in high-density environments.
TL;DR: In this article, the first calculation of these form factors from lattice quantum chromodynamics is presented, which uses a larger-than-physical value of the light quark mass corresponding to the energy-momentum tensor.
Abstract: A future electron-ion collider will enable the gluon contributions to the gravitational form factors of the proton to be constrained experimentally for the first time. Here, the first calculation of these form factors from lattice quantum chromodynamics is presented. The calculations use a larger-than-physical value of the light quark mass corresponding to ${m}_{\ensuremath{\pi}}\ensuremath{\sim}450\text{ }\text{ }\mathrm{MeV}$. All three form factors, which encode the momentum dependence of the lowest moment of the spin-independent gluon generalized parton distributions and are related to different components of the energy-momentum tensor, are resolved. In particular, the gluon $D$-term form factor, related to the pressure distribution inside the nucleon, is determined for the first time. The gluon contributions to the two gravitational form factors of the pion are also determined, and are compared to existing lattice determinations of the quark contributions to the gravitational form factors and to phenomenology.
TL;DR: The first determination of the nuclear parton distribution functions (nPDF) based on the NNPDF methodology: nNNPDF1.0. as mentioned in this paper was performed up to NNLO in QCD calculations with heavy quark mass effects.
Abstract: We present a first determination of the nuclear parton distribution functions (nPDF) based on the NNPDF methodology: nNNPDF1.0. This analysis is based on neutral-current deep-inelastic structure function data and is performed up to NNLO in QCD calculations with heavy quark mass effects. For the first time in the NNPDF fits, the $\chi^2$ minimization is achieved using stochastic gradient descent with reverse-mode automatic differentiation (backpropagation). We validate the robustness of the fitting methodology through closure tests, assess the perturbative stability of the resulting nPDFs, and compare them with other recent analyses. The nNNPDF1.0 distributions satisfy the boundary condition whereby the NNPDF3.1 proton PDF central values and uncertainties are reproduced at $A=1$, which introduces important constraints particularly for low-$A$ nuclei. We also investigate the information that would be provided by an Electron-Ion Collider (EIC), finding that EIC measurements would significantly constrain the nPDFs down to $x\simeq 5\times 10^{-4}$. Our results represent the first-ever nPDF determination obtained using a Monte Carlo methodology consistent with that of state-of-the-art proton PDF fits, and provide the foundation for a subsequent global nPDF analyses including also proton-nucleus data.
TL;DR: In this paper, single and double-differential cross-section measurements for the production of top-quark pairs, in the lepton + jets channel at particle and parton level, are presented.
Abstract: Single- and double-differential cross-section measurements are presented for the production of top-quark pairs, in the lepton + jets channel at particle and parton level. Two topologies, resolved a ...
TL;DR: The first determination of the nuclear parton distribution functions (nPDF) based on the NNPDF methodology: nNNPDF1.0 as mentioned in this paper was performed up to NNLO in QCD calculations with heavy quark mass effects.
Abstract: We present a first determination of the nuclear parton distribution functions (nPDF) based on the NNPDF methodology: nNNPDF1.0. This analysis is based on neutral-current deep-inelastic structure function data and is performed up to NNLO in QCD calculations with heavy quark mass effects. For the first time in the NNPDF fits, the $$\chi ^2$$
minimization is achieved using stochastic gradient descent with reverse-mode automatic differentiation (backpropagation). We validate the robustness of the fitting methodology through closure tests, assess the perturbative stability of the resulting nPDFs, and compare them with other recent analyses. The nNNPDF1.0 distributions satisfy the boundary condition whereby the NNPDF3.1 proton PDF central values and uncertainties are reproduced at $$A=1$$
, which introduces important constraints particularly for low-A nuclei. We also investigate the information that would be provided by an Electron-Ion Collider (EIC), finding that EIC measurements would significantly constrain the nPDFs down to $$x\simeq 5\times 10^{-4}$$
. Our results represent the first-ever nPDF determination obtained using a Monte Carlo methodology consistent with that of state-of-the-art proton PDF fits, and provide the foundation for a subsequent global nPDF analyses including also proton-nucleus data.
TL;DR: In this paper, the Higgsstrahlung process was performed in the geneva Monte Carlo framework and the beam thrust resummation was carried out to next-to-next-leading logarithmic accuracy, consistently incorporating all singular virtual and real NNLO corrections.
Abstract: We present results for the Higgsstrahlung process within the geneva Monte Carlo framework. We combine the fully differential next-to-next-to-leading order (NNLO) calculation with the higher-order resummation in the 0-jettiness resolution variable (beam thrust). The resulting parton-level events are further showered and hadronized by pythia8. The beam thrust resummation is carried out to next-to-next-to-leading logarithmic accuracy, consistently incorporating all singular virtual and real NNLO corrections. It thus provides a natural perturbative connection between the NNLO calculation and the parton shower regime, including a systematic assessment of perturbative uncertainties. In this way, observables which are inclusive over the additional radiation are correct to NNLO, while the description of 0-jet -like resummation variables is improved beyond the parton shower approximation. We provide predictions for the 13 TeV LHC.
TL;DR: In this article, the sub-leading color effects in the single-jet inclusive double-differential cross sections are indeed negligible as far as phenomenological applications are concerned, and the necessary modifications of the sector-improved residue subtraction scheme that made this work possible.
Abstract: Next-to-next-to-leading order QCD predictions for single-, double- and even triple-differential distributions of jet events in proton-proton collisions have recently been obtained using the NNLOjet framework based on antenna subtraction. These results are an important input for Parton Distribution Function fits to hadron-collider data. While these calculations include all of the partonic channels occurring at this order of the perturbative expansion, they are based on the leading-color approximation in the case of channels involving quarks and are only exact in color in the pure-gluon channel. In the present publication, we verify that the sub-leading color effects in the single-jet inclusive double- differential cross sections are indeed negligible as far as phenomenological applications are concerned. This is the first independent and complete calculation for this observable. We also take the opportunity to discuss the necessary modifications of the sector-improved residue subtraction scheme that made this work possible.
TL;DR: In this paper, the Wigner and Husimi distributions of protons in the color glass condensate framework were derived for a proposed electron-ion collider by computing coherent diffractive dijet production cross sections in e+p collisions within the same framework.
Abstract: Experimental processes that are sensitive to parton Wigner distributions provide a powerful tool to advance our understanding of proton structure. In this work, we compute gluon Wigner and Husimi distributions of protons within the color glass condensate framework, which includes a spatially dependent McLerran-Venugopalan initial configuration and the explicit numerical solution of the Jalilian-Marian–Iancu–McLerran–Weigert–Leonidov–Kovner equations. We determine the leading anisotropy of the Wigner and Husimi distributions as a function of the angle between the impact parameter and transverse momentum. We study experimental signatures of these angular correlations at a proposed electron-ion collider by computing coherent diffractive dijet production cross sections in e+p collisions within the same framework. Specifically, we predict the elliptic modulation of the cross section as a function of the relative angle between the nucleon recoil and dijet transverse momentum for a wide kinematical range. We further predict its dependence on the collision energy, which is dominated by the growth of the proton with decreasing x.
TL;DR: In this article, the transverse momentum dependent (TMD) parton distributions obtained from the parton branching (PB) method are combined with next-to-leading-order (NLO) calculations of Drell-Yan (DY) production.
Abstract: Transverse momentum dependent (TMD) parton distributions obtained from the parton branching (PB) method are combined with next-to-leading-order (NLO) calculations of Drell-Yan (DY) production. We apply the MC@NLO method for the hard process calculation and matching with the PB TMDs. We compute predictions for the transverse momentum, rapidity, and ${\ensuremath{\phi}}^{*}$ spectra of Z-bosons. We find that the theoretical uncertainties of the predictions are dominated by the renormalization and factorization scale dependence, while the impact of TMD uncertainties is moderate. The theoretical predictions agree well, within uncertainties, with measurements at the Large Hadron Collider (LHC). In particular, we study the region of lowest transverse momenta at the LHC, and comment on its sensitivity to nonperturbative TMD contributions.
TL;DR: In this paper, the authors presented the first numerical results for the two-loop helicity amplitudes for the scattering of four partons and a W-boson in QCD.
Abstract: We present the first numerical results for the two-loop helicity amplitudes for the scattering of four partons and a W-boson in QCD. We use a finite field sampling method to reduce directly from Feynman diagrams to the coefficients of a set of master integrals after applying integration-by-parts identities. Since the basis of master integrals is not yet fully known analytically, we identify a set of master integrals with a simple divergence structure using local numerator insertions. This allows for accurate numerical evaluation of the amplitude using sector decomposition methods.
DOI•
03 Dec 2019
TL;DR: In this article, the future opportunities for high-density QCD studies with ion and proton beams at the LHC are presented, and four major scientific goals are identified: the characterisation of the macroscopic long wavelength Quark-Gluon Plasma (QGP) properties with unprecedented precision, the investigation of the microscopic parton dynamics underlying QGP properties, the development of a unified picture of particle production and QCD dynamics from small (pp) to large (nucleus--nucus) systems, the exploration of parton densities in nuclei in
Abstract: The future opportunities for high-density QCD studies with ion and proton beams at the LHC are presented. Four major scientific goals are identified: the characterisation of the macroscopic long wavelength Quark-Gluon Plasma (QGP) properties with unprecedented precision, the investigation of the microscopic parton dynamics underlying QGP properties, the development of a unified picture of particle production and QCD dynamics from small (pp) to large (nucleus--nucleus) systems, the exploration of parton densities in nuclei in a broad ($x$, $Q^2$) kinematic range and the search for the possible onset of parton saturation. In order to address these scientific goals, high-luminosity Pb-Pb and p-Pb programmes are considered as priorities for Runs 3 and 4, complemented by high-multiplicity studies in pp collisions and a short run with oxygen ions. High-luminosity runs with intermediate-mass nuclei, for example Ar or Kr, are considered as an appealing case for extending the heavy-ion programme at the LHC beyond Run 4. The potential of the High-Energy LHC to probe QCD matter with newly-available observables, at twice larger center-of-mass energies than the LHC, is investigated.
TL;DR: Using lattice configurations for quantum chromodynamics (QCD) generated with three domain-wall fermions at a physical pion mass, the authors obtained a parameter-free prediction of QCD's renormalisation-group-invariant process-independent effective charge.
Abstract: Using lattice configurations for quantum chromodynamics (QCD) generated with three domain-wall fermions at a physical pion mass, we obtain a parameter-free prediction of QCD's renormalisation-group-invariant process-independent effective charge, $\hat\alpha(k^2)$. Owing to the dynamical breaking of scale invariance, evident in the emergence of a gluon mass-scale, this coupling saturates at infrared momenta: $\hat\alpha(0)/\pi=0.97(4)$. Amongst other things: $\hat\alpha(k^2)$ is almost identical to the process-dependent (PD) effective charge defined via the Bjorken sum rule; and also that PD charge which, employed in the one-loop evolution equations, delivers agreement between pion parton distribution functions computed at the hadronic scale and experiment. The diversity of unifying roles played by $\hat\alpha(k^2)$ suggests that it is a strong candidate for that object which represents the interaction strength in QCD at any given momentum scale; and its properties support a conclusion that QCD is a mathematically well-defined quantum field theory in four dimensions.
TL;DR: In this article, the pion and the kaon parton distribution functions were obtained from the eigenstates of a light front effective Hamiltonian in the constituent quark-antiquark representation suitable for low-momentum scale applications.
Abstract: We obtain the pion and the kaon parton distribution functions from the eigenstates of a light front effective Hamiltonian in the constituent quark-antiquark representation suitable for low-momentum scale applications. By taking these scales as the only free parameters, the valence quark distribution functions of the pion, after QCD evolution, are consistent with the data from the FNAL-E615 experiment. The ratio of the up quark distribution of the kaon to that of the pion also agrees with the CERN-NA3 experiment. Supplemented by known parton distribution functions for the nucleons, we further obtain the cross section consistent with experimental data for the ${\ensuremath{\pi}}^{\ensuremath{-}}\text{nucleus}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}X$ Drell-Yan process.
TL;DR: In this paper, a set of nuclear parton distribution functions (nPDFs) at next-to-leading order and nextto-next-toleading order were presented.
Abstract: We present new sets of nuclear parton distribution functions (nPDFs) at next-to-leading order and next-to-next-to-leading order. Our analyses are based on deeply inelastic scattering data with charged-lepton and neutrino beams on nuclear targets. In addition, a set of proton baseline PDFs is fitted within the same framework with the same theoretical assumptions. The results of this global QCD analysis are compared to existing nPDF sets and to the fitted cross sections. Also, the uncertainties resulting from the limited constraining power of the included experimental data are presented. The published work is based on an open-source tool, xfitter, which has been modified to be applicable also for a nuclear PDF analysis. The required extensions of the code are discussed as well.
TL;DR: In this paper, an algorithm that evolves hard processes at the amplitude level by dressing them iteratively with (massless) quarks and gluons is presented. But it is not specified at leading logarithmic accuracy, the framework should be sufficient to go beyond.
Abstract: We present an algorithm that evolves hard processes at the amplitude level by dressing them iteratively with (massless) quarks and gluons. The algorithm interleaves collinear emissions with soft emissions and includes Coulomb/Glauber exchanges. It includes all orders in Nc, is spin dependent and is able to accommodate kinematic recoils. Although it is specified at leading logarithmic accuracy, the framework should be sufficient to go beyond. Coulomb exchanges make the factorisation of collinear and soft emissions highly non-trivial. In the absence of Coulomb exchanges, we show how factorisation works out and how a partial factorisation is manifest in the presence of Coulomb exchanges. Finally, we illustrate the use of the algorithm by deriving DGLAP evolution and computing the resummed thrust, hemisphere jet mass and gaps-between-jets distributions in e+e−.
TL;DR: In this paper, the relation between Parton Distribution Functions (PDFs) and matrix elements computable from lattice QCD, focusing on the quasi-Parton distribution functions (qPDFs), was investigated, and the potentiality of such lattice data in constraining PDFs was investigated.
Abstract: We revise the relation between Parton Distribution Functions (PDFs) and matrix elements computable from lattice QCD, focusing on the quasi-Parton Distribution Functions (qPDFs) approach. We exploit the relation between PDFs and qPDFs in the case of the unpolarized isovector parton distribution to obtain a factorization formula relating the real and imaginary part of qPDFs matrix elements to specific nonsinglet distributions, and we propose a general framework to extract PDFs from the available lattice data, treating them on the same footing as experimental data. We implement the proposed approach within the NNPDF framework, and we study the potentiality of such lattice data in constraining PDFs, assuming some plausible scenarios to assess the unknown systematic uncertainties. We finally extract the two nonsinglet distributions involved in our analysis from a selection of the available lattice data.
TL;DR: In this paper, the feasibility of a measurement of Weizsacker-Williams transverse momentum dependent (TMD) gluon distributions through dijet production in DIS on a nucleus at an electron-ion collider is analyzed.
Abstract: The production of a hard dijet with small transverse momentum imbalance in semi-inclusive DIS probes the conventional and linearly polarized Weizsacker-Williams (WW) transverse momentum dependent (TMD) gluon distributions. The latter, in particular, gives rise to an azimuthal dependence of the dijet cross section. In this paper we analyze the feasibility of a measurement of these TMDs through dijet production in DIS on a nucleus at an electron-ion collider. We introduce the mcdijet Monte Carlo generator to sample quark-antiquark dijet configurations based on leading-order parton level cross sections with WW gluon distributions that solve the nonlinear small-x QCD evolution equations. These configurations are fragmented to hadrons using PYTHIA, and final-state jets are reconstructed. We report on background studies and on the effect of kinematic cuts introduced to remove beam jet remnants. We estimate that with an integrated luminosity of 20 fb−1/nucleon one can determine the distribution of linearly polarized gluons with a statistical accuracy of approximately 5%.