Nuclear Parton Distribution Research Articles

Measurement of prompt photon production in [formula omitted] TeV p + Pb collisions with ATLAS

The inclusive production rates of isolated, prompt photons in p+Pb collisions at sNN=8.16 TeV are studied with the ATLAS detector at the Large Hadron Collider using a dataset with an integrated luminosity of 165 nb−1 recorded in 2016. The cross-section and nuclear modification factor RpPb are measured as a function of photon transverse energy from 20 GeV to 550 GeV and in three nucleon–nucleon centre-of-mass pseudorapidity regions, (−2.83,−2.02), (−1.84,0.91), and (1.09,1.90). The cross-section and RpPb values are compared with the results of a next-to-leading-order perturbative QCD calculation, with and without nuclear parton distribution function modifications, and with expectations based on a model of the energy loss of partons prior to the hard scattering. The data disfavour a large amount of energy loss and provide new constraints on the parton densities in nuclei.

Nuclear parton distributions from lepton-nucleus scattering and the impact of an electron-ion collider

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 xsimeq 5times 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.

Probing nuclear modifications of parton distribution functions through the isolated prompt photon production at energies available at the CERN Large Hadron Collider

An accurate knowledge of nuclear parton distribution functions (nPDFs) is an essential ingredient of high energy physics calculations when the processes are involving nuclei in the initial state. It is well known now that the prompt photon production both in hadronic and nuclear collisions is a powerful tool for exploring the parton densities in the nucleon and nuclei especially of the gluon. In this work, we are going to perform a comprehensive study of the isolated prompt photon production in $ p $-Pb collisions at backward rapidities to find the best kinematic regions in which the experimental measurements have most sensitivity to the nuclear modifications of parton densities. Most emphasis will be placed on the antishadowing nuclear modification. To this aim, we calculate and compare various quantities at different values of center-of-mass energy covered by the LHC and also different rapidity regions to realize which one is most useful.

Constraints on nuclear parton distributions from dijet photoproduction at the LHC

Using QCD calculations of the cross section of inclusive dijet photoproduction in Pb–Pb ultraperipheral collisions in the LHC kinematics as pseudo-data, we study the effect of including these data using the Bayesian reweighting technique on nCTEQ15, nCTEQ15np, and EPPS16 nuclear parton distribution functions (nPDFs). We find that, depending on the assumed error of the pseudo-data, it leads to a significant reduction of the nPDF uncertainties at small values of the momentum fraction x_A. Taking the error to be 5%, the uncertainty of nCTEQ15 and nCTEQ15np nPDFs reduces approximately by a factor of two at x_A=10^{-3}. At the same time, the reweighting effect on EPPS16 nPDFs is much smaller due to the higher value of the tolerance and a more flexible parametrization form.

The non-singlet structure function of light and heavy nuclei up to next-to-leading order analysis at low x region

We compute the non-singlet structure function of some various light and heavy nuclei using the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) evolution equation at leading and next-to-leading order analysis at low x and low and moderate Q2 in x≤0.05 and 1≤Q2≤12 GeV2 region. Using the bounded nuclear parton distribution functions from the HKN parametrization group, we obtain the nuclear non-singlet structure function. Its dynamical behavior as well as nuclear factor are investigated. We compare our results with the new nCTEQ15 set of nuclear parton distribution functions. Our results are in reasonably good agreement with this new global fit.

Probing cold nuclear matter effects with the productions of isolated- and +jet in p+Pb collisions at 8.16 TeV * * Supported by the NSFC of China (11435004, 11322546, 11805167) and partly Supported by China University of Geosciences (Wuhan) (162301182691)

We investigate cold nuclear matter (CNM) effects on the productions of isolated prompt photons and jet in proton-lead collisions at TeV under next-to-leading order perturbative quantum chromodynamics calculations with four parameterizations for nuclear parton distribution functions (nPDFs), i.e., DSSZ, EPPS16, nCTEQ15, and nIMParton. Our theoretical calculations provide good descriptions of the pp baseline in the ATLAS collaboration and make predictions for future experimental results at + collisions. We calculate the dependence of the nuclear modification factor of isolated prompt photons on transverse momentum and pseudo-rapidity at very forward and backward rapidity regions, and we demonstrate that the forward-to-backward yield asymmetries as a function of with different nPDF parameterizations have diverse behaviors. Furthermore, the nuclear modification factor of isolated- jet as a function of jet's pseudo-rapidity at different average transverse momenta has been discussed. This can facilitate a tomographic study of CNM effects with precise locations in a rather wide kinematic region by varying the transverse momenta and rapidities of both isolated photons and jets in p+A collisions.

Prompt \u039bc+ production in pPb collisions at sqrt{s_{mathrm{NN}}}=5.02 TeV

The prompt production of Λc+ baryons is studied in proton-lead collisions collected with the LHCb detector at the LHC. The data sample corresponds to an integrated luminosity of 1.58 nb−1 recorded at a nucleon-nucleon centre-of-mass energy of sqrt{s_{mathrm{NN}}}=5.02 TeV. Measurements of the differential cross-section and the forwardbackward production ratio are reported for Λc+ baryons with transverse momenta in the range 2 < pT < 10 GeV/c and rapidities in the ranges 1.5 < y∗ < 4.0 and −4.5 < y∗ < −2.5 in the nucleon-nucleon centre-of-mass system. The ratio of cross-sections of Λc+ baryons and D0 mesons is also reported. The results are compared with next-to-leading order calculations that use nuclear parton distribution functions.

Nuclear Parton Distributions from Neural Networks

In this contribution we present a status report on the recent progress towards an analysis of nuclear parton distribution functions (nPDFs) using the NNPDF methodology. We discuss how the NNPDF fitting approach can be extended to account for the dependence on the atomic mass number $A$, and introduce novel algorithms to improve the training of the neural network parameters within the NNPDF framework. Finally, we present preliminary results of a nPDF fit to neutral current deep-inelastic lepton-nucleus scattering data, and demonstrate how one can validate the new fitting methodology by means of closure tests.

The study of gluon energy loss in cold nuclear matter from J/ψ production**Supported by National Natural Science Foundation of China (11405043) and Natural Science Foundation of Hebei Province (A2018209269)

The energy loss effects of the incident quark, gluon, and the color octet on J/ψ suppression in p-A collisions are studied by means of the experimental data at E866, RHIC, and LHC energy. We extracted the transport coefficient for gluon energy loss from the E866 experimental data in the middle xF region (0.20 < xF > 0.65) based on the Salgado-Wiedemann (SW) quenching weights and the recent EPPS16 nuclear parton distribution functions together with nCTEQ15. It was determined that the difference between the values of the transport coefficient for light quark, gluon, and heavy quark in cold nuclear matter is very small. The theoretical results modified by the parton energy loss effects are consistent with the experimental data for E866 and RHIC energy, and the gluon energy loss plays a remarkable role on J/ψ suppression in a broad variable range. Because the corrections of the nuclear parton distribution functions in the J/ψ channel are significant at LHC energy level, the nuclear modification due to the parton energy loss is minimal. It is worth noting that we use the color evaporation model (CEM) at leading order to compute the p-p baseline, and the conclusion in this paper is CEM model dependent.

EMC effect in the next-to-leading order approximation based on the Laplace transformation

In this article, using Laplace transformation , an analytical solution is obtained for the DGLAP evolution equation at the next-to-leading order of perturbative QCD. The technique is also employed to extract, in the Laplace $s$-space, an analytical solution for the nuclear structure function, $F_2^A(x, Q^2)$. Firstly, the results for separate nuclear parton distributions for all parton types are presented which include valence quark densities, the anti-quark and strange sea PDFs and finally the gluon distribution. Based on the Laplace transformation, the obtained parton distribution functions and the nuclear structure function in the $x$-space are compared with the results from the [Phys.\ Rev.\ C series 86, 064301 (2012)] model. Our calculations are in good agreement with the available DIS experimental data as well as theoretical models which contain both small and large values of $x$-Bjorken variable. We compare our nuclear PDFs sets with those from other recent collaborations, in particular with the [ nCTEQ15 ] and [ HKN07] sets. The comparison between our results and those from the literature indicates a good agreement .

Constraining Gluon Distributions in Nuclei Using Dijets in Proton-Proton and Proton-Lead Collisions at sqrt[s_{NN}]=5.02 TeV.

The pseudorapidity distributions of dijets as functions of their average transverse momentum (p_{T}^{ave}) are measured in proton-lead (pPb) and proton-proton (pp) collisions. The data samples were collected by the CMS experiment at the CERN LHC, at a nucleon-nucleon center-of-mass energy of 5.02TeV. A significant modification of the pPb spectra with respect to the pp spectra is observed in all p_{T}^{ave} intervals investigated. The ratios of the pPb and pp distributions are compared to next-to-leading order perturbative quantum chromodynamics calculations with unbound nucleon and nuclear parton distribution functions (PDFs). These results give the first evidence that the gluon PDF at large Bjorken x in lead ions is strongly suppressed with respect to the PDF in unbound nucleons.

Nuclear modification factors of strange and multi-strange particles in pPb collisions with the CMS experiment

Recent observation of collective effects in high-multiplicity pp and pA collisions raise the question of whether QGP can also be formed in the smaller systems. Systematic studies of the strange particle abundance and nuclear modification factors can shed light on this issue. The CMS experiment has excellent strange-particle reconstruction capabilities over a broad kinematic range in pp and pPb collisions. The spectra of and hadrons have been measured in various rapidity regions as a function of pT in pp and pPb collisions at 5.02 TeV. Based on the measurements of these spectra, nuclear modification factors of , and in mid-rapidity are measured. Since pPb is an asymmetric system, the nuclear modification factor of , and in Pb-going direction are compared to those in p-going direction. These final results for nuclear modification factors measured out to high-pT can be helpful in discussing the implications of collective behavior and energy loss. In addition, the measurement of the forward-backward rapidity yield asymmetries of and Λ as a function of pT provide sensitivity to initial state effects, such as shadowing in the nuclear parton distributions.

Pion nucleus Drell\u2013Yan process and parton transverse momentum in the pion

We present a thorough analysis of unpolarized Drell–Yan (DY) pair production in pion–nucleus scattering. On the nucleus side, we use nuclear parton distributions along with parametrisations of the nucleon partonic transverse distribution available in the literature. Partonic longitudinal and transverse distributions of the pion are those obtained in a recent calculation in a Nambu–Jona Lasinio (NJL) framework, with Pauli–Villars regularization. The scale of the NJL model is determined with a minimisation procedure comparing NLO predictions based on NJL evolved pion distributions to rapidity differential DY cross sections data. The resulting distributions are then used to describe, up to next-to-leading logarithmic accuracy, the transverse momentum spectrum of dilepton pairs up to a transverse momentum of 2 GeV. With no additional parameters, fair agreement is found with available pion–nucleus data, confirming the virtues of the NJL description of pion parton structure. We find sizable evolution effects on the shape of the distributions and on the generated average transverse momentum of the dilepton pair. We furthermore discuss the possibility of gaining information about the behavior of the pion unpolarized transverse momentum dependent parton distribution from pion nucleus DY data.

Probing high-momentum protons and neutrons in neutron-rich nuclei

The atomic nucleus is one of the densest and most complex quantum-mechanical systems in nature. Nuclei account for nearly all the mass of the visible Universe. The properties of individual nucleons (protons and neutrons) in nuclei can be probed by scattering a high-energy particle from the nucleus and detecting this particle after it scatters, often also detecting an additional knocked-out proton. Analysis of electron- and proton-scattering experiments suggests that some nucleons in nuclei form close-proximity neutron-proton pairs1-12 with high nucleon momentum, greater than the nuclear Fermi momentum. However, how excess neutrons in neutron-rich nuclei form such close-proximity pairs remains unclear. Here we measure protons and, for the first time, neutrons knocked out of medium-to-heavy nuclei by high-energy electrons and show that the fraction of high-momentum protons increases markedly with the neutron excess in the nucleus, whereas the fraction of high-momentum neutrons decreases slightly. This effect is surprising because in the classical nuclear shell model, protons and neutrons obey Fermi statistics, have little correlation and mostly fill independent energy shells. These high-momentum nucleons in neutron-rich nuclei are important for understanding nuclear parton distribution functions(the partial momentum distribution of the constituents of the nucleon) and changes in the quark distributions of nucleons bound in nuclei (the EMC effect)1,13,14. They are also relevant for the interpretation of neutrino-oscillation measurements15 and understanding of neutron-rich systems such as neutron stars3,16.

Coherent deeply virtual Compton scattering off He4

Coherent Deeply virtual Compton scattering off the $^4$He nucleus is studied in impulse approximation. A convolution formula for the nuclear Generalized Parton Distribution (GPD) is derived in terms of the $^4$He non-diagonal spectral function and of the GPD of the struck nucleon. A model of the nuclear non diagonal spectral function, based on the momentum distribution corresponding to the AV18 nucleon-nucleon interaction, is used in the actual calculation. Typical impulse approximation results are reproduced, in proper limits, for the nuclear form factor and for nuclear parton distributions. The nuclear generalized parton distribution and the Compton form factor are evaluated using, as nucleonic ingredient, a well known generalized parton distribution model. An overall very good agreement is found with the data recently published by the EG6 experiment at the Jefferson Laboratory (JLab). More refined nuclear calculations are addressed and will be necessary for the expected improved accuracy of the next generation of experiments at JLab, with the 12 GeV electron beam and high luminosity.

Nuclear parton density functions from dijet photoproduction at the EIC

We study the potential of dijet photoproduction measurements at a future electron-ion collider (EIC) to better constrain our present knowledge of the nuclear parton distribution functions. Based on theoretical calculations at next-to-leading order and approximate next-to-next-to-leading order of perturbative QCD, we establish the kinematic reaches for three different EIC designs, the size of the parton density function modifications for four different light and heavy nuclei from He-4 over C-12 and Fe-56 to Pb-208 with respect to the free proton, and the improvement of EIC measurements with respect to current determinations from deep-inelastic scattering and Drell-Yan data alone and when also considering data from existing hadron colliders.

Nuclear Parton Distribution Functions (nPDFs) and Their Uncertainties in the LHC Era

.

Predictions for cold nuclear matter effects in p+Pb collisions at [formula omitted] TeV

Predictions for cold nuclear matter effects on charged hadrons, identified light hadrons, quarkonium and heavy flavor hadrons, Drell–Yan dileptons, jets, photons, gauge bosons and top quark pairs produced in p+Pb collisions at sNN=8.16 TeV are compiled and, where possible, compared to each other. Predictions of the normalized ratios of p+Pb to p+p cross sections are also presented for most of the observables, providing new insights into the expected role of cold nuclear matter effects. In particular, the role of nuclear parton distribution functions on particle production can now be probed over a wider range of phase space than ever before.

Impact of LHC Heavy-Flavour Data on Nuclear Gluon PDF

We use for the first time experimental data for the inclusive heavyquark production in proton–lead collisions at the LHC in order to improve our knowledge of the gluon distribution in nuclei. We first check that the two most recent global nuclear parton distribution analyses (nCTEQ15 and EPPS16) provide a good overall description of the data, and then use these data in a PDF reweighing analysis. We find a first clear confirmation of gluon shadowing at small x. Additionally, it demonstrates that the inclusion of such heavy-flavour data in a global fit would significantly reduce the uncertainty on the gluon density down to x ' 7 × 10−6 while keeping an agreement with the other data of the global fits. Our study accounts for the factorisation scale uncertainties which become the largest for the charm(onium) sector.

Nuclear structure functions at a future electron-ion collider

The quantitative knowledge of heavy nuclei's partonic structure is currently limited to rather large values of momentum fraction $x$ -- robust experimental constraints below $x \sim 10^{-2}$ at low resolution scale $Q^2$ are particularly scarce. This is in sharp contrast to the free proton's structure which has been probed in deep inelastic scattering (DIS) measurements down to $x \sim 10^{-5}$ at perturbative resolution scales. The construction of an Electron-Ion Collider (EIC) with a possibility to operate with a wide variety of nuclei, will allow one to explore the low-$x$ region in much greater detail. In the present paper we simulate the extraction of the nuclear structure functions from measurements of inclusive and charm reduced cross sections at an EIC. The potential constraints are studied by analyzing simulated data directly in a next-to-leading order global fit of nuclear parton distribution functions based on the recent EPPS16 analysis. A special emphasis is placed on studying the impact an EIC would have on extracting the nuclear gluon PDF, the partonic component most prone to non-linear effects at low $Q^2$. In comparison to the current knowledge, we find that the gluon PDF can be measured at an EIC with significantly reduced uncertainties.