Nuclear Parton Distribution Research Articles

Impact of nuclear gluon distributions on leptoquark production by neutrinos

We investigate non-standard interactions of neutrinos with atomic nuclei through excitations of leptoquarks. A leptoquark term in the Lagrangian admits the possibility that neutrinos interact with gluons. The current lower limits on the leptoquark masses are of the order of 1 TeV depending on the leptoquark quantum numbers and couplings. Such heavy states can be produced in ultra-high energy cosmic neutrino scattering processes. The four-momentum transfer squared and the Bjorken variable simultaneously probed in these processes may reach values kinematically inaccessible at present collider experiments. We study the impact of the gluon density in a nucleus on the cross section for the neutrino-induced leptoquark production. We show that taking into account the nuclear parton distributions shifts the production threshold to significantly lower neutrino energies. As a particular case we consider the interaction with oxygen, which is abundant in water targets used in neutrino detection experiments.

Coupled Boltzmann transport equations of heavy quarks and quarkonia in quark-gluon plasma

We develop a framework of coupled transport equations for open heavy flavor and quarkonium states, in order to describe their transport inside the quark-gluon plasma. Our framework is capable of studying simultaneously both open and hidden heavy flavor observables in heavy-ion collision experiments and can account for both, uncorrelated and correlated recombination. Our recombination implementation depends on real-time open heavy quark and antiquark distributions. We carry out consistency tests to show how the interplay among open heavy flavor transport, quarkonium dissociation and recombination drives the system to equilibrium. We then apply our framework to study bottomonium production in heavy-ion collisions. We include ϒ(1S), ϒ(2S), ϒ(3S), χb(1P) and χb(2P) in the framework and take feed-down contributions during the hadronic gas stage into account. Cold nuclear matter effects are included by using nuclear parton distribution functions for the initial primordial heavy flavor production. A calibrated 2 + 1 dimensional viscous hydrodynamics is used to describe the bulk QCD medium. We calculate both the nuclear modification factor RAA of all bottomonia states and the azimuthal angular anisotropy coefficient v2 of the ϒ(1S) state and find that our results agree reasonably with experimental measurements. Our calculations indicate that correlated cross-talk recombination is an important production mechanism of bottomonium in current heavy-ion experiments. The importance of correlated recombination can be tested experimentally by measuring the ratio of RAA(χb(1P)) and RAA(ϒ(2S)).

QCD hidden-color hexadiquark in the core of nuclei

Hidden-color configurations are a key prediction of QCD with important physical consequences. In this work we examine a QCD color-singlet configuration in nuclei formed by combining six scalar [ud] diquarks in a strongly bound SU(3)C channel. The resulting hexadiquark state is a charge-2, spin-0, baryon number-4, isospin-0, color-singlet state. It contributes to alpha clustering in light nuclei and to the additional binding energy not saturated by ordinary nuclear forces in He4 as well as the alpha-nuclei sequence of interest for nuclear astrophysics. We show that the strongly bound combination of six scalar isospin-0 [ud] diquarks within the nuclear wave function - relative to free nucleons - provides a natural explanation of the EMC effect measured by the CLAS collaboration's comparison of nuclear parton distribution function ratios for a large range of nuclei. These experiments confirmed that the EMC effect; i.e., the distortion of quark distributions within nuclei, is dominantly identified with the dynamics of neutron-proton (“isophobic”) short-range correlations within the nuclear wave function rather than proton-proton or neutron-neutron correlations.

Nuclear modification factor of isolated prompt photons in pp and PbPb collisions at [formula omitted] with the CMS detector

High-transverse-momentum prompt photons, as colorless objects, do not interact strongly with the medium and provide a direct way to test perturbative QCD and the nuclear parton distribution functions (nPDFs). The transverse energy spectra and the nuclear modification factors of isolated photons are measured in pp and PbPb collisions at sNN=5.02TeV using the CMS detector. The data are compared to jetphox next-to-leading-order calculations and found to be consistent with the prediction of the nuclear modification factor. The measurements significantly improved the accuracy compared to the previous CMS results at sNN=2.76TeV. No significant modification of isolated photon cross-sections in PbPb collisions with respect to pp collisions is observed in the pseudorapidity range |η| < 1.44 for EγT between 25 and 200 GeV at various collision centrality intervals.

NNNPDF2.0: quark flavor separation in nuclei from LHC data

We present a model-independent determination of the nuclear parton distribution functions (nPDFs) using machine learning methods and Monte Carlo techniques based on the NNPDF framework. The neutral-current deep-inelastic nuclear structure functions used in our previous analysis, nNNPDF1.0, are complemented by inclusive and charm-tagged cross-sections from charged-current scattering. Furthermore, we include all available measurements of W and Z leptonic rapidity distributions in proton-lead collisions from ATLAS and CMS at sqrt{s} = 5.02 TeV and 8.16 TeV. The resulting nPDF determination, nNNPDF2.0, achieves a good description of all datasets. In addition to quantifying the nuclear modifications affecting individual quarks and antiquarks, we examine the implications for strangeness, assess the role that the momentum and valence sum rules play in nPDF extractions, and present predictions for representative phenomenological applications. Our results, made available via the LHAPDF library, highlight the potential of high-energy collider measurements to probe nuclear dynamics in a robust manner.

Cross section of ultrahigh-energy neutrinos: Estimation of the uncertanties due to parton distribution functions

In this work we analyze the effect of uncertainties in parton distribution functions upon the calculation of cross sections for ultrahigh-energy neutrinos. As a detector we choose $^{12}\mathrm{C}$. The elementary diagrams for the neutrino-proton and neutron interactions have been calculated at quark level using nuclear parton distribution functions and their corresponding error bars, as given by the Jyvaskyla group. The leptonic sector of the current-current interactions includes active-sterile neutrino mixing. From the calculated cross section we have set limits on the values of the mixing parameters.

Can we fit nuclear PDFs with the high-x CLAS data?

Nuclear parton distribution functions (nuclear PDFs) are non-perturbative objects that encode the partonic behaviour of bound nucleons. To avoid potential higher-twist contributions, the data probing the high-x end of nuclear PDFs are sometimes left out from the global extractions despite their potential to constrain the fit parameters. In the present work we focus on the kinematic corner covered by the new high-x data measured by the CLAS/JLab collaboration. By using the Hessian re-weighting technique, we are able to quantitatively test the compatibility of these data with globally analyzed nuclear PDFs and explore the expected impact on the valence-quark distributions at high x. We find that the data are in a good agreement with the EPPS16 and nCTEQ15 nuclear PDFs whereas they disagree with TuJu19. The implications on flavour separation, higher-twist contributions and models of EMC effect are discussed.

A QCD analysis of LHCb D-meson data in p+Pb collisions

We scrutinize the recent LHCb data for D0-meson production in p+Pb collisions within a next-to-leading order QCD framework. Our calculations are performed in the SACOT-mT variant of the general-mass variable-flavour-number scheme (GM-VFNS), which has previously been shown to provide a realistic description of the LHC p+p data. Using the EPPS16 and nCTEQ15 nuclear parton distribution functions (PDFs) we show that a very good agreement is obtained also in the p+Pb case both for cross sections and nuclear modification ratios in the wide rapidity range covered by the LHCb data. Encouraged by the good correspondence, we quantify the impact of these data on the nuclear PDFs by the Hessian reweighting technique. We find compelling direct evidence of gluon shadowing at small momentum fractions x, with no signs of parton dynamics beyond the collinear factorization. We also compare our theoretical framework to a fixed-order calculation supplemented with a parton shower. While the two frameworks differ in the absolute cross sections, these differences largely cancel in the nuclear modification ratios. Thus, the constraints for nuclear PDFs appear solid.

Diffractive dissociation of alpha particles as a test of isophobic short-range correlations inside nuclei

The CLAS collaboration at Jefferson Laboratory has compared nuclear parton distributions for a range of nuclear targets and found that the EMC effect measured in deep inelastic lepton-nucleus scattering has a strongly “isophobic” nature. This surprising observation suggests short-range correlations between neighboring n and p nucleons in nuclear wavefunctions that are much stronger compared to p−p or n−n correlations. In this paper we propose a definitive experimental test of the nucleon-nucleon explanation of the isophobic nature of the EMC effect: the diffractive dissociation on a nuclear target A of high energy He4 nuclei to pairs of nucleons n and p with high relative transverse momentum, α+A→n+p+A′+X. The comparison of n−p events with p−p and n−n events directly tests the postulated breaking of isospin symmetry. The experiment also tests alternative QCD-level explanations for the isophobic EMC effect. In particular it will test a proposal for hidden-color degrees of freedom in nuclear wavefunctions based on isospin-zero [ud] diquarks.

Probing impact-parameter dependent nuclear parton densities from double parton scatterings in heavy-ion collisions

We propose a new method to determine the spatially or impact-parameter dependent nuclear parton distribution functions (nPDFs) using the double parton scattering (DPS) processes in high-energy heavy-ion (proton-nucleus and nucleus-nucleus) collisions. We derive a simple generic DPS formula in nuclear collisions by accommodating both the nuclear collision geometry and the spatially dependent nuclear modification effect, under the assumption that the impact-parameter dependence of nPDFs is only related to the nuclear thickness function. While the geometric effect is widely adopted, the impact of the spatially dependent nuclear modification on DPS cross sections has been overlooked so far, which can, however, be significant when the initial nuclear modification is large. In turn, the DPS cross sections in heavy-ion collisions can provide useful information on the spatial dependence of nPDFs. They can be, in general, obtained in minimum-bias nuclear collisions, featuring the virtue of independence of Glauber modeling.

Cross sections of ultrahigh-energy neutrinos: Comparison with ICE-CUBE data

Ultrahigh energetic neutrino events have been recorded by ICE-CUBE. The detection of these events indicates the extragalactic origin of the neutrinos. In order to characterized the elementary mechanisms responsible for the emission of these neutrinos it is necessary to determine their energy distributions and flavor compositions. In this work we are presenting the results of neutrino-nucleon cross section for the interaction with nucleons of a detector. The elementary diagrams for the neutrino-proton (or neutron) interactions have been calculated at the quark level adding the corresponding form factors for quarks in nucleons, as well as nuclear parton distribution functions. The leptonic sector of the current-current interactions have been calculated including neutrino oscillations and the mixing with a sterile neutrino.

The study of the incoming parton energy loss effect on the NLO nuclear Drell–Yan ratios

At a next-to-leading order (NLO) calculation, the initial-state energy loss of the quark is investigated by means of the Drell–Yan experimental data including the new E906 measurements. Furthermore, the incoming gluon energy loss effect embedded in the Compton scattering subprocess of the nuclear Drell–Yan process is also examined. The NLO computations for Drell–Yan ratios are carried out with the SW quenching weights (provided by Salgado and Wiedemann to evaluate the probability distribution that quark (gluon) loses the energy), as well as the nuclear parton distributions (obtained by only fitting the existing experimental data on nuclear structure functions). It is found that the obtained calculations that consider the incoming quark energy loss agree well with the experimental data particularly for E906 data. In addition, the incoming gluon energy loss embodied in the primary NLO Compton scattering subprocess is not obvious, which may be due to the form of the Drell–Yan differential cross section ratio as a function of the quark momentum fraction () minimizes the influence of the QCD NLO correction.

Nuclear-modification factor of charged hadrons at forward and backward rapidity in p + Al and p + Au collisions at sNN=200GeV

The PHENIX experiment has studied nuclear effects in $p$$+$Al and $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV on charged hadron production at forward rapidity ($1.4<\eta<2.4$, $p$-going direction) and backward rapidity ($-2.2<\eta<-1.2$, $A$-going direction). Such effects are quantified by measuring nuclear modification factors as a function of transverse momentum and pseudorapidity in various collision multiplicity selections. In central $p$$+$Al and $p$$+$Au collisions, a suppression (enhancement) is observed at forward (backward) rapidity compared to the binary scaled yields in $p$+$p$ collisions. The magnitude of enhancement at backward rapidity is larger in $p$$+$Au collisions than in $p$$+$Al collisions, which have a smaller number of participating nucleons. However, the results at forward rapidity show a similar suppression within uncertainties. The results in the integrated centrality are compared with calculations using nuclear parton distribution functions, which show a reasonable agreement at the forward rapidity but fail to describe the backward rapidity enhancement.

Improvement of heavy flavor production in a multiphase transport model updated with modern nuclear parton distribution functions

Recently we have updated a multi-phase transport (AMPT) model with modern parton distribution functions of nuclei (nPDFs). Here we study open charm production in the updated AMPT model and compare to the experimental data from $pp$ and $AA$ collisions over a wide range of collision energies. Besides the update of nPDFs, we have removed the transverse momentum cutoff on initial heavy quark productions and also included the resultant heavy flavor cross section into the total minijet cross section in the initial condition as described by the HIJING model. We show that the AMPT model with these updates provides a much better description of the yields and transverse momentum spectra of various open charm hadrons in comparison with the experimental data. This lays the foundation for further heavy flavor studies within the transport model approach.

Parton and Valon Distributions in the Nuclei

In the valon model a nucleon is assumed to be a bound state of three valence quark clusters (valons). On this base the nucleus structure function which are made from nucleons are analysed. Here in the frame work of perturbative quantum chromodynamics (pQCD) the structure of the valons are described at next to leading-order (NLO) approximation. Using the constituent-quark model, structure function of nucleon is largely investigated in the nuclear medium. For a series of nuclei with the nuclear mass A such that 2 ≤ A ≤ 40 within the kinetic region 0.0001 ≤ x ≤ 0.9 which covers a wide range of x Bjorken variable, the theoretical predications are in good agreement with the existing lepton-nucleus data. A better and fair description of experimental data can be done, considering the shadowing effect for small values of x i.e. x < 0.01. Using the valon model, we do an analysis at the NLO approximation while shadowing effect is employed. A comparison between our theoretical prediction for nuclear parton distribution functions (nPDFs) and the results from recent collaborations like AT12 and HKN07 sets have been done. The results of our calculations are in good agreement with the experimental data in deep inelastic scattering (DIS) processes which covers an extended region of x-variable, including the small and also large x amounts and also with some theoretical models.

Z boson production in Pb+Pb collisions at [formula omitted] TeV measured by the ATLAS experiment

The production yield of Z bosons is measured in the electron and muon decay channels in Pb+Pb collisions at sNN=5.02 TeV with the ATLAS detector. Data from the 2015 LHC run corresponding to an integrated luminosity of 0.49 nb−1 are used for the analysis. The Z boson yield, normalised by the total number of minimum-bias events and the mean nuclear thickness function, is measured as a function of dilepton rapidity and event centrality. The measurements in Pb+Pb collisions are compared with similar measurements made in proton–proton collisions at the same centre-of-mass energy. The nuclear modification factor is found to be consistent with unity for all centrality intervals. The results are compared with theoretical predictions obtained at next-to-leading order using nucleon and nuclear parton distribution functions. The normalised Z boson yields in Pb+Pb collisions lie 1–3σ above the predictions. The nuclear modification factor measured as a function of rapidity agrees with unity and is consistent with a next-to-leading-order QCD calculation including the isospin effect.

Probing parton saturation with forward Z0-boson production at small transverse momentum in p+p and p+A collisions

We calculate and compare the differential cross sections for forward Z0-boson production at small transverse momentum, in proton-proton and proton-nucleus collisions, using both the collinear and dilute-dense factorization frameworks. In both cases, we implement a Sudakov resummation of the large logarithms generated by soft-gluon emissions, which is essential in order to describe the transverse momentum distribution of forward Z0 bosons measured at the Tevatron and the LHC. We further compute the nuclear modification factor in the dilute-dense framework, hoping to single out signals of saturation effects at small values of x. Our predictions are compared with those obtained in the collinear factorization framework, using two different nuclear parton distribution functions.

A New Heavy Flavor Program for the Future Electron-Ion Collider

The proposed high-energy and high-luminosity Electron–Ion Collider (EIC) will provide one of the cleanest environments to precisely determine the nuclear parton distribution functions (nPDFs) in a wide x–Q2 range. Heavy flavor production at the EIC provides access to nPDFs in the poorly constrained high Bjorken-x region, allows us to study the quark and gluon fragmentation processes, and constrains parton energy loss in cold nuclear matter. Scientists at the Los Alamos National Laboratory are developing a new physics program to study heavy flavor production, flavor tagged jets, and heavy flavor hadron-jet correlations in the nucleon/nucleus going direction at the future EIC. The proposed measurements will provide a unique way to explore the flavor dependent fragmentation functions and energy loss in a heavy nucleus. They will constrain the initial-state effects that are critical for the interpretation of previous and ongoing heavy ion measurements at the Relativistic Heavy Ion Collider and the Large Hadron Collider. We show an initial conceptual design of the proposed Forward Silicon Tracking (FST) detector at the EIC, which is essential to carry out the heavy flavor measurements. We further present initial feasibility studies/simulations of heavy flavor hadron reconstruction using the proposed FST.

Open-source QCD analysis of nuclear parton distribution functions at NLO and NNLO

We present new sets of nuclear parton distribution functions (nPDFs) at next-to-leading order (NLO) and next-to-next-to-leading order (NNLO). 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.

Measurement of W^pm boson production in Pb+Pb collisions at sqrt{s_{mathrm{NN}}} = 5.02~text {Te}text {V} with the ATLAS detector

A measurement of W^pm boson production in Pb+Pb collisions at sqrt{s_mathrm {NN}} = 5.02~text {Te}text {V} is reported using data recorded by the ATLAS experiment at the LHC in 2015, corresponding to a total integrated luminosity of 0.49;mathrm {nb^{-1}}. The W^pm bosons are reconstructed in the electron or muon leptonic decay channels. Production yields of leptonically decaying W^pm bosons, normalised by the total number of minimum-bias events and the nuclear thickness function, are measured within a fiducial region defined by the detector acceptance and the main kinematic requirements. These normalised yields are measured separately for W^+ and W^- bosons, and are presented as a function of the absolute value of pseudorapidity of the charged lepton and of the collision centrality. The lepton charge asymmetry is also measured as a function of the absolute value of lepton pseudorapidity. In addition, nuclear modification factors are calculated using the W^pm boson production cross-sections measured in pp collisions. The results are compared with predictions based on next-to-leading-order calculations with CT14 parton distribution functions as well as with predictions obtained with the EPPS16 and nCTEQ15 nuclear parton distribution functions. No dependence of normalised production yields on centrality and a good agreement with predictions are observed for mid-central and central collisions. For peripheral collisions, the data agree with predictions within 1.7 (0.9) standard deviations for W^- (W^+) bosons.