Parton Distributions In Nuclei Research Articles

Hunting for an EMC–like effect for antiquarks

We argue that the Drell–Yan process in the xA≥0.15 kinematics recently studied at FNAL by the E906/SeaQuest experiment may allow to observe an analogous of the EMC effect for antiquarks. The effects of Fermi motion and energy loss are considered. The preliminary E906/SeaQuest data are inconsistent with the growth of the σA/AσN ratio expected in the Fermi motion scenario at xA≥0.25. The pattern of the xA dependence of the ratio seems also inconsistent with a scenario in which the dominant nuclear effect is a suppression of the cross section due to the energy loss experienced by a quark of the projectile proton involved in the Drell–Yan process. All together the data suggest the possibility of a modification of the antiquark parton distributions in nuclei, with a pattern similar to the one observed in the EMC effect. We argue that optimal kinematics to look for an antiquark EMC–like effect would be to measure the σADY/AσNDY ratios for xA = 0.2 – 0.4 and xp ≈ constant.

Imaging nuclear modifications on parton distributions with triple-differential dijet cross sections in proton-nucleus collisions

Dijet production in proton-nucleus ($p$A) collisions at the LHC provides invaluable information on the underlying parton distributions in nuclei, especially the gluon distributions. Triple-differential dijet cross sections enable a well-controlled kinematic scan (over momentum fraction $x$ and probing scale $Q^2$) of the nuclear parton distribution functions (nPDFs), i.e., $f^\textrm{A}_i(x,Q^2)$. In this work, we study several types of triple-differential cross sections for dijet production in proton-proton ($pp$) and proton-lead ($p$Pb) collisions at the LHC, to next-to-leading order within the framework of perturbative quantum chromodynamics (pQCD). Four sets of nPDF parametrizations, EPPS16, nCTEQ15, TUJU19, and nIMParton16 are employed in the calculations for $p$Pb collisions. We show that the observable nuclear modification factor $R_{p\textrm{Pb}}$ of such cross sections can serve as a nice image of the nuclear modifications on parton distributions, quantified by the ratio $r^{\textrm{A}}_i(x,Q^2)\!=\!f^\textrm{A,proton}_i(x,Q^2)/f^\textrm{proton}_i(x,Q^2)$. Considerable differences among the $R_{p\textrm{Pb}}$ predicted by the four nPDF sets can be observed and intuitively understood. Future measurements of such observables are expected to not only constrain the nPDF parametrizations, but also help confirm various nuclear effects, e.g., shadowing, anti-shadowing, EMC, and Fermi motion in different regions of $x$ and their variations with probing scale $Q^2$.

Probing superfast quarks in nuclei through dijet production at the LHC

We investigate dijet production from proton-nucleus collisions at the Large Hadron Collider (LHC) as a means for observing superfast quarks in nuclei with Bjorken $x>1$. Kinematically, superfast quarks can be identified through directly measurable jet kinematics. Dynamically, their description requires understanding several elusive properties of nuclear QCD, such as nuclear forces at very short distances, as well as medium modification of parton distributions in nuclei. In the present work, we develop a model for nuclear parton distributions at large $x$ in which the nuclear dynamics at short distance scales are described by two- and three-nucleon short range correlations (SRCs). Nuclear modifications are accounted for using the color screening model, and an improved description of the EMC effect is reached by using a structure function parametrization that includes higher-twist contributions. We apply QCD evolution at the leading order to obtain nuclear parton distributions in the kinematic regime of the LHC, and based on the obtained distributions calculate the cross section for dijet production. We find not only that superfast quarks can be observed at the LHC, but also that they provide sensitivity to the practically unexplored three-nucleon SRCs in nuclei. Additionally, the LHC can extend our knowledge of the EMC effect to large $Q^2$ where higher-twist effects are negligible.

Unintegrated parton distributions in nuclei

We study how unintegrated parton distributions in nuclei can be calculated from the corresponding integrated partons using the EPS09 parametrization. The role of nuclear effects is presented in terms of the ratio $R^A=\text{uPDF}^A/A\cdot \text{PDF}^N$ for both large and small $x$ domains.

Conceptual design and performance simulations of super-compact electromagnetic calorimeter

Measurements of particle production at forward rapidities in high energy p-p, p-A and A-A collisions provide access to physics processes at very low Bjorken x. These measurements will allow to study the gluon saturation scale and improve our knowledge of parton distribution in nuclei. Specific requirements must be fulfilled for a calorimeter to successfully operate in high-multiplicity forward region within often stringent space limits. Here we present a study of a conceptual design of super-compact electromagnetic calorimeter being developed at Czech Technical University in Prague. The design of the sampling calorimeter is based on a sandwich structure of thin tungsten and scintillator layers oriented in parallel to the beam. Used optical readout of individual scintillator pads guaranties the required high radiation hardness of the detector. We present simulation of the expected performance of the optical pad readout together with overall detector performance. It is aimed for the detector to allow measuring of high energy photons (1<E<300 GeV) in forward rapidities (2.5 < η < 4).

Nuclear parton densities and their uncertainties at the next-to-leading order

Considering the nuclear reactions which involve specific features at certain regions of the $x$-Bjorken variable which was first observed by the EMC group, we indicate that the parton distributions in nuclei are not simply as the parton densities in nucleons. In addition to the most commonly analyzed data sets for deep-inelastic scattering of charged leptons off nuclei, we also analyze the Drell-Yan dilepton production. We investigate parametrizations of nuclear parton distributions at the next-to-leading order (NLO) of ${\ensuremath{\alpha}}_{s}$. Finally, optimum nuclear structure functions are determined by a ${\ensuremath{\chi}}^{2}$ analysis of experimental data for the nuclear structure function ${F}_{2}^{A}$/${F}_{2}^{{A}^{\ensuremath{'}}}$ and Drell-Yan cross-section ratios. The related uncertainties are estimated by the Hessian method. Our results are in good agreement with the available experimental data and better than the results of some other fitting parametrization methods.

Cold nuclear matter effects on dijet productions in relativistic heavy-ion reactions at LHC

We investigate the cold nuclear matter (CNM) effects on dijet productions in high-energy nuclear collisions at LHC with the next-to-leading order perturbative QCD. The nuclear modifications for dijet angular distributions, dijet invariant mass spectra, dijet transverse momentum spectra and dijet momentum imbalance due to CNM effects are calculated by incorporating EPS, EKS, HKN and DS parametrization sets of parton distributions in nucleus. It is found that dijet angular distributions and dijet momentum imbalance are insensitive to the initial-state CNM effects and thus provide optimal tools to study the final-state hot QGP effects such as jet quenching. On the other hand, the invariant mass spectra and the transverse momentum spectra of dijet are generally enhanced in a wide region of the invariant mass or transverse momentum due to CNM effects with a feature opposite to the expected suppression because of the final-state parton energy loss effect in the QGP. The difference of EPS, EKS, HKN and DS parametrization sets of nuclear parton distribution functions is appreciable for dijet invariant mass spectra and transverse momentum spectra at p+Pb collisions, and becomes more pronounced for those at Pb+Pb reactions.

J/ψproduction in proton-induced collisions at energies available at the GSI Facility for Antiproton and Ion Research

We have examined the production of J/$\psi$ mesons in high energy proton-proton and proton-nucleus collisions at beam energies in the range from 158 GeV to 920 GeV, available from different fixed target experiments. In the employed model J/$\psi$ production in hadronic collisions is assumed to be a factorisable two step process: (i) production of a $c\bar{c}$ pair which can be reliably described by perturbative QCD, and (ii) formation of J/$\psi$ resonance from the $c\bar{c}$ pair, which can be conveniently parameterized incorporating different existing physical mechanisms of color neutralization. We show that, for lower collision energies, J/$\psi$ production through quark-anti-quark annihilation gives larger contribution at higher $x_F$, while gluon-gluon fusion dominates the production at smaller $x_F$. For proton-nucleus collisions the model takes into account both the initial state modification of parton distributions in nuclei and the final state interaction of the produced $c\bar{c}$ pairs with the target nucleons. The model is found to give reasonable description of data on J/$\psi$ production in protonic and proton-nucleus collisions, for different existing fixed target experiments. In case of proton-nucleus collisions, our calculations show a non-negligible dependence of the final state nuclear dissociation of J/$\psi$ mesons on the energy of the incident proton beam. The model has been applied to predict the J/$\psi$ production and suppression expected in proton-nucleus collisions at energies relevant to FAIR, the upcoming accelerator facility at Darmstadt, Germany. The amount of suppressions, for different mechanisms of J/$\psi$ hadronization has been found to be distinguishably different which might help an experimental settlement of the much controversial issue of color neutralization.

Photon parton distributions in nuclei and the EMC effect

Photons, as well as quarks and gluons, are constituents of the infinite momentum frame (IMF) wave function of an energetic particle. They are mostly equivalent photons whose amplitude follows from the Lorentz transformation of the particle rest frame Coulomb field into the IMF and from the conservation of the electromagnetic current. We evaluate in a model independent way the dominant photon contribution $\ensuremath{\propto}{\ensuremath{\alpha}}_{\mathrm{em}}({Z}^{2}/{A}^{4/3})\mathrm{ln}(1/{R}_{A}{m}_{N}x)$ to the nuclear structure functions as well as the term $\ensuremath{\propto}{\ensuremath{\alpha}}_{\mathrm{em}}Z/A$. In addition we show that the definition of $x$ consistent with the exact kinematics of $\mathit{eA}$ scattering (with exact sum rules) works in the same direction as the nucleus field of equivalent photons. Combined, these effects account for the bulk of the hadronic European Muon Collaboration (EMC) effect for $x\ensuremath{\leqslant}0.5$ where Fermi motion effects are small. In particular, for these $x$ the hadronic mechanism contribution to the EMC effect does not exceed $~3%$ for all nuclei. Also, the $A$ dependence of the hadronic mechanism of the EMC effect for $x&gt;0.5$ is significantly modified.

Unitarity bound for gluon shadowing

Although at small Bjorken x gluons originated from different nucleons in a nucleus overlap in the longitudinal direction, most of them are still well separated in the transverse plane, therefore cannot fuse. For this reason the gluon density in nuclei cannot drop at small x below a certain bottom bound, which we evaluated in a model independent manner assuming the maximal strength of gluon fusion. We also calculated gluon shadowing in the saturated regime using on the Balitsky-Kovchegov equation, and found the nuclear ratio to be well above the unitarity bound. The recently updated analysis of parton distributions in nuclei \cite{eps08} including RHIC data on high-pT hadron production at forward rapidities, led to astonishingly strong gluon shadowing, which is far beyond the unitarity bound. This indicates a misconception in the interpretation of the nuclear suppression observed at HRIC.

THE NUCLEON STRUCTURE AND THE EQUATION OF STATE FOR NUCLEAR MATTER

We show the density dependent corrections to the nucleon structure function in the frame of nuclear Relativistic Mean Field (RMF) models. These corrections are connected with the modifications of the parton distribution in nuclei emerging from generalized nuclear Fermi motion and final state interactions between the nucleon and the rest of the nucleus. The medium effects concern the nucleon structure, namely the changes in the nucleon rest energy, the enhancement of sea quark contribution (simulated with "nuclear pions") and the modifications of the transverse parton momentum distribution inside Nuclear Matter (NM). The sea parton distributions are described by the modified cloud of virtual pions in order to saturate the nuclear energy-momentum sum rule. The description of theses features are in good agreement with experimental data; the EMC effect for x &gt; 0.15 and nuclear lepton pair production data has been described essentialy without free parameters. The influence of these medium modifications to the nucleon structure function within the equation of state in RMF models of NM will be discussed.

Single particle sum rules in the nuclear deep-inelastic region

We have modeled the parton distribution in nuclei using a suitably modified nuclear Fermi motion. The modifications concern the nucleon rest energy which changes the Bjoerken x in a nuclear medium. We also introduce final state interactions between the scattered nucleon and the rest of the nucleus. The energy-momentum sum rule is saturated. Good agreement with experimental data of the EMC effect for x>0.15 and nuclear lepton pair production data has been obtained.

Single particle sum rules and the EMC effect

We have modelled parton distribution in nuclei using a suitably modified nuclear Fermi motion. The modifications concern the nucleon rest energy which changes the Bj\"orken x in nuclear medium. We also introduce final state interactions between the scattered nucleon and the rest of the nucleus. The energy-momentum sum rule is saturated. Good agreement with experimental data of the EMC effect for $x> 0.15$ and nuclear lepton pair production data has been obtained.

How the nuclear Fermi motion plus a simple statistical model explains the EMC effect

We present calculation of influence caused by nucleon Fermi motion on the parton distributions in nuclei. Our approach is based on the model where momenta of valence partons have some primordial distribution inside the hadron at rest, which is either provided by a statistical considerations or calculated using spherically symmetric Gaussian distribution with a width derived from the Heisenberg uncertainty relation. The sea parton contribution emerges from the similar Gaussian distribution with a width dictated by the presence of virtual pions in hadron. We show that the influence of Fermi motion changes substantially the nucleonic structure function inside the nucleus in the right direction and therefore should be considered seriously in all attempts devoted to explain the experimentally observed EMC effect for x Bj > 0.1.

Multiple parton scattering in nuclei: parton energy loss

Multiple parton scattering and induced parton energy loss are studied in deeply inelastic scattering (DIS) off nuclei. The effect of multiple scattering of a highly off-shell quark and the induced parton energy loss is expressed in terms of the modification to the quark fragmentation functions. We derive such modified quark fragmentation functions and their QCD evolution equations in DIS using the generalized factorization of higher twist parton distributions. We consider double-hard and hard-soft parton scattering as well as their interferences in the same framework. The final result, which depends on both the diagonal and off-diagonal twist-four parton distributions in nuclei, demonstrates clearly the Landau–Pomeranchuk–Migdal interference features and predicts a unique nuclear modification of the quark fragmentation functions.

A model for the parton distribution in nuclei

We have extended recently proposed model of parton distributions in nucleons to the case of nucleons in nuclei.

Charmonium suppression from shadowing effects

The extend to which geometrical effects contribute to the production and suppression of the $J/\psi$ and $q\bar{q}$ minijet pairs in general is investigated for high energy heavy ion collisions at SPS, RHIC and LHC energies. For the energy range under investigation, the geometrical effects referred to are shadowing and anti-shadowing, respectively. Due to those effects, the parton distributions in nuclei deviate from the naive extrapolation from the free nucleon result; $f_{A}\neq A f_{N}$. The strength of the shadowing/anti-shadowing effect increases with the mass number. The consequences of gluonic shadowing effects for the $x_F$ distribution of $J/\psi$'s at $\sqrt s =20$ GeV, $\sqrt s =200$ GeV and $\sqrt s =6$ TeV are calculated for some relevant combinations of nuclei, as well as the $p_T$ distribution of minijets at midrapidity for $N_f=4$ in the final state.

Partonic picture of nuclear shadowing at small x

We find that coherent multiple scatterings of the partonic fluctuations of the virtual photon with the nucleons inside the nucleus result in the large nuclear shadowing effect for parton distributions in nuclei. We predict the gluon shadowing effect at small x and for Q 2 ≥ 3GeV 2.

Parton Distributions in Nuclei : Overview and Prospect

Parton Distributions in Nuclei : Overview and Prospect

Initial and final state effects in the charmonium suppression in hadron-nucleus and nucleus-nucleus collisions.

The suppression of the J/\ensuremath{\Psi} production is attributed to shadowing in the initial state parton distributions in nuclei due to multiquark cluster formation and to the dissociation of the final cc\ifmmode\bar\else\textasciimacron\fi{} state in the nuclear medium. The model agrees with the hadron-nucleus and nucleus-nucleus collision data and predicts enhancement of the effect at the Relativistic Heavy Ion Collider energies. Nuclear transparency at these energies is also considered and found to increase the suppression. \textcopyright{} 1996 The American Physical Society.