Nuclear EMC Effect Research Articles

Electromagnetic form factors for nucleons in short-range correlations

Recent experimental studies have led to the suggestion that short-range correlations may be a major contributor to the nuclear EMC effect. This hypothesis requires that the structure function for nucleons involved in short-range correlations should be heavily suppressed compared to that of a free nucleon. Based on calculations performed within an AdS/QCD motivated, light-front quark-diquark model, we find that this large suppression of the nucleon structure function leads to a strong suppression of the nucleon elastic form factors.

The European Muon Collaboration effect from short-range correlated nucleons in a x-rescaling model* *Supported by the National Natural Science Foundation of China (12005266) and the Strategic Priority Research Program of Chinese Academy of Sciences (XDB34030301). N.-N. Ma is supported by the National Natural Science Foundation of China (12105128). T.-F. Wang is supported by the National Natural Science Foundation of China

In this paper, we examine the hypothesis that the nuclear EMC effect arises merely from the N-N SRC pairs inside the nucleus and that the properties of the N-N SRC pair are universal among the various nuclei, using the conventional x-rescaling model for the EMC effect. With the previously determined effective mass of the short-range correlated nucleon and the number of N-N SRC pairs estimated, we calculated the EMC effect of various nuclei within the x-rescaling approach. According to our calculations, the nuclear EMC effect due to the mass deficits of the SRC nucleons is not sufficient to reproduce the observed EMC effect in experiments. We speculate that the internal structure of the mean-field single nucleon is also clearly modified. Alternatively, there can be more origins of the EMC effect beyond the N-N SRC configuration (such as the α cluster), or the universality of N-N SRC pair is significantly violated from light to heavy nuclei.

Impact of EMC effect on D meson modification factor in equilibrating QGP

In this article we employ the nuclear EMC effect to extract the parton distribution functions (PDFs) inside the Lead (Pb) and Gold (Au) nuclei. Extracted PDFs are utilized to obtain the transverse momentum dependent (TMD) ones, using the computing codes like Pythia 8 or MCFM-10. Through this procedure TMDPDFs for charm and bottom quarks in Au at $\sqrt{s_{NN}}=200\;GeV$, Pb at $\sqrt{s_{NN}}=2.76\;TeV$ and $\sqrt{s_{NN}}=5.02\;TeV$ are calculated. To evaluate the validity of results and investigate the influence of nuclear EMC effect, the numerated TMDs are used as input to estimate heavy quark modification factor $R_{AA}$ at transverse plane $P_T$. This observable is calculated through numerical solution of the Fokker-Planck equation. For this purpose we need to extract the drag and diffusion coefficients, using the hard thermal loop correction. It is done in the frame work of the relativistic hydrodynamics up to the third order approximation of gradient expansion. The results are compared with same solutions when the input PFDs are considered inside the unbounded protons where the nuclear effect is not included. The comparison indicates a significant improvement of computed $R_{AA}$ with available experimental data when the EMC effect is considered.

Isovector EMC Effect from Global QCD Analysis with MARATHON Data

We report the results of a MonteCarlo global QCD analysis of unpolarized parton distribution functions (PDFs), including for the first time constraints from ratios of ^{3}He to ^{3}H structure functions recently obtained by the MARATHON experiment at Jefferson Lab. Our simultaneous analysis of nucleon PDFs and nuclear effects in A=2 and A=3 nuclei reveals the first indication for an isovector nuclear EMC effect in light nuclei. We find that while the MARATHON data yield relatively weak constraints on the F_{2}^{n}/F_{2}^{p} neutron to proton structure function ratio and on the d/u PDF ratio, they suggest an enhanced nuclear effect on the d-quark PDF in the bound proton, questioning the assumptions commonly made in nuclear PDF analyses.

Short-range correlations and the nuclear EMC effect in deuterium and helium-3

The EMC effect in deuterium and helium-3 is studied using a convolution formalism that allows isolating the impact of high-momentum nucleons in short-ranged correlated (SRC) pairs. We assume that the modification of the structure function of bound nucleons is given by a universal (i.e. nucleus independent) function of their virtuality, and find that the effect of such modifications is dominated by nucleons in SRC pairs. This SRC-dominance of nucleon modifications is observed despite the fact that the bulk of the nuclear inelastic scattering cross-section comes from interacting with low-momentum nucleons. These findings are found to be robust to model details including nucleon modification function parametrization, free nucleon structure function and treatment of nucleon motion effects. While existing data cannot discriminate between such model details, we present predictions for measured, but not yet published, tritium EMC effect and tagged nucleon structure functions in deuterium that are sensitive to the neutron structure functions and bound nucleon modification functions.

Do Short-Range Correlations Cause the Nuclear EMC Effect in the Deuteron?

The relative contributions to the valence nuclear European Muon Collaboration (EMC) effect in the deuteron arising from nucleon off-shell effects and Fermi motion are examined in models which include nuclear binding and off-shell effects. Contrary to expectations, the effect of Fermi motion overwhelms the off-shell effects for nucleons in short-range correlations (SRCs), calling into question the hypothesized causal connection between SRCs and the EMC effect.

Polarized EMC effect in the QMC model

The ratios of the in-medium to free nucleon structure functions for the unpolarized and polarized cases are obtained using the MIT bag model for the free case, along with the QMC model to incorporate the in-medium modifications of the structure functions. As discussed in earlier work, the observed nuclear EMC effect is reasonably well described. This gives us confidence to investigate the predictions of the model for the polarized EMC effect, the ratio g1⁎(x)/g1(x) for a bound proton to that of a free proton. This ratio is found to be substantially different from unity and very similar in shape and size to that found in the unpolarized case. This prediction of such a fundamental change in the valence structure of a bound nucleon needs to be tested experimentally at the earliest opportunity.

Nuclear EMC effect through Λ¯/Λ production in semi-inclusive deep-inelastic scattering processes

We calculate $\mathrm{\ensuremath{\Lambda}}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}$ hadron production cross sections in charged lepton semi-inclusive deep-inelastic scattering off nuclear target ($A$, using iron $\mathrm{Fe}$ as an example) and deuteron ($D$) target. The results show that the ratio $({\overline{\mathrm{\ensuremath{\Lambda}}}}^{A}/{\mathrm{\ensuremath{\Lambda}}}^{A})/({\overline{\mathrm{\ensuremath{\Lambda}}}}^{D}/{\mathrm{\ensuremath{\Lambda}}}^{D})$ is sensitive to the sea quark content of the nucleus. We adopt three different models to take the nuclear EMC effect into account. The ratio $({\overline{\mathrm{\ensuremath{\Lambda}}}}^{A}/{\mathrm{\ensuremath{\Lambda}}}^{A})/({\overline{\mathrm{\ensuremath{\Lambda}}}}^{D}/{\mathrm{\ensuremath{\Lambda}}}^{D})$ is predicted to be different by these different models.

Measurement of the structure function of the nearly free neutron using spectator tagging in inelasticH2(e,e′ps)Xscattering with CLAS

Much less is known about neutron structure than that of the proton due to the absence of free neutron targets. Neutron information is usually extracted from data on nuclear targets such as deuterium, requiring corrections for nuclear binding and nucleon off-shell effects. These corrections are model dependent and have significant uncertainties, especially for large values of the Bjorken scaling variable x. The Barely Off-shell Nucleon Structure (BONuS) experiment at Jefferson Lab measured the inelastic electron deuteron scattering cross section, tagging spectator protons in coincidence with the scattered electrons. This method reduces nuclear binding uncertainties significantly and has allowed for the first time a (nearly) model independent extraction of the neutron structure function. A novel compact radial time projection chamber was built to detect protons with momentum between 70 and 150 MeV/c. For the extraction of the free neutron structure function $F_{2n}$, spectator protons at backward angle and with momenta below 100 MeV/c were selected, ensuring that the scattering took place on a nearly free neutron. The scattered electrons were detected with Jefferson Lab's CLAS spectrometer. The extracted neutron structure function $F_{2n}$ and its ratio to the deuteron structure function $F_{2d}$ are presented in both the resonance and deep inelastic regions. The dependence of the cross section on the spectator proton momentum and angle is investigated, and tests of the spectator mechanism for different kinematics are performed. Our data set can be used to study neutron resonance excitations, test quark hadron duality in the neutron, develop more precise parametrizations of structure functions, as well as investigate binding effects (including possible mechanisms for the nuclear EMC effect) and provide a first glimpse of the asymptotic behavior of d/u as x goes to 1.

Constraints on the large-xd/uratio from electron-nucleus scattering atx>1

Recently the ratio of neutron to proton structure functions ${F}_{2}^{n}/{F}_{2}^{p}$ was extracted from a phenomenological correlation between the strength of the nuclear EMC effect and inclusive electron--nucleus cross section ratios at $x>1$. Within conventional models of nuclear smearing, this ``in-medium correction'' (IMC) extraction constrains the size of nuclear effects in the deuteron structure functions, from which the neutron structure function ${F}_{2}^{n}$ is usually extracted. The IMC data determine the resulting proton $d/u$ quark distribution ratio, extrapolated to $x=1$, to be $0.23\ifmmode\pm\else\textpm\fi{}0.09$ with a 90% confidence level. This is well below the SU(6) symmetry limit of $1/2$ and significantly above the scalar diquark dominance limit of 0.

Light flavor asymmetry of nucleon sea

The light flavor antiquark distributions of the nucleon sea are calculated in the effective chiral quark model and compared with experimental results. The contributions of the flavor-symmetric sea-quark distributions and the nuclear EMC effect are taken into account to obtain the ratio of Drell–Yan cross sections σ pD/2σ pp, which can match well with the results measured in the FermiLab E866/NuSea experiment. The calculated results also match the $\bar{d}(x)-\bar{u}(x)$ measured in different experiments, but unmatch the behavior of $\bar{d}(x)/\bar{u}(x)$ derived indirectly from the measurable quantity σ pD/2σ pp by the FermiLab E866/NuSea Collaboration at large x. We suggest to measure again $\bar{d}(x)/\bar{u}(x)$ at large x from precision experiments with careful treatment of the experimental data. We also propose an alternative procedure for experimental data treatment.

Structure functions for light nuclei

We discuss the nuclear EMC effect with particular emphasis on recent data for light nuclei including $^{2}\mathrm{H}$, $^{3}\mathrm{He}$, $^{4}\mathrm{He}$, $^{9}\mathrm{Be}$, $^{12}\mathrm{C}$, and $^{14}\mathrm{N}$. In order to verify the consistency of available data, we calculate the ${\ensuremath{\chi}}^{2}$ deviation between different data sets. We find a good agreement between the results from the NMC, SLAC E139, and HERMES experiments. However, our analysis indicates an overall normalization offset of about $2%$ in the data from the recent JLab E03-103 experiment with respect to previous data for nuclei heavier than $^{3}\mathrm{He}$. We also discuss the extraction of the neutron/proton structure function ratio ${F}_{2}^{n}/{F}_{2}^{p}$ from the nuclear ratios $^{3}\mathrm{He}$/$^{2}\mathrm{H}$ and $^{2}\mathrm{H}$/$^{1}\mathrm{H}$. Our analysis shows that the E03-103 data on $^{3}\mathrm{He}$/$^{2}\mathrm{H}$ require a renormalization of about $3%$ in order to be consistent with the ${F}_{2}^{n}/{F}_{2}^{p}$ ratio obtained from the NMC experiment. After such a renormalization, the $^{3}\mathrm{He}$ data from the E03-103 and HERMES experiments are in a good agreement. Finally, we present a detailed comparison between data and model calculations, which include a description of the nuclear binding, Fermi motion, and off-shell corrections to the structure functions of bound proton and neutron, as well as the nuclear pion and shadowing corrections. Overall, a good agreement with the available data for all nuclei is obtained.

Parton distribution functions and nuclear EMC effect in a statistical model

A new and simple statistical approach is performed to calculate the parton distribution functions (PDFs) of the nucleon in terms of light-front kinematic variables. Analytic expressions of x-dependent PDFs are obtained in the whole x region. And thereafter, we treat the temperature T as a parameter of the atomic number A to explain the nuclear EMC effect in the region x ∊[0.2,0.7]. We give the predictions of PDF ratios, and they are very different from those by other models, thus experiments aiming at measuring PDF ratios are suggested to provide a discrimination of different models.

Nuclear EMC effect in a statistical model

A simple statistical model in terms of light-front kinematic variables is used to explain the nuclear EMC effect in the range x ∈ [ 0.2 , 0.7 ] , which was constructed by us previously to calculate the parton distribution functions (PDFs) of the nucleon. Here, we treat the temperature T as a parameter of the atomic number A, and get reasonable results in agreement with the experimental data. Our results show that the larger A, the lower T thus the bigger volume V, and these features are consistent with other models. Moreover, we give the predictions of the quark distribution ratios, i.e., q A ( x ) / q D ( x ) , q ¯ A ( x ) / q ¯ D ( x ) , and s A ( x ) / s D ( x ) , and also the gluon ratio g A ( x ) / g D ( x ) for iron as an example. The predictions are different from those by other models, thus experiments aiming at measuring the parton ratios of anti-quarks, strange quarks, and gluons can provide a discrimination of different models.

Implications of the nuclear EMC effect

The discovery more than twenty years ago, by the EMC Collaboration, that the deep-inelastic-scattering DIS structure functions are influenced by the nuclear environment stunned the nuclear physics community and brought quarks and gluons into the field with great impact. A great length of time has passed, but despite a semi-infinite number of papers on the subject, there is no explanation that is universally accepted. Many models (related in one way or another to QCD) have been successful in reproducing data for deep inelastic scattering on nuclear targets, but fewer have described both the DIS and nuclear Drell-Yan experiments. Although there are some positive indications, no model has been used to predict correctly and unambiguously new independent phenomena. We review the history and discuss the best experimental prospects for future discovery.

Chiral solitons in nuclei: saturation, EMC effect, and Drell-Yan experiments.

The chiral quark-soliton model of the nucleon contains a mechanism for an attractive interaction between nucleons. This, along with the exchange of vector mesons between nucleons, is used to compute the saturation properties of infinite nuclear matter. This provides a new way to assess the effects of the nuclear medium on a nucleon that includes valence and sea quarks. We show that the model simultaneously describes the nuclear EMC effect and the related Drell-Yan experiments.

Quark-hadron duality and the nuclear EMC effect

Recent data on polarized proton knockout reactions off He-4 nuclei suggest a small but nonzero modification of proton electromagnetic form factors in medium. Using model independent relations derived on the basis of quark-hadron duality, we relate the medium modification of the form factors to the modification at large x of the deep-inelastic structure function of a bound proton. This places strong constraints on models of the nuclear EMC effect which assume a large deformation of the intrinsic structure of the nucleon in medium.

Quark-hadron duality and the nuclear EMC effect

Recent data on polarized proton knockout reactions off 4He nuclei suggest a small but nonzero modification of proton electromagnetic form factors in medium. Using model-independent relations derived on the basis of quark-hadron duality, we relate the medium modification of the form factors to the modification at large x of the deep-inelastic structure function of a bound proton. This places strong constraints on models of the nuclear EMC effect which assume a large deformation of the intrinsic structure of the nucleon in medium.

Effect of non-perturbative QCD in the structure functions at small x and at large x in all Q2

Effect of non-perturbative QCD in the nucleon structure function is discussed based on our recent works to obtain a universal mass scale in non-perturbative effect of QCD through a fit to the structure functions in a wide kinematic range including the region at small x and at large x in all Q 2. The physical consequences are then discussed in relation to the nuclear EMC-effect at small x as well as to the local duality with pomerons in hadron dynamics.

Probing the origin of the EMC effect via tagged structure functions of the deuteron

We demonstrate that measurement of tagged structure functions of the deuteron in (e, e′N) semi-inclusive reactions can discriminate between different hypotheses on the origin of the nuclear EMC effect. By choosing extreme backward kinematics for the spectator nucleon to minimize effects from the deuteron wave function and final state interactions, one can isolate the modifications in the structure of the bound nucleon within the impulse approximation. The same reaction can be used to extract the large-x neutron to proton structure function ratio.