Change of confinement scale in nuclei and the EMC effect

We study a model in which the confinement scale of quarks in a nucleus of massA changes asA ^1/3. This explains theA dependence of structure functions (emc effect) as seen in muon, electron and antineutrino deep inelastic scattering from nuclear targets. We also investigate a prediction of this model—anA dependence of theqcd scale parameter A.     

Color screening and deconfinement for bound states of heavy quarks

We study the binding and deconfinement of heavy quarks in a thermal environment, using a non-relativistic confinement potential model with color screening. As a result, we obtain the dependence of the dissociation energies, the binding radii and the masses of heavy quark resonances (charmonium and bottonium states) on the color screening lengthr _D of the medium, and we determine for the different resonances those values ofr _D below which no more binding is possible. Finally, we consider the implication of our results on resonance suppression as signal for deconfinement.     

EMC and polarized EMC effects in nuclei

We determine nuclear structure functions and quark distributions for ⁷Li, ¹¹B, ¹⁵N and ²⁷Al. For the nucleon bound state we solve the covariant quark–diquark equations in a confining Nambu–Jona-Lasinio model, which yields excellent results for the free nucleon structure functions. The nucleus is described using a relativistic shell model, including mean scalar and vector fields that couple to the quarks in the nucleon. The nuclear structure functions are then obtained as a convolution of the structure function of the bound nucleon with the light-cone nucleon distributions. We find that we are readily able to reproduce the EMC effect in finite nuclei and confirm earlier nuclear matter studies that found a large polarized EMC effect.     

An overview of quarks in nuclei

In this report, the relevance and possible manifestations of quark-gluon degrees of freedom in nuclear systems are discussed. An attempt is made to view the subject in the context of the modern theory of fundamental interactions based on the concept of local gauge invariance. How this gives rise to the quantum chromodynamics of colored quarks and gluons is recounted. An introduction is given to the concepts of asymptotic freedom, color confinement, lattice gauge theory, and chiral symmetry. The nature, the successes and limitations of phenomenological quark models of hadrons are described. The possible appearance of exotic hadrons, and of quark effects in nuclear forces, in high-energy nuclear reactions and in nuclear properties are discussed. The expected properties and possible production and detection of unusual quark-gluon phases of matter are reviewed. Both the experimental detection and the theoretical treatment of quark effects in nuclei appear difficult, but the question of how nuclear physics fits into the panorama which is the modern theory of fundamental interactions appears to represent a worthwhile intellectual challenge.     

Energy loss of fast quarks in nuclei

We report an analysis of the nuclear dependence of the yield of Drell-Yan dimuons from the 800 GeV/c proton bombardment of 2H, C, Ca, Fe, and W targets. Employing a new formulation of the Drell-Yan process in the rest frame of the nucleus, this analysis examines the effect of initial-state energy loss and shadowing on the nuclear-dependence ratios versus the incident proton's momentum fraction and dimuon effective mass. The resulting energy loss per unit path length is -dE/dz = 2.32+/-0.52+/-0.5 GeV/fm. This is the first observation of a nonzero energy loss of partons traveling in a nuclear environment.     

Chiral symmetry restoration and deconfinement in the contact interaction model of quarks with parallel electric and magnetic fields

We study the impact of steady,homogeneous,and external parallel electric and magnetic field strengths(eE ‖ eB) on the chiral symmetry breaking-restoration and confinement-deconfinement phase transition.We also sketch the phase diagram of quantum chromodynamics(QCD) at a finite temperature T and in the presence of background fields.The unified formalism for this study is based on the Schwinger-Dyson equations,symmetry preserving vector-vector contact interaction model of quarks,and an optimal time regularization scheme.At T=0,in the purely magnetic case(i.e.,eE→0),we observe the well-known magnetic catalysis effect.However,in a pure electric field background(eB→0),the electric field tends to restore the chiral symmetry and deconfinement above the pseudo-critical electric field eE_c~(χ,C).In the presence of both eE and eB,we determine the magnetic catalysis effect in the particular region where eB dominates over eE,whereas we observe the chiral inhibition(or electric chiral rotation) effect when eE overshadows eB.At finite T,in the pure electric field case,the phenomenon of inverse electric catalysis appears to exist in the proposed model.Conversely,for a pure magnetic field background,we observe the magnetic catalysis effect in the mean-field approximation and inverse magnetic catalysis with eB-dependent coupling.The combined effects of eE and eB on the pseudo-critical T_c~(χ,C) yields an inverse electromagnetic catalysis,with and without an eB-dependent effective coupling of the model.The findings of this study agree well with the already predicted results obtained via lattice simulations and other reliable effective models of QCD.     

Quark deconfinement in antiproton annihilation at rest on light nuclei

The production of low-energy antimatter provides unique opportunities to search for new physics in an unexplored regime. Testing gravitational interactions with antimatter is one such opportunity. Here a scenario based on Lorentz and CPT violation in the Standard-Model Extension is considered in which anomalous gravitational effects in antimatter could arise.     

From ‘mesons in nuclei’ to ‘quarks in nuclei’

The mesonic aspects of nuclear magnetic moments are discussed with the author's personal proximity and reminiscence. It is emphasized that the observed anomaly in orbitalg factors, traditionally related to the meson exchange effect, may have to be reconsidered from the viewpoint of quark structure of nuclei.     

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.     

Density of quarks in heavy spherical nuclei using NRQSM

A nonrelativistic quark shell model (NRQSM) is used to derive an expression for the density of quarks in heavy spherical nuclei. It is shown that quark density is related in a simple way with the probability of finding a nucleon in a nucleus. The quark density is used to determine the ratio of average distance between two quarks to the average distance between two nucleons.     

Opportunities with Drell-Yan scattering: Probing sea quarks in the nucleon and nuclei

The large ˉ(x)/ˉ(x) ratio observed by Fermilab E866/NuSea convincingly demonstrated that the sea is not simply a result of pQCD. Moreover, meson cloud models also failed to explain fully the observed kinematic dependence. The Drell-Yan mechanism offers a unique, selective probe of antiquarks in the nucleon. Fermilab has approved a new Drell-Yan experiment, E906, that will exploit this feature to probe ˉ(x)/ˉ(x) by measuring the ratio of cross-sections for the proton-induced Drell-Yan process on hydrogen to deuterium. When the nucleon is contained in a nucleus, the nucleon's parton distributions should to be modified; although this effect was not seen in the sea quark distributions obtained by Fermilab E772 with Drell-Yan scattering. The upcoming E906 Drell-Yan experiment will provide much more precise measurements over a wider kinematic range in order to guide and challenge the theoretical models.     

On the EMC effect

A simple model of deep inelastic scattering of nuclei is discussed. It is argued that at least a substantial part of the EMC effect reflects the physics on hadronic rather than quark level.Invited talk presented at Hamburg University on 4 December 1986.     

Origins of the EMC effect

The conventional nuclear (pion) and dynamical rescaling models of nuclear effects in deep inelastic scattering give similar results. We explain this, compare the two models, and find that dynamical rescaling mimics binding effects in the conventional model.     

Partons and the EMC spin effect

We focus on the parton model and the role of the axial anomaly in polarised deep inelastic scattering. We show that the axial anomaly is relevant to each of the higher moments of the spin dependent structure functiong ₁ (x) and not just the first moment. This result implies that the factorisation of mass singularities is not sufficient to define the parton model in spin dependent QCD. (It is certainly a necessary condition.) We also need to consider the locality of the photon parton interaction. The anomaly is observed over allx in the EMCg ₁ (x) data.     

Thermodynamical analysis of the EMC effect

Assuming that the sea quark distribution vanishes for x > 0.3, we analyse the F₂^Fe(x, Q²) and F₂^D(x, Q²) structure functions measured by the European Muon Collaboration in the framework of a thermodynamical model of the valence quarks. The experimental ratio $\text{F}{}_2{}^{\mathrm{<mtext>Fe</mtext>}}\text{(x)}\text{F}{}_2{}^{\mathrm{<mtext>D</mtext>}}\text{(x)}$ is well reproduced over the whole x range by the ratio of two valence quark distributions at different temperatures T and confinement volumes V. We obtain T_D?T_Fe≈3 MeV and $\text{V}{}_{\mathrm{<mtext>Fe</mtext>}}\text{V}{}_{\mathrm{<mtext>D</mtext>}}\text{≈ 1.3}$.     

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.     

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.     

Studies of the EMC effect in the QCD framework with the EMC iron and hydrogen structure functions

The QCD evolution of the measured structure functions F₂ in iron and hydrogen have been analysed to show that the momentum distribution of the valence quarks is lower in iron than in hydrogen and that the opposite is true for the gluon and sea quark distributions in the region of 0.1<x<0.65.