Nuclear Quark Research Articles

Skyrmions at high density

The phase diagram of quantum chromodynamics is conjectured to have a rich structure containing at least three forms of matter: hadronic nuclear matter, quarkyonic matter and quark–gluon plasma. We justify the origin of the quarkyonic phase transition in a chiral-quark model and describe its formulation in terms of Skyrme crystals.

Short-range correlations and their implications for isospin-dependent modification of nuclear quark distributions

The past decade has provided a much clearer picture of the structure of high-momentum components in nucleons, associated with hard, short-distance interactions between pairs of nucleons. Recent Jefferson Lab data on light nuclei suggest a connection between these so-called 'short-range correlations' and the modification of the quark structure of nucleons in the nuclear environment. In light of this discovery that the detailed nuclear structure is important in describing the nuclear quark distributions, we examine the potential impact of the isospin-dependent structure of nuclei to see at what level this might yield flavor-dependent effects in nuclear quark distributions.

A mesonic analog of the deuteron

Using the LAMP model for nuclear quark structure, we calculate the binding energy and quark structure of a B meson merging with a D meson. Our variational calculation shows that a molecular, deuteron-like state structure changes rather abruptly, as the separation between the two mesons decreases, and at a separation of about 0.14 fm, the hadronic system transforms into a four-quark bound state, although one maintaining an internal structure rather than that of a four-quark "bag." Unlike the deuteron, pion exchange does not provide any contribution to the ≈ 150 MeV binding.

Process dependent nucleark⊥broadening effect

We study the process dependent nuclear $k_\perp$ broadening effect by employing the transverse momentum dependent(TMD) factorization approach in combination with the Mclerran-Venugopalan(MV) model. More specifically, we investigate how the parton transverse momentum distributions are affected by the process dependent gauge links in cold nuclear matter. In particular, our analysis also applies to the polarized cases including the nuclear quark Boer-Mulders function and the linearly polarized gluon distribution. Our main focus is on the nuclear TMDs at intermediate or large x.

Detailed study of the nuclear dependence of the EMC effect and short-range correlations

Background: The density of the nucleus has been important in explaining the nuclear dependence of the quark distributions, also known as the EMC effect, as well as the presence of high-momentum nucleons arising from short-range correlations (SRCs). Recent measurements of both of these effects on light nuclei have shown a clear deviation from simple density-dependent models.Purpose: A better understanding of the nuclear quark distributions and short-range correlations requires a careful examination of the experimental data on these effects to constrain models that attempt to describe these phenomena.Methods: We present a detailed analysis of the nuclear dependence of the EMC effect and the contribution of SRCs in nuclei, comparing to predictions and simple scaling models based on different pictures of the underlying physics. We also make a direct, quantitative comparison of the two effects to further examine the connection between these two observables related to nuclear structure.Results: We find that, with the inclusion of the new data on light nuclei, neither of these observables can be well explained by common assumptions for the nuclear dependence. The anomalous behavior of both effects in light nuclei is consistent with the idea that the EMC effect is driven by either the presence of high-density configurations in nuclei or the large virtuality of the high-momentum nucleons associated with these configurations.Conclusions: The unexpected nuclear dependence in the measurements of the EMC effect and SRC contributions appear to suggest that the local environment of the struck nucleon is the most relevant quantity for explaining these results. The common behavior suggests a connection between the two seemingly disparate phenomena, but the data do not yet allow for a clear preference between models which aim to explain this connection.

Pion-induced Drell-Yan processes and the flavor-dependent EMC effect

Pion-induced Drell-Yan processes are proposed as a promising tool with which to measure the flavor dependence of the European Muon Collaboration (EMC) effect, that is, the flavor-dependent modification of quark distributions in the nuclear medium. Existing pionic Drell-Yan data are compared with calculations using a recent model for nuclear quark distributions that incorporates flavor-dependent nuclear effects. While no firm conclusions can yet be drawn, we find that existing Drell-Yan data likely imply a flavor dependence of the EMC effect. We demonstrate that pion-induced Drell-Yan experiments on nuclear targets can access new aspects of the EMC effect not probed in deep inelastic scattering and can therefore provide important new constrains on the nuclear quark distributions. Predictions for possible future pion-induced Drell-Yan experiments are also presented.

Exact solutions to a schematic nuclear quark model and colorless superconductivity

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Exact solutions to a schematic nuclear quark model and colorless superconductivity

Exact solutions are found to the equations of a standard nuclear quark model exemplified by the Bonn model which is defined in terms of an effective pairing force. We show, by symmetry arguments, that, in general, the ground state of this model is not color neutral. In particular, color-neutral states have, in general, higher energy than the ground state. A novel BCS-type formalism, which is able to describe exactly color symmetrical BCS states, is used to show that the model admits, but only as excited states, color-neutral superconductivity. Therefore, such a model, with just a pairing force, is unrealistic as a model for the color-neutral confined phase which prevails at normal nuclear densities. Finally, the paper shows that there exists a color-neutral superconducting phase independently of whether the model is based on the pairing force or a more realistic three-body string force.

Nuclear shadowing in deep inelastic scattering on nuclei: leading twist versus eikonal approaches

We use several diverse parameterizations of diffractive parton distributions, extracted in leading twist QCD analyses of the HERA diffractive deep inelastic scattering (DIS) data, to make predictions for leading twist nuclear shadowing of nuclear quark and gluon distributions in DIS on nuclei. We find that the HERA diffractive data are sufficiently precise to allow us to predict large nuclear shadowing for gluons and quarks, unambiguously. We performed detailed studies of nuclear shadowing for up and charm sea quarks and gluons within several scenarios of shadowing and diffractive slopes, as well as at central impact parameters. We compare these leading twist results with those obtained from the eikonal approach to nuclear shadowing (which is based on a very different space-time picture) and observe sharply contrasting predictions for the size and Q^2-dependence of nuclear shadowing. The most striking differences arise for the interaction of small dipoles with nuclei, in particular for the longitudinal structure function F_{L}^{A}.

Quark condensate at finite baryon density.

We discuss a recently derived, model-independent relation that expresses the value of a medium-modified quark condensate in terms of the vacuum value of the condensate and the value of the nucleon sigma term, [sigma][sub [ital N]]. Our goal is to calculate the value of the quark condensate in nuclear matter, [l angle]NM[vert bar][ital [bar q]](0)[ital q](0)[vert bar]NM[r angle], using some standard many-body techniques. Here, we comment on the mean-field calculations of Cohen, Furnstahl, and Griegel and others. We then calculate the value of the nuclear matter quark condensate using linear response theory. In a sigma-dominance model, the linear response calculation relates the modification of the vacuum condensate to the matrix element of the operator [ital [bar q]q] taken between a state of the sigma meson and the vacuum. (That matrix element may be used to define a sigma decay constant, [ital f][sub [sigma]].) We also provide some additional insight into the relation between the dynamics of the quark condensate and the scalar fields of relativistic nuclear physics.

Nuclear shadowing in deep-inelastic scattering and Drell-Yan production

More support is provided for the parton description of shadowing in deep-inelastic scattering by demonstrating that a joint description of these data and Drell-Yan data is feasible with thesame modifications to the quark distributions in nuclei. The nuclear quark distributions obtained are used to modify the estimate ofJ/Ψ suppression in nucleus-nucleus collisions due to initialstate effects. A small change is found for currently-available energies but it could be much more dramatic at higher energies.

Direct photons, dimuons and nuclear effects

LargepT direct photon and dimuon production in experiments using nuclear targets have been studied in detail. We show that, by a careful choice of kinematic configurations in these experiments, very good estimates of nuclear quark and gluon densities can be obtained. Information on the cumulative region of the nuclear structure functions i.e. the regionx>1 can also be obtained by considering semi-integrated cross sections.

On the nuclear dependence of electroproduction

We discuss the theoretical description of electroproduction with nuclear targets. Models which convolute the probability for finding light constituents within the nucleus with their electroproduction cross sections are found to large corrections. Instead it is desirable to determine directly the nuclear quark and gluon distributions. We consider ways in which they may be estimated in QCD and their reliability. We present a detailed estimate for the expected A dependence of these distributions.