Misak M. Sargsian

Evidence for strong dominance of proton-neutron correlations in nuclei.

We analyze recent data from high-momentum-transfer (p, pp) and (p, ppn) reactions on carbon. For this analysis, the two-nucleon short-range correlation (NN-SRC) model for backward nucleon emission is extended to include the motion of the NN pair in the mean field. The model is found to describe major characteristics of the data. Our analysis demonstrates that the removal of a proton from the nucleus with initial momentum 275-550 MeV/c is 92(+8/-18) % of the time accompanied by the emission of a correlated neutron that carries momentum roughly equal and opposite to the initial proton momentum. This indicates that the probabilities of pp or nn SRCs in the nucleus are at least a factor of 6 smaller than that of pn SRCs. Our result is the first estimate of the isospin structure of NN-SRCs in nuclei, and may have important implication for modeling the equation of state of asymmetric nuclear matter.

Hard probes of short-range nucleon-nucleon correlations

Abstract One of the primary goals of nuclear physics is providing a complete description of the structure of atomic nuclei. While mean-field calculations provide detailed information on the nuclear shell structure for a wide range of nuclei, they do not capture the complete structure of nuclei, in particular the impact of small, dense structures in nuclei. The strong, short-range component of the nucleon–nucleon potential yields hard interactions between nucleons which are close together, generating a high-momentum tail to the nucleon momentum distribution, with momenta well in excess of the Fermi momentum. This high-momentum component of the nuclear wave-function is one of the most poorly understood parts of nuclear structure. Utilizing high-energy probes, we can isolate scattering from high-momentum nucleons, and use these measurements to examine the structure and impact of short-range nucleon–nucleon correlations. Over the last decade we have moved from looking for evidence of such short-range structures to mapping out their strength in nuclei and examining their isospin structure. This has been made possible by high-luminosity and high-energy accelerators, coupled with an improved understanding of the reaction mechanism issues involved in studying these structures. We review the general issues related to short-range correlations, survey recent experiments aimed at probing these short-range structures, and lay out future possibilities to further these studies.

RECENT OBSERVATION OF SHORT-RANGE NUCLEON CORRELATIONS IN NUCLEI AND THEIR IMPLICATIONS FOR THE STRUCTURE OF NUCLEI AND NEUTRON STARS

Novel processes probing the decay of nucleus after removal of a nucleon with momentum larger than Fermi momentum by hard probes nally proved unambiguously the evidence for long sought presence of short-range correlations (SRCs) in nuclei. In combination with the analysis of large Q 2 , A(e,e’)X processes at x > 1 they allow us to conclude that (i) practically all nucleons with momenta 300 MeV/c belong to SRCs, consisting mostly of two nucleons, ii) probability of such SRCs in medium and heavy nuclei is 25%, iii) a fast removal of such nucleon practically always leads to emission of correlated nucleon with approximately opposite momentum, iv) proton removal from twonucleon SRCs in 90% of cases is accompanied by a removal of a neutron and only in 10% by a removal of another proton. We explain that observed absolute probabilities and the isospin structure of two nucleon SRCs conrm the

Hadrons in the nuclear medium

Quantum chromodynamics (QCD), the microscopic theory of strong interactions, has not yet been applied to the calculation of nuclear wavefunctions. However, it certainly provokes a number of specific questions and suggests the existence of novel phenomena in nuclear physics which are not part of the traditional framework of the meson–nucleon description of nuclei. Many of these phenomena are related to high nuclear densities and the role of colour in nucleonic interactions. Quantum fluctuations in the spatial separation between nucleons may lead to local high-density configurations of cold nuclear matter in nuclei, up to four times larger than typical nuclear densities. We argue here that experiments utilizing the higher energies available upon completion of the Jefferson Laboratory energy upgrade will be able to probe the quark–gluon structure of such high-density configurations and therefore elucidate the fundamental nature of nuclear matter. We review three key experimental programmes: quasi-elastic electro-disintegration of light nuclei, deep inelastic scattering from nuclei at x > 1 and the measurement of tagged structure functions. These interrelated programmes are all aimed at the exploration of the quark structure of high-density nuclear configurations. The study of the QCD dynamics of elementary hard processes is another important research direction and nuclei provide a unique avenue to explore these dynamics. In particular, we argue that the use of nuclear targets and large values of momentum transfer at energies available with the Jefferson Laboratory upgrade would allow us to determine whether the physics of the nucleon form factors is dominated by spatially small configurations of three quarks. Similarly, one could determine if hard two-body processes such as exclusive vector meson electroproduction are dominated by production of mesons in small-size q configurations.

SELECTED TOPICS IN HIGH ENERGY SEMI-EXCLUSIVE ELECTRO-NUCLEAR REACTIONS

We review the present status of the theory of high energy reactions with semi-exclusive nucleon electro-production from nuclear targets. We demonstrate how the increase of transferred energies in these reactions opens a completely new window for study of the microscopic nuclear structure at small distances. The simplifications in theoretical descriptions associated with the increase in the energies are discussed. The theoretical framework for calculation of high energy nuclear reactions based on the effective Feynman diagram rules is described in detail. The result of this approach is the generalized eikonal approximation (GEA), which is reduced to the Glauber approximation when nucleon recoil is neglected. The method of GEA is demonstrated in the calculation of high energy electro-disintegration of the deuteron and A=3 targets. Subsequently, we generalize the obtained formulae for A>3 nuclei. The relation of GEA to the Glauber theory is analyzed. Then, based on the GEA framework we discuss some of the phenomena which can be studied in exclusive reactions: nuclear transparency and short-range correlations in nuclei. We illustrate how light-cone dynamics of high-energy scattering emerge naturally in high energy electro-nuclear reactions.

QCD Rescattering and High Energy Two-Body Photodisintegration of the Deuteron

Photon absorption by a quark in one nucleon followed by its high-momentum transfer interaction with a quark in the other may produce two final-state nucleons with high relative momentum. We sum the relevant quark rescattering diagrams to show that the scattering amplitude depends on a convolution between the large angle pn scattering amplitude, the hard photon-quark interaction vertex, and the low-momentum deuteron wave function. The computed absolute values of the cross section are in reasonable agreement with the data.

New properties of the high-momentum distribution of nucleons in asymmetric nuclei

Based on the recent experimental observations of the dominance of tensor interaction in the 250-600 MeV/c momentum range of nucleons in nuclei, the existence of two new properties for high-momentum distribution of nucleons in asymmetric nuclei is suggested. The rst property is the approximate scaling relation between proton and neutron high-momentum distributions weighted by their relative fractions in the nucleus. The second property is the inverse proportionality of the strength of the high-momentum distribution of protons and neutrons to the same relative fractions. Based on these two properties the high-momentum distribution function for asymmetric nuclei has been modeled and demonstrated that it describes reasonably well the high-momentum characteristics of light nuclei. However, the most surprising result is obtained for neutron rich nuclei with large A, for which a substantial relative abundance of high-momentum protons as compared to neutrons is predicted. For example, the model predicts that in Au the relative fraction of protons with momenta above kF 260 MeV/c is 50% more than that of neutrons. Such a situation may have many implications for dierent observations in nuclear physics related to the properties of a proton in neutron rich nuclei.

Final-state interactions in semi-inclusive deep inelastic scattering off the Deuteron

Semi-inclusive deep inelastic scattering off the deuteron with production of a slow nucleon in recoil kinematics is studied in the virtual nucleon approximation, in which the final-state interaction (FSI) is calculated within generalized eikonal approximation. The cross section is derived in a factorized approach, with a factor describing the virtual photon interaction with the off-shell nucleon and a distorted spectral function accounting for the final-state interactions. One of the main goals of the study is to understand how much the general features of the diffractive high-energy soft rescattering accounts for the observed features of FSI in deep inelastic scattering (DIS). Comparison with the Jefferson Lab data shows good agreement in the covered range of kinematics. Most importantly, our calculation correctly reproduces the rise of the FSI in the forward direction of the slow nucleon production angle. By fitting our calculation to the data we extracted the W and Q(2) dependencies of the total cross section and slope factor of the interaction of DIS products, X, off the spectator nucleon. This analysis shows the XN-scattering cross section rising with W and decreasing with an increase of Q(2). Finally, our analysis points at a largely suppressed off-shell part of the rescattering amplitude.

Exclusive electrodisintegration of He 3 at high Q 2 . II. Decay function formalism

Based on the theoretical framework of generalized eikonal approximation we study the two-nucleon emission reactions in high

Hard photodisintegration of a proton pair in 3He

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