arXiv subjects

M. M. Sargsian

Publications and source records attributed to M. M. Sargsian.

11 recordsLinked to original sources

arXiv2015

Correlated Fermion Pairs in Nuclei and Ultracold Atomic Gases

Background: The high momentum distribution of atoms in two spin-state ultra-cold atomic gases with strong short-range interactions between atoms with different spins, which can be described using Tan's contact, are dominated by short range pairs of different fermions and decreases as $k^{-4}$. In atomic nuclei the momentum distribution of nucleons above the Fermi momentum ($k>k_F \approx 250$ Mev/c) is also dominated by short rangecorrelated different-fermion (neutron-proton) pairs. Purpose: Compare high-momentum unlike-fermion momentum distributions in atomic and nuclear systems. Methods: We show that, for $k>k_F$ MeV/c, nuclear momentum distributions are proportional to that of the deuteron. We then examine the deuteron momentum distributions derived from a wide variety of modern nucleon-nucleon potentials that are consistent with $NN$-scattering data. Results: The high momentum tail of the deuteron momentum distribution, and hence of the nuclear momentum distributions appears to decrease as $k^{-4}$. This behavior is shown to arise from the effects of the tensor part of the nucleon-nucleon potential. In addition, when the dimensionless interaction strength for the atomic system is chosen to be similar to that of atomic nuclei, the probability for finding a short range different-fermion pair in both systems is the same. Conclusions: Although nuclei do not satisfy all of the conditions for Tan's contact, the observed similarity of the magnitude and $k^{-4}$ shape of nuclear and atomic momentum distributions is remarkable because these systems differ by about $20$ orders of magnitude in density. This similarity may lead to a greater understanding of nuclei and the density dependence of nuclear systems.

nucl-ex↗

arXiv2013

Measurement of the neutron electric to magnetic form factor ratio at Q2 = 1.58 GeV2 using the reaction 3He(e,e'n)pp

A measurement of beam helicity asymmetries in the reaction 3He(e,e'n)pp has been performed at the Mainz Microtron in quasielastic kinematics in order to determine the electric to magnetic form factor ratio of the neutron, GEn/GMn, at a four momentum transfer Q2 = 1.58 GeV2. Longitudinally polarized electrons were scattered on a highly polarized 3He gas target. The scattered electrons were detected with a high-resolution magnetic spectrometer, and the ejected neutrons with a dedicated neutron detector composed of scintillator bars. To reduce systematic errors data were taken for four different target polarization orientations allowing the determination of GEn/GMn from a double ratio. We find mu_n GEn/GMn = 0.250 +/- 0.058(stat.) +/- 0.017 (sys.).

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arXiv2006

Coherent Vector Meson Photo-Production from Deuterium at Intermediate Energies

We analyze the cross section for vector meson photo-production off a deuteron for the intermediate range of photon energies starting at a few GeVs above the threshold and higher. We reproduce the steps in the derivation of the conventional non-relativistic Glauber expression based on an effective diagrammatic method while making corrections for Fermi motion and intermediate energy kinematic effects. We show that, for intermediate energy vector meson production, the usual Glauber factorization breaks down and we derive corrections to the usual Glauber method to linear order in longitudinal nucleon momentum. The purpose of our analysis is to establish methods for probing interesting physics in the production mechanism for phi-mesons and heavier vector mesons. We demonstrate how neglecting the breakdown of Glauber factorization can lead to errors in measurements of basic cross sections extracted from nuclear data.

hep-ph↗

arXiv2005

Measurement of 2- and 3-Nucleon Short Range Correlation Probabilities in Nuclei

The ratios of inclusive electron scattering cross sections of He4, C12 and Fe56 to He3 have been measured at 1 < x_B < 3. At Q^2 > 1.4 GeV^2, the ratios exhibit two separate plateaus, at 1.5 < x_B < 2 and at x_B > 2.25. This pattern is predicted by models that include 2- and 3-nucleon short-range correlations (SRC). Relative to A=3, the per-nucleon probabilities of 3-nucleon SRC are 2.3, 3.2, and 4.6 times larger for A=4, 12 and 56. This is the first measurement of 3-nucleon SRC probabilities in nuclei. \\

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arXiv2005

Exclusive Electro-Disintegration of 3He at High Q2: I. Generalized Eikonal Approximation

We develop a theoretical framework for calculation of high $Q^2$ exclusive electro-disintegration of A=3 systems. The main result of this work is the calculation of the final state interaction of the struck energetic nucleon with recoil nucleons within the generalized eikonal approximation (GEA) which allows us to account for the finite and relatively large momenta of the bound nucleons in the nucleus. The important advantage of this approach is the possibility to study in a self-consistent way the short range correlations in nuclei. This is due to the fact that GEA does not require a stationary condition for recoil nucleons as conventional Glauber approximation does. As a result GEA treats explicitly the Fermi motion of recoil nucleons in the nucleus.

nucl-th↗

arXiv2005

Exclusive Electro-Disintegration of 3He at high Q2: II. Decay Function Formalism

Based on the theoretical framework of generalized eikonal approximation we study the two-nucleon emission reactions in high $Q^2$ electro-disintegration of $^3He$. Main aim is to investigate those features of the reaction which can be unambiguously identified with the short range properties of the ground state nuclear wave function. To evaluate the differential cross section we work in the formalism of the decay function which characterizes the property of the ground state wave function as well as the decay properties of final two nucleon spectator system. Our main motivation here is to explore the accessibility of two-- and three--nucleon short range correlations in $^3$He as well as to isolate unambiguously single and double rescattering processes in the reaction dynamics. Our analysis allowed us also to identify new approaches for investigating the role of the practically unknown three-nucleon forces in the ground state wave function of $^3He$.

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arXiv2002

Hadrons in the Nuclear Medium

Quantum Chromodynamics, the microscopic theory of strong interactions, has not yet been applied to the calculation of nuclear wave functions. 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 the traditional framework of the meson-nucleon description of nuclei. Many of these phenomena are related to high nuclear densities and the role of color 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 programs: quasi-elastic electro-disintegration of light nuclei, deep inelastic scattering from nuclei at $x>1$, and the measurement of tagged structure functions. These interrelated programs 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. We argue that the use of nuclear targets and large values of momentum transfer at would allow us to determine whether the physics of the nucleon form factors is dominated by spatially small configurations of three quarks.

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arXiv2002

Neutron structure function and inclusive DIS from H-3 and He-3 at large Bjorken-x

A detailed study of inclusive deep inelastic scattering (DIS) from mirror A = 3 nuclei at large values of the Bjorken variable x is presented. The main purpose is to estimate the theoretical uncertainties on the extraction of the neutron DIS structure function from such nuclear measurements. On one hand, within models in which no modification of the bound nucleon structure functions is taken into account, we have investigated the possible uncertainties arising from: i) charge symmetry breaking terms in the nucleon-nucleon interaction, ii) finite Q**2 effects neglected in the Bjorken limit, iii) the role of different prescriptions for the nucleon Spectral Function normalization providing baryon number conservation, and iv) the differences between the virtual nucleon and light cone formalisms. Although these effects have been not yet considered in existing analyses, our conclusion is that all these effects cancel at the level of ~ 1% for x < 0.75 in overall agreement with previous findings. On the other hand we have considered several models in which the modification of the bound nucleon structure functions is accounted for to describe the EMC effect in DIS scattering from nuclei. It turns out that within these models the cancellation of nuclear effects is expected to occur only at a level of ~ 3%, leading to an accuracy of ~ 12 % in the extraction of the neutron to proton structure function ratio at x ~ 0.7 -0.8$. Another consequence of considering a broad range of models of the EMC effect is that the previously suggested iteration procedure does not improve the accuracy of the extraction of the neutron to proton structure function ratio.

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arXiv2002

Neutron structure function and inclusive DIS from H-3 and He-3 targets at large Bjorken-x

A detailed study of inclusive deep inelastic scattering from mirror A = 3 nuclei at large values of Bjorken-x is presented. The main purpose is to estimate the theoretical uncertainties on the extraction of F2n from such measurements. Within the convolution approach we confirm the cancellation of nuclear effects at the level of ~1 % for x < 0.75 in overall agreement with previous findings. However, within models in which modifications of the bound nucleon structure functions are accounted for to describe the EMC effect in nuclei, we find that the nuclear effects may be canceled at a level of ~ 3 % only, leading to an accuracy of ~ 12 % in the extraction of F2n / F2p at x ~ 0.7 - 0.8. Another consequence of bound nucleon modifications is that the iteration procedure does not improve the accuracy of the extraction of F2n / F2p.

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arXiv1997

Feynman Graphs and Generalized Eikonal Approach to High Energy Knock-Out Processes

The cross section of hard semi-exclusive $A(e,e'N)(A-1)$ reactions for fixed missing energy and momentum is calculated within the eikonal approximation. Relativistic dynamics and kinematics of high energy processes are unambiguously accounted for by using the analysis of appropriate Feynman diagrams. A significant dependence of the final state interactions on the missing energy is found, which is important for interpretation of forthcoming color transparency experiments. A new, more stringent kinematic restriction on the region where the contribution of short-range nucleon correlations is enhanced in semi-exclusive knock-out processes is derived. It is also demonstrated that the use of light-cone variables leads to a considerable simplification of the description of high-energy knock-out reactions.

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arXiv1996

On the Possibility to Study Color Transparency in the Large Momentum Transfer Exclusive $d(p,2p)n$ Reaction

The deuteron disintegration at high energies and large angles in the $d(p,2p)n$ reaction, is calculated in kinematical conditions where the dominant contributions are due to soft rescatterings of the initial and final nucleons, which accompany the hard $pp$ reaction. The eikonal approximation, which accounts for relativistic kinematics as dictated by Feynman diagrams, reveals the important role played by the initial and final state interactions in the angular and momentum dependences of the differential cross section. Based on these results, we propose a new and effective test, at moderate energies, of the physics relevant for the color transparency phenomenon in hadron-initiated exclusive hard processes.

hep-ph↗