Shalev Gilad

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.

Vertical drift chambers for the Hall A high-resolution spectrometers at Jefferson Lab

The High Resolution Spectrometers in Hall A at Jefferson Laboratory have been instrumented with state-of-the-art Vertical Drift Chambers designed and constructed by the Nuclear Interactions Group at MITLNS in conjunction with the Physics Division at Jefferson Lab. These chambers rely on a unique, high cell-density design made possible by the absence of field-shaping wires. Each chamber has an inert per-plane resolution for 5-cell cosmic ray track of 145 mu-m FWHM when operated on the bench at -4.8 kV with argon-isobutane gas, and 225 mu-m FWHM for 5-cell electron tracks when operated in the High Resolution Spectrometer detector stack at -4.0 kV with argon-ethane gas. The design and construction facilities wire placement and replacement to 50 mu-m, very low dark current, and no crosswalk. The detectors have been in almost continuous use since April 1996, providing reliable, high-resolution charged-particles tracking data for the hall A physics program.

Performance of a compact detector package for the out-of-plane spectrometer system

Abstract We report on the design and performance of compact detector packages currently installed in the four magnetic out-of-plane spectrometers for electron scattering experiments at the MIT-Bates Linear Accelerator Center. The detector packages have been designed to meet the mechanical requirements arising from out-of-plane particle detection. They offer good trajectory and momentum reconstruction, particle identification and time-of-flight measurements for electrons, pions, protons, and deuterons with large momentum bites and in broad kinematical ranges and high luminosities. The detectors have so far been used with great success in out-of-plane measurements of 12 C ( e → , e ′ p ) , 2 H ( e → , e ′ p ) , virtual Compton scattering below pion threshold and in studies of the N → Δ transition in both exclusive reaction channels 1 H ( e → , e ′ p ) π 0 and 1 H ( e → , e ′ π + )n .

Measured properties of an out-of-plane spectrometer

Abstract We report the results of measurements of the properties of a prototype out-of-plane magnetic spectrometer (OOPS). This spectrometer is one of four identical modules which, together with a support structure, comprise the OOPS cluster. The performance of the spectrometer was found to closely match its design characteristics.

Quasielastic C-12(e,e ' p) reaction at high momentum transfer

We measured the {sup 12}C(e,e{sup {prime}}p) cross section as a function of missing energy in parallel kinematics for (q,{omega})=(970 MeV/c, 330 MeV) and (990 MeV/c, 475 MeV). At {omega}=475 MeV, at the maximum of the quasielastic peak, there is a large continuum (E{sub m}{gt}50 MeV) cross section extending out to the deepest missing energy measured, amounting to almost 50{percent} of the measured cross section. The ratio of data to distorted-wave impulse approximation (DWIA) calculation is 0.4 for both {ital p} and {ital s} shells. At {omega}=330 MeV, well below the maximum of the quasielastic peak, the continuum cross section is much smaller and the ratio of data to DWIA calculation is 0.85 for the {ital p} shell and 1.0 for the {ital s} shell. We infer that one or more mechanisms that increase with {omega} transform some of the single-nucleon knockouts into a multinucleon knockout, decreasing the valence knockout cross section and increasing the continuum cross section. {copyright} {ital 1999} {ital The American Physical Society}

Measurement of the Vector and Tensor Asymmetries at Large Missing Momentum in Quasielastic ( e → , e ′ p ) Electron Scattering from Deuterium

We report the measurement of the beam-vector and tensor asymmetries A_{ed}^{V} and A_{d}^{T} in quasielastic (e[over →],e^{}p) electrodisintegration of the deuteron at the MIT-Bates Linear Accelerator Center up to missing momentum of 500u2009u2009MeV/c. Data were collected simultaneously over a momentum transfer range 0.1<Q^{2}<0.5u2009u2009(GeV/c)^{2} with the Bates Large Acceptance Spectrometer Toroid using an internal deuterium gas target polarized sequentially in both vector and tensor states. The data are compared with calculations. The beam-vector asymmetry A_{ed}^{V} is found to be directly sensitive to the D-wave component of the deuteron and has a zero crossing at a missing momentum of about 320u2009u2009MeV/c, as predicted. The tensor asymmetry A_{d}^{T} at large missing momentum is found to be dominated by the influence of the tensor force in the neutron-proton final-state interaction. The new data provide a strong constraint on theoretical models.

Recent Studies of Nucleon Electromagnetic Form Factors

The electromagnetic form factors of nucleons are fundamental quantities in nucleon structure. As such, they have been studied extensively both theoretically and experimentally. Significant progress has been made with new measurements at Jlab, MAMI and MIT‐Bates, with emphases on expanding the momentum‐transfer range and on higher precision. In this paper, we describe the status of this field and present new results from measurements at both low and high momentum transfers. We also compare the experimental data to model predictions, and mention possible implications of the new results to other fields.

Recent Studies Of Correlations In Nuclei Using (e,e'p) Reactions

New measurements of the reaction 12C(e,e′p) at high missing energies and momenta, at Q2u2009=u20092(GeV/c)2, and at xB>1 are described. Preliminary cross sections for a large range of missing energies and momenta are presented and display a relation between the missing energies and momenta which takes into account the recoil and excitation of the residual nucleus. Distorted spectral functions were extracted. They display badly‐broken factorization and raise doubt about knowledge of the elementary off‐shell electron‐nucleon cross sections for xB>1.

Polarization transfer in the {sup 2}H(e-vector,e{sup '}p-vector)n reaction up to Q{sup 2}=1.61 (GeV/c){sup 2}

The recoil proton polarization was measured in the d(epol,e ppol)n reaction in Hall A of the Thomas Jefferson National Accelerator Facility (JLab). The electron kinematics were centered on the quasielastic peak (x{sub Bj} {approx} 1) and included three values of the squared four-momentum transfer, Q{sup 2}=0.43, 1.00 and 1.61 (GeV/c){sup 2}. For Q{sup 2}=0.43 and 1.61 (GeV/c){sup 2}, the missing momentum, p{sub m}, was centered at zero while for Q{sup 2}=1.00 (GeV/c){sup 2} two values of p{sub m} were chosen: 0 and 174 MeV/c. At low p{sub m}, the Q{sup 2} dependence of the longitudinal polarization, P{sub z}, is not well described by a state-of-the-art calculation. Further, at higher p{sub m}, a 3.5 sigma discrepancy was observed in the transverse polarization, P{sub x}. Understanding the origin of these discrepancies is important in order to confidently extract the neutron electric form factor from the analogous d(epol,e npol)p experiment.

Polarization transfer in the d(epol,e' ppol)n reaction up to Q^2=1.61 (GeV/c)^2

The recoil proton polarization was measured in the d(epol,e ppol)n reaction in Hall A of the Thomas Jefferson National Accelerator Facility (JLab). The electron kinematics were centered on the quasielastic peak (x{sub Bj} {approx} 1) and included three values of the squared four-momentum transfer, Q{sup 2}=0.43, 1.00 and 1.61 (GeV/c){sup 2}. For Q{sup 2}=0.43 and 1.61 (GeV/c){sup 2}, the missing momentum, p{sub m}, was centered at zero while for Q{sup 2}=1.00 (GeV/c){sup 2} two values of p{sub m} were chosen: 0 and 174 MeV/c. At low p{sub m}, the Q{sup 2} dependence of the longitudinal polarization, P{sub z}, is not well described by a state-of-the-art calculation. Further, at higher p{sub m}, a 3.5 sigma discrepancy was observed in the transverse polarization, P{sub x}. Understanding the origin of these discrepancies is important in order to confidently extract the neutron electric form factor from the analogous d(epol,e npol)p experiment.