Electron-induced proton knockout from neutron rich nuclei

We study the evolution of the \eep cross section on nuclei with increasing asymmetry between the number of neutrons and protons. The calculations are done within the framework of the nonrelativistic and relativistic distorted-wave impulse approximation. In the nonrelativistic model phenomenological Woods-Saxon and Hartree-Fock wave functions are used for the proton bound-state wave functions, in the relativistic model the wave functions are solutions of Dirac-Hartree equations. The models are first tested against experimental data on $^{40}$Ca and $^{48}$Ca nuclei, and then they are applied to a set of spherical calcium isotopes.Comment: 5 pages, 2 figures. contribution to the XIX International School on Nuclear Physics, Neutron Physics and Applications, Varna (Bulgaria) September 19-25, 201

Spectral Function of Quarks in Quark Matter

We investigate the spectral function of light quarks in infinite quark matter using a simple, albeit self-consistent model. The interactions between the quarks are described by the SU(2) Nambu--Jona-Lasinio model. Currently mean field effects are neglected and all calculations are performed in the chirally restored phase at zero temperature. Relations between correlation functions and collision rates are used to calculate the spectral function in an iterative process.Comment: final version, published in PRC; 15 pages, RevTeX

Effects of Short-Range Correlations in (e,e'p) reactions and nuclear overlap functions

A study of the effects of short-range correlations over the (e,e'p) reaction for low missing energy in closed shell nuclei is presented. We use correlated, quasi-hole overlap functions extracted from the asymptotic behavior of the one-body density matrix, containing central correlations of Jastrow type, up to first-order in a cluster expansion, and computed in the very high asymptotic region, up to 100 fm. The method to extract the overlap functions is checked in a simple shell model, where the exact results are known. We find that the single-particle wave functions of the valence shells are shifted to the right due to the short-range repulsion by the nuclear core. The corresponding spectroscopic factors are reduced only a few percent with respect to the shell model. However, the (e,e'p) response functions and cross sections are enhanced in the region of the maximum of the missing momentum distribution due to short-range correlations.Comment: 45 pages, 15 figure

Measurement of mechanical vibrations excited in aluminium resonators by 0.6 GeV electrons

We present measurements of mechanical vibrations induced by 0.6 GeV electrons impinging on cylindrical and spherical aluminium resonators. To monitor the amplitude of the resonator's vibrational modes we used piezoelectric ceramic sensors, calibrated by standard accelerometers. Calculations using the thermo-acoustic conversion model, agree well with the experimental data, as demonstrated by the specific variation of the excitation strengths with the absorbed energy, and with the traversing particles' track positions. For the first longitudinal mode of the cylindrical resonator we measured a conversion factor of 7.4 +- 1.4 nm/J, confirming the model value of 10 nm/J. Also, for the spherical resonator, we found the model values for the L=2 and L=1 mode amplitudes to be consistent with our measurement. We thus have confirmed the applicability of the model, and we note that calculations based on the model have shown that next generation resonant mass gravitational wave detectors can only be expected to reach their intended ultra high sensitivity if they will be shielded by an appreciable amount of rock, where a veto detector can reduce the background of remaining impinging cosmic rays effectively.Comment: Tex-Article with epsfile, 34 pages including 13 figures and 5 tables. To be published in Rev. Scient. Instr., May 200

Nuclear structure studies with the 7Li(e,e'p) reaction

Experimental momentum distributions for the transitions to the ground state and first excited state of 6He have been measured via the reaction 7Li(e,e'p)6He, in the missing momentum range from -70 to 260 MeV/c. They are compared to theoretical distributions calculated with mean-field wave functions and with variational Monte Carlo (VMC) wave functions which include strong state-dependent correlations in both 7Li and 6He. These VMC calculations provide a parameter-free prediction of the momentum distribution that reproduces the measured data, including its normalization. The deduced summed spectroscopic factor for the two transitions is 0.58 +/- 0.05, in perfect agreement with the VMC value of 0.60. This is the first successful comparison of experiment and ab initio theory for spectroscopic factors in p-shell nuclei.Comment: 4 pages, 3 figure

Meson exchange currents in electromagnetic one-nucleon emission

The role of meson exchange currents (MEC) in electron- and photon-induced one-nucleon emission processes is studied in a nonrelativistic model including correlations and final state interactions. The nuclear current is the sum of a one-body and of a two-body part. The two-body current includes pion seagull, pion-in-flight and the isobar current contributions. Numerical results are presented for the exclusive 16O(e,e'p)15N and 16O(\gamma,p)15N reactions. MEC effects are in general rather small in (e,e'p), while in (\gamma,p) they are always large and important to obtain a consistent description of (e,e'p) and (\gamma,p) data, with the same spectroscopic factors. The calculated (\gamma,p) cross sections are sensitive to short-range correlations at high values of the recoil momentum, where MEC effects are larger and overwhelm the contribution of correlations.Comment: 9 pages, 6 figure

Effects of Short Range Correlations on Ca Isotopes

The effect of Short Range Correlations (SRC) on Ca isotopes is studied using a simple phenomenological model. Theoretical expressions for the charge (proton) form factors, densities and moments of Ca nuclei are derived. The role of SRC in reproducing the empirical data for the charge density differences is examined. Their influence on the depletion of the nuclear Fermi surface is studied and the fractional occupation probabilities of the shell model orbits of Ca nuclei are calculated. The variation of SRC as function of the mass number is also discussed.Comment: 11 pages (RevTex), 6 Postscript figures available upon request at [email protected] Physical Review C in prin

One Body Density Matrix, Natural Orbits and Quasi Hole States in 16O and 40Ca

The one body density matrix, momentum distribution, natural orbits and quasi hole states of 16O and 40Ca are analyzed in the framework of the correlated basis function theory using state dependent correlations with central and tensor components. Fermi hypernetted chain integral equations and single operator chain approximation are employed to sum cluster diagrams at all orders. The optimal trial wave function is determined by means of the variational principle and the realistic Argonne v8' two-nucleon and Urbana IX three-nucleon interactions. The correlated momentum distributions are in good agreement with the available variational Monte Carlo results and show the well known enhancement at large momentum values with respect to the independent particle model. Diagonalization of the density matrix provides the natural orbits and their occupation numbers. Correlations deplete the occupation number of the first natural orbitals by more than 10%. The first following ones result instead occupied by a few percent. Jastrow correlations lower the spectroscopic factors of the valence states by a few percent (~1-3%) and an additional ~8-12% depletion is provided by tensor correlations. It is confirmed that short range correlations do not explain the spectroscopic factors extracted from (e,e'p) experiments. 2h-1p perturbative corrections in the correlated basis are expected to provide most of the remaining strength, as in nuclear matter.Comment: 25 pages, 9 figures. Submitted to Phys.Rev.

Evidence for Quark-Hadron Duality in the Proton Spin Asymmetry A1A_1

Spin-dependent lepton-nucleon scattering data have been used to investigate the validity of the concept of quark-hadron duality for the spin asymmetry $A_1$. Longitudinally polarised positrons were scattered off a longitudinally polarised hydrogen target for values of $Q^2$ between 1.2 and 12 GeV$^2$ and values of $W^2$ between 1 and 4 GeV$^2$. The average double-spin asymmetry in the nucleon resonance region is found to agree with that measured in deep-inelastic scattering at the same values of the Bjorken scaling variable $x$. This finding implies that the description of $A_1$ in terms of quark degrees of freedom is valid also in the nucleon resonance region for values of $Q^2$ above 1.6 GeV$^2$.Comment: 5 pages, 1 eps figure, table added, new references added, in print in Phys. Rev. Let

Measurement of Longitudinal Spin Transfer to Lambda Hyperons in Deep-Inelastic Lepton Scattering

Spin transfer in deep-inelastic Lambda electroproduction has been studied with the HERMES detector using the 27.6 GeV polarized positron beam in the HERA storage ring. For an average fractional energy transfer = 0.45, the longitudinal spin transfer from the virtual photon to the Lambda has been extracted. The spin transfer along the Lambda momentum direction is found to be 0.11 +/- 0.17 (stat) +/- 0.03 (sys); similar values are found for other possible choices for the longitudinal spin direction of the Lambda. This result is the most precise value obtained to date from deep-inelastic scattering with charged lepton beams, and is sensitive to polarized up quark fragmentation to hyperon states. The experimental result is found to be in general agreement with various models of the Lambda spin content, and is consistent with the assumption of helicity conservation in the fragmentation process.Comment: 8 pages, 3 figures; new version has an expanded discussion and small format change