Abstract:
The attempts to investigate correlations in electromagnetically induced one- and two-nucleon knockout are reviewed. The theoretical framework for cross section calculations is outlined and some results are presented for the exclusive $^{16}$O($e,e'p)^{15}$N and $^{16}$O($e,e'pp)^{14}$C reactions. For the ($e,e'p$) reaction attention is focussed on extracting the spectroscopic factors. For the ($e,e'pp$) reaction the possibility of obtaining direct and clear information on short-range correlations is discussed.

Abstract:
Cross sections and photon asymmetries of the exclusive 16O(\gamma,pn)14N and 16O(\gamma,pp)14C knockout reactions are calculated for transitions to the low-lying discrete final states of the residual nucleus in the photon-energy range between 100 and 400 MeV. Exclusive reactions may represent a test of reaction mechanisms and a promising tool for investigating the dynamics of nucleon pairs in different states. Cross sections and asymmetries for both (\gamma,pn) and (\gamma,pp) turn out to be only slightly affected by short-range correlations and dominated by two-body currents. Therefore, two-nucleon knockout reactions induced by real photons appear well suited to investigate the nuclear current and the selectivity of individual transitions to its different components.

Abstract:
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.

Abstract:
The general formalism of nucleon recoil polarization in the (${\vec e},e'{\vec N}N$) reaction is given. Numerical predictions are presented for the components of the outgoing proton polarization and of the polarization transfer coefficient in the specific case of the exclusive $^{16}$O(${\vec e},e'{\vec p}p$)$^{14}$C knockout reaction leading to discrete states in the residual nucleus. Reaction calculations are performed in a direct knockout framework where final-state interactions and one-body and two-body currents are included. The two-nucleon overlap integrals are obtained from a calculation of the two-proton spectral function of $^{16}$O where long-range and short-range correlations are consistently included. The comparison of results obtained in different kinematics confirms that resolution of different final states in the $^{16}$O(${\vec e},e'{\vec p}p$)$^{14}$C reaction may act as a filter to disentangle and separately investigate the reaction processes due to short-range correlations and two-body currents and indicates that measurements of the components of the outgoing proton polarization may offer good opportunities to study short-range correlations.

Abstract:
Nucleon recoil polarization in electromagnetic reactions with two-nucleon emission is discussed for both (${\vec e},e'{\vec N}N$) and (${\vec \gamma},{\vec N}N$). Numerical results are given for exclusive two-nucleon knockout reactions from $^{16}$O in a theoretical model where final-state interactions, one-body and two-body currents, and the effect of correlations in the initial pair wave function are included.

Abstract:
The formalism of (${\vec \gamma},{\vec N}N$) reactions is given where the incident photon is polarized and the outgoing nucleon polarization is detected. Sixteen structure functions and fifteen polarization observables are found in the general case, while only eight structure functions and seven polarizations observables survive in coplanar kinematics. Numerical examples are presented for the $^{16}$O($\gamma,pn$) and $^{16}$O($\gamma,pp$) reactions. The transitions to the ground state of $^{14}$C and $^{14}$N are calculated in a model where realistic short-range and tensor correlations are taken into account for the $pn$ pair, while short-range and long-range correlations are included in a consistent way for $pp$ pairs. The effects of the one-body and two-body components of the nuclear current and the role of correlations in cross sections and polarizations are studied and discussed.

Abstract:
We propose new techniques to implement numerically the overlap-Dirac operator which exploit the physical properties of the underlying theory to avoid nested algorithms. We test these procedures in the two-dimensional Schwinger model and the results are very promising. We also present a detailed computation of the spectrum and chiral properties of the Schwinger Model in the overlap lattice formulation.

Abstract:
We present the results of a computation of the sum of the strange and average up-down quark masses with overlap fermions in the quenched approximation. Since the overlap regularization preserves chiral symmetry at finite cutoff and volume, no additive quark mass renormalization is required and the results are O(a) improved. Our simulations are performed at beta=6.0 and volume V=16^3X32, which correspond to a lattice cutoff of ~2 GeV and to an extension of ~1.4 fm. The logarithmically divergent renormalization constant has been computed non-perturbatively in the RI/MOM scheme. By using the K-meson mass as experimental input, we obtain (m_s + m_l)^RI(2 GeV) = 120(7)(21) MeV, which corresponds m_s^MS (2 GeV) = 102(6)(18) MeV if continuum perturbation theory and ChiPT are used. By using the GMOR relation we also obtain ^MS(2 GeV)/N_f = - 0.0190(11)(33) GeV^3 = - [267(5)(15) MeV]^3.

Abstract:
We present results of a quenched QCD simulation with overlap fermions on a lattice of volume V = 16^3X32 at beta=6.0, which corresponds approximatively to a lattice cutoff of 2 GeV and an extension of 1.4 fm. From the two-point correlation functions of bilinear operators we extract the pseudoscalar meson masses and the corresponding decay constants. From the GMOR relation we determine the chiral condensate and, by using the K-meson mass as experimental input, we compute the sum of the strange and average up-down quark masses (m_s + \hat m). The needed logarithmic divergent renormalization constant Z_S is computed with the RI/MOM non-perturbative renormalization technique. Since the overlap preserves chiral symmetry at finite cutoff and volume, no divergent quark mass and chiral condensate additive renormalizations are required and the results are O(a) improved.