Electromagnetic proton-neutron knockout off 16O: new achievements in theory

Results for the cross sections of the exclusive 16O(e,e'pn)14N and 16O(gamma,pn)14N knockout reactions are presented and discussed in different kinematics. In comparison with earlier work, a complete treatment of the center-of-mass (CM) effects in the nuclear one-body current is considered in connection with the problem of the lack of orthogonality between initial bound and final scattering states. The effects due to CM and orthogonalization are investigated in combination with different treatments of correlations in the two-nucleon overlap function and for different parametrizations of the two-body currents. The CM effects lead in super-parallel kinematics to a dramatic increase of the 16O(e,e'pn) cross section to the 1_2^+ excited state (3.95 MeV) of 14N. In all the situations considered the results are very sensitive to the treatment of correlations. A crucial role is played by tensor correlations, but also the contribution of long-range correlations is important.Comment: 13 pages, 10 figure

Photon-induced two-nucleon knockout reactions to discrete final states

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.Comment: 21 pages, 16 postscript figure

Finite size scaling of meson propagators with isospin chemical potential

We determine the volume and mass dependence of scalar and pseudoscalar two-point functions in N_f-flavour QCD, in the presence of an isospin chemical potential and at fixed gauge-field topology. We obtain these results at second order in the \epsilon-expansion of Chiral Perturbation Theory and evaluate all relevant zero-mode group integrals analytically. The virtue of working with a non-vanishing chemical potential is that it provides the correlation functions with a dependence on both the chiral condensate, \Sigma, and the pion decay constant, F, already at leading order. Our results may therefore be useful for improving the determination of these constants from lattice QCD calculations. As a side product, we rectify an earlier calculation of the O(\epsilon^2) finite-volume correction to the decay constant appearing in the partition function. We also compute a generalised partition function which is useful for evaluating U(N_f) group integrals

NN final-state interaction in two-nucleon knockout from 16O^{16}O

The influence of the mutual interaction between the two outgoing nucleons (NN-FSI) in electro- and photoinduced two-nucleon knockout from $^{16}O$ has been investigated perturbatively. It turns out that the effect of NN-FSI depends on the kinematics and on the type of reaction considered. The effect is generally larger in pp- than in pn-knockout and in electron induced than in photoinduced reactions. In superparallel kinematics NN-FSI leads in the $(e,e'pp)$ channel to a strong increase of the cross section, that is mainly due to a strong enhancement of the $\Delta$-current contribution. In pn-emission, however, this effect is partially cancelled by a destructive interference with the seagull current. For photoreactions NN-FSI is considerably reduced in superparallel kinematics and can be practically negligible in specific kinematics.Comment: 16 pages, 9 figure

The light-quark contribution to the leading HVP term of the muon g−2g - 2 from twisted-mass fermions

We present a lattice calculation of the leading Hadronic Vacuum Polarization (HVP) contribution of the light u- and d-quarks to the anomalous magnetic moment of the muon, $a_\mu^{\rm HVP}(ud)$, adopting the gauge configurations generated by the European Twisted Mass Collaboration with $N_f = 2+1+1$ dynamical quarks at three values of the lattice spacing with pion masses in the range 210 - 450 MeV. Thanks to several lattices at fixed values of the light-quark mass and scale but with different sizes we perform a careful investigation of finite-volume effects (FVEs). In order to remove FVEs we develop an analytic representation of the vector correlator, which describes the lattice data for time distances larger than $\simeq 0.2$ fm. The representation is based on quark-hadron duality at small and intermediate time distances and on the two-pion contributions in a finite box at larger time distances. After extrapolation to the physical pion point and to the continuum limit we obtain $a_\mu^{\rm HVP}(ud) = 619.0~(17.8) \cdot 10^{-10}$. Adding the contribution of strange and charm quarks, obtained by ETMC, and an estimate of the isospin-breaking corrections and quark-disconnected diagrams from the literature we get $a_\mu^{\rm HVP}(udsc) = 683~(19) \cdot 10^{-10}$, which is consistent with recent results based on dispersive analyses of the experimental cross section data for $e^+ e^-$ annihilation into hadrons. Using our analytic representation of the vector correlator, taken at the physical pion mass in the continuum and infinite volume limits, we provide the first eleven moments of the polarization function and we compare them with recent results of the dispersive analysis of the $\pi^+ \pi^-$ channels. We estimate also the light-quark contribution to the missing part of $a_\mu^{\rm HVP}$ not covered in the MUonE experiment.Comment: 34 pages, 20 figures, 7 tables. Version to appear in PR

Neutrino-Nucleus Quasi-Elastic Scattering in a Relativistic Model

A relativistic distorted-wave impulse-approximation model is applied to neutral-current and charged-current quasi-elastic neutrino-nucleus scattering. The effects of final state interactions are investigated and the sensitivity of the results to the strange nucleon form factors is discussed in view of their possible experimental determinationComment: 7 pages, 3 figures Proc. of the XXV International Workshop on Nuclear Theory, 26 June- 1 July, 2006 Rila, Bulgari

Non-perturbative renormalisation of left-left four-fermion operators with Neuberger fermions

We outline a general strategy for the non-perturbative renormalisation of composite operators in discretisations based on Neuberger fermions, via a matching to results obtained with Wilson-type fermions. As an application, we consider the renormalisation of the four-quark operators entering the Delta S=1 and Delta S=2 effective Hamiltonians. Our results are an essential ingredient for the determination of the low-energy constants governing non-leptonic kaon decays.Comment: 14 pages, 3 figure

Proton recoil polarization in exclusive (e,e'pp) reactions

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.Comment: 12 pages, 6 figure

Finite-size scaling for the left-current correlator with non-degenerate quark masses

We study the volume dependence of the left-current correlator with non-degenerate quark masses to next-to-leading order in the chiral expansion. We consider three possible regimes: all quark masses are in the $\epsilon$-regime, all are in the $p$-regime and a mixed-regime where the lighest quark masses satisfy $m_v \Sigma V \leq 1$ while the heavier $m_s \Sigma V \gg 1$. These results can be used to match lattice QCD and the Chiral Effective Theory in a large but finite box in which the Compton wavelength of the lightest pions is of the order of the box size. We consider both the full and partially-quenched results.Comment: 27 pages, 4 figure

Lattice quark masses: a non-perturbative measurement

We discuss the renormalization of different definitions of quark masses in the Wilson and the tree-level improved SW-Clover fermionic action. For the improved case we give the correct relationship between the quark mass and the hopping parameter. Using perturbative and non-perturbative renormalization constants, we extract quark masses in the \MSbar scheme from Lattice QCD in the quenched approximation at $\beta=6.0$, $\beta=6.2$ and $\beta=6.4$ for both actions. We find: \bar{m}^{\MSbar}(2 GeV)=5.7 \pm 0.1 \pm 0.8 MeV, m_s^{\MSbar}(2GeV)= 130 \pm 2 \pm 18 MeV and m_c^{\MSbar}(2 GeV) = 1662\pm 30\pm 230 MeV.Comment: 21 pages, 4 figures, typos corrected, no result change