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Recent ab-initio calculations showed that oversized solute atoms (OSA) in BCC and FCC iron exhibit a very strong attraction with a nearby vacancy (V) at first neighbour distance (1NN). The attraction it is so large that the 1NN pair OSA+V is no longer stable and relaxes spontaneously towards a new configuration where the OSA sits in the middle of the bond, the two ends of which are decorated with two half-vacancies (V/2). Taking into account the formation of this complex V/2+OSA+V/2 in the migration process has never been done before: it requires a new formulation of correlation effects. This is the aim of the present contribution which establishes the expression of the diffusion coefficient of the OSA and evaluates exactly the correlation factor in BCC and FCC lattices, thanks to the introduction of macrojumps. The formalism is applied to the case of Y in BCC iron and its results are compared to those obtained by previous authors. It is shown that the smallness of the correlation factor is not due to the transport mechanism of the OSA but only to the jump frequencies of the vacancy in the vicinity of the OSA. This smallness of the correlation factor is however overcompensated by a large macrojump frequency, and, as a result, the Y atom diffuses more rapidly than iron at all temperatures by orders of magnitude in the two structures., Comment: 47 pages, 8 figures, 8 tables, 7 appendices

The $\Sigma$ beam asymmetry in $\eta^{\prime}$ photoproduction off the proton was measured at the GrAAL polarised photon beam with incoming photon energies of 1.461 and 1.480 GeV. For both energies the asymmetry as a function of the meson production angle shows a clear structure, more pronounced at the lowest one, with a change of sign around 90$^o$. The observed behaviour is compatible with P-wave D-wave (or S-wave F-wave) interference, the closer to threshold the stronger. The results are compared to the existing state-of-the-art calculations that fail to account for the data., Comment: 6 pages, 4 figures, accepted for publication on Eur. Phys. J A

The correlation factor for diffusion by the divacancy mechanism in an FCC lattice has been the target of a long quest. Amounting to 0.54 at the beginning, its value was progressively decreased down to 0.4582 with a confidence interval of 0.0005. The limitations of previous studies came from the matrix method at work which confined the defect trajectories to a volume of finite extension around the tracer atom, or, more basically, from the availability of computing time for the numerical simulations. The present study determines the exact value thanks to a double Laplace-Fourier transform of the transport equation, which takes into account all the divacancy trajectories without any limitation of their lengths. The exact value is lower than all the preceding evaluations, but falls within the confidence interval which was estimated from the best evaluations based on simulations. One interesting byproduct of our analytical approach is the proof of the equality t11=t22., Comment: 33 pages; 3 figures; one main section; 5 appendices

A formalism using a double Laplace Fourier transform of the transport equation yields the return probabilities of the vacancy in the vicinity of the tracer atom in the presence of solute-vacancy interactions of arbitrary extension. Studying model cases, it is shown that taking into account the full range of the interaction may change noticeably the correlation factor. The latter depends tightly on the pattern of migration barriers which is chosen to describe the vacancy jumps around the tracer atom. A thorough ab initio evaluation of all barriers is rarely available in the literature. It is shown that approximations often used to overcome this lack of information can be misleading. The examination of dilute systems recently studied shows that the interactions within the first three neighbour shells dictate the final value with a good precision. The main improvement of the modelling comes from dropping the restrictive assumption which impose an equal value to the jump frequencies leading to a dissociation of the solute-vacancy pair., Comment: 45 pages; 8 figures

The $\Sigma$ beam asymmetry in the photoproduction of negative pions from quasi-free neutrons in a deuterium target was measured at Graal in the energy interval 700 - 1500 MeV and a wide angular range, using polarized and tagged photons. The results are compared with recent partial wave analyses., Comment: 8 pages, 9 figures, 1 table, resubmitted to Phys. Rev. C

The possibility of anisotropies in the speed of light relative to the limiting speed of electrons is considered. The absence of sidereal variations in the energy of Compton-edge photons at the ESRF's GRAAL facility constrains such anisotropies representing the first non-threshold collision-kinematics study of Lorentz violation. When interpreted within the minimal Standard-Model Extension, this result yields the two-sided limit of 1.6 x 10^{-14} at 95% confidence level on a combination of the parity-violating photon and electron coefficients kappa_{o+} and c. This new constraint provides an improvement over previous bounds by one order of magnitude., Comment: 4 pages, 4 figures

When the electrons stored in the ring of the European Synchrotron Radiation Facility (ESRF, Grenoble) scatter on a laser beam (Compton scattering in flight) the lower energy of the scattered electron spectra, the Compton Edge (CE), is given by the two body photon-electron relativistic kinematics and depends on the velocity of light. A precision measurement of the position of this CE as a function of the daily variations of the direction of the electron beam in an absolute reference frame provides a one-way test of Relativistic Kinematics and the isotropy of the velocity of light. The results of GRAAL-ESRF measurements improve the previously existing one-way limits, thus showing the efficiency of this method and the interest of further studies in this direction., Comment: Proceed. MG12 meeting, Paris, July, 2009

Beam asymmetry and differential cross section for the reaction gamma+p->eta+p were measured from production threshold to 1500 MeV photon laboratory energy. The two dominant neutral decay modes of the eta meson, eta->2g and eta->3pi0, were analyzed. The full set of measurements is in good agreement with previously published results. Our data were compared with three models. They all fit satisfactorily the results but their respective resonance contributions are quite different. The possible photoexcitation of a narrow state N(1670) was investigated and no evidence was found., Comment: 18 pages, 14 figures, 4 tables Submitted to EPJA

The measurement of the Compton edge of the scattered electrons in GRAAL facility in European Synchrotron Radiation Facility (ESRF) in Grenoble with respect to the Cosmic Microwave Background dipole reveals up to 10 sigma variations larger than the statistical errors. We now show that the variations are not due to the frequency variations of the accelerator. The nature of Compton edge variations remains unclear, thus outlining the imperative of dedicated studies of light speed anisotropy.

We have studied the angular fluctuations in the speed of light with respect to the apex of the dipole of Cosmic Microwave Background (CMB) radiation using the experimental data obtained with GRAAL facility, located at the European Synchrotron Radiation Facility (ESRF) in Grenoble. The measurements were based on the stability of the Compton edge of laser photons scattered on the 6 GeV monochromatic electron beam. The results enable to obtain a conservative constraint on the anisotropy in the light speed variations \Delta c(\theta)/c < 3 10^{-12}, i.e. with higher precision than from previous experiments., Comment: The published version: reported at the workshop "Testing the Equivalence Principle in Space and on Ground" (STEP), Pescara, Italy, September, 2004

A new experiment is proposed with the aim to study eta-mesic nuclei and low-energy interactions of eta with nuclei. Two decay modes of eta produced by a photon beam inside a nucleus will be observed, namely a collisional decay \eta N \to \pi N inside the nucleus and the radiative decay \eta \to \gamma \gamma outside. In addition, a collisional decay of stopped S_{11}(1535) resonance inside the nucleus, S_{11}(1535) N \to N N, will be studied. The experiment can be performed using the tagged photon beam at ESRF with the end-point energy 1000 MeV and the GRAAL detector which includes a high-resolution BGO calorimeter and a large acceptance lead-scintillator time-of-flight wall. Some results of simulation and estimates of yields are given., Comment: 20 pages, 19 figures

The Sigma beam asymmetry for positive pion photoproduction on the proton has been measured over an angular range of 40-170 deg at photon energies from 0.8 to 1.5 GeV. The resulting data set includes 237 accurate points, 136 of these belonging to an almost unexplored domain above 1.05 GeV. Data of such high precision provide severe constraints for partial wave analyses. The influence of this experiment on the GW multipole analysis is demonstrated. Significant changes are found in multipoles connected to the S31(1620) and P13(1720) resonances. Comparisons using the MAID analysis are also presented., Comment: 11 pages, 4 eps figures. to be published in Physics Letters B

The differential cross section for the reaction p(gamma, eta p) has been measured from threshold to 1100 MeV photon laboratory energy. For the first time, the region of the S11(1535) resonance is fully covered in a photoproduction experiment and allows a precise extraction of its parameters at the photon point. Above 1000 MeV, S-wave dominance vanishes while a P-wave contribution is observed whose nature will have to be clarified. These high precision data together with the already measured beam asymmetry data will provide stringent constraints on the extraction of new couplings of baryon resonances to the eta meson., Comment: 10 pages, 5 figures, submitted to Phys. Letters B. Typos corrected. Some more information on the S11(1535) parameters