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M. Guidal

Publications and source records attributed to M. Guidal.

At least 91 records · Page 5Linked to original sources
arXiv2010

Constraints on the $\tilde{H}$ Generalized Parton Distribution from Deep Virtual Compton Scattering Measured at HERMES

We have analyzed the longitudinally polarized proton target asymmetry data of the Deep Virtual Compton process recently published by the HERMES collaboration in terms of Generalized Parton Distributions. We have fitted these new data in a largely model-independent fashion and the procedure results in numerical constraints on the $\tilde{H}_\mathrm{Im}$ Compton Form Factor. We present its $t-$ and $ξ-$ dependencies. We also find improvement on the determination of two other Compton Form Factors, $H_\mathrm{Re}$ and $H_\mathrm{Im}$.

hep-ph↗
arXiv2010

Measurement of Single and Double Spin Asymmetries in Deep Inelastic Pion Electroproduction with a Longitudinally Polarized Target

We report the first measurement of the transverse momentum dependence of double spin asymmetries in semi-inclusive production of pions in deep inelastic scattering off the longitudinally polarized proton. Data have been obtained using a polarized electron beam of 5.7 GeV with the CLAS detector at the Thomas Jefferson National Accelerator Facility (JLab). A significant non-zero $\sin2ϕ$ single spin asymmetry was also observed for the first time indicating strong spin-orbit correlations for transversely polarized quarks in the longitudinally polarized proton. The azimuthal modulations of single spin asymmetries have been measured over a wide kinematic range.

hep-ex↗
arXiv2010

Generalized Parton Distributions from Deep Virtual Compton Scattering at CLAS

We have analyzed the beam spin asymmetry and the longitudinally polarized target spin asymmetry of the Deep Virtual Compton Scattering process, recently measured by the Jefferson Lab CLAS collaboration. Our aim is to extract information about the Generalized Parton Distributions of the proton. By fitting these data, in a largely model-independent procedure, we are able to extract numerical values for the two Compton Form Factors $H_{Im}$ and $\tilde{H}_{Im}$ with uncertainties, in average, of the order of 30%.

hep-ph↗
arXiv2009

Generalized Parton Distributions from Deeply Virtual Compton Scattering at HERMES

The HERMES collaboration has recently published a set of (correlated) beam charge, beam spin and target spin relative asymmetries for the Deeply Virtual Compton Scattering process. This reaction allows in principle to access the Generalized Parton Distributions of the nucleon. We have fitted, in the QCD leading-order and leading-twist handbag approximation, but in a model independent way, this set of data and we report our results for the extracted Compton Form Factors. In particular, we are able to extract constrains on the $H$ GPD.

hep-ph↗
arXiv2009

Nucleon Form Factors from Generalized Parton Distributions

We discuss the links between Generalized Parton Distributions (GPDs) and elastic nucleon form factors. These links, in the form of sum rules, represent powerful constraints on parametrizations of GPDs. A Regge parametrization for GPDs at small momentum transfer, is extended to the large momentum transfer region and it is found to describe the basic features of proton and neutron electromagnetic form factor data. This parametrization is used to estimate the quark contribution to the nucleon spin.

hep-ph↗
arXiv2008

A fitter code for Deep Virtual Compton Scattering and Generalized Parton Distributions

We have developped a fitting code based on the leading-twist handbag Deep Virtual Compton Scattering (DVCS) amplitude in order to extract the Generalized Parton Distributions (GPD) information from DVCS observables in the valence region. In a first stage, with simulations and pseudo-data, we show that the full GPD information can be recovered from experimental data if enough observables are measured. If only part of these observables are measured, valuable information can still be extracted, certain observables being particularly sensitive to certain GPDs. In a second stage, we make a practical application of this code to the recent DVCS Jefferson Lab Hall A data from which we can extract numerical constraints for the two $H$ GPD Compton Form Factors.

hep-ph↗
arXiv2008

Electroproduction of $ϕ(1020)$ mesons at $1.4\leq Q^2\leq$ GeV$^2$ measured with the CLAS spectrometer

Electroproduction of exclusive $ϕ$ vector mesons has been studied with the CLAS detector in the kinematical range $1.4\leq Q^2\leq 3.8$ GeV$^{2}$, $0.0\leq t^{\prime}\leq 3.6$ GeV$^{2}$, and $2.0\leq W\leq 3.0$ GeV. The scaling exponent for the total cross section as $1/(Q^2+M_ϕ^2)^n$ was determined to be $n=2.49\pm 0.33$. The slope of the four-momentum transfer $t'$ distribution is $b_ϕ=0.98 \pm 0.17$ GeV$^{-2}$. Under the assumption of s-channel helicity conservation (SCHC), we determine the ratio of longitudinal to transverse cross sections to be $R=0.86 \pm 0.24$. A 2-gluon exchange model is able to reproduce the main features of the data.

nucl-ex↗
arXiv2008

Measurement of beam-recoil observables Ox, Oz and target asymmetry for the reaction gamma p -> K Lambda

The double polarization (beam-recoil) observables Ox and Oz have been measured for the reaction gamma p -> K Lambda from threshold production to Egamma = 1500 MeV. The data were obtained with the linearly polarized beam of the GRAAL facility. Values for the target asymmetry T could also be extracted despite the use of an unpolarized target. Analyses of our results by two isobar models tend to confirm the necessity to include new or poorly known resonances in the 1900 MeV mass region.

nucl-ex↗
arXiv2008

Lowering the Light Speed Isotropy Limit: European Synchrotron Radiation Facility Measurements

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.

astro-ph↗
arXiv2008

Discussion on the recent proton-DVCS results of Jefferson Lab

We present the recent data issued from the Halls A and B of Jefferson Laboratory for the Deep Virtual Compton Scattering Process on the proton. An important set of data for beam spin asymmetries, unpolarized cross sections and differences of polarized cross sections have been obtained. We modestly attempt a first "global" analysis of these three observables at a single $ $, $ $ and $<-t>$ kinematic point. We find that it is extremely challenging to describe simultaneously these data in the framework of a few Generalized Parton Distributions models.

hep-ph↗
arXiv2007

Measurement of eta photoproduction on the proton from threshold to 1500 MeV

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.

nucl-ex↗
arXiv2005

Eta photoproduction off the neutron at GRAAL

The gamma n -> eta n quasi-free cross section reveals a resonant structure at W ~ 1.675 GeV. This structure may be a manifestation of a baryon resonance. A priori its properties, the possibly narrow width and the strong photocoupling to the neutron, look surprising. This structure may also signal the existence of a narrow state.

hep-ex↗
arXiv2005

Photoproduction of the Theta^+ resonance on the nucleon in a Regge model

We estimate the reaction mechanisms for the photoproduction of the Theta^+(1540) resonance on the nucleon, through K and K^* Regge exchanges. We compare the size of the cross sections for the gamma n -> K^- Theta^+ and gamma p -> K^0 bar Theta^+ reactions, and investigate their sensitivity to the spin-parity assignments J^P = 1/2^\pm, 3/2^\pm for the Theta^+ resonance. The model allows to estimate the cross sections corresponding with a given upper bound on the width of the Theta^+. Within this model, the cross sections on the neutron are found to be around a factor 5 larger than the ones on the proton, due to the presence of charged K exchange for the reaction on a neutron target. Furthermore, the photon asymmetry is found to display a pronounced sensitivity to the parity of the Theta^+, making it a very promising observable to help determining the quantum numbers of the Theta^+ resonance.

hep-ph↗
arXiv2005

Probing the Light Speed Anisotropy with respect to the Cosmic Microwave Background Radiation Dipole

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 Δc(θ)/c < 3 10^{-12}, i.e. with higher precision than from previous experiments.

astro-ph↗
arXiv2004

Power corrections to the process $γ^{*}γ\to ππ$ in the running coupling method

Power-suppressed corrections coming from the end-point integration regions in the amplitude of the process $γ^{*}γ\to ππ$ at large $Q^2$ and small squared center-of-mass energy $W^2$ are calculated in the QCD hard-scattering approach where the amplitudes factorize in a hard perturbatively calculable part and a generalized distribution amplitude. The running coupling method and the technique of infrared renormalon calculus are applied to obtain Borel resummed expressions for the two main components of the process amplitude. Numerical estimates for these power corrections are presented. They are sizeable when $Q^2 < 10 GeV^2$.

hep-ph↗
arXiv2004

Deep Virtual Compton Scattering and the Nucleon Generalized Parton Distributions

In the following, the subject of Deep Virtual Compton Scattering on the nucleon and its relation to the recently introduced concept of Generalized Parton Distributions are briefly reviewed. The general theoretical framework and the links between theory and experiment will be outlined and the recently published data which look promising for the development of this field will be discussed. Finally, the experimental prospectives of the domain will be presented.

hep-ph↗