GRAAL Experiment Research Articles

The light speed versus the observer: the Kennedy\u2013Thorndike test from GRAAL-ESRF

High precision tests of the light speed constancy for all observers as a empirical basis of Special Relativity have continuously been among the goals of advanced experimental studies. Based on the Compton edge method proposed by us (Gurzadyan and Margarian in Phys Scr 53:513, 1996), a constraint on the one-way light speed isotropy and Lorentz invariance violation has been obtained at the dedicated GRAAL experiment at European Synchrotron Radiation Facility (ESRF, Grenoble) (Gurzadyan et al. in Mod Phys Lett A 20:1, 2005, Nuovo Cimento 122:515, 2007, Proceedings of the XII M. Grossmann meeting on general relativity, vol B. World Scientific, p 1495. arXiv:1004.2867, 2012; Bocquet et al. in Phys Rev Lett 104:241601, 2010). Using the GRAAL data we now get a new constraint on one of the key tests of Special Relativity—the Kennedy–Thorndike experiment (Kennedy and Thorndike in Phys Rev 42:400, 1932) in probing light speed invariance with respect to the velocity of the observer (apparatus). Our analysis takes advantage of GRAAL’s setup where two separate energy scales are involved: first, via the position of the Compton edge determining the light speed in the reference frame of incident 6 GeV electrons within the tagging system, second, in the calorimeter via the 1.27 MeV photons of the ^{22}Na source. The two energy scales are engaged to each other through production of eta mesons by tagged laser Compton backscattered gamma -rays. The accuracy of the calibration and stability of energies reached in each section enable us to obtain the limit of 7 times 10^{-12} for the Kennedy–Thorndike test, which improves the currently existing limits by three orders of magnitude.

Nuclear multifragmentation by 700–1500 MeV photons: New data of GRAAL experiment

The cross sections of carbon nucleus photodisintegration into protons and neutrons with high multiplicity for photon energies from 700 to 1500 MeV were measured. The experiment was performed at the tagged photon beam of the GRAAL setup using the wide-aperture detector LAGRANγE. It was shown that multifragmentation up to complete disintegration into separate nucleons is initiated by elementary reactions of meson photoproduction with a subsequent intranuclear cascade.

Measurement of theΣbeam asymmetry for theωphotoproduction off the proton and the neutron at the GRAAL experiment

We report on new measurements of the beam asymmetry for \omega\ photo-production on proton and neutron in Hydrogen and Deuterium targets from the GRAAL collaboration. The beam asymmetry values are extracted from the reaction threshold (E_{\gamma} = 1.1 GeV in the free nucleon kinematics) up to 1.5 GeV of incoming photon energy. For the first time both the radiative and the three-pion decay channels are simultaneously investigated on the free proton. Results from the two decay channels are in agreement and provide important constraints for the determination of resonant state contributions to the \omega\ production mechanism. First experimental results on the deuteron allow the extraction of the \Sigma\ beam asymmetry on quasi-free nucleons. Comparison of the results for free and quasi-free kinematics on the proton shows a generally reasonable agreement, similar to the findings in pseudo-scalar meson photo-production reactions. For the first time measurements on quasi-free neutrons are available, showing that both the strength and the angular distributions of the beam asymmetry are sensibly different from the results on the proton target.

Analysis of theη′photoproduction off the proton and preliminary beam asymmetry results at the GRAAL experiment

Differential and total cross section measurements on eta' photoproduction were published by the CLAS Collaboration (M. Dugger et al., Phys. Rev. Lett. 96, 062001 (2006) and M. Williams et al., Phys.Rev.C80, 045213 (2009)) for center-of-mass energies from near the threshold up to 2.84 GeV, and by the CB-ELSA-TAPS Collaboration (V. Crede et al., Phys. Rev. C80, 055202 (2009)) up to 2.36 GeV and also making a precise threshold scan of the differential cross section in the 1446 -1527.4 MeV gamma beam energy range. However, the wide information about reaction cross sections are not sufficient to understand the role of resonances involved in the process. Different theoretical works stressed the importance to have also polarization observables in order to solve the ambiguity in the choice of the parameters used in their models. We present the analysis of the eta' photoproduction off the proton, identifying the meson via the gamma gamma, pi(0)pi(0)eta, and pi(+)pi(-)eta decay modes by using the GRAAL apparatus; and we show the preliminary GRAAL results on the beam asymmetry Sigma from the threshold (1.446 GeV) up to 1.5 GeV.

Constraints on spacetime anisotropy and Lorentz violation from the GRAAL experiment

The GRAAL experiment could constrain the variations of the speed of light. The anisotropy of the speed of light may imply that the spacetime is anisotropic. Finsler geometry is a reasonable candidate to deal with the spacetime anisotropy. In this paper, the Lorentz invariance violation (LIV) of the photon sector is investigated in the locally Minkowski spacetime. The locally Minkowski spacetime is a class of flat Finsler spacetime and refers a metric with the anisotropic departure from the Minkowski one. The LIV matrices used to fit the experimental data are represented in terms of these metric deviations. The GRAAL experiment constrains the spacetime anisotropy to be less than \(10^{-14}\). In addition, we find that the simplest Finslerian photon sector could be viewed as a geometric representation of the photon sector in the minimal standard model extension (SME).

A theoretical diagnosis on light speed anisotropy from GRAAL experiment

The light speed anisotropy, i.e., the variation of the light speed with respect to the direction in an “absolute” reference frame, is a profound issue in physics. The one-way experiment, performed at the GRAAL facility of the European Synchrotron Radiation Facility (ESRF) in Grenoble, reported results on the light speed anisotropy by Compton scattering of laser photons on high-energy electrons. So far, most articles concerned with the GRAAL data have established only the upper bounds on the anisotropy parameters based on available theories. We use a new theory of the Lorentz invariance violation to analyse the available GRAAL data and obtain the stringent upper limit of the order 2.4×10-14 on the Lorentz violation parameters. In the meantime, we also can reproduce the allowed light speed anisotropy appearing in the azimuthal distribution of the GRAAL experimental data, and find that the best-fit parameters are compatible with the competitive upper bounds.

THE BGO-OPENDIPOLE SETUP AT ELSA

At the electron accelerator facility ELSA of the University of Bonn presently the new BGO-OpenDipole experiment is being setup. It is optimized for meson photoproduction final states of mixed charges. The detector combines hermetic coverage for photon detection through the BGO ball of the former GRAAL experiment with high resolution detection of forward going charged particles in the Open Dipole spectrometer. This setup is well suited for many different channels, including pseudoscalar, vectormeson and associated strangeness photoproduction. This contribution highlights the strangeness channels associated with the Λ and Σ ground states and the Λ(1405) excited state.

PRELIMINARY RESULTS ON η′ PHOTOPRODUCTION AT GRAAL

The γp → η′p reaction was studied near the threshold, in the 1.44 GeV > Eγ < 1.55 GeV energy range with the tagged photons of the former GRAAL experiment. The two neutral decays γγ(Γi/Γ = 2.10 ± 0.12%) and π0π0η(Γi/Γ = 20.7 ± 1.2%) were analized and preliminary results of the beam asymmetry were extracted together with the invariant masses.

New methods for studying the interaction of unstable mesons with nuclei

Recent results of the GRAAL experiment and simulation of meson photoproduction on nuclei are presented.

Eta-meson photoproduction on protons, neutrons, and light nuclei in the nucleon-resonance region

Eta-meson photoproduction on protons, neutrons, and light nuclei is reviewed. New results obtained for photoproduction on protons and neutrons at the GRAAL facility are analyzed in detail. The experiment in question was performed in a gamma-ray beam that was obtained by means of Compton backscattering of laser photons on electrons at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). A comparative analysis of data on the differential cross sections and the asymmetries for eta-meson photoproduction allowed us to reveal some special features and to study the mechanisms of eta-meson photoproduction on nucleons in detail. Data available from the literature on eta-meson photoproduction on light nuclei are discussed in detail. Results that suggest the existence of bound states (etamesic nuclei) are presented. Particular attention is given to the newmethod of “taggedmesons,” which, according to the GRAAL experiment and to simulated results, makes it possible to study thoroughly the interaction of unstable mesons with intranuclear nucleons.

Vertex identification of events in photonuclear reactions by cylindrical multiwire proportional chambers

In the framework of investigation of meson photo-production on proton and neutron in a deuterium target at the GRAAL experiment, we present a method to identify the vertex of interaction when in reaction products there are at least two charged particles, by the information provided by the cylindrical multiwire proportional chambers. By this device, we are able to obtain the direction of charged particles that leave the target after a photo-production reaction. For each charged particle, the device provides the coordinates of two points in the laboratory reference system, and the z-axis represents the beam direction. We calculate the trajectory of each charged particle and its cross point, representing the vertex of the reaction.

Simultaneous Photoproduction ofηandπ0Mesons on the Proton

The analysis of the gammap-->etapi(0)p reaction has been performed using data from the GRAAL experiment. The total and differential cross sections and the beam asymmetry have been obtained from threshold up to 1.5 GeV of beam energy. The two resonances S11(1535) and Delta(1700) are expected to be excited in the intermediate states of this reaction. The results are used to test predictions based on the assumption that both resonances are dynamically generated from the meson-baryon interaction provided by chiral Lagrangians. The term involving the Delta(1700) excitation, followed by the decay into etaDelta(1232), is found to be dominant.

MESON PHOTOPRODUCTION ON THE NEUTRON AT GRAAL

Using a Deuterium target, the GRAAL experiment has collected meson photoproduction data. The data have been analysed in the quasi free regime. The results of the quasi free process on the proton reproduce quite well the results previously obtained with a Hydrogen target. The preliminary results on the quasi free neutron for π0 and η photoproduction are presented and discussed.

A new [formula omitted] resonance in the [formula omitted] reaction

We study the η-photoproduction focusing on the new nucleon resonance which was observed at s=1675MeV with a narrow decay width (∼10MeV) in the recent GRAAL experiment. Using an effective Lagrangian approach, we compute differential cross sections for the η-photoproduction. In addition to N∗(1675), we employ three other nucleon resonances, i.e., N∗(1535), N∗(1650) and N∗(1710), and vector meson exchanges which are the most relevant ones to this reaction process. As a result, we can reproduce the GRAAL data qualitatively well and observe obvious isospin asymmetry between the transition magnetic moments of N∗(1675): μγnn∗≫μγpp∗.

Very high-energy γ-ray observations of the Crab nebula and other potential sources with the GRAAL experiment

The “γ-ray astronomy at Almeria” (GRAAL) experiment uses 63 heliostat-mirrors with a total mirror area of ≈2500 m 2 from the CESA-1 field at the “Plataforma Solar de Almeria” to collect Cherenkov light from air showers. The detector is located in a central solar tower and detects photon-induced showers with an energy threshold of 250±110 GeV and an asymptotic effective detection area of about 15 000 m 2. A comparison between the results of detailed Monte-Carlo simulations and data is presented. Data sets taken in the period September 1999–September 2000 in the direction of the Crab pulsar, the active galaxy 3C 454.3, the unidentified γ-ray source 3EG J1835+59 and a “pseudosource” were analyzed for high energy γ-ray emission. Evidence for a γ-ray flux from the Crab pulsar with an integral flux of 2.2±0.4 ( stat) +1.7 −1.3( syst)×10 −9 cm −2 s −1 above threshold and a significance of 4.5 σ in a total measuring time of 7 h and 10 min on source was found. No evidence for emission from the other sources was found. Some difficulties with the use of heliostat fields for γ-ray astronomy are pointed out. In particular the effect of field-of-view restricted to the central part of a detected air shower on the lateral distribution and timing properties of Cherenkov light are discussed. Upon restriction the spread of the timing front of proton-induced showers sharply decreases and the reconstructed direction becomes biased towards the pointing direction. This is shown to make efficient γ-hadron separation difficult.

Results from the Graal experiment

The features of the Graal experiment at ESRF are presented, The obtained results are discussed and future developments outlined

GRAAL Experiment at the ESRF

GRAAL Experiment at the ESRF

The data acquisition system for the GRAAL experiment at the ESRF in Grenoble

A specific data acquisition system System Acquisition Graal Asic, (SAGA) has been implemented for a multidetector experiment. Its main feature is an hardware event builder associated with compact and programmable electronics based on two kinds of ASIC circuits linked by a bus, located on the detectors. The transfer rate on this bus is about 23 Mbyte/s.