Beam asymmetry in ′ photoproduction off the proton at the GRAAL experiment

G. Mandaglio , V. Bellini, J. P. Bocquet, M. Capogni , F. Curciarello, A. D’Angelo, V. De Leo, J.P. Didelez, R. Di Salvo, A. Fantini, D. Franco , G. Gervino, F. Ghio, G. Giardina, B. Girolami, A.M. Lapik, P. Levi Sandri, A. Lleres, F. Mammoliti, M. Manganaro , D. Moricciani, A.N. Mushkarenkov, V.G. Nedorezov, D. Rebreyend, N.V. Rudnev, C. Schaerf, M.L. Sperduto, M.C. Sutera, A. Turinge, V. Vegna and I. Zonta


Introduction
The investigation of meson photoproduction off nucleon with polarized photons is a powerful tool to identify the contribution of the broad and widely of the overlap baryon resonances, not easily accessible with the differential cross section measurements only.Polarization observables ( , T , P ) are sensitive, via the interference between complex helicity amplitudes, and consequently allow to reveal small resonance contributions which remain hidden under some dominant contribution in the differential cross section [1][2][3][4].The detailed description of the photon-nucleon interaction requires a complete data set containing, at least, eight independent observables: the cross differential section, the three single polarization observables (beam, target and recoil nucleon) and four, appropriately chosen, double polarization observables [5].The meson photoproduction is a very interesting subject of investigation in baryon spectroscopy because it offers the possibility to extract information on N * nucleon resonances, in the less explored N * mass region.Recently, the CLAS experiment at Jlab and the CB-ELSA-TAPS in Bonn have produced a rich amount of cross section data on the proton [6][7][8] from threshold (1.447 GeV) up to 2.84 GeV.This huge experimental effort established that the N channel couples mainly to S 11 (1535) and P 11 (1710).A marginal role is played by J=3/2 resonances, namely P 13 (1720) and D 13 (1520) [6]; g NN = 1.3-1.5, a value consistent with existing theoretical estimates [6]; above 2 GeV, where the process is dominated by and exchange, the dynamics of photoproduction are similar to those of photoproduction [8].
Different theoretical approaches developed to describe these data [9,10], stressed the importance to have also polarization observables like beam asymmetry to solve the ambiguity on the parameters used in their models.
In this work, we present the preliminary beam asymmetry of photoproduction off proton just above the threshold at Graal.

GRAAL experimental set-up
The GRAAL experiment was located at the ESRF (Grenoble, France) and has been taking data until the end of 2008.The experiment consisted of a polarized photon beam (600-1550 MeV), a unpolarized Hydrogen and Deuterium liquid target and a 4 solid angle detector LAGRAN E (Large Acceptance GRaal-beam Apparatus for Nuclear Experiments).The peculiarities of the -ray beam of the experiment, produced by the backward Compton scattering of laser photons on the relativistic electrons (6.03 GeV) circulating in the storage ring, are the very high degree polarization and the good energy resolution.The energy resolution of the tagged beam is limited by the optics of the ESRF magnetic lattice and is 16 MeV (FWHM) over the entire spectrum.The energy of the -rays is provided by the tagging set-up which is located inside the ESRF shielding, attached to the ESRF vacuum system.The correlation between photon energy and polarization is calculated with QED and it is about 96% in the investigated energy above the photoproduction threshold (1.446 GeV).
The Graal apparatus has been described in several papers [11][12][13][14][15][16].The LAGRAN E detector can be schematically divided into three polar angle region; forward ( ≤ 25 chambers (MWPC) measuring the charged particle angles ( ∼ 1.5 • , ∼ 2 • , FWHM), a double layer scintillator walls (26 vertical × 26 horizontal bars) and a shower wall (16 vertical modules) placed at about 3 and 3.3 meters from the target, respectively.The first scintillator wall is able to measure charged particle position, specific ionization and time of flight, while the other one, made of a sandwich of scintillators and lead, is able to detect and identify charged particles, -rays and neutrons measuring their angles, time of flight and energy loss.In the central region, the target is surrounded by two cylindrical-MWPCs measuring with high efficiency charged particle angles ( ∼ 3.5 • , ∼ 4 • , FWHM), 32 scintillator bars (barrel) measuring the energy loss and the azimuthal angle of charged particles and 480 BGO crystals (15 crowns of 32 crystals) constituting the electromagnetic calorimeter, which is able to measure angles and energy of photons, protons, electrons, and to detect pions and neutrons measuring their angles only.Backward angles, > 155 • , are covered by two disks of plastic scintillators separated by 6 mm of lead to detect charged particles and gamma-rays escaping in the backward direction, and used for veto purposes.At the end of beamline, two flux monitoring detectors are used.

Data analysis and results
The analysis of photoproduction in the , 0 0 and + − decays channels was performed by using the large number of quantities measured with the LAGRAN E detector.We were able to identify protons and charged pions at all angles, photons and neutrons in forward direction < 25 • ; to measure angles and energy of protons, angles and energy of photons in the BGO-calorimeter, angles of charged pions with MWPCs.The preliminary event selection was defined by the following conditions: i) to detect at least two neutral particles in the BGO calorimeter, in order to be able to reconstruct the invariant mass; ii) the energy of the beam has to overcome the photoproduction threshold (E ≥ 1.446 GeV); iii) the events are only detected in the acceptance region defined by the relation between the energy of the beam and the polar angle of proton showed in Fig. 1a.
In Fig. 1b we present the missing mass from the detected proton, in the hypothesis of two particles in the final state reaction, by applying the condition i), (dash-dotted line), the conditions i) and ii) (dashed line), all conditions i-iii) (solid line).Solid line in Fig. 1c clearly shows the presence of the mass peak (mass = 0.957 GeV) dominant over a small and flat background.
The residual background was suppressed by additional constraints on the decay products of meson, like good and invariant mass reconstruction, presence (or absence) of identified charged pions etc.In particular, we were able to identify the decay channels + − (B.R. 42.6%), 0 0

Figure 1 .
Figure 1.Panel a): energy of the beam versus the proton polar angle distribution for a simulated p → p reaction.Panels b)and c) experimental proton missing mass with the cuts defined in condition i) (dash-dotted line); i) and ii) (dashel line), and all conditions i)-iii) (solid line).