Results from polarized experiments at LEGS and GRAALA. D'Angelo, K. Ardashev, C. Bade, O. Bartalini, V. Bellini et al. Citation: AIP Conf. Proc. 1432, 56 (2012); doi: 10.1063/1.3701189 View online: http://dx.doi.org/10.1063/1.3701189 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1432&Issue=1 Published by the American Institute of Physics. Additional information on AIP Conf. Proc.Journal Homepage: http://proceedings.aip.org/ Journal Information: http://proceedings.aip.org/about/about_the_proceedings Top downloads: http://proceedings.aip.org/dbt/most_downloaded.jsp?KEY=APCPCS Information for Authors: http://proceedings.aip.org/authors/information_for_authors

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Results from polarized experiments at LEGS andGRAAL

A. D’Angeloa,b, K. Ardashevc,d, C. Badee, O. Bartalinia,b, V. Bellinif,g,M. Blecherh, J. -P. Bocqueti, M. Capognia,j, A. Caracappak, L. E. Casanob,

M. Castoldil, R. Di Salvob, A. Fantinia,b, D. Francoa,m, G. Gervinon,F. Ghioo, G. Giardinap,q, C. Gibsond, B. Girolamio, A. Giusaf,q, H. Glüklerr,

K. Hickse, S. Hoblitc,k, A. Honigs, T. Kageyak,t, M. Khandakeru,t, O.C. Kistnerk, S. Kizilgule, S. Kucukerc, A. Lapikv, A. Lehmannd, P. Levi

Sandriw, A. Lleresi, M. Lowryk,t, M. Lucase, J. Mahone, F. Mammolitif,q,G. Mandagliop,q, M. Manganarop,q, L. Micelik, D. Moriccianib,

A. Mushkarenkovv, V. Nedorezovv, B. Norumc, M. Papr, B. Preedomd,H. Seyfarthr, C. Randierif,q, D. Rebreyendi, N. Rudnevv, G. Russof,q,

A. Sandorfik,t, C. Schaerfa,b, M. -L. Sperdutof,q, H. Stroherr,M. -C. Suteraf,q, C. E. Thornk, A. Turingev, V. Vegnaa,b, C. S. Whisnantx,

K. Wangc and X. Weik,t

aDip. di Fisica, Universitá di Roma "Tor Vergata", Via della Ricerca Scientifica 1, I-00133 Roma,Italy

bINFN Sezione di Roma Tor Vergata, Via della Ricerca Scientifica 1, I-00133 Roma, ItalycDept. of Physics, University of Virginia, Charlottesville, Virginia 22901, USA

dDept. of Physics, University of South Carolina, Columbia, South Carolina 29208, USAeDept. of Physics, Ohio University, Athens Ohio 45701, USA

fDip. di Fisica, Universitá di Catania , Via Santa Sofia 44, I-95123 Catania, ItalygINFN Laboratori Nazionali del Sud, Via Santa Sofia 44, I-95123 Catania, Italy

hPhysics Dept., Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061,USA

iIN2P3, Laboratoire de Physique Subatomique et de Cosmologie, 38026 Grenoble, FrancejPresent affiliation: ENEA - C.R. Casaccia, via Anguillarese 301, I-00060 Roma, Italy

kPhysics Dept., Brookhaven National Laboratory, Upton, New York 11973, USAlDip. di Fisica, Università degli Studi di Genova, via Dodecaneso 33, I-16146 Genova, Italy

mPresent affiliation: Albert Einstein Center for Fundamental Physics - LHEP - University of Bern,Switzerland

nDip. di Fisica Sperimentale, Università degli Studi di Torino and INFN Sezione di Torino, viaPietro Giuria 1, I-10125 Torino, Italy

oIstituto Superiore di Sanità, viale Regina Elena 299, I-00161 Roma, Italy and INFN Sezione diRoma, piazzale Aldo Moro 2, I-00185 Roma, Italy

pDip. di Fisica, Università di Messina, salita Sperone 31, I-98166 Messina, ItalyqINFN Sezione di Catania, via Santa Sofia 64, I-95123 Catania, Italy

rForschungszentrum Jülich GmbH, D-52425 Jülich, GermanysDept. of Physics, Syracuse University, Syracuse, New York 13210, USA

tPresent affiliation:Thomas Jefferson National Accelerator Facility, Newport News, Virginia23606, USA

uNorfolk State University, Norfolk, Virginia 23606, USAvInstitute for Nuclear Research, 60-letiya Oktyabrya prospekt 7a, 117312 Moscow, Russia

wINFN Laboratori Nazionali di Frascati, via E. Fermi 40, I-00044 Frascati, ItalyxJames Madison University, Harrisonburg, Virginia 22807, USA

The 8th International Workshop on the Physics of Excited NucleonsAIP Conf. Proc. 1432, 56-61 (2012); doi: 10.1063/1.3701189

© 2012 American Institute of Physics 978-0-7354-1018-3/$30.00

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Abstract. Compton backscattering gamma ray beams are characterized by a high degree of linearand circular polarization with low unpolarized backgrounds and have proven to provide very precisemeasurements of polarization observables. Latest results from LEGS and GRAAL experiments onproton and deuteron targets are presented. The Σ beam asymmetry for ω photoproduction has beenmeasured by the GRAAL collaboration for both the ω → π0γ and the ω → π+π−π0 decay channelson the proton target; single and double polarization asymmetries have been provided also for the Kphotoproduction channel. E and G double polarization asymmetries for single pion photoproductionon the proton and deuteron have been measured at LEGS using a frozen spin HD target.

Keywords: Photoreactions, polarized photon beams, polarized targets.PACS: 25.20-x,24.70+s

1. INTRODUCTION

Constituent Quark Models (CQM) have been used in the last decades to predict theresonant structure of the nucleon[1, 2, 3]. Excited baryon states (N∗) are classified byisospin, parity and spin within each oscillator band. Comparison with experimentaldata shows, however, that only the lowest few resonances in each band have beenseen in πN reactions. Moreover the g(πN) couplings are predicted to decrease rapidlywith increasing mass in each oscillator band and higher levels are expected to havelarger couplings to KΛ, KΣ,ππN,ηN and ωN channels. Therefore, unseen or missingresonances may show their presence in other reaction channels such as K,η ,ω andmulti-pion photo-production on the nucleons.

An alternative explanation comes from the di-quark model in which two quarks in thenucleon are assumed to be quasi-bound in a color isotriplet; the baryon states are madeof diquark-quark combinations that are net color iso-singlet. If all possible internal di-quark excitations are considered the full spectrum of CQM is obtained. But if internaldi-quark excitations are frozen out (having both spin and and isospin equal to zero)a large reduction in the number of baryon degrees of freedom is predicted, providingabout the same number of N∗ states, seen in πN reactions.

Unravelling the N∗ spectrum is still a challenge. Photo-nuclear reactions on theproton, in the resonance energy region, have been deeply investigated in recent yearsand detailed knowledge of differential and total cross sections is presently availablefor most of the possible reaction channels. However amplitudes extraction from datahave never been accomplished because they require measurements from polarizationobservables, whose data set is much sparser. Models have conjectured resonances andadjusted couplings to compare with limited data. An experiment capable of providingenough single and double polarization observable on both proton and neutron targets, sothat a model independent extraction of all spin- and isospin- dependent amplitudes maybe performed from the data, is called complete experiment[4, 5]. It requires linearly andcircularly polarized photon beams and longitudinally and transversely polarized targets.A large effort is going on in worldwide facilities to realize this goal. The most recentresults from the LEGS and the GRAAL experiments are here shown, providing newsingle and double polarization measurements on single pion, omega and kaon photo-production channels.

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FIGURE 1. Preliminary results on the Σ beam asymmetry of the ω photo-production reaction channelon a proton target from the GRAAL collaboration. See text for legend.

2. THE LEGS AND GRAAL EXPERIMENTS

Both LEGS and GRAAL experiments used Compton backscattered photon beams, pro-duced by the interaction of an Ion-Argon Laser with the electrons circulating inside astorage ring: the National Synchrotron Radiation Facility at the Brookhaven NationalLaboratory (USA), in the case of LEGS, and the European Synchrotron Radiation Fa-cility in Grenoble (France), in the case of GRAAL. The two tagged photon beams hadcomplementary energy ranges: (220-480) MeV, covering the first nucleon resonance re-gion, in the case of LEGS; (550-1480)MeV, corresponding to the second and third nu-cleon resonance regions, in the case of GRAAL. Since relativistic electrons conservehelicity in their scattering process with the incoming Laser photon beam, also backscat-tered photons retain their initial degree of polarization. Therefore if the Laser light islinearly or circularly polarized, the Compton backscattered photon beam is also linearlyor circularly polarized to a high degree. The polarization degree slowly decreases as afunction of the electron scattering angle and scattered photon energy, but it could be kepthigher than 90% at LEGS and 60% at GRAAL, over the entire tagged energy range, bychanging the Ion-Argon Laser line. Both experiments were equipped with a large solidangle detector. In the case of the LEGS experiment a NaI crystal box was used to detectphotons and charged particles for polar angles between 20◦ and 160◦. It was alterna-tively equipped with an internal neutron barrel, made of plastic scintillators, optimizedfor neutron detection or with a Time Projection Chamber (TPC), for charged particletracking. At forward angles two walls, made of plastic scintillators and of Lead Glasscrystals, respectively, allowed to detect both charged and neutral particles. A polarizedfrozen-spin HD target completed the experimental set-up. Both proton and deuteronscould be independently polarized, with polarization degrees up to 60% and 35% forprotons and deuterons, respectively.

The GRAAL set-up was complemented by the LAGRANγE detector. It was made of aBGO ball covering polar angles between 25◦ and 155◦, internally equipped with a plasticscintillator barrel and a double layer of cylindrical Multi Wire Proportional Chambers(MWPC). The detector was optimized for photon detection up to 1.5 GeV and for protondetection up to 300 MeV. At forward angles a double wall of planar MWPC was backed

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FIGURE 2. Results on the Ox and Oz double polarization observables for the γ + p → K+Λ reactionfrom the GRAAL collaboration. Results are compared with Ghent Isobar (Regge plus resonance) model(dashed curve) and the Bonn-Gatchina model (soulid curves).

by a double wall of plastic scintillators for charged particles detection and a shower wallmade of sandwiches of plastic scintillators and lead, for neutral particles detection.

Both experiments were characterized by very low levels of backgrounds, due tounpolarized or untagged photons impinging the target. Since only H or D targets wereused, very low backgrounds coming from heavier nuclear target components (suchas Carbon in buthanol targets or Nitrogen in ammonia targets) contaminated the dataacquisition, allowing for excellent signal/background separation.

3. RESULTS FROM THE GRAAL COLLABORATION

Σ beam asymmetries of pseudo-scalar meson photo-production reactions on the nucle-ons have been systematically measured by the GRAAL collaboration[6] and comparedto partial wave analysis predictions from SAID [7] and MAID[8]. The new preciseGRAAL data required a revision of both SAID and MAID solutions leading to severalchanges in the updated versions.

A sample of preliminary results on the Σ beam asymmetry of the ω photo-productionon a proton target from the GRAAL collaboration is shown in Fig.1. Full circles and fullsquares correspond to the analysis of the ω → π0γ and ω → π+π−π0 decay channels,respectively. Results are compared with existing data: open circles are from radiativeomega decay channel from[9]; oped squares are from the omega three pions decaychannel from[10]. Data are also compared with prediction from[11]: solid and dottedcurves correspond to the calulation including or not the P13(1720) resonance. New Graalresults are lower than existing measurements but show a very good agreement amongthe two ω meson decay channels and with the full theoretical prediction from [11].

The study of the γ + p → K++Λ reaction is particularly interesting because the weakdecay of the Λ recoil allows for the measurement of the P recoil asymmetry, from theΛ → π− + p decay distribution. If a linearly polarized photon beam is used, the Oxand the Oz double polarization asymmetries may be measured. A sample of the resultsobtained by the GRAAL collaboration on these observables[12] are shown in Fig.2,together with Ghent Isobar (Regge plus resonance) model (dashed curve)[13] and the

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FIGURE 3. Measurements of the E polarization observable for the γ p → π+n (left two panels), andγ p → π0 p (right two panels) at LEGS.

Bonn-Gatchina model (solid curves)[14]. A general good agreement is found betweendata and the Bonn-Gatchina Model, which requires the introduction of new or poorlyknown resonances in the 1900 MeV mass region (P and/or D) to correctly reproduce thedata.

4. RESULTS FROM THE LEGS COLLABORATION

A circularly polarized photon beam has been used on a longitudinally polarized HDtarget to measure the E observable for both γ p → π+n and γ p → π0 p reactions, in theenergy range between 220 and 480 MeV, by the LEGS experiment. The E observablemay be directly related to the difference of cross sections for parallel and anti-parallelpolarizations of beam and target, as it appears in the GDH spin sum rule. The results areshown in Fig. 3 for a few energy bins, together with predictions from the SAID[FA07k]solution of partial wave analysis[7]. The curves slightly overestimate the experimentalresults and LEGS measurements allowed for a new evaluation of the GDH sum rule[15],closer to the theoretical prediction. Using linearly polarized photons on a longitudinallypolarized HD target it was also possible to measure the G observable for the same pionphoto-production channels on the proton. Some preliminary results are shown in Fig. 4together with previous data from [16] and predictions form the SAID[FA07k] solution.The new data show that while a reasonable agreement exists with both existing resultsand predictions for the charged pion production, opposite sign and higher strength thanpredicted are found for the neutral pion channel. This effect is still under investigationand it may indicate that a larger D-wave component of the ∆(1232) than currentlyestablished is necessary to describe the data.

5. CONCLUSIONS

The latest results from the LEGS and GRAAL collaboration have been reported in theaim of enlarging the experimental data-set on single and double polarization observablesfor meson photo-production reactions, in the nucleon resonance energy region. Similar

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FIGURE 4. Measurements of the G polarization observable for the γ p → π+n (left panel), and γ p →π0 p (right panel) at LEGS. Results are compared with previous measures at Mainz and with SAID[SP09k]partial wave analysis predictions.

efforts are currently made by other collaborations such as CLAS at the Thomas JeffersonNational Accelerator Facility, the CB-ELSA collaboration at the Bonn University, Mamiat Mainz and LEPS at Spring8 in Japan. A very attractive reaction candidate is theγN → K + Λ reaction, because of the "self-analyzing" properties of the Λ recoil. Thesereactions are being measured by the CLAS collaboration on both buthanol and HDpolarized targets with polarized photon beams. The goal of performing the first completeexperiment is very close to being achieved.

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