First measurement of the helicity asymmetry E in photoproduction on the proton

I Senderovich,B Mckinnon,A El Alaoui,G Fedotov,S Strauch,V Sytnik,J Zhang,V Credé,K Hafidi,N Sparveris,S Stepanyan,D Ho,P Peng,X Wei,N Zachariou,I Zonta,D Keller,M Ungaro,T Mineeva,B Garillon,V Batourine,M Osipenko,D Jenkins,P Stoler,O Cortes,S Niccolai,E Pasyuk,S Pisano,G Rosner,S Fegan,D Rönchen,V Mokeev,M Guidal,A Kim,M Holtrop,D Adikaram,E Voutier,I Niculescu,L Zana,E Golovatch,F Sabatié,N Dashyan,W Gohn,K Livingston,A Movsisyan,M Battaglieri,Y Prok,G Niculescu,L El Fassi,O Pogorelko,E Seder,R De Vita,K Hicks,Iu Skorodumina,G Charles,S Park,P Mattione,A Filippi,I Bedlinskiy,D Schott,V Kubarovsky,A Simonyan,J Ball,A Fradi,M Dugger,L Elouadrhiri,Y Ilieva,H Voskanyan,M Contalbrigo,K Park,P Eugenio,M Ripani,S Chandavar,M Khandaker,M Hattawy,L Guo,P Roy,Z Akbar,S Joosten,Ye Tian,E De Sanctis,J Brock,A Celentano,P Collins,P Lenisa,H Egiyan,G Khachatryan,L Colaneri,W Phelps,A Rizzo,Y Ghandilyan,C Carlin,H Hakobyan,C Djalali,S Boiarinov,S Anefalos Pereira,C Salgado,C Hanretty,A Deur,K L Giovanetti ,J W Price ,B Morrison ,A D’angelo ,D I Glazier ,D Meekins ,L A Net ,Michael Kunkel ,Michael Döring ,H S Jo ,W J Briscoe ,G D Smith ,I J D Macgregor ,Ross Tucker ,Barry Ritchie ,M H Wood ,R Dupré ,D G Ireland ,G P Gilfoyle ,P L Cole ,B S Ishkhanov ,D I Sober ,C Keith ,C Muñoz Camacho ,J T Goetz ,K A Griffioen ,S M Hughes ,C A Meyer ,F J Klein ,N K Walford ,Z W Zhao ,J A Fleming ,I I Strakovsky ,W K Brooks ,K P Adhikari ,P Nadel-Turoński ,R A Montgomery ,R W Gothe ,Hao Jiang ,D S Carman ,F X Girod ,R Badui ,H Y Lu ,A J R Puckett ,A S Biselli ,R A Schumacher ,N A Baltzell ,K S Joo ,Volker Burkert ,M L Seely

doi:10.1016/j.physletb.2016.01.044

Abstract

Results are presented for the first measurement of the double-polarization helicity asymmetry E for the $\eta$ photoproduction reaction $\gamma p \rightarrow \eta p$. Data were obtained using the FROzen Spin Target (FROST) with the CLAS spectrometer in Hall B at Jefferson Lab, covering a range of center-of-mass energy W from threshold to 2.15 GeV and a large range in center-of-mass polar angle. As an initial application of these data, the results have been incorporated into the J\"ulich model to examine the case for the existence of a narrow $N^*$ resonance between 1.66 and 1.70 GeV. The addition of these data to the world database results in marked changes in the predictions for the E observable using that model. Further comparison with several theoretical approaches indicates these data will significantly enhance our understanding of nucleon resonances.

Highlights

Much activity is being devoted to establishing the details of the excitation spectrum of the nucleon in order to deepen our understanding of that fundamental strongly-interacting three-quark system
Results are presented for the first measurement of the double-polarization helicity asymmetry E for the η photoproduction reaction γ p → ηp
While some resonances are well established, fewer states have been observed than most constituent quark models and Lattice QCD calculations predict [1]

Summary

Introduction

Much activity is being devoted to establishing the details of the excitation spectrum of the nucleon in order to deepen our understanding of that fundamental strongly-interacting three-quark system. Polarization observables involve interferences between sets of amplitudes, so their measurement can provide stringent tests for predictions of the photoproduction process and help sort out ambiguities in the theoretical description of the reaction in terms the resonances involved. One such polarization observable is the helicity asymmetry E in pseudoscalar meson photoproduction, which is the normalized difference in photoproduction yield when spins of the incident photon and a longitudinally-polarized target are parallel and anti-parallel. The η photoproduction process on the proton acts as an “isospin filter” for the nucleon resonance spectrum, resulting in a useful tool for disentangling the different states, and is especially important in finding and investigating states that do not couple strongly to pions

The experiment

Analysis

Results and discussion