Threshold [formula omitted] photoproduction on transverse polarised protons at MAMI

S Schumann,S Garni,W Gradl,M Dieterle,R Kondratiev,M Wolfes,J Mancel,A Polonski,V Lisin,V Sokhoyan,K Livingston,M Unverzagt,R Beck,D Hornidge,C Collicott,Th Strub,I Supek,B Krusche,P Hall-Barrientos,T Rostomyan,B Oussena,D Howdle,G Reicherz,M Ostrick,O Steffen,M Korolija,P Pedroni,S Cherepnya,I Keshelashvili,M Oberle,L Tiator,N S Borisov ,I I Strakovsky ,W Meyer,D G Middleton ,S Prakhov,D M Manley ,A M Bernstein ,C Fernández-Ramírez,L.v Filʼkov ,E F Mcnicoll ,D Watts ,A B Lazarev ,A Fix,H J Arends ,C S Akondi ,D I Glazier ,P P Martel ,B M K Nefkens ,D Paudyal,Yu A Usov ,L Witthauer,S Costanza,I J D Macgregor ,P B Otte ,G M Gurevich ,R Miskimen,E J Downie ,Philipp Sebastian Ott ,D Hamilton,M H Sikora ,A J Sarty ,V V Polyanski ,A B Neganov ,A Thomas,A Nikolaev,W J Briscoe ,J R M Annand ,A Braghieri,V L Kashevarov ,G M Huber ,H Ortega,D Werthmüller,A Mushkarenkov

doi:10.1016/j.physletb.2015.09.015

Abstract

Polarisation-dependent differential cross sections σT associated with the target asymmetry T have been measured for the reaction γp→→pπ0 with transverse target polarisation from π0 threshold to photon energies of 190 MeV. The data were obtained using a frozen-spin butanol target with the Crystal Ball / TAPS detector set-up and the Glasgow photon tagging system at the Mainz Microtron MAMI. Results for σT have been used in combination with our previous measurements of the unpolarised cross section σ0 and the beam asymmetry Σ for a model-independent determination of S- and P-wave multipoles in the π0 threshold region, which includes for the first time a direct determination of the imaginary part of the E0+ multipole.

Highlights

Recent accurate threshold π0 photoproduction [1] and pionic H and D atom experiments [2, 3] have supported the concept that low-energy πN dynamics reflect the sponta-September 11, 2015 neous chiral symmetry breaking in QCD and the quasi Nambu-Goldstone boson nature of the pion [4–6]
In this Letter, we present the first results in the nearthreshold region for the γp → π0p transverse polarised target cross section σT = σ0T, where T is the polarised target asymmetry and σ0 is the unpolarised differential cross section [16]
The reaction γp → pπ0 on a transverse polarised target has been measured at the Mainz Microtron (MAMI) electron accelerator facility [26] using the Glasgow tagging spectrometer [27, 28] and the Crystal Ball / TAPS detector set-up

Summary

Introduction

Recent accurate threshold π0 photoproduction [1] and pionic H and D atom experiments [2, 3] have supported the concept that low-energy πN dynamics reflect the sponta-. With our past measurement of the γp → π0p reaction [1] using linear polarised photons and an unpolarised liquid H2 target we have provided the first assessment of the energy range where ChPT can be applied to this reaction [1, 7–9] These are first steps towards an accurate measurement of the energy dependence of the pion photoproduction multipoles. Isospin breaking leads to a slightly higher threshold for nπ+, which produces a unitary cusp in the E0+ multipole for γp → pπ0 This cusp causes a rapid rise in the imaginary part of ImE0+ for photon energies ω just above the γp → nπ+ reaction threshold [22, 23], which has been directly observed for the first time in this experiment. The magnitude of the decrease is model dependent so it cannot be completely specified by unitarity

Experimental set-up

Experimental results

Multipole extraction

Conclusion and outlook