First measurement of helicity-dependent cross sections in π0η photoproduction from quasi-free nucleons

A Käser,S Kay,I Keshelashvili,V Lisin,K Spieker,A Neiser,S Abt,C Sfienti,D Hornidge,F Afzal,G Ron,A Fix,O Steffen,Th Strub,V Sokhoyan,P Drexler,M Unverzagt,R Miskimen,W Gradl,J Wettig,I Supek,M Korolija,S Garni,F Cividini,Z Ahmed,J Ahrens,A Denig,S Costanza,L Zana,G Reicherz,R Beck,D Werthmüller,S Lutterer,A Mushkarenkov,W Meyer,M Bashkanov,C Collicott,P Achenbach,T Rostomyan,M Ostrick,A Thiel,R Kondratiev,L Heijkenskjöld,M Biroth,M Thiel,E Mornacchi,P Adlarson,D Paudyal,L Witthauer,A Polonski,A Thomas,K Livingston,P Pedroni,M Wolfes,B Krusche,M Oberle,I Gorodnov,V Metag,A Braghieri,M Dieterle,Yu A Usov ,G M Huber ,W J Briscoe ,E J Downie ,H J Arends ,V L Kashevarov ,I J D Macgregor ,A J Sarty ,P B Otte ,S.r Wagner ,G M Gurevich ,A B Lazarev ,D P Watts ,A B Neganov ,N K Walford ,S Prakhov ,D M Manley ,Michael Günther ,P P Martel ,D I Glazier ,J R M Annand ,N S Borisov ,I I Strakovsky

doi:10.1016/j.physletb.2018.10.006

Abstract

The helicity-dependent cross sections for the photoproduction of π0η pairs have been measured for the first time. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball – TAPS calorimeter. The experiment used a polarized deuterated butanol target and a circularly polarized photon beam. This arrangement allowed the σ1/2 (photon and target spin antiparallel) and σ3/2 (parallel spins) components to be measured for quasi-free production of π0η pairs off protons and neutrons. The main finding is that the two helicity components contribute identically, within uncertainties, for both participant protons and neutrons. The absolute couplings for protons and neutrons are also identical. This means that nucleon resonances contributing to this reaction in the investigated energy range have almost equal electromagnetic helicity couplings, A1/2n,p and A3/2n,p. Identical couplings for protons and neutrons are typical for Δ resonances and identical A1/2 and A3/2 components are only possible for J≥3/2 states, which constrains possible contributions of nucleon resonances.

Highlights

Excited states of the nucleon decay almost exclusively by the emission of mesons
The absolute couplings for protons and neutrons are identical. This means that nucleon resonances contributing to this reaction in the investigated energy range have almost equal electromagnetic helicity couplings, An1,/p2 and An3,/p2
The first measurement of the helicity dependence shows in addition that for both target nucleons the asymmetry E is consistent with zero. This means that contributions from the two helicity states must be exactly balanced over the full energy range explored

Summary

Introduction

Excited states of the nucleon decay almost exclusively by the emission of mesons. Photoproduction of mesons is one of the principal tools for studying the nucleon excitation spectrum, which is crucial for the understanding of the strong interaction in the non-perturbative regime. Two-body final states, such as π N, ηN, ..., are still the backbone of most partial wave analyses, the progress in accelerator and detector techniques over the last two decades allows studies of multimeson production reactions with comparable statistical and systematic uncertainties. This advance has opened a new window on spectroscopy that provides access to new questions about the excitation spectrum of nucleons. The obvious nucleon resonances of interest are those states that, due to their internal structure, have only small branching ratios for direct decays to the nucleon ground state Rather they decay predominantly in cascades involving at least one intermediate excited state. Interesting are final states with neutral mesons for which non-resonant background contributions are small

Methods

Results

Conclusion