Eur. Phys. J. A (2023) 59: 217
https://doi.org/10.1140/epja/s10050-023-01123-3

Regular Article - Experimental Physics

Measurement of the helicity asymmetry $\mathbb{E}$ for the $\overrightarrow{\gamma }\overrightarrow{p}\to p{\pi }^0$ reaction in the resonance region

The CLAS Collaboration

¹ Argonne National Laboratory, 60439, Argonne, IL, USA
² Arizona State University, 85287, Tempe, AZ, USA
³ California State University, Dominguez Hills, 90747, Carson, CA, USA
⁴ Canisius College, 14208, Buffalo, NY, USA
⁵ Carnegie Mellon University, 15213, Pittsburgh, PA, USA
⁶ The Catholic University of America, 20064, Washington, DC, USA
⁷ IRFU, CEA, Universitè Paris-Saclay, 91191, Gif-sur-Yvette, France
⁸ Christopher Newport University, 23606, Newport News, VA, USA
⁹ University of Connecticut, 06269, Storrs, CT, USA
¹⁰ Duquesne University, 15282, Pittsburgh, PA, USA
¹¹ Fairfield University, 06824, Fairfield, CT, USA
¹² Università di Ferrara, 44121, Ferrara, Italy
¹³ Florida International University, 33199, Miami, FL, USA
¹⁴ Florida State University, 32306, Tallahassee, FL, USA
¹⁵ The George Washington University, 20052, Washington, DC, USA
¹⁶ GSI Helmholtzzentrum fur Schwerionenforschung GmbH, 64291, Darmstadt, Germany
¹⁷ INFN, Sezione di Ferrara, 44100, Ferrara, Italy
¹⁸ INFN, Sezione di Genova, 16146, Genova, Italy
¹⁹ INFN, Sezione di Roma Tor Vergata, 00133, Rome, Italy
²⁰ INFN, Sezione di Torino, 10125, Torino, Italy
²¹ INFN, Sezione di Pavia, 27100, Pavia, Italy
²² Institut für Kernphysik, Johannes Gutenberg University of Mainz, 55099, Mainz, Germany
²³ Kyungpook National University, 41566, Daegu, South Korea
²⁴ Lamar University, 77710, Beaumont, TX, USA
²⁵ Massachusetts Institute of Technology, 02139, Cambridge, MA, USA
²⁶ Mississippi State University, 39762, Mississippi State, MS, USA
²⁷ National Research Centre Kurchatov Institute, Institute for Theoretical and Experimental Physics, 117259, Moscow, Russia
²⁸ New Mexico State University, 88003, Las Cruces, NM, USA
²⁹ Norfolk State University, 23504, Norfolk, VA, USA
³⁰ National Research Centre Kurchatov Institute, Petersburg Nuclear Physics Institute, 188300, Gatchina, Russia
³¹ Ohio University, 45701, Athens, OH, USA
³² Old Dominion University, 23529, Norfolk, VA, USA
³³ II Physikalisches Institut, Justus Liebig University Giessen, 35392, Giessen, Germany
³⁴ Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, 119234, Moscow, Russia
³⁵ Temple University, 19122, Philadelphia, PA, USA
³⁶ Thomas Jefferson National Accelerator Facility, 23606, Newport News, VA, USA
³⁷ Universitè Paris-Saclay, CNRS/IN2P3, IJCLab, 91405, Orsay, France
³⁸ University of New Hampshire, 03824, Durham, NH, USA
³⁹ University of Richmond, 23173, Richmond, VA, USA
⁴⁰ Università di Roma Tor Vergata, 00133, Rome, Italy
⁴¹ University of South Carolina, 29208, Columbia, SC, USA
⁴² Universidad Tècnica Federico Santa María, Casilla 110-V, Valparaìso, Chile
⁴³ Università degli Studi di Brescia, 25123, Brescia, Italy
⁴⁴ University of California Riverside, 92521, Riverside, CA, USA
⁴⁵ University of Glasgow, G12 8QQ, Glasgow, UK
⁴⁶ University of York, YO10 5DD, York, UK
⁴⁷ Virginia Polytechnic Institute and State University, 24061, Blacksburg, VA, USA
⁴⁸ University of Virginia, 22901, Charlottesville, VA, USA
⁴⁹ College of William and Mary, 23187, Williamsburg, VA, USA
⁵⁰ Yerevan Physics Institute, 375036, Yerevan, Armenia
⁵¹ Montgomery College, 20876, Germantown, MD, USA
⁵² University of Nebraska, 68198, Omaha, NE, USA

^b nick.zachariou@york.ac.uk

Received: 11 May 2023
Accepted: 31 August 2023
Published online: 26 September 2023

Abstract

The double-spin-polarization observable $\mathbb{E}$ for $\overrightarrow{\gamma }\overrightarrow{p}\to p{\pi }^0$ has been measured with the CEBAF Large Acceptance Spectrometer (CLAS) at photon beam energies $E_{\gamma }$ from 0.367 to $2.173\text{GeV}$ (corresponding to center-of-mass energies from 1.240 to $2.200\text{GeV}$) for pion center-of-mass angles, $cos{\theta }_{{\pi }^0}^{c.m.}$, between $-$ 0.86 and 0.82. These new CLAS measurements cover a broader energy range and have smaller uncertainties compared to previous CBELSA data and provide an important independent check on systematics. These measurements are compared to predictions as well as new global fits from The George Washington University, Mainz, and Bonn-Gatchina groups. Their inclusion in multipole analyses will allow us to refine our understanding of the single-pion production contribution to the Gerasimov-Drell-Hearn sum rule and improve the determination of resonance properties, which will be presented in a future publication.