Eur. Phys. J. A (2021) 57: 342
https://doi.org/10.1140/epja/s10050-021-00625-2

Regular Article - Experimental Physics

Progress and opportunities in backward angle (u-channel) physics

¹ Mississippi State University, 39762, Starkville, MS, USA
² Institute of Computer Science, Pedagogical University of Krakow, 30-084, Kraków, Poland
³ Justus Liebig Universität Gießen, 35390, Gießen, Germany
⁴ University of Connecticut, 06269, Storrs, CT, USA
⁵ University of California, 95616, Davis, CA, USA
⁶ Thomas Jefferson National Accelerator Facility, 23606, Newport News, VA, USA
⁷ University of Regina, S4S 0A2, Regina, SK, Canada
⁸ Lawrence Berkeley National Laboratory, 94720, Berkeley, CA, USA
⁹ William & Mary, 23185, Williamsburg, VA, USA
¹⁰ Departamento de Física Teórica, Universidad Complutense de Madrid and IPARCOS, 28040, Madrid, Spain
¹¹ Departament de Física Quàntica i Astrofísica and Institut de Ciències del Cosmos, Universitat de Barcelona, E08028, Barcelona, Spain
¹² Hampton University Proton Therapy Institute, 40 Enterprise Parkway, 23666, Hampton, VA, USA
¹³ National Yang Ming Chiao Tung University, 30010, Hsinchu, Taiwan
¹⁴ CPHT, CNRS, Ecole polytechnique, 91128, I.P. Paris, Palaiseau, France
¹⁵ National Research Centre Kurchatov Institute: Petersburg Nuclear Physics Institute, 188300, Gatchina, Russia
¹⁶ Higher School of Economics, National Research University, 194100 St, Petersburg, Russia
¹⁷ National Centre for Nuclear Research, NCBJ, 02-093, Warsaw, Poland
¹⁸ Korea Aerospace University, 10540, Goyang, South Korea

^k billlee@jlab.org

Received: 15 July 2021
Accepted: 4 November 2021
Published online: 31 December 2021

Abstract

Backward angle (u-channel) scattering provides complementary information for studies of hadron spectroscopy and structure, but has been less comprehensively studied than the corresponding forward angle case. As a result, the physics of u-channel scattering poses a range of new experimental and theoretical opportunities and questions. We summarize recent progress in measuring and understanding high energy reactions with baryon charge exchange in the u-channel, as discussed in the first Backward angle (u-channel) Physics Workshop. In particular, we discuss backward angle measurements and their theoretical description via both hadronic models and the collinear factorization approach, and discuss planned future measurements of u-channel physics. Finally, we propose outstanding questions and challenges for u-channel physics.