Eur. Phys. J. A (2015) 51: 77
https://doi.org/10.1140/epja/i2015-15077-0

Letter

First measurement of the beam asymmetry in photoproduction off the proton near threshold

¹ INFN, Laboratori Nazionali di Frascati, 00044, Frascati, Italy
² Dipartimento di Fisica e di Scienze della Terra, Università di Messina, 98166, Messina, Italy
³ INFN, Gruppo collegato di Messina, 98166, Messina, Italy
⁴ Dipartimento di Matematica e Fisica dell’Università “Roma Tre”, INFN, Sezione di Roma Tre, Roma, Italy
⁵ INFN, Sezione di Roma Tor Vergata, 00133, Roma, Italy
⁶ Dipartimento di Fisica, Università degli Studi di Roma Tor Vergata, 00133, Roma, Italy
⁷ Dipartimento di Fisica, Università degli Studi di Catania, 95123, Catania, Italy
⁸ INFN, Sezione di Catania, 95123, Catania, Italy
⁹ LPSC, Université Grenoble-Alpes, CNRS/IN2P3, Grenoble IPN, F-38026, Grenoble, France
¹⁰ IN2P3, Institut de Physique Nucléaire, 91406, Orsay, France
¹¹ INFN, Sezione di Torino, 10125, Torino, Italy
¹² Dipartimento di Fisica Sperimentale, Università degli Studi di Torino, I-10125, Torino, Italy
¹³ INFN, Sezione di Roma, 00185, Roma, Italy
¹⁴ Istituto Superiore di Sanità, I-00161, Roma, Italy
¹⁵ Institute for Nuclear Research, 60-letiya Oktyabrya prospekt 7a, 117312, Moscow, Russia

^* e-mail: gmandaglio@unime.it

Received: 16 April 2015
Revised: 16 June 2015
Accepted: 18 June 2015
Published online: 8 July 2015

Abstract

The beam asymmetry in photoproduction off the proton was measured at the GrAAL polarised photon beam with incoming photon energies of 1.461 and 1.480 GeV. For both energies the asymmetry as a function of the meson production angle shows a clear structure, more pronounced at the lowest one, with a change of sign around . The observed behaviour is compatible with P-wave D-wave (or S-wave F-wave) interference, the closer to threshold the stronger. The results are compared to the existing state-of-the-art calculations that fail to account for the data.