Observation of a narrow enhancement in the pp mass distribution from the reaction γd → pp π−

Abstract

Studying the reaction γd → pp π −, a narrow resonance structure in the pp invariant mass distribution has been observed at 2014 ± 2 MeV/ c 2 with a statistical significance of 4.7 standard deviations. Using a bremsstrahlung photon beam ( k 0 = 520 MeV), the kinematics of the reaction were completely determined by detecting both outgoing protons in two large solid angle time-of-flight spectrometers. By accepting only proton momenta greater than ∼300 MeV/ c, the predominant contribution from quasi-free pion production was suppressed.