Non-Strange Dibaryons Studied In Coherent Double Neutral-Meson Photoproduction On The Deuteron

The B = 2 bound /resonance state (dibaryon) is an interesting object, which can be a molecule consisting of two baryons or a spatially compact hexaquark hadron object. The gamma d -> pi(0)pi(0)d reaction has been experimentally investigated at incident energies ranging from 0.58 to 1.15 GeV to study non-strange dibaryons. The angular distributions of deuteron emission in the gamma d center-of-mass energy cannot be reproduced by quasi-free production of two neutral pions followed by deuteron coalescence. Additionally a 2.14-GeV peak is observed in the pi(0)d invariant mass distribution. These suggest a sequential process such as gamma d -> R-IS -> pi R-0(IV) -> pi(0)pi(0)d with an isoscalar dibaryon R-IS and an isovector dibaryon R-IV. Since the mass of the observed isoscalar dibaryons are close to the sum of the nucleon (N) and nucleon resonance (N*) masses, an S-wave NN* molecule may play a role as a doorway to a dibaryon.