On the existence of the $\it NN$-decoupled dibaryon $d^\ast_1$(1956)

Abstract

We present strong experimental evidence for the existence of a nonstrange $\it NN$-decoupled dibaryon with a mass of about 1956 MeV stable against strong decay, which is a very likely candidate for long-sought multiquark hadrons with the baryon number B=2. We start by presenting the first evidence for the existence of this dibaryon called $d^{\ast }_{1}$(1956), that was found in the energy spectrum of coincident photons emitted at $\pm 90^0$ from the reaction $pp \to pp \gamma\gamma$ at an energy of 216 MeV measured at JINR. We then show its signatures found in several experimental energy spectra of single photons and invariant mass spectra of photon pairs from photon production processes induced by nucleon-nucleon and nucleon-nucleus collisions at intermediate energies.