Experimental evidence for the exotic dibaryon d 1 ∗ $d^{\ast }_{1}$

Abstract

We provide strong experimental evidence for the existence of a nonstrange exotic dibaryon with a mass of about 1956 MeV called \(d^{\ast }_{1}\)(1956). This dibaryon is expected to be stable against strong decay and decays predominantly into two nucleons (NN) via the isospin-conserving radiative process \(d^{\ast }_{1} \to NN \gamma \). First, we present the experimental evidence for the \(d^{\ast }_{1}\)(1956) found in the energy spectrum of the coincident photons emitted at ±900 from the reaction pp → ppγγ at 216 MeV. Then we give an explanation why the WASA/CELSIUS Collaboration did not find signatures of this dibaryon in its proton-proton bremsstrahlung data measured at 310 and 200 MeV. We also present signatures of this dibaryon found in experimental invariant mass spectra of photon pairs from the pp → ppγγ reaction measured by this collaboration at 1360 and 1200 MeV. These signatures provide very substantial confirmation of the existence of the \(d^{\ast }_{1}\)(1956).