Dibaryons – a new state of matter ?

New results in the field of dibaryons are discussed concentrating on the d∗(2380) resonance with I(JP) = 0(3), which has been observed first in the doublepionic fusion to deuterium and meanwhile also in neutron-proton scattering — establishing it thus as a genuine s-channel resonance. Double-pionic fusion reactions to the He isotopes demonstrate that this resonance survives also in a nuclear surrounding. It also has been shown to contribute to dilepton production helping thus to resolve the so-called DLS puzzle. Moreover, the existence of dibaryons has also an impact on the equation of state for nuclear matter.


Introduction
Despite their long painful and unsuccessful history -for a review see Refs.[1][2][3] -dibaryon searches have recently received new interest, in particular by the recognition that there are more complex quark configurations than just the familiar q q and qqq systems.The "hidden color" aspect makes dibaryons a particularly interesting object in QCD [4].
In general the term dibaryon means just a state with baryon number B=2 -independent of its internal structure.It may be a genuine six-quark state with all quarks confined in a single bag or just a baryonic molecule like, e.g., the deuteron representing a sort of trivial dibaryon from the QCD point of view.Of course, the first category appears to be more exciting.

d * (2380) in nuclei
Since in the double-pionic fusion reactions to 3 He [27] and 4 He [28] the signature of this resonance is observed too, it obviously is robust enough to survive even in a nuclear surrounding, which may have interesting consequences for nuclear matter under extreme conditions.
The enhancement in the dilepton spectrum observed in heavy-ion collisions for invariant electronpositron masses in the range 0.15 GeV < M e + e − < 0.6 GeV has recently been traced back to a corresponding enhancement in pn collisions relative to pp collisions [29].Whereas the dilepton spectra from pp collisions are understood quantitatively, theoretical descriptions fail to account for the much higher dilepton rate in pn collisions -in particular regarding the region M e + e − > 0.3 GeV at beam energies below 2 GeV ("DLS Puzzle" [30,31]).In Ref. [32] it has been shown that the missing strength can be attributed to ρ 0 -channel π + π − -production, which is dominated by conventional ΔΔ excitation due to t-channel meson exchange and contributions from d * (2380).

d * (2380) in theoretical calculations
The first prediction for such a resonance has been given by Dyson & Xuong [33] back in 1964shortly after the appearance of the quark model.As it turns out now, its mass prediction has been already amazingly close to the now obtained experimental result.New state-of-the-art three-body [34,35] and quark-model [36][37][38] calculations reproduce the mass quite well, partly also the width.
Dibaryons are bosons, hence not Pauli-blocked and as such allow for higher densities of compressed nuclear matter.The effect of dibaryons on the equation of state for nuclear matter has been considered in various theoretical investigations, see e.g.Refs.[39][40][41], most recently also in Ref. [42].