WASA-at-COSY

Despite their long painful history dibaryon searches (where dibaryon means a baryon number $B=2$ state independently on the internal structure: genuine six-quark state/baryonic-molecule) have recently received new interest, in particular by the recognition that there are more complex quark configurations than just the familiar $q\bar{q}$ and $qqq$ systems. The "hidden color" aspect makes dibaryons a particularly interesting object in QCD. A resonance like structure recently observed in double-pionic fusion to deuteron, at $M=2.38 GeV$ with $\Gamma=70 MeV$ and $I(J^p)=0(3^+)$ meanwhile proved to be the so called “inevitable dibaryon” $d^$. To investigate its structure we have measured its decay branches into the $d\pi^0\pi^0$, $d\pi^+\pi^-$, $pp\pi^-\pi^0$, $pn\pi^0\pi^0$ and $pn$ channels by $pd$ and $dp$ collisions in the quasi-free reaction mode, utilizing the WASA detector setup at COSY. The $pn$ decay channel was measured by use of polarized deuterons in inverse kinematics. These new $np$ analyzing power data exhibit a pronounced resonance effect in their energy dependence. The SAID partial-wave analysis with inclusion of these data reveals a pole in the complex plane of the $^3D_3$ partial wave at $(2380\pm10) MeV-i(40\pm5) MeV$ in accordance with the $d^$ resonance hypothesis. Since in the double-pionic fusion reactions to $^3He$ and $^4He$ 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 the nuclear matter under extreme conditions. In addition to the Wasa-at-Cosy results also the latest results in this field as well as the influence of dibaryons on other areas of physics, including heavy ion collisions, will be discussed. The enhancement in the dilepton spectrum observed in heavy-ion collisions for invariant electron-positron masses in the range 0.15 GeV $< M_{e^+e^-} <$ 0.6 GeV has recently been traced back to a corresponding enhancement in proton-neutron (pn) collisions relative to $pp$ collisions. 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"). We show that the missing strength can be attributed to $\rho^0$-channel $\pi^+\pi^-$-production, which is dominated by $t$-channel $\Delta\Delta$ excitation and the recently found isoscalar dibaryonic resonance $d^*$ at $M \approx$ 2.37 GeV.

Supported by COSY-FFE (FZ Jülich).