Task 4.3 - Dibaryon structure and parameter determination


    Coordinator: Daniel Watts 

    Contributors: Mikhail Bashkanov, Nicholas Zachariou



    General description:


    Task 4.3 will focus on study and searches for non-standard (hexaquark) configurations for hadronic matter. The task will primarily focus on the light quark sector (u,d and s quarks), exploiting polarisation observables and beam facilities at the frontiers of intensity to provide new levels of precision in searching for and establishing hexaquark states. A key aim of the network will be to analyse current data and develop new experiments to better establish the structure and basic properties of the recently discovered d*(2380) hexaquark and to search for its expected SU(3) partners. Beyond collaboration with the hadron theoretical and experimental communities, we also aim to develop new interdisciplinary networks with nuclear theorists, astrophysicists, astronomers and cosmologists to explore the potential impact of hexaquarks in these areas.



    Progress [1-18 months]:


    There was substantial progress in the dibaryon field during the first 18 months of the network. For JLAB, exploratory analysis of single-, double-, and triple-strangeness members of the d* SU(3) multiplet are underway based on electroproduction measurements at CLAS12. For d_s (single strangeness (s=1) hexaquark state) a new decay branch into members of the baryon octet was identified for sensitivity studies. These [GC20], utilised polarisation observables (and build on event generator frameworks developed for Task 3). Studies for s=3 indicate modest and smooth background in the region of the d_sss state [MN20]. First calculations of d* multiplet width and branching ratios were performed [B20], indicating strong suppression of dsss decay to Ξ*Σ*, while dsss → ΔΩ and dsss → ΞΣ are “golden” decay channels. The Kaon beam facility at JLAB was recently approved with a science case including hexaquark studies [KLF20].

    At MAMI, new deuterium photodisintegration measurements with polarised photons indicate dominant magnetic octupole and suppressed electric quadrupole moments for the d* [BWZ19, BWP19, BWZ20], consistent with compact hexaquark hypotheses. Recoil polarisation observables indicated both proton and neutron reach full polarisation at the d* mass [BWZ20]. A new proposal for polarised deuteron beamtime was approved by MAMI PAC with “A”rating [MAMI20].

    At Wasa, the latest results on d*(2380) excitation in pn-elastic scattering significantly improve the partial wave analysis fit uncertainty, narrowing down the error in the d* mass/width determination [WASA20].

    Networks, collaborations and research output with astrophysicists have been setup. Earlier work [NS18] indicated the d* could impact on neutron stars (e.g. mass limits). The parameter space for its (poorly known) interaction was explored [NS20]. In cosmology, a paper speculating on Bose-Einstein condensed d* [BW20] as a new dark matter candidate instigated astronomical studies [BEC20a,BEC20b] and was mentioned in the case for next generation gamma-ray telescopes [Egor2020]

    Invited conference presentations include Nstar19 (Watts), Hadron20 postponed, Meson20 postponed, FewBody19. Invited seminars with cosmology and astrophysics groups include QMU UK 2020. We organised an exotic hadron spectroscopy workshop at York which attracted 50 international participants.

    We did not as yet draw on the EU funds as the planned in person-meetings were impacted by Covid19. However, focused collaboration meetings have been held online.







             Dark Matter citations: 













    This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 824093.

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