Search for strange four-quark states in the charmonium sector with BESIII

• Applicant: Professor Dr. Alfons Khoukaz
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 443159800

The search for exotic hadrons, states that do not fit the conventional assignment as either three quark states for baryons or quark-antiquark states for mesons, is a highly topical issue in hadron spectroscopy. We propose to search for the Z_cs, the missing strange partner of the established exotic four-quark Z_c states, using new and existing data of the BESIII experiment. In a recent review, the search for these Z_cs states was named as one of the most important topics in experimental hadron spectroscopy in the coming years. Both mass and decay widths for four decay channels of the lightest Z_cs with J^P=1^+ have been predicted by theory using QCD sum rules. Two of these decays include open charm mesons while the other two channels include hidden charm mesons. We intend to search for the production of the Z_cs in combination with one charged kaon in electron-positron annihilations via the identification of these four decay modes. The proposed complex analyses are planned to be performed by two PhD students. While the first one will analyze final states including two D-mesons, the second one will focus on final states with charmonia. Therefore, the analyses will be complementary to each other and will require a close cooperation in the final interpretation. With the BESIII experiment, we already collected about 17 1/fb of e+e- collision data in the center-of-mass energy range of 4.0 GeV to 4.6 GeV thus far, dedicated to the spectroscopy of exotic states in the charmonium sector. After an upgrade of the Beijing Electron Positron Collider II (BEPCII), that has already been finished, new data in the center-of-mass region 4.6 GeV to 4.7 GeV most relevant for the studies proposed here will be gathered in the upcoming measurement period between October 2019 and June 2020.Assuming that the relative strength of a hypothetical Z_cs contribution to the e+e- → J/ψ K+K- channel is of the same magnitude as for the Z_c(3900) in e+e- → J/ψ π+π-, an observation of the Z_cs via its decay into a J/ψ and a charged kaon is estimated. This would mark the first ever observation of the Z_cs and a discovery that is equally groundbreaking as the original discovery of the Z_c(3900), which was selected as one of the standout discoveries of 2013 by APS Physics and is still the most cited BESIII publication. Moreover, the other three channels are equally important, as they do not only pose significant discovery potential concerning the Z_cs, but also allow to search for new vector resonances formed in the initial e+e- annihilation.

• DFG Programme: Research Grants