Decays of an exotic 1-+ hybrid meson resonance in QCD

Decays of an exotic 1-+ hybrid meson resonance in QCD

Using cutting-edge lattice Quantum Chromodynamics (QCD) methods, in a recent calculation [Phys. Rev. D103, 054502 (2021), arXiv:2009.10034] we demonstrated the presence of an exotic hybrid meson resonance for the first time. The results suggest that it is related to the  resonance observed by the COMPASS experiment at CERN.

Most observed hadrons (strongly bound clusters of quarks like protons and neutrons) can be explained in models as three-quark and quark-antiquark states. However, in recent years experiments have found a number of puzzling hadrons that do not appear to fit with this picture. One suggestion is that some are ‘hybrid’ mesons containing an excited gluonic field along with a quark and an antiquark. While hybrids have been studied in phenomenological models, this work was the first time a hybrid had been investigated from first principles in Quantum Chromodynamics, the fundamental theory of strongly-interacting matter, taking into account that it is a resonance, i.e. it can decay to lighter hadrons. We demonstrated the presence of a resonance containing light quarks with an exotic combination of spin , parity  and charge-conjugation , , that cannot arise from solely a quark-antiquark pair, and identified it as a hybrid meson. The calculation used unphysically heavy quarks; extrapolating to physical quark masses, the results are compatible with the  resonance observed by COMPASS in  and, interestingly, suggest that the dominant decay mode may actually be via , very relevant for ongoing and planned experiments.

Excited and exotic bottomonium spectroscopy from lattice QCD

In another study [JHEP 02 (2021) 214, arXiv:2008.02656], we computed the spectrum of bottomonium mesons containing a bottom quark and a bottom antiquark. Investigating a variety of  quantum numbers, many excited states were determined, as shown in the figure, including some with exotic quantum numbers ( ) that were identified as hybrid mesons. In models, hybrids can also arise with non-exotic quantum numbers and we observed states that could be identified as such hybrids, e.g. with  where experimental candidates exist. The pattern of hybrid mesons, highlighted in red and blue in the figure, suggests that the physics of bottomonium hybrids is similar to that of hybrid mesons and baryons with light, strange and charm quarks. These calculations neglected the unstable nature of the hybrids and further studies are required to understand these interesting states in more detail, like the investigation of the light hybrid meson resonance described above.

This work was made possible in part through the DiRAC Data Intensive Service hosted at the University of Cambridge.