Task 3.1 - Search for and study of light exotic mesons, charmonium and strangeonium


    Coordinator: Vincent Mathieu

    Contributors: Derek Glazier , Bryan McKinnon , Miriam Fritsch , Marco De Stefanis , Ulrike Thoma , Sebastian Neubert , Angela Badala, K.Peters, Nora Brambilla, Isabella Garzia, Gianluigi Cibinetto, Giulio Mezzadri, Ilaria Balossino, Marco Scodeggio



    General description:


    In the recent years, high-energy physics experiments have delivered a lot of unexpected exotic hadron resonances, that challenge the quark model of mesons with a quark-antiquark pair. Candidates for tetraquarks, molecules, and for hadrons with gluonic degrees of freedom have been found. In the heavy quark sector, their unknown nature has led to the denomination of "XYZ states". In fixed target experiments, these possible resonances are often produced in association with baryon resonances. In this case, the desired meson final states need to be either separated from the meson signal by kinematical cuts or parameterized jointly with the baryonic structures. With the meson final states single out, establishing the existence of isolated resonances that go beyond the minimal quark model require performing coupled-channels partial-wave analysis at a level never achieved before and comparing these measurements to modern theoretical predictions. A joint effort between theorists and experimentalists is therefore needed to embed theoretical calculation into Monte-Carlo simulations and developing new models to analyse experimental data.
    The GlueX and CLAS collaborations at the Jefferson Lab have an ongoing program to detect light exotic meson in electro- and photo-production. Their programs complement the COMPASS effort at CERN who recently observed the lightest hybrid meson π1 decaying in both ηπ-  and  η'π- final states. These channels, as well as their neutral partners η(')π0  are currently under investigation by the GlueX collaboration to confirm the production of exotic meson in photoproduction. The forthcoming data obtained in electro-production by the CLAS detector should provide further inside on the gluonic degrees of freedom of the π1 wave function.
    The BESIII collaboration collected a total of 10 billion of J/ψ in 2019, which can be used to improve previous analyses and to study more in detail and with an unprecedented accuracy the light hadron spectrum. The new scan data collected between 4.29 and 4.42 GeV in the center of mass energy, together with the new data set between 4.6 and 4.7 GeV, will be helpful to better investigate the "XYZ states". LHCb is also actively studying the properties of "XYZ states" produced diffractively and in B meson decays.



    Progress [1-18 months]:


    In the last 18 months, studies on exotic mesons and  the light and charme quarks sector progressed with a significant experimental activity: light quark hadron spectroscopy using  photoproduction and electroproduction with GlueX and CLAS at Jefferson Lab; pion-production with COMPASS at CERN (who recently observed the lightest hybrid meson π1 decaying in both ηπ- and η'π- final states);  charmonium 'XYZ' exotic states in e+e- collisions at BESIII and further investigation using B meson decay at LHCb.

    In Ref. [Ma19], the formalism to extract polarized observables in two-meson photoproduction has been developed. A new observable, sensitive to exotic meson, has been proposed. This formalism is currently implemented in several GlueX and CLAS12 analyses. Most of XYZ candidates are observed in three-body final states. In Ref. [Mi20], the general formalism to analyse any three-body is presented. A code, ready for direct implementation in data analysis, is available online on the JPAC website [Jpac]. Theoretical cross sections of exclusive XYZ production at the future EIC has been calculated in Ref. [Al20]. The code available from this article has in addition been incorporated into an event generator for electroproduction of XYZ which will be used to plan for an EIC spectroscopy program [Gl21]. Using a non relativistic effective field theory description the spin structure of hybrids has been investigated [Br20].





    • [Ma19] V. Mathieu et al. [JPAC], "Moments of angular distribution and beam asymmetries in ηπphotoproduction at GlueX", Phys. Rev. D 100, no.5, 054017 (2019)
    • [Mi20] M. Mikhasenko et al. [JPAC], Phys. Rev. D 101 (2020) no.3, 034033 doi:10.1103/PhysRevD.101.034033
    • [Jpac] https://ceem.indiana.edu/jpac
    • [Al20] M. Albaladejo et al. [JPAC], "XYZ spectroscopy at electron-hadron facilities: Exclusive processes", [arXiv:2008.01001 [hep-ph]].
    • [Gl21] D.I. Glazier, J. Stevens, D. Winney, https://github.com/dglazier/elSpectro
    • [Br20] N. Brambilla, W. K. Lai, J. Segovia and J. Tarrùs Castellà, Phys. Rev. D 101 (2020) no.5, 054040 doi:10.1103/PhysRevD.101.054040 [arXiv:1908.11699 [hep-ph]].


    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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