The Multiquark States in LHCb

Heavy quarks have been unlocking secrets of hadrons (i.e., strongly interacting particles), for nearly half a century. The discovery of the J/ψ , and of the other members of the charmonium family, solidified the quark model of hadrons [1]. The lower mass charmonium states line up to the mass spectrum, which can be well reproduced in nonrelativistic quantum mechanics as bound states of charmed–anticharmed quarks (cc ). Their large masses reflect mostly heaviness of charmed quarks, while their much smaller mass-differences reflect various radial and orbital-momentum excitations, with the positronium-like fine and hyperfine structures testifying to the fermionic nature of quarks. Their masses are well defined (i.e., they have narrow widths), as their decays proceed via OZI suppressed processes (disjoint quark diagrams) or electromagnetic transitions. Adding beauty to the charm, the bottomonium family bb ( ) was discovered, with even heavier constituent inside [1]. Previously known hadrons, made out of light down (d), up (u), and strange (s) quarks, lined up to more confusing mass patterns, complicated by near equality of masses of different quarks [source of the isospin and of the SU(3) flavor symmetries] and the excitation energies exceeding masses of the constituents, making the light quark mesons (qq ) and baryons qqq ( ) highly relativistic systems. Most of the excited states are wide, as they are quite unstable, decaying via OZI allowed processes, which makes quantitative theoretical description of them more complicated. In the previous decade, + − e e colliders operating with the collision energy near the → + − e e bb threshold (the Belle and the BaBar experiments) dominated the research into heavy quarks, not only b, but also c, produced either promptly or via weak → b c decays. While motivated mostly by searches for new fundamental forces in heavy quark decays mediated by loop diagrams, these machines provided an ample source of hadrons with heavy quarks inside. This led to discoveries of several heavy mesons with properties, which did not fit the expectations for either QQ or Qq states, where = Q c b , and = q u d s , , . Such states are often called exotic hadrons. Most of them were relatively narrow and with masses near heavy meson–meson thresholds, Qq Qq ( )( ). This fueled suggestions that these are loosely bound systems of meson pairs, in analogy with deuteron taken as a bound state of proton and neutron. Such four-quark states are usually referred to as “molecular,” since the binding is often described by exchange of light quarks in form of lowmass qq mesons. Notable examples include the X (3872) state (aka χ (3872)) c1 at the D D 0 *0 threshold, cu cu ( )( ),