Abstract
Heavy hadron spectroscopy was well understood within the naive quark model until the end of the past century. However, in 2003, the X(3872) was discovered, with puzzling properties difficult to understand in the simple naive quark model picture. This state made clear that excited states of heavy mesons should be coupled to two-meson states in order to understand not only the masses but, in some cases, unexpected decay properties. In this work, we will give an overview of a way in which the naive quark model can be complemented with the coupling to two hadron thresholds. This program has been already applied to the heavy meson spectrum with the chiral quark model, and we show some examples where thresholds are of special relevance.
Highlights
Heavy hadron spectroscopy started in November 1974, when Brookhaven National Laboratory announced the discovery of a new particle called J [1] and, at the same time, the Stanford Linear Accelerator reported the existence of another new particle, called ψ [2]
Heavy Quark Spin Symmetry (HQSS) and Heavy Flavor Symmetry (HFS) are good approximate symmetries of QCD, so one would expect them to be realized in the heavy hadron spectrum
The naive quark model has been very successful in describing heavy hadron phenomenology for a very long time
Summary
Heavy hadron spectroscopy started in November 1974, when Brookhaven National Laboratory announced the discovery of a new particle called J [1] and, at the same time, the Stanford Linear Accelerator reported the existence of another new particle, called ψ [2] Very soon, both particles were seen as the same state, which we know as the J/ψ state of the charmonium spectrum. In 2003, only 14 states were identified [6], while again, during the last 17 years, 33 new states have been included [7] This impressive development of the heavy hadron spectrum has been possible thanks in part to the so called B-factories, like Belle and BaBar, which are electron–positron colliders tuned to the center of mass energy of the Υ(4S) that decays into two B mesons.
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