Speaker
Description
In recent years various near-threshold narrow
peaks were observed in heavy quarknoium mass regions, such as $X(3872)$, $Z_c(3900)$, $Z_c(4020)$, $Z_b(10610)$ and $Z_b(10650)$, which are located close to the $D\bar D^*$, $D\bar D^*$, $D^*\bar D^*$, $B\bar B^*$ and $B^*\bar B^*$
thresholds in an $S$ wave, respectively. Once there is a strongly coupled $S$-wave threshold nearby, the Breit-Wigner parametrisation does not work anymore. In this talk, we introduction a practical parametrisation for the line shapes of these near-threshold states which satisfies the unitarity and analyticity requirement of $S$ matrix. This parametrisation scheme can accommodate arbitrary number of elastic and inelastic channels, which is easy and reliable for both experimental and theoretical analysis. As an application, the line shapes of the two $Z_b$ states can be very well described. Furthermore, we also discuss whether their line shapes can be interpreted as the cusp effects.
