# International Workshop on Partial Wave Analyses and Advanced Tools for Hadron Spectroscopy

Mar 13 – 17, 2017
Europe/Zurich timezone

## Parametrisation for near-threshold states

Mar 16, 2017, 6:00 PM
30m

### Seminar Room

Topic 3: Theoretical Constraints on Amplitude Analyses

### Speaker

Dr Qian Wang (FORSCHUNGSZENTRUM Jülich)

### 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.

### Primary author

Dr Qian Wang (FORSCHUNGSZENTRUM Jülich)