Transverse momentum fluctuation in ultra-central Pb+Pb collision at the LHC

The ATLAS collaboration has recently observed that the variance of the transverse momentum per particle ($[p_t]$), when measured as a function of the collision multiplicity ($N_{ch}$) in Pb+Pb collisions, decreases by a factor $2$ for the largest values of $N_{ch}$, corresponding to ultra-central collisions. We show that this phenomenon is naturally explained by invoking impact parameter ($b$) fluctuations, which contribute to the variance, and gradually disappear in ultra-central collisions. It implies that $N_{ch}$ and $[p_t]$ are strongly correlated at fixed $b$, which is explained by the local thermalization of the QGP medium.