Speaker
Description
It is a long standing puzzle that hydrodynamic calculations give larger
elliptic flow for ultra-central Pb+Pb events than experiments. One explanation
is that sampling from a single particle distribution in Glauber model generates
too much shape fluctuation, which can be expected to be modified by NN
correlations. We study the fluctuation of shape-deformation as currently
implemented in MC-Glauber-like models and its consequences on the magnitude of
eccentricities. We show that for both deformed and spherical species, there are
large fluctuations in the deformation that emerge event-by-event. For each
nucleonic configuration, we characterize the deformation with parameters
$\beta$ and $\gamma$, calculated using quadruple moments. The second order
eccentricity $\varepsilon_2$ shows a strong correlation with the E-by-E
deformation $\beta$'s for ultra-central collisions. We further show, via
acoustic scaling, with a moderate reduction of $\beta$ by rescaling, the
eccentricities we get then agree with the measured values of $v_n$, for all
experimentally available centrality bins. Therefore, the model provides
eccentricities that describe experimental data, both where geometry dominates
and where fluctuation dominates. We further show that this modification has important implication for the novel observable, the dependence of mean $p_T$ of $v_2$, for ultra-central collisions.
