Elucidating the event-by-event flow fluctuations in heavy ion collisions via the event-shape selection and twisting techniques

20 May 2014, 16:30
spectrum (darmstadtium)



Board: I-15
Poster Correlations and Fluctuations Poster session


Mr Peng Huo (Stony Brook University)


Recent discovery of significant higher-order flow harmonics and theoretical study of the non-linear hydrodynamics have led to the realization of a large set of new event-by-event (EbyE) flow observables that can be measured in heavy-ion collisions. These observables can be expressed generally as a joint probability distribution of flow coefficients $v_n$ and their phases $\Phi_n$, $p(v_n, v_m, ..., \Phi_n,\Phi_m....)$. Initial measurements of a small subset of these flow observables, namely $p(v_n)$ and event-plane correlations $p(\Phi_n,\Phi_m....)$ at LHC, have already provided new insights on the geometry fluctuations in the initial state and the non-linear hydrodynamics in the final state. In a recent work, we propose an experimental method to further probe other flow observables based on the recently proposed event-shape selection technique a new event-shape twisting technique. In the first method (arXiv:1311.7091), events in fixed centrality are further divided in bins of $v_n$ values in the forward pseudorapidity ($\eta_F$). This allows us to measure how the $v_m$ and event-plane correlation respond to the change in $v_n$, which provide access to two classes of observables not studied before: $p(v_n, v_m)$ and $p(v_n, \Phi_n,\Phi_m....)$. Furthermore our method also allows a quantitative study of the correlation of $v_n$ in different rapidity: $p(v_n(\eta_F),v_n(\eta))$. The robustness of this method is demonstrated using the AMPT model, which is known to describe reasonably the centrality dependence of $v_n$ and event-plane correlations. Strong positive correlations are observed among all even harmonics v2, v4, and v6, between v2 and v5 and between v3 and v5, consistent with the effects of non-linear hydrodynamic response, while a significant anti-correlation is observed between v2 and v3. The $v_n(\eta)$ for events selected on $v_n(\eta_F)$ shows a significant forward/backward asymmetry, which indicates the de-correlation of event planes over pseudorapidity. In the second method (paper to be submitted), a twist angle between the event planes in the forward and backward rapidity is enforced by cutting on $\Delta\Phi = \Phi_n(\eta_F)-\Phi_n(\eta_B)$. This twisting procedure leads to a non-zero $\eta$-dependence $<\sin(n(\phi-\Psi_n))>$ component in between, which can be used to calculate the event plane angle as a function of $\eta$. This allows us to distinguish between two competing mechanisms for the rapidity de-correlation: a systematic rotation versus a random fluctuation of event plane angles along the $\eta$ direction. Indeed, a significant fraction of the observed rapidity de-correlation in the AMPT model is found to arise from a systematic rotation of event plane angles along the $\eta$ direction. Our studies show that the event-shape selection and event-shape twisting techniques promise to provide unprecedented details on the EbyE flow fluctuations. The prospects of applying these techniques to Au+Au/Pb+Pb collisions at RHIC and LHC are discussed.

Primary author

Mr Peng Huo (Stony Brook University)


Prof. Jiangyong Jia (Stony Brook University) Dr Soumya Mohapatra (State University of New York (US))

Presentation Materials