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

20 May 2014, 16:30
2h
spectrum (darmstadtium)

spectrum

darmstadtium

Board: I-15
Poster Correlations and Fluctuations Poster session

Speaker

Mr Peng Huo (Stony Brook University)

Description

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)

Co-authors

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

Presentation Materials