Nuclear Physics & RIKEN Theory Seminar

It was suggested that fluctuations in the initial geometry of ultrarelativistic heavy ion collisions can provide an explanation for the surprisingly large elliptic flow values observed in central Cu+Cu collisions. Measurement of elliptic flow fluctuations in Au+Au collisions has confirmed the existence of such large geometry fluctuations. In this talk, I will discuss how the same principle of initial geometry fluctuations may also be the key to understanding the "ridge" and "broad away side" structures present in two particle correlation measurements.

I will introduce the concepts of participant triangularity and triangular flow, analogous to the definitions of participant eccentricity and elliptic flow. The participant triangularity characterizes the triangular anisotropy of the initial nuclear overlap geometry and arises from event-by-event fluctuations in the participant-nucleon collision points. In studies using a multi-phase transport model (AMPT), a triangular flow signal is observed that is proportional to the participant triangularity and corresponds to a large third Fourier coefficient in two-particle azimuthal correlation functions. Ratios of the second and third Fourier coefficients of two particle azimuthal correlations are observed to exhibit similar trends in experimental data and AMPT calculations as a function of centrality and transverse momentum, suggesting a similar origin. Triangular flow provides a new handle on the initial collision geometry and collective expansion dynamics in heavy-ion collisions.