FEMTOSCOPY IN RELATIVISTIC HEAVY ION COLLISIONS: Two Decades of Progress

Abstract

Analyses of two-particle correlations have provided the chief means for determining spatio-temporal characteristics of relativistic heavy ion collisions. We discuss the theoretical formalism behind these studies and the experimental methods used in carrying them out. Recent results from RHIC are put into context in a systematic review of correlation measurements performed over the past two decades. The current understanding of these results is discussed in terms of model comparisons and overall trends.

Fitted HBT radii versus space-time variances in flow-dominated models

“…[1] for details). C(q q q, K K K) is the modification to the conditional probability for measuring particle b with momentum p p p b = K K K− 1 2 q q q if particle a has been measured with momentum p p p a = K K K+ 1 2 q q q, due to two-particle effects sensitive to space-time separation.…”

“…Indeed, HBT radii calculated with Boltzmann/cascade models which are based on Gaussian fits to the simulated correlation functions agree somewhat better with measurements than do radii based on an extraction of spacetime variances from hydrodynamic calculations [1]. Whether this is due to a more realistic modeling of the collision in the Boltzmann/cascade approach or the shortcomings of the comparison of variances with HBT radii in the hydrodynamic case is unclear.…”

“…Focus exclusively on the "HBT Puzzle," per se, may be misplaced, since the transition from deconfined matter (sQGP) to confined matter is likely a crossover transition rather than a strong first-order one; in other words, there is no large latent heat associated with the transition. In this case, the large HBT radii initially predicted by models with a strong phase transition [10], are not expected after all [1]. In any event, our task here is to evaluate the degree of validity of methods of comparing model calculations to data.…”

“…In this paper we do not take up this issue. Instead, we note that most experimental studies in heavyion physics to date have used the Gaussian ansatz [1], and we explore some ways in which HBT radii obtained in this way from data may be compared to model calculations.…”

“…This raises the question to what extent some of the persistently observed discrepancies between model predictions and measurements of the HBT radii [1]-the so-called "RHIC HBT Puzzle"-might be due to such an "apples-with-oranges" comparison. Indeed, HBT radii calculated with Boltzmann/cascade models which are based on Gaussian fits to the simulated correlation functions agree somewhat better with measurements than do radii based on an extraction of spacetime variances from hydrodynamic calculations [1].…”