Notes on correlation femtoscopy
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ELEMENTARY PARTICLES AND FIELDS Experiment
Notes on Correlation Femtoscopy* R. Lednicky** Joint Institute for Nuclear Research, Dubna, Russia; Institute of Physics, Czech Academy of Sciences, Praha, Czech Republic Received March 6, 2008
Abstract—The particle correlations at small relative velocities are sensitive to the space–time characteristics of the production processes on a femtometer scale owing to the effects of quantum statistics and finalstate interaction. Though the final-state interaction complicates the correlation analysis, it is an important source of information allowing for coalescence femtoscopy, correlation femtoscopy with unlike particles including access to the relative space–time asymmetries in particle production, and a study of strong interactions between specific particles. I will briefly discuss the history and basics of correlation femtoscopy, some of the recent results from femtoscopy of relativistic heavy-ion collisions, and their consequences. PACS numbers: 25.75.-q, 25.75.Gz DOI: 10.1134/S1063778808090123
1. INTRODUCTION The momentum correlations of two or more particles at small relative momenta in their centerof-mass (c.m.) system are widely used to study space–time characteristics of the production processes on a level of 1 fm = 10−15 m, thus serving as a correlation femtoscopy tool. Particularly, for noninteracting identical particles, like photons or, to some extent, pions, these correlations result from the interference of the production amplitudes due to the symmetrization requirement of quantum statistics (QS) [1–5]. The momentum QS correlations were first observed as an enhanced production of the pairs of identical pions with small opening angles (GGLP effect [1]). Later on, Kopylov and Podgoretsky settled the basics of correlation femtoscopy in more than 20 papers (see review [5]) and developed it as a practical tool; particularly, they suggested studying the interference effect in terms of the correlation function, proposed the mixing techniques to construct an uncorrelated reference sample, and clarified the role of the space–time characteristics of particle production in various physical situations. There exists [2–10] an analogy of the momentum QS correlations of photons with the space–time correlations of the intensities of classical electromagnetic fields used in astronomy to measure the angular radii ∗ **
The text was submitted by the author in English. E-mail: [email protected]
of stellar objects based on the superposition principle (HBT effect) [11]. This analogy is sometimes misunderstood and the momentum correlations are confused with the space–time (HBT) correlations in spite of the fact that their orthogonal character and thus the absence of the former in astronomy measurements due to extremely large space–time extent of stellar objects (and vice versa) was already pointed out in an early paper [8] (see also [9, 10]). The momentum correlations of particles emitted at nuclear distances are also influenced by the effect of final-state interaction (FSI) [12–17]. Thus, the effect
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