Status and promise of particle interferometry in heavy-ion collisions

Abstract

After five years of running at RHIC, and on the eve of the LHC heavy-ion program, we highlight the status of femtoscopic measurements. We emphasize the role interferometry plays in addressing fundamental questions about the state of matter created in such collisions, and present an enumerated list of measurements, analyses and calculations that are needed to advance the field in the coming years.Keywords: Theoretical and experimental identical-particle correlations; Theoretical and experimental femtoscopy; Particleantiparticle correlationsThe first two-pion interferometric measurements of the collider era emerged within a year of RHIC becoming operational. To the surprise of many in the field, the measurements were remarkably similar to those recorded at the AGS and the SPS. The analyses were inconsistent, both qualitatively and quantitatively, with dynamic models incorporating first-order phase transitions in general. In fact, parametric explanations of the data, the forms of which are often motivated by solutions to dynamical equations, suggest that the matter explodes violently, growing from a radius of 6 fm to 13 fm in only 10 fm/c. The surprisingly strong acceleration required for such behavior and the associated failure of many of the field's most sophisticated models became known as the "HBT Puzzle".Five years later, the field has made steady progress on a variety of fronts. Theoretically, sophisticated dynamic models have more successfully reproduced experimental results (though the very sophistication of these models has made it difficult to ascertain which aspects of the models are being validated by the comparison). New techniques have been applied to the analysis of experimental correlation functions, revealing greater detail about the size and shape of the emission region. During our discussions at the workshop, it was clear that a remarkable consensus had developed among the practitioners of the field. Although this agreement by no means represented a final conclusion, we found numerous points that could be stated without dissent. In this white paper we will first list the points concerning the current status of the field, then further below, enumerate points where participants agreed were important for further progress. Achievements:• Remarkable agreement has been observed between the RHIC experiments, PHENIX, PHOBOS and STAR. All three have produced high-statistics high-quality pion correlations, whose apparent source sizes are consistent to a few tenths of a fm. A similar consistency was observed among measurements performed at the top SPS energy; at lower SPS energies the maximum deviations are on the level of 20%.• Femtoscopic studies are highly multi-dimensional. Even the simplest and most common case of twoidentical pion correlations depend on six independent variables, which have only been fully explored within the past few years. This includes extracting characteristic source sizes as function of transverse momentum, rapidity and the angle with respect to the reaction plane for off-axis co...

Lévy-stable two-pion Bose-Einstein correlations in sNN=200 GeV Au+Au collisions

“…In addition to settings in the correlation function measurement, we have performed fit range studies by varying the initial and the final Q bin locations (Q min and Q max ). The results were remarkably stable for adding or removing the first few (1)(2)(3)(4)(5) or the last few (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) data points at the beginning or the end of the fit. In fact we used this stability criteria to define the beginning and the end points of the fitted range.…”

“…31, then it follows that the pair distribution (D) shown in Eq. (19), is a sum of the three components,…”

“…The approximate transverse mass (m T ) dependence of the measured Gaussian source radii (R Gauss ) is R −2 Gauss ∝ a + bm T (where a and b are constants), which is almost universal across collision centrality, particle type, colliding energy and colliding system size [12,13]. This is a direct consequence of a strong longitudinal as well as radial hydrodynamical expansion [14][15][16][17][18][19][20]. Directional Hubble flows seem to be a crucial property of the sQGP formation in heavy ion collisions, or Little…”

“…Bangs [14][15][16][17]. The so-called RHIC HBT puzzle, the apparent contradiction between several hydrodynamical model predictions and the observed ratio of the HBT radii [8,9], also turned out to be resolvable in a hydrodynamical picture with more realistic physics conditions and refined models of three dimensional Hubble flows [15,18,19,[21][22][23]. For a more detailed introduction and review of Bose-Einstein correlations and their application in high energy heavy ion collisions, see the review papers in Refs.…”

Lévy-stable two-pion Bose-Einstein correlations in $\sqrt{s_{_{NN}}}=200$ GeV Au$+$Au collisions

“…The approximate transverse mass (m T ) dependence of the measured Gaussian source radii (R Gauss ) is R −2 Gauss ∝ a+bm T (where a and b are constants), which is almost universal across collision centrality, particle type, colliding energy and colliding system size [12,13]. This is a direct consequence of a strong longitudinal as well as radial hydrodynamical expansion [14][15][16][17][18][19][20]. Directional Hubble flows seem to be a crucial property of the sQGP formation in heavy ion collisions, or Little Bangs [14][15][16][17].…”