Freeze-out radii extracted from three-pion cumulants in pp, p–Pb and Pb–Pb collisions at the LHC

Abstract

In high-energy collisions, the spatio-temporal size of the particle production region can be measured using the Bose-Einstein correlations of identical bosons at low relative momentum. The source radii are typically extracted using two-pion correlations, and characterize the system at the last stage of interaction, called kinetic freeze-out. In low-multiplicity collisions, unlike in high-multiplicity collisions, two-pion correlations are substantially altered by background correlations, e.g. mini-jets. Such correlations can be suppressed using three-pion cumulant correlations. We present the first measurements of the size of the system at freeze-out extracted from three-pion cumulant correlations in pp, p-Pb and Pb-Pb collisions at the LHC with ALICE. At similar multiplicity, the invariant radii extracted in p-Pb collisions are found to be 5-15% larger than those in pp, while those in Pb-Pb are 35-55% larger than those in p-Pb. Our measurements disfavor models which incorporate substantially stronger collective expansion in p-Pb as compared to pp collisions at similar multiplicity.

Two-pion femtoscopy inp-Pb collisions atsNN=5.02TeV

“…A transverse momentum dependence of the radii similar to A-A was observed for the asymmetric d-Au collision system at the BNL Relativistic Heavy Ion Collider (RHIC) [19,20]. The one-dimensional radii extracted from the ALICE three-pion cumulant analysis were also investigated in pp, p-Pb, and Pb-Pb collisions at the LHC [21]. For the p-Pb system, at a given multiplicity, the radii were found to be 5%-15% larger than those in pp, while the radii in Pb-Pb were 35%-55% larger than those in p-Pb.…”

Multipion Bose-Einstein correlations inpp,p-Pb, and Pb-Pb collisions at energies available at the CERN Large Hadron Collider

“…We also extracted G with several other multipion correlations which is shown in a separate note [41]. In pp and p-Pb collisions, where non-negligible nonfemtoscopic backgrounds exist, we only use the second build technique because three-pion correlations have a larger signal-to-background ratio [42].…”

“…We apply muon corrections similar to those of Refs. [24,42]. We assign a 2% uncertainty to the muon correction procedure.…”

Centrality dependence of pion freeze-out radii in Pb-Pb collisions atsNN=2.76 TeV

“…This work extends this measurement to other centralities and compares the obtained radii to recent hydrodynamic calculations in order to check their validity in a large range of event multiplicities. A measurement of one-dimensional radii was also performed by using the two-pion and three-pion cumulants [37]. This work extends the two-pion measurement to several ranges of pair transverse momentum.…”