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.
Highlights
The role of initial and final-state effects in interpreting differences between Pb–Pb and pp collisions is expected to be clarified with p–Pb collis√ions Pb collisions at sNN [1]
For pp and p–Pb, owing to the larger background present for twopion correlations, we extend the fit range to q = 1.2 GeV/c for the upper variation
Compared to the radii from two-pion correlations, the radii from three-pion cumulant correlations are less susceptible to non-femtoscopic background correlations due to the increased quantum statistics (QS)
Summary
The role of initial and final-state effects in interpreting differences between Pb–Pb and pp collisions is expected to be clarified with p–Pb collis√ions Pb collisions at sNN [1]. A CGC initial state model (IP-Glasma), without a hydrodynamic phase, predicts similar freeze-out radii in p–Pb and pp collisions [17]. The extraction of freeze-out radii can be achieved using identical boson correlations at low relative momentum, which are dominated by quantum statistics (QS) and final-state Coulomb and strong interactions (FSIs). Both FSIs and QS correlations encode information about the femtoscopic space–time structure of the particle emitting source at kinetic freeze-out [18,19,20]. The calculation of FSI correlations allows for the isolation of QS correlations
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