Abstract
We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb--Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV as a function of collision centrality and the average transverse momentum of the pair $k_{\rm T}$. Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one-dimensional radii are extracted. The radii decrease with $k_{\rm T}$, following a power-law behavior. This is qualitatively consistent with expectations from a collectively expanding system, produced in hydrodynamic calculations. The radii also scale linearly with $\left< \mathrm{d}N_{\rm ch}/\mathrm{d}\eta \right>^{1/3}$. This behaviour is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller $\sqrt{s_{\rm NN}}$, a decrease in the $R_{\rm out}/R_{\rm side}$ ratio is seen, which is in qualitative agreement with specific predictions from hydrodynamic models. The results provide further evidence for the production of a collective, strongly coupled system in heavy-ion collisions at the LHC.
Highlights
Collisions of lead ions at √sNN = 2.76 TeV have been recorded by A Large Ion Collider Experiment (ALICE) at the Large Hadron Collider (LHC) at CERN
We report on the centrality and pair kT dependence of the three-dimensional pion femtoscopic arnaddiidiirnectPiobn--Pabvercaoglelidsioonnse-daitme√nssNioNn=al
Simple linear scaling predictions are not valid when the collision energy is increased by an order of magnitude
Summary
Strong hydrodynamic collective flow in the longitudinal and transverse directions results in the decrease of the apparent size of the system with increasing kT This is because longitudinal- and transverse-velocity boosts cause particles emitted from spatially separated parts of the collision region to move away from one another. The decrease of the size with kT is observed in experimental data from heavy-ion collisions at all centralities, various collision energies and colliding system types, and is well described quantitatively in hydrodynamic models [11,30] and qualitatively in hadronic rescattering codes [32]. The radius along the pair transverse momentum (called Rout) is decreased by the correlation with respect to the other transverse radius (called Rside), which decreases the ratio Rout/Rside All of those effects have been observed in the first measurement for central (0%–5%) collisions at the LHC [36].
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