Abstract
Bose-Einstein correlations between identified charged pions are measured for $p$+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV using data recorded by the ATLAS detector at the LHC corresponding to a total integrated luminosity of $28$ $\mathrm{nb}^{-1}$. Pions are identified using ionization energy loss measured in the pixel detector. Two-particle correlation functions and the extracted source radii are presented as a function of collision centrality as well as the average transverse momentum ($k_{\mathrm{T}}$) and rapidity ($y^{\star}_{\pi\pi}$) of the pair. Pairs are selected with a rapidity $-2 < y^{\star}_{\pi\pi} < 1$ and with an average transverse momentum $0.1 < k_{\mathrm{T}} < 0.8$ GeV. The effect of jet fragmentation on the two-particle correlation function is studied, and a method using opposite-charge pair data to constrain its contributions to the measured correlations is described. The measured source sizes are substantially larger in more central collisions and are observed to decrease with increasing pair $k_{\mathrm{T}}$. A correlation of the radii with the local charged-particle density is demonstrated. The scaling of the extracted radii with the mean number of participating nucleons is also used to compare a selection of initial-geometry models. The cross-term $R_\mathrm{ol}$ is measured as a function of rapidity, and a nonzero value is observed with $5.1\sigma$ combined significance for $-1 < y^{\star}_{\pi\pi} < 1$ in the most central events.
Highlights
To address the topics and questions discussed above, this paper presents measurements of correlations between identified charged pions in 5.02 TeV p+Pb collisions which were performed by the ATLAS experiment at the Large Hadron Collider (LHC)
The p+Pb collision centrality is characterized using ETPb, the total transverse energy measured in the Pb-going forward calorimeter (FCal) [45]
The measurements presented in this paper have been performed using the inner detector, minimum-bias trigger scintillators (MBTS), FCal, zero-degree calorimeter (ZDC), and the trigger and data acquisition systems
Summary
Studies of multiparticle correlations in proton-lead (p+Pb) [1,2,3,4,5] and proton-proton (pp) [6] collisions at the CERN Large Hadron Collider (LHC) and in deuteron-gold (d+Au) [7,8,9] and helium-3–gold (3He+Au) [10] collisions at the BNL Relativistic Heavy Ion Collider (RHIC) have shown that these correlation functions exhibit features similar to those observed in nucleus-nucleus collisions [11,12,13,14,15,16] that are attributed to collective dynamics of the strongly coupled quark-gluon plasma. The inherent asymmetry of p+Pb collisions seen, for example, in the charged-particle pseudorapidity distributions [44,45], provides a unique opportunity to study the correlations between source sizes and the pair’s rapidity, collision centrality, or the local (in rapidity) charged-particle density The results of such a study may provide insight into or constrain theoretical models of the underlying dynamics responsible for producing the final-state particles. While femtoscopic methods have already been applied to p+Pb systems at the LHC [46,47], this paper presents a new data-driven technique to constrain the significant background contribution from jet fragmentation, referred to in this paper as the “hard process” background It provides new measurements of the dependence of the source radii on the pair’s rapidity yππ , calculated assuming both particles have the mass of the pion, over the range −2 < yππ < 1. Using the measured centrality dependence of the source radii, the scaling of the system size with the number of nucleon participants Npart is investigated, using a generalization of the Glauber model [48]
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