Measurement of flow harmonics correlations with mean transverse momentum in lead–lead and proton–lead collisions at $$\sqrt{s_{\mathrm{NN}}} = 5.02~\hbox {TeV}$$ with the ATLAS detector

Abstract

To assess the properties of the quark-gluon plasma formed in ultrarelativistic ion collisions, the ATLAS experiment at the LHC measures a correlation between the mean transverse momentum and the flow harmonics. The analysis uses data samples of lead-lead and proton-lead collisions obtained at the centre-of-mass energy per nucleon pair of 5.02 TeV, corresponding to total integrated luminosities of 22 μb −1 and 28 nb −1 , respectively. The measurement is performed using a modified Pearson correlation coefficient with the charged-particle tracks on an event-by-event basis. The modified Pearson correlation coefficients for the 2nd-, 3rd-, and 4th-order flow harmonics are measured in the lead-lead collisions as a function of event centrality quantified as the number of charged particles or the number of nucleons participating in the collision. The measurements are performed for several intervals of the charged-particle transverse momentum. The correlation coefficients for all studied harmonics exhibit a strong centrality evolution, which only weakly depends on the charged-particle momentum range. In the proton-lead collisions, the modified Pearson correlation coefficient measured for the 2nd-order flow harmonics shows only weak centrality dependence. The lead-lead data is qualitatively described by the predictions based on the hydrodynamical model.

Recent ATLAS measurements of correlations in $pp$ and $p$+Pb collisions

“…The denominator includes the dynamical variance of v n {2} 2 [15] and the [p T ] variance calculated by the dynamical p T fluctuation magnitude c k [16,17]. The ρ coefficient is obtained for the 5.02 TeV MB p+Pb and Pb+Pb data samples with an integrated luminosity of 28 nb −1 and 22 µb −1 , respectively [9] . In the Pb+Pb collisions the modified Pearson correlation coefficients for the v 2 , v 3 and v 4 harmonics are measured as a function of event centrality quantified as the number of charged particles or the number of nucleons participating in the collision.…”

“…Event-by-event correlations of flow harmonics and the mean transverse momentum are expected to be sensitive to the initial conditions in nuclear collisions. The v n − p T correlation in p+Pb collisions at √ s NN = 5.02 TeV is presented in this report [9]. To study the effect of initial conditions the charged hadrons elliptic flow in photo-nuclear interactions in ultra-peripheral 5.02 TeV Pb+Pb collisions is compared to the pp and p+Pb results [10].…”

“…The modified Pearson's coefficient, ρ(v 2 2 , [p T ])as a function of N ch in 5.02 TeV p+Pb collisions for different p T intervals[9].…”

Correlation of flow harmonics and mean transverse momentum in 5.02 TeV p+Pb and Pb+Pb collisions and event-plane dependence of HBT radii in high-multiplicity p+Pb collisions with the ATLAS detector

“…The existence of such correlation indicates a link between the initialstate quantity, [p T ], and the v n encoding the system evolution. In the ATLAS measurement a Pearsons correlation coefficient is obtained [3]. The coefficient depends among others on detector performance or selection of kinematic requirements applied on charged particles.…”

“…For more details about this study and the above equations see [3]. The measurements are performed for three intervals of p T to probe different regions of spectra: 0.5 < p T < 2 GeV which is well described by hydrodynamics, p T < 5 GeV to observe an impact of the energy loss on ρ and the region with the increased lower threshold from 0.5 to 1 GeV ( 0.3 to 0.5 GeV in case of p+Pb) to test sensitivity to significant change in multiplicity.…”