Investigation of the nonflow corrections in $p-$Pb collisions at the ALICE acceptance

Abstract

Long-range two-particle azimuthal correlations have been used to study collective behaviors of produced particles in high-energy heavy-ion collisions. In small collision systems like pp and pA, it is essential to estimate and subtract correlations from jets and particle decays called nonflow effects. Experiments at the LHC have developed several techniques to deal with the nonflow effects. Details of the methods are different, so it is crucial to understand how it works in other circumstances where it was initially designed, such as kinematic regions and multiplicity ranges. In the ALICE experiment, the TPC acceptance ($|\eta |<0.9$) is quite limited, so other detectors at forward rapidity like FMD can be used for the long-range two-particle correlation study. We perform a detailed study on how different nonflow subtraction methods work at the ALICE kinematic region using Monte Carlo event generators AMPT and PYTHIA. The results in events without flow indicate that the template fit method better estimates the nonflow contribution than the peripheral subtraction method. In the case of events with both flow and nonflow, the flow coefficients obtained using the peripheral subtraction method are lower than the template fit method because it includes uncorrelated pair yields in low multiplicity events when calculating the flow coefficients.

Appendix A: Two-particle correlations in AMPT events without parton/hadron scatterings

We study AMPT events without parton/hadron scatterings as another case of events including only nonflow. Figure 10 shows the comparison of correlation functions, and the correlation functions of 60–100% events are vertically shifted to match the minimum value of the correlation functions of 0–20% events. Figures 11 and 12 show the results of nonflow subtraction with the template fit and peripheral subtraction methods.

Fig. 10

1D correlation functions in 0–20% and 60–100% $p-$Pb events from AMPT without parton/hadron scatterings. The correlation functions in 60–100% events are shifted to match the minimum yield of the correlation functions in 0–20% events to compare the correlated part

Fig. 11

Template fit results with correlation functions in 0–20% $p-$Pb events from AMPT without parton/hadron scatterings. Red lines represent the template fit results, and blue lines represent the flow components. Blue lines are shifted vertically for a better visibility

Fig. 12

Peripheral subtraction results with correlation functions in 0–20% $p-$Pb events from AMPT without parton/hadron scatterings. Red lines represent the Fourier fit results, and blue lines are results after scaling. Blue lines are shifted vertically to the minimum value of the correlation function ($\mathrm {Y}^\mathrm {HM-LM}_\mathrm {min}(\Delta \phi )$) for a better visibility