Transverse momentum and process dependent azimuthal anisotropies in $\sqrt{s_{\mathrm{NN}}}=8.16$ TeV $p$+Pb collisions with the ATLAS detector

TL;DR

This paper measures azimuthal anisotropies in proton-lead collisions at 8.16 TeV using the ATLAS detector, revealing different behaviors at low and high transverse momentum and challenging current theoretical models.

Contribution

It provides the first detailed measurement of azimuthal anisotropies across a wide $p_T$ range in $p$+Pb collisions, highlighting process-dependent effects.

Findings

Anisotropies at low $p_T$ align with hydrodynamic flow.

Significant anisotropies at high $p_T$ are not explained by existing theories.

Differences in anisotropies between minimum-bias and jet-triggered events.

Abstract

The azimuthal anisotropy of charged particles produced in $\sqrt{s_{\mathrm{NN}}}=8.16$ TeV $p$+Pb collisions is measured with the ATLAS detector at the LHC. The data correspond to an integrated luminosity of $165$ $\mathrm{nb}^{-1}$ that was collected in 2016. Azimuthal anisotropy coefficients, elliptic $v_2$ and triangular $v_3$, extracted using two-particle correlations with a non-flow template fit procedure, are presented as a function of particle transverse momentum ($p_\mathrm{T}$) between $0.5$ and $50$ GeV. The $v_2$ results are also reported as a function of centrality in three different particle $p_\mathrm{T}$ intervals. The results are reported from minimum-bias events and jet-triggered events, where two jet $p_\mathrm{T}$ thresholds are used. The anisotropies for particles with $p_\mathrm{T}$ less than about $2$ GeV are consistent with hydrodynamic flow expectations, while the significant non-zero anisotropies for $p_\mathrm{T}$ in the range $9$-$50$ GeV are not explained within current theoretical frameworks. In the $p_\mathrm{T}$ range $2$-$9$ GeV, the anisotropies are larger in minimum-bias than in jet-triggered events. Possible origins of these effects, such as the changing admixture of particles from hard scattering and the underlying event, are discussed.