Measurement of Azimuthal Anisotropy of Charged Particles in PbPb Collisions with the ATLAS Detector

Heavy-ion collisions at the LHC produce a dense, hot medium known as the quark-gluon plasma (QGP). The QGP can be described by nearly ideal hydrodynamics, which results in its initial geometry being converted into azimuthal anisotropy in final state particles with low momentum in the plane transverse to the beam direction. High-momentum partons from the hard scatterings in these collisions lose energy as they traverse in the QGP. Because of the initial geometry of the QGP, partons produced at different angles, with respect to the collision event plane, traverse different path lengths of the medium leading to azimuthal-angle dependence of the yields of high transverse momentum ($p_T$) final-state particles. This study uses the scalar-product method to measure the $p_T$-dependent Fourier coefficients $v_n$, $ngeq2$, of the angular modulation of charged-particle distribution in PbPb collisions at $sqrt{s_{NN}}=5.02$ TeV, using the 2018 data recorded by the ATLAS detector. The low-$p_T$ $v_{n}$ are key parameters in constraining hydrodynamic models of the QGP, and the high-$p_T$ $v_{n}$ provide new information about the path-length dependence of the energy loss mechanism. This result is compared with current azimuthal-anisotropy measurements using charged particles and jets. With improved statistics, this measurement will explore a higher $p_T$ range than current measurements.