Published November 17, 2024
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Azimuthal anisotropies of high-pT charged particles in Pb+Pb collisions at 5.02 TeV with the ATLAS detector
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The quark gluon plasma (QGP) is a novel state of matter that forms under extreme temperature and density, in which the normally bounded hadronic states of matter “melt” into constituent quarks and gluons. Understanding the short-distance interactions within the QGP gives us a probe to one of the fundamental forces of nature, strong force. The QGP can be created in ultra-relativistic heavy ion collisions in collider facilities such as the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). By colliding accelerated beams of heavy ions and measuring the collision products, bulk properties and short-
distance interaction mechanisms of the QGP can be probed. Soft and hard QCD processes both happen in these collisions, the former giving rise to the QGP, and the latter gives high-momentum partons that traverse through and interact with the QGP. It has been observed that the QGP exhibit collective expansion, and significant anisotropies exist in the particle production along the azimuthal direction. In lower momentum particles, studies of this anisotropy lead to the understanding of the transport properties of the QGP, which explain how the initial geometry of the QGP is converted into the final-state particles. In higher momentum particles, this anisotropy reflects the parton energy loss mechanism to the QGP. This is done by colliding accelerated beams of heavy nuclei and measuring the products of the collisions with a particle detector. In particle collisions, partons can scatter off each other with a large momentum transfer, and when they come out of the collisions, they create collimated sprays of particles that are called jets. Therefore, the study of azimuthal anisotropies in the high-momentum sector is powerful probe for the short-distance interactions within the QGP.
This thesis presents a measurement of azimuthal anisotropy coefficients v2 and v3 of charged particles produced in Pb+Pb collisions at $\sqrt{sNN}$ = 5.02 TeV using a data set corresponding to an integrated luminosity of 435 $\mub^{−1}$ collected with the ATLAS detector at the LHC in 2018. The values of v2 and v3 are measured in intervals of centrality covering up to the 60% most central events and a charged-particle transverse momentum (pT) range of 1–400 GeV using the scalar product and multi-particle cumulant methods. These methods are sensitive to event-by-event fluctuations and non-flow effects in the measurements of azimuthal anisotropies. Positive values of v2 are observed up to 100 GeV from both methods across all centrality intervals. Positive values of v3 are observed up to approximately 25 GeV using the scalar product method, and the values of v3 for pT > 10 GeV using the multi-particle cumulant method show a strong centrality dependence. At high pT, charged particles dominantly come from the fragmentation of jets, and the results of this study are sensitive to the path-length dependence of the energy loss of the jets as they traverse the quark-gluon plasma produced in Pb+Pb collisions.
distance interaction mechanisms of the QGP can be probed. Soft and hard QCD processes both happen in these collisions, the former giving rise to the QGP, and the latter gives high-momentum partons that traverse through and interact with the QGP. It has been observed that the QGP exhibit collective expansion, and significant anisotropies exist in the particle production along the azimuthal direction. In lower momentum particles, studies of this anisotropy lead to the understanding of the transport properties of the QGP, which explain how the initial geometry of the QGP is converted into the final-state particles. In higher momentum particles, this anisotropy reflects the parton energy loss mechanism to the QGP. This is done by colliding accelerated beams of heavy nuclei and measuring the products of the collisions with a particle detector. In particle collisions, partons can scatter off each other with a large momentum transfer, and when they come out of the collisions, they create collimated sprays of particles that are called jets. Therefore, the study of azimuthal anisotropies in the high-momentum sector is powerful probe for the short-distance interactions within the QGP.
This thesis presents a measurement of azimuthal anisotropy coefficients v2 and v3 of charged particles produced in Pb+Pb collisions at $\sqrt{sNN}$ = 5.02 TeV using a data set corresponding to an integrated luminosity of 435 $\mub^{−1}$ collected with the ATLAS detector at the LHC in 2018. The values of v2 and v3 are measured in intervals of centrality covering up to the 60% most central events and a charged-particle transverse momentum (pT) range of 1–400 GeV using the scalar product and multi-particle cumulant methods. These methods are sensitive to event-by-event fluctuations and non-flow effects in the measurements of azimuthal anisotropies. Positive values of v2 are observed up to 100 GeV from both methods across all centrality intervals. Positive values of v3 are observed up to approximately 25 GeV using the scalar product method, and the values of v3 for pT > 10 GeV using the multi-particle cumulant method show a strong centrality dependence. At high pT, charged particles dominantly come from the fragmentation of jets, and the results of this study are sensitive to the path-length dependence of the energy loss of the jets as they traverse the quark-gluon plasma produced in Pb+Pb collisions.
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