The Quark-Gluon Plasma (QGP) produced in heavy-ion collisions has been shown to behave like a nearly perfect fluid, characterized by a very low ratio of shear viscosity to entropy density. Significant measurements in large collision systems have improved the constraints on the value of π/π . However, the precise temperature dependence of π/π still remains unknown. The interplay between viscous effects and initial geometry fluctuations is important that requires further investigation. Another key open question in the study of multi-particle production is the relationship between the βridgeβ β observed azimuthal correlations between particles in the underlying event that extend over all rapidities β and hard or semi-hard scattering processes in small collision systems. In particular, it is not known whether jets or their soft fragments are correlated with particles in the underlying event.
This dissertation presents two analyses. The first analysis measures flow harmonics π2βπ6 in 3 πb^-1 of Xe+Xe collisions at βπ ππ = 5.44 TeV using the ATLAS detector at the LHC. The centrality, multiplicity, and π_T dependence of the πn values obtained using two-particle correlations and template-fit procedure are presented, and the measurements are compared with those in Pb+Pb collisions and π+Pb collisions at 5.02 TeV. The πn values in Xe+Xe collisions are observed to be larger than those in Pb+Pb collisions for n = 2, 3, and 4 in the most central events. However, the πn values in Xe+Xe collisions become smaller than those in Pb+Pb collisions with decreasing centrality or increasing harmonic order n. The πn in Xe+Xe and Pb+Pb collisions are also compared as a function of the mean number of participating nucleons and the measured charged-particle multiplicity in the detector. The π3 values in Xe+Xe and Pb+Pb collisions are observed to be similar at the same β¨π_partβ© or multiplicity, but the other harmonics are significantly different.
The second analysis studies two-particle correlations in pp collisions at 13 TeV using data collected by the ATLAS experiment at the LHC, with an integrated luminosity of 15.8 pbβ»ΒΉ, in two different configurations. In the first case, charged particles associated with jets are excluded from the correlation analysis, while in the second case, correlations are measured between particles within jets and charged particles from the underlying event. Second-order flow coefficients, π2, are presented as a function of event multiplicity and transverse momentum. These measurements show that excluding particles associated with jets does not affect the measured correlations. Moreover, particles associated with jets do not exhibit any significant azimuthal correlations with the underlying event, ruling out hard processes contributing to the ridge.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/zh6m-3z76 |
| Date | January 2023 |
| Creators | Yin, Pengqi |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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