Incoherent Photoproduction of $\phi$ Mesons in Ultra-Peripheral Pb+Pb Collisions at $\sqrt{s_{NN}}$ = 5.02 TeV
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Authors
Gautam, Amrit
Issue Date
2019-08-12
Volume
Issue
Type
Dissertation
Language
en_US
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Abstract
The Large Hadron Collider (LHC) can accelerate fully ionized lead nuclei to near the speed of light. A Large Ion Collider Experiment (ALICE) is one of the detectors located at the LHC which observes the lead+lead collisions.
During Run 2 at the LHC, a center of mass energy of 5.02 TeV per nucleon was achieved. An ultra peripheral collision occurs when the lead nuclei miss each other, having an impact parameter greater than twice the radius of the nucleus. The interaction, in this case, is electromagnetic. These electromagnetic interactions can be treated as the exchange of virtual photons. In a photonuclear interaction, the photon from one nucleus interacts with the other nucleus, with a single vector meson the most common final state. The $\phi$ meson is a vector meson composed of a strange and a anti-strange quark, with $\phi\rightarrow K^+ K^-$ as the dominant decay mode. The photoproduction of $\phi$ mesons has been studied in photon+proton collisions at HERA. However, no observation of $\phi$ meson photoproduction has been made in ultra peripheral collisions of nuclei. This thesis presents the first observation of incoherently produced $\phi$ mesons in ultra peripheral heavy ion collisions. We analyzed 2018 run 2 ALICE data collected with the CCUP29 trigger to observe the incoherent photoproduction of $\phi$ meson in the $K^+ K^-$ decay channel in Pb+Pb collisions at an energy of 5.02 TeV per nucleon. We observed 23$\pm$5 $\phi$ meson candidates in the mass range 1.015 to 1.2 GeV/$c^2$. Efficiency correction was done with the STARLIGHT Monte Carlo. The efficiency-corrected mass distribution was fitted with a Gaussian for the signal and a linear function for the background. The mean and width of the $\phi$ meson were measured to be 1.0181$\pm3\times 10^{-4}$ GeV/$c^2$ and 1.5$\times10^{-3}\pm$2$\times10^{-4}$ GeV/$c^2$ respectively, in agreement with Particle Data Group values. The data analyzed was collected with a trigger requirement that the opening angle between the tracks in the transverse plane should be greater than 0.63 radians (36 degrees). However, we observed that only 13$\%$ of the $\phi$ candidates satisfied this trigger condition, indicating a very high rate of false triggers and leading to a very large uncertainty in the measured cross section. We measured the cross section of incoherent photonuclear production of the $\phi$ meson within the acceptance of the ALICE central barrel
at an energy of 5.02 TeV per nucleon to be $19^{+6}_{-19}$ mb.
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Publisher
Creighton University
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Copyright is retained by the Author. A non-exclusive distribution right is granted to Creighton University and to ProQuest following the publishing model selected above.