Direct photon production at low transverse momentum in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Direct photons are unique probes to study quark-gluon plasma (QGP) created in ultra-relativistic heavy-ion collisions, as they are emitted from the whole stages of the space-time evolution with negligible final state interaction. They carry undistorted medium information such as thermodynamic properties. Direct photon measurements in pp collisions serve as a vacuum baseline for the studies in heavy-ion collisions and can be used as a test of perturbative QCD (pQCD) calculation. On the other hand, recent measurements in pp collisions with high event activity exhibit similarities to heavy-ion collisions, such as collective phenomena of hadrons. This motivates a search for the thermal photons and the creation of the QGP in small systems to better understand the underlying dynamics in such collisions. This thesis presents direct photon production in pp collisions at $\sqrt{s}$ = 13 TeV in minimum-bias and high-multiplicity event multiplicity classes to test pQCD and search thermal photons in small systems. This work is performed via the internal conversion technique, the ratio of direct to inclusive photons can be extracted from the dielectron continuum. Compared to the previous study, the statistical and systematic uncertainties were significantly improved and the direct-photon signal was extracted in the range $1 < p_T < 6 \, \text{GeV}/ c$, for both event multiplicity classes. It was found that direct photon yields in inelastic and high-multiplicity pp collisions show clear multiplicity dependence.