Search for the Heavy Ion Physics Signatures in Small Collision Systems with the ATLAS Detector at the LHC

Quark-Gluon Plasma is the state of matter in which the quarks and gluons are not bound into hadrons. This form of matter is observed in large systems of particles that can be produced in collisions of relativistic Heavy Ions, for example, at the LHC at CERN. Recent measurements reveal the effects which are considered the signatures of the QGP also in much smaller proton-proton collisions, where they have no clear explanation. The thesis includes two independent analyses that may shed light on this novel phenomenon. The analysis of the multiplicity and kinematic distributions of charged particles produced in association with an $\Upsilon$ meson measured in proton-proton collisions uses the data collected by the ATLAS experiment at the LHC. The analysis uses a full Run-2 data set obtained at $\sqrt{s} = 13 TeV$, corresponding to the integrated luminosity of 139 $\mathrm{fb}^{−1}$. At zero $\Upsilon$ transverse momentum, the associated charged-particle multiplicity drastically differs for different $\Upsilon$ states. It is by $17\pm4$% fewer for $\Upsilon(3S)$ and by $12\pm1$% fewer for $\Upsilon (2S)$ than for $\Upsilon(1S)$. These differences are associated with the underlying event of collisions and decrease with increasing transverse momentum of the $\Upsilon$ states. This measurement is a direct suggestion of bottomonia suppression in $p+p$ collisions at the LHC. A global study of the momentum distributions of the mesons at LHC energies uses an empirical transverse mass scaling approach. This study demonstrates patterns in the spectral properties of mesons related to their quark content and is instrumental in working out the differences in the spectral shapes of particles with identical quark content and close masses. Based on the transverse mass scaling assumption, the excited bottomonia states are found to be suppressed with respect to the ground state by a factor of 1.6 and 2.4 for $\Upsilon (2S)$ and $\Upsilon (3S)$ respectively. The two measurements must be related to the same physics mechanism and have to be understood together.