CERN Accelerating science

Several unforeseen collective phenomena have been observed in high-multiplicity (HM) final states of small collision systems that resemble the well-established signatures of the quark-gluon plasma (QGP) formation in heavy-ion collisions. However, jet quenching has not been observed in small collision systems. Quantification or setting limits on the magnitude of jet quenching in small systems is essential for understanding the limits of the QGP formation. This thesis discusses the outcomes of a search for jet quenching effects in HM proton-proton collisions at $\sqrt{s} = 13$ TeV collected by the ALICE detector. The multiplicity of charged particles in an event is characterized by the signal amplitude from the V0 forward scintillator detector. The analysis measures the semi-inclusive azimuthal acoplanarity distribution of charged-particle jets recoiling from a trigger hadron with high transverse momentum $(p_{\text{T, trig}})$. Propagating through the hot and dense medium, hard scattered parton is deflected relative to its original direction, thus resulting in the broadening of the hadron-jet acoplanarity compared to a vacuum. Jet reconstruction is performed utilizing the anti-$k_{\text{T}}$ algorithm with cone radius $R = 0.4$. The analysis performs a data-driven subtraction of contributions from uncorrelated trigger-recoil jet pairs, including contributions from multiple partonic interactions. The measured acoplanarity distribution for HM events exhibits suppression and broadening compared to minimum-bias events. These peculiar features are also seen in pp events simulated by the PYTHIA 8 Monte Carlo event generator, which does not incorporate jet quenching. Further studies of PYTHIA simulations reveal that the observed suppression and broadening originate from a bias induced by the ALICE high-multiplicity trigger. The identification of this bias has implications for all jet quenching searches in small collision systems where selection is made on the event activity.