Speaker
Description
High-multiplicity (HM) final states of small collision systems exhibit collective phenomena which resemble the well-established signatures of the quark--gluon plasma (QGP) formed 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 collision systems is crucial for understanding the limits of the QGP formation. This talk discusses the outcomes of a search for jet quenching effects in HM pp collisions at $\sqrt{s} = 13$ TeV collected by the ALICE detector. The multiplicity of charged particles in a collision is characterized by the signal amplitude from the 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. Provided that the QGP medium is created in HM events, it should deflect produced hard partons relative to their original direction, thus resulting in a broadening of the hadron-jet acoplanarity distribution compared to minimum bias (MB) events. The measured HM acoplanarity distributions exhibit suppression and broadening compared to MB events. These peculiar features are also observed in pp events simulated by the PYTHIA 8, 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. Identifying this bias has implications for all jet quenching searches in small collision systems where selection is made on the event activity.
