Strong Constraints on Jet Quenching in Centrality-Dependent p + Pb Collisions at 5.02 TeV from ATLAS

Abstract

<p>Jet quenching is the process of color-charged partons losing energy via interactions with quark-gluon<br>plasma droplets created in heavy-ion collisions. The collective expansion of such droplets is well described<br>by viscous hydrodynamics. Similar evidence of collectivity is consistently observed in smaller collision<br>systems, including pp and p þ Pb collisions. In contrast, while jet quenching is observed in Pb þ Pb<br>collisions, no evidence has been found in these small systems to date, raising fundamental questions about<br>the nature of the system created in these collisions. The ATLAS experiment at the Large Hadron Collider<br>has measured the yield of charged hadrons correlated with reconstructed jets in 0.36 nb−1 of p þ Pb and<br>3.6 pb−1 of pp collisions at 5.02 TeV. The yields of charged hadrons with pchT > 0.5 GeV near and<br>opposite in azimuth to jets with pjetT > 30 or 60 GeV, and the ratios of these yields between p þ Pb and pp<br>collisions, IpPb, are reported. The collision centrality of p þ Pb events is categorized by the energy<br>deposited by forward neutrons from the struck nucleus. The IpPb values are consistent with unity within a<br>few percent for hadrons with pchT > 4 GeV at all centralities. These data provide new, strong constraints<br>that preclude almost any parton energy loss in central p þ Pb collisions.</p>