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LHC Seminar

Probing jet energy redistribution and broadening in pp and Pb-Pb collisions with ALICE

by Jaime Norman (University of Liverpool (GB))

222/R-001 (CERN)



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Jets are generated in heavy-ion collisions, and their interactions with the Quark-Gluon Plasma (QGP) produced in such collisions provide unique probes of QGP properties. Experimentally observable consequences of jet-QGP interactions include jet energy loss, modification of intra-jet structure, and jet angular deflection. Jet deflection can occur via multiple soft scatterings or single-hard Moliere scatterings off QGP constituents, and its observation will probe the nature of QGP quasi-particles. Such effects are expected to be most prominent at low jet transverse momentum ($p_\mathrm{T}$), which is where jet measurements are also sensitive to wake effects due to response of the QGP itself to the jet-medium energy transfer. Measurements that are differential in jet $p_\mathrm{T}$, jet radius $R$, and acoplanarity (deflection angle) may be able to disentangle these various mechanisms.

This talk presents new measurements of the semi-inclusive distribution of charged-particle jets recoiling from a trigger hadron in pp and central Pb--Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02$ GeV. The measurement is performed using a background-subtraction technique which provides a precise data-driven subtraction of the large uncorrelated background contaminating the measurement, uniquely enabling the measurement of jet production and acoplanarity over a wide phase space, including the low jet $p_\mathrm{T}$ region for large $R$. This measurement in pp collisions provides a test of pQCD predictions for this process. We observe a striking medium-induced enhancement in recoil jet yield at low $p_\mathrm{T,jet}$, whose R-dependence enables a clear discrimination of in-medium jet scattering and QGP wake effects. The in-medium suppression of recoil yield at higher $p_\mathrm{T,jet}$ provides new insight into the mechanisms underlying jet quenching. Comparison to model calculations and their dependence on jet $R$ will be shown to interpret the results.

Refreshments will be served at 10:30

Organized by

Michelangelo Mangano, Jan Fiete Grosse-Oetringhaus and Pedro Silva

There is a live webcast for this event