Search for medium effects with jet-like hadron correlations at ALICE at the LHC

We present an analysis of the properties of jets in proton-proton and lead-lead collisions at the ALICE experiment at the CERN Large Hadron Collider, using the technique of two-particle angular correlations. The data sample comprises proton collisions at energies of $\sqrt{s}=7\text{ TeV}$ and $\sqrt{s}=2.76\text{ TeV}$, and lead collisions at a nucleon-nucleon collision energy of $\sqrt{s_\mathrm{NN}}=2.76\text{ TeV}$, from data taking periods in 2010 and 2011. The jet width and transverse momentum imbalance, quantified by the jet fragmentation transverse momentum $j_\mathrm{T}$ and the net partonic transverse momentum $k_\mathrm{T}$ respectively, were studied in minimum bias proton collisions. The width was found to be independent of hadron transverse momentum, with a value of $\sqrt{\langle j_\mathrm{T}^2 \rangle}\approx 0.8\text{ GeV}$. This is higher than measurements from earlier experiments at lower beam energies, consistent with theoretical expectations. $\sqrt{\langle k_\mathrm{T}^2 \rangle}$ was found to increase with collision energy and also with the transverse momentum of the hadrons in the jet. The dependence of jet yield on the produced multiplicity was studied in proton and ion collisions, with the multiplicity providing an estimator for the ion collision centrality. The ratio of awayside to nearside yield was studied, with a view to identifying suppression of jet momentum through interaction with a deconfined quark-gluon plasma. This suppression was identied in lead collisions as expected from previous results, but no suppression was observed in high multiplicity proton collisions. The jet width and transverse momentum imbalance were also studied as a function of multiplicity, with the width quantified by $j_\mathrm{T}$ and the imbalance quantified by the correlation function awayside peak width. In proton collisions, the jet width was measured to be $\sqrt{\langle j_\mathrm{T}^2 \rangle}\approx 0.8\text{ GeV}$ independently of multiplicity; this was consistent with the measurements in minimum bias collisions. No firm evidence of medium effects was observed in measurements of the awayside peak width in proton or lead collisions, but the jet width was observed to decrease in lead collisions of increasing centrality. This was interpreted as a suppression of the high momentum hadrons in the jets.