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2011 QCD and High Energy Interactions - Rencontres de Moriond ...

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Figure 13: Particle <strong>de</strong>nsities (left) <strong>and</strong> ellipticity (right) measured by ALICE in comparison with lower<br />

energy experiments.<br />

Figure 14: η-φ correlations between particles measured at RHIC by the STAR experiment in domains<br />

of increasing PT: PT > 0.15 GeV (left), 1.7 GeV (middle) <strong>and</strong> 2.5 GeV (right).<br />

A domain of particular interest is the influence of the QGP medium on parton flow, studied from<br />

the yields of individual high-PT hadrons or hadronic jets as function of centrality. In central<br />

collisions, ALICE measures a stronger quenching of high-PT hadrons as at RHIC (figure 15). As<br />

regards hadronic jets, a qualitatively new measurement, ma<strong>de</strong> possible by the high centre of mass<br />

energy <strong>and</strong> the 4π calorimetric coverage of the <strong>de</strong>tectors, is the observation by ATLAS <strong>and</strong> CMS of<br />

direct jet quenching, visible from event by event dijet asymmetries increasing with centrality. The<br />

study of the azymuthal energy flow as function of this dijet asymmetry shows that the quenched<br />

energy is released in softer particles emitted away from the direction of the initial high-PT parton<br />

(figure 16). This feature is not <strong>de</strong>scribed by conventional PYTHIA radiation.

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