ALICE

High-energy scattering of partons (quarks and gluons) produces collimated cones of particles called jets, the production rate of which can be calculated using perturbative QCD techniques. In heavy-ion collisions, partons lose energy in the hot, dense quark–gluon plasma (QGP), leading to a modification of the jet-energy distribution. Measurement of the jet characteristics can therefore be used to probe QGP properties.

In non-central heavy-ion collisions, the overlap region between the two nuclei where nucleon–nucleon scattering takes place has a roughly elliptic shape, resulting in a longer average path length – and therefore larger energy loss – for jets and particles that are emitted along the major axis, than for those emitted along the minor axis of the interaction region. The resulting variation of the azimuthal jet distribution can be expressed as jet ν2, the second coefficient of a Fourier expansion of the angular distribution. The magnitude of jet ν2 depends on the path-length dependence of the jet energy loss, which differs among proposed energy-loss mechanisms and can be studied via model comparisons.

The figure shows the new jet ν2 measurement of ALICE, using only charged particles for jet reconstruction, in semi-central collisions (30–50% collision centrality) compared with earlier jet ν2 results of ATLAS, using both charged and neutral fragments, and ν2 of ALICE and CMS, using single charged particles. The ALICE measurement covers the pT range between the charged-particle results and the ATLAS jet measurements. Jet measurements in this momentum range (20–50 GeV/c) are challenging due to the large background of soft particles in the event. This background is itself subject to azimuthal variations, which have to be carefully separated from the jet ν2 signal.

The significant positive ν2 for both jets and single charged particles indicates that in-medium parton energy loss is large, and that sensitivity to the collision geometry persists up to high pT. For a more quantitative interpretation in terms of density and path-length dependence, the experimental results will need to be interpreted within theoretical models that include the effects of parton energy loss as well as jet fragmentation. Larger data samples from Run 2 will further improve the measurement, giving more precise information about the nature of the QGP and its interactions with high-momentum quarks and gluons.