by Marta Verweij. Published: 13 October 2012

Measurement of jet spectra with charged particles in Pb-Pb collisions at ?sNN=2.76 TeV with the ALICE detector


Marta Verweij presented a flash talk this year in the Quark Matter conference focusing on her recent work on jet spectra

One of the important physical processes in hadron collisions is jet fragmentation in which highly energetic quarks and gluons (partons) that are produced by hard scatterings of incoming partons, radiate gluons. These gluons then split into quarks and anti-quarks, which finally combine and emerge as a collimated spray of detectable hadrons (pions, kaons, protons) from the collisions.This process is sensitive to the presence of the hot medium created in heavy-ion collisions and can thus be used to probe the properties of the medium.

High energetic partons created when two lead nuclei collide interact with the strongly interacting medium while traversing it. Due to the interaction with the medium the partons loose energy by radiating gluons: jet quenching. It is expected that due to jet quenching the structure of the spray of hadrons is modified with respect to the vacuum case.


Marta Verweij in the QM12 conference

In the experiment the measured hadrons are grouped together by a jet algorithm. The kinematic properties of the jets can be associated to the kinematic properties of the original hard parton from the hard process. ALICE has measured inclusive transverse momentum spectra of jets Pb-Pb collisions at ?sNN=2.76 TeV. For this analysis jets are reconstructed from charged particles detected in the central tracking detectors (Time Projection Chamber and Inner Tracking System). The charged particles are the constituents of the jets and are measured with a high efficiency down to very low transverse momenta (150 MeV/c). The challenge in heavy-ion collisions is to separate the particles originating from the hard parton and from the soft underlying event. This is done by estimating the average underlying event momentum density for each event which then is subtracted from the jets in the same event. Statistical and region-to-region fluctuations of the underlying event are quantified by embedding different high pT probes into heavy-ion data. These fluctuations have a significant effect on the reconstructed jet energy and need to be corrected. Also missing energy due to detector inefficiency needs to be corrected. These two corrections are done on a statistical basis by unfolding the measured jet spectrum.

The jet cross section measured in central Pb-Pb events is compared to the jet cross section in peripheral and PYTHIA events by taking the ratio: RAA,jet = Njets,central PbPb / Njets, reference. A strong jet suppression similar to the suppression of of inclusive charged particles is measured in central events. This indicates that gluons are radiated outside the cone of the jet and not all the energy is captured by the jets. It is expected that by looking at larger cone radii part or all of the lost energy can be recovered. For this analysis jet spectra with cone radii R=0.2 and R=0.3 have been measured and compared by taking the ratio of the cross-sections. Within experimental uncertainties we do not observe a modified structure of the jets.


Suppression of inclusive jets and jets triggered by a high pT hadron in central heavy-ion collisions.

In the near future we plan to also study the jet spectrum for larger cone radii and to have a closer a look at the jet structure.


Read also: Plenary Flash Talk by Marta Verweij