by C. Klein-Boesing and M.van Leeuwen - Coordinators of the PWG-Jets of ALICE. Published: 13 October 2012

The Quark Matter 2012 saw several highlights in the physics of reconstructed jets, and ALICE contributed some unique additions to the field. The main results have been presented in one plenary and two parallel talks, supported by five Posters, out of which one has been chosen for presentation as Flash Talk in the closing plenary session on Saturday.

The motivation for reconstructing jets in heavy-ion collisions is to gain a more direct access to the properties of quarks and gluons that have been scattered with a large momentum transfer in the early reaction phase prior to the formation of a medium. These hard scattered partons provide a controled probe for the later stages since their production rates are well understood from the baseline (vacuum) proton-proton measurements. During its propagation through the medium a scattered parton loses energy via elastic or inelastic collisions with the constituents of the expanding medium. That way, the energy of the original parton is distributed to observable particles (fragmentation) in a different way than in the vacuum.

In general, jet reconstruction algorithms aim to revert this fragmentation and recombine the particles in a certain angular region to a jet as a proxy for the original parton properties. The size of this angular region is defined by a distance parameter R, which is a measure for the maximum separation in the ??-plane between the center of a jet and a jet constituent (red entries in Figure 1). Such procedures are well defined in elementary reactions, but the high particle density of heavy ion collisions poses a challenge to many algorithms. The amount of background particles from the uncorrelated underlying events can add up to several tens of GeV (depending on the radius and momentum). Furthermore, the subtraction of this background can only be done on average and the challenge is to correct for the remaining fluctuations of the jet energy introduced by the underlying event. At the same time the impact of the underlying event on jet structure observables has to be carefully separated from possible medium effects.

Figure 1: Illustration of jet finding in central PbPb: The red entries show the constituents of a reconstructed jet with R = 0.4. The subtraction of the average underlying background in the event yields the corrected jet momentum.

For this demanding task, the ALICE-detector is particularly well suited, since it allows to incorporate particles down to very low momentum in the jet measurement, on the one hand to search for medium effects on jet fragmentation in a momentum region not covered by any other LHC experiment, on the other hand to explore systematically the impact of different (low) momentum cut offs on the jet reconstruction in heavy-ion collisions.

This year’s Quark Matter marked the starting point of reconstructed jet physics with ALICE. We presented the first results on reconstructed jets using charged and neutral information thanks to the complete installation of the ALICE EMCal in early 2011. This completion happened right in time for the data taking in the pp reference run at 2.76 TeV. The resulting jet spectra include particles down to a transverse momentum of 150 MeV/c and are shown in Figure 2. They demonstrate a good agreement with state-of- the art calculations and generators and provide the basic reference for jet measurement in heavy-ion collisions.

Figure 2: Differential cross section of fully reconstructed jets with R = 0.4 in proton-proton collisions at √s = 2.76 TeV. First jet measurement with the EMCal.

The first study of the attenuation of jets in the medium as presented at Quark Matter is shown in Figure 3. It is quantified via the scaled ratio of reconstructed jets in central and peripheral Pb-Pb collisions, which is expected to be unity in the absence of any medium effects. These results are based on jets from charged particles only and complement the measurements of ATLAS and CMS with lower jet transverse momentum. In addition, it is remarkable that despite the exceptionally low momentum cut-off on the jet constituents a similar magnitude of jet yield suppression is observed in the momentum region where the three experiments overlap. This indicates that the parton energy is redistributed to larger angles than currently covered in this observable.

Figure 3. Compariosn of jets reconstructed with charged particles and R =0.3 in different centralities of Pb–Pb collisions at √sN N = 2.76 TeV.

At Quark Matter we also introduced a new way to circumvent some of the limitations in extracting a jet signal at low transverse momentum with increased angular extend. In the standard measurement as described above, which considers all jets, this is hindered by the dominance of fake jets from the underlying event. In the new method, we take advantage of the fact that jets occur back-to-back in ? and require a single track with large transverse momentum opposite to the reconstructed jets. By this tagging the recon- structed jets can be divided experimentally into two classes: Correlated jets, where the shape of the spectrum depends on the choice of transverse momentum used for the tagging, and random correlations which are independent of the latter. The random contributions are subtracted out when taking the difference of two distributions with different trigger momentum ranges. This method enables the study of jets in central heavy-ion collisions with R = 0.4 down to 20 GeV/c. The ratio of this observable in Pb-Pb to a simulated pp reference is shown in Figure 4 and it was seen at Quark Matter that this observable poses a challenge to some models describing well the suppression observed in the inclusive jet spectrum.

Figure 4. Comparison of the difference spectrum of jets tagged with recoiling high pT hadrons in pp and central Pb–Pb collisions at 2.76 TeV for R = 0.4

Already for the Hot Quarks conference next month we plan to show new jet measurements with the inclusion of the EMCal and further, differential study of jet structure and of course the preliminary results from Quark Matter will be finalized in the next months for publication.

Further Reading

Plenary Talk by Andreas Morsch
Plenary Flash Talk by Marta Verweij
Parallel Talk by Rosi Reed
Parallel Talk by Leticia Cunqueiro