Current and future measurements of semi-inclusive hadron+jet distributions with ALICE
The measurement of jets recoiling from a trigger hadron in heavy-ion collisions can be used to understand the properties of the Quark Gluon Plasma. Jet-medium interactions cause jets to lose energy in the medium and may modify the jet structure. Jet deflection towards large angles due to scattering off of quasi-particles in the Quark-Gluon Plasma may also occur, which can be studied through a measurement of the hadron-jet acoplanarity. These phenomena can be studied through the semi-inclusive distribution of track-based jets recoiling from a trigger hadron. This contribution presents the latest measurements and prospects of semi-inclusive hadron+jet distributions with ALICE. Constraints on energy loss in p–Pb collisions and future prospects to measure energy loss in smaller systems are shown. A jet shape measurement of N-subjettiness using recoil jets is outlined. Finally, prospects for hadron+jet acoplanarity measurements with ALICE are presented.
Current and future measurements of semi-inclusive hadron+jet distributions with ALICE
Jaime Norman††thanks: Speaker. , for the ALICE collaboration
Laboratoire de Physique Subatomique et Cosmologie, Grenoble, France
Measurements of jets created in ultrarelativistic heavy-ion collisions provide unique probes to characterise the hot and dense QCD medium created in these collisions. The measurement of inclusive jets (see e.g.  for recent ALICE results) show a significant suppression in heavy-ion collisions with respect to pp collisions, indicating that partons lose energy while travelling through and interacting with the QCD medium.
The measurement of jets recoiling from a trigger hadron is being employed to further study jet quenching effects. ALICE has measured the trigger-normalised semi-inclusive yield of jets recoiling from a trigger hadron , differential in jet transverse momentum . A variable is then defined as the difference between the trigger-normalised recoil jet distributions in ‘reference’ and ‘signal’ trigger track intervals and :
where accounts for the combined effects of invariance of total jet yield with trigger track . With this observable one removes entirely the background from uncorrelated reconstructed jets, giving the possibility of extending jet measurements to low- and high jet resolution parameter . The jet population measured with this technique is not biased in terms of jet fragmentation pattern. As a trigger-normalised and semi-inclusive quantity one can also avoid model-dependent assumptions to relate event activity to event geometry, leading to greater systematic sensitivity to jet quenching effects in small systems . It is noted that the measurements shown here use jets reconstructed from charged tracks only, i.e. ‘track-based jets’.
2 Constraints on jet quenching in smaller systems
The measurement of the trigger-normalised recoil jet distributions in Pb–Pb collisions indicates that jets lose a significant amount of energy in Pb–Pb collisions, and that this energy is predominantly radiated to angles greater than . A similar analysis was performed in p–Pb collisions in different event-activity classes, defined using the signal magintude in the V0A detectors of ALICE, to test whether jets are quenched in smaller systems . Figure 1 (left) shows the ratio in high/low event activity classes for recoil jets with from . The ratio is consistent with unity, indicating minimal jet quenching in p–Pb collisions, and a limit of out-of-cone energy loss for jets in this range is set (90% CL).
The sensitivity to jet quenching in small systems in Run 3/4 of the LHC has also been assessed, based on PYTHIA simulations to estimate the expected number of charged hadron triggers and trigger normalised recoil jet spectrum for a given integrated luminosity. Figure 1 (right) shows the projection of the same observable for jets in pp collisions at with an integrated luminosity of 200 p, where the ratio of ‘central’ and ‘peripheral’ events corresponds to the 0–0.1% centrality percentile and 50–100% centrality percentile respectively. Here no event-activity shift is included, and the statistical limit (90% CL) on a measurement of out-of-cone energy loss is 175 for this system. For p–Pb collisions where the high-EA is set to the 0–5% percentile, this limit is 70 . While the corresponding systematic uncertainties are not estimated, it is noted that the statistical uncertainties were dominant in the Run 1 measurement.
3 Substructure of recoil jets
The measurement of jet shapes and their modification in heavy-ion collisions can give further insight into jet quenching mechanisms. For example, the role of colour coherence  can be probed by studying how 2-pronged jets are modified in heavy-ion collisions. The N-prongness of jets is measured through the N-subjettiness observable . For this observable, jets are reclustered using an exclusive clustering algorithm, and ‘subjet’ axes are found by unwinding the last clustering step. is then defined as
where is the distance between track and subjet , is the of the -th jet constituent and is the jet resolution parameter. The ratio is a measure of how 2-pronged a jet is.
In order to get a combinatorial background-free distribution of low- jets with low fragmentation bias, a similar technique as described in Section 1 is used. As shown in figure 2 (left) a ‘reference’ trigger track recoil jet distribution is subtracted from a ‘signal’ trigger track distribution to obtain the distribution of true jets, free from fragmentation bias. Figure 2 (right) shows the distribution in Pb–Pb collisions in comparison with the same distribution in PYTHIA. No modification of two-prongness with respect to PYTHIA is seen within the experimental uncertainties. Different reclustering algorithms are also explored which are sensitive to different properties of the jet splitting, see  for more information.
4 Di-jet azimuthal correlations
The interaction of jets with the Quark-Gluon Plasma can be further studied by measuring the azimuthal correlation of dijets, or in this case, the azimuthal correlation between a trigger hadron and a corresponding recoiling jet. This is measured by the angle between the trigger hadron and recoiling jet, denoted . The motivation for studying this observable is two-fold:
The broadening of the peak of the away side distribution (at ) with respect to vacuum expectation is sensitive to soft multiple scattering in-medium. Since the angular deflection can be related to the change in the momentum transverse to the direction of the initial parton, this could give direct constrains to the transport coefficienct .
The shape of the tails of the distribution at large angles away from can be used to study the rate of large angle scattering in the QGP. This can arise from resolving the weak degrees of freedom in the Quark-Gluon Plasma, and evidence of large angle scattering could give evidence of a quasiparticle nature of the plasma .
The background-subtracted hadron+jet azimuthal distribution was measured in Pb–Pb collisions at ALICE with Run 1 data . The rate of large angle scattering showed no deviation from the vacuum expectation within the experimental uncertainties, though this measurement was statistically limited. Recent theoretical work (see e.g. [7, 8]) has suggested that low- jets are most sensitive to such effects and additional, higher statistics measurements are underway.
The reach of a hadron+jet measurement in Run 3/4 of the LHC has been assessed. Central Pb–Pb and pp collisions are simulated with the JEWEL model . Figure 3 (left) shows the background-corrected azimuthal distribution of jets recoiling from a high- trigger hadron, with the expected statistics of Run 3/4. The difference between the large-angle recoil jet yield in pp and Pb–Pb collisions is then studied by integrating this distribution at large angles away from , between and a theshold angle , defining .
Figure 3 (right) shows the distribution in Pb–Pb and pp collisions in JEWEL, and the ratio between the two systems. The statistical precision of the ratio at is around 5% (dominated by the uncertainty of the pp reference), so a deviation in the large-angle yield from vacuum will be able to be resolved to approximately this accuracy. Theoretical calculations predict deviations of similar magnitude , so a measurement in Run 3/4 promises to resolve different pictures of the micro-structure of the medium.
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