Contributing 35 talks and almost 100 posters, the ALICE Collaboration has had a strong participation in QM2018, which took place in Venice from 13 to 19 May, where an ample set of new results was presented.
The 27th edition of the Quark Matter conference took place in Venice in the week 13-19 May. In ALICE, preparations for the conference started months ago, with new analyses in all the Physics Working Groups. More than 70 new preliminary results were reviewed and approved for the conference and 16 new papers were made available on the preprint server right before the start of the conference.
In the opening session, Alexander Kalweit presented the ALICE highlights talk, which covered many of the new results, and provided pointers to the 35 contributed talks and almost 100 posters that ALICE presented in the following days. The new results from ALICE covered a broad set of topics, including particle production in pp, p-Pb, and Pb-Pb collisions, and for the first time also Xe-Xe collisions, which were produced by the LHC in a short test run in October 2017. In the following, we will highlight a few of the new results that were presented by ALICE at Quark Matter.
Alexander Kalweit giving his talk on the ALICE highlights.
For the small system collisions, pp and p-Pb, we reported on recent measurements on production of resonances and (anti-)nuclei as a function of the total charged particle multiplicity. These results show an intriguing dependence of the production of high-momentum particles on the overall multiplicity, which is likely due to the occurrence of multiple semi-hard scatterings in a single proton-proton collision. The goal of these measurements is to study the onset of effects such as strangeness enhancement and radial and elliptic flow, which are typically associated with Quark-Gluon Plasma formation.
New results on heavy flavour production in p-Pb collisions show that the charmed baryon production rate is much larger than was expected from electron-positron collisions, and the baryon-to-meson ratio is characterized by a maximum at intermediate pT, which is also seen for light flavour baryon-to-meson ratios. This suggests that there is a common production mechanism for light flavour baryons such as protons and Λ baryon and for the charmed Λc baryon. A first result of Λc baryon production in Pb-Pb collisions was presented, which also shows a large baryon/meson ratio. Improving the precision of these measurements is one of the goals of the detector upgrade programme, which was discussed in another session.
New results on production of the quarkonia J/Ψ and Ψ(2s) in p-Pb collisions at √sNN = 8.2 TeV provide more precise information on the density distributions of quarks and gluons in the nucleus. The production of Ψ(2s) is significantly suppressed with respect to expectations from proton-proton collisions, even in the proton-going direction where no suppression is seen for the lower-mass J/Ψ. This suppression is not fully understood yet, but may be coming from final state interactions with light particles at similar momenta.
In October last year, the LHC collided Xe nuclei for a few hours and ALICE recorded about 2 M collisions, which allow studying the dependence of particle production and QGP effects on the size of the colliding nuclei: the isotope of Xe that was used has 129 nucleons, whereas Pb has 208 nucleons. The total multiplicity (number of produced particles) in Xe-Xe collisions is found to be similar to that in Pb-Pb with the same number of participating nucleons, except for very central Xe-Xe collisions, where an increase of particle production per participant pair is found. The elliptic and triangular flow in Xe-Xe and Pb-Pb collisions are very similar when comparing analogous centralities, as expected, because of the similarity of the initial shape of the system. The smaller number of nucleons in Xe leads to larger fluctuations of the initial geometry, which in turn lead to a larger flow signal in central events; the measured values agree with model calculations. The relative abundances of light flavour hadrons in the new Xe-Xe data confirm the previously-established picture that particle chemistry depends mostly on final state particle multiplicity at LHC energies. Finally, for high-momentum particle production, we observe a similar nuclear modification factor in Xe and Pb when comparing collisions with the same multiplicity. This is qualitatively in line with expectations, since parton energy loss depends on the density and the volume of the system, but more detailed model comparisons are being pursued.
A dedicated study of the nuclear modification factor of peripheral Pb-Pb collisions shows that, while the suppression of high-momentum particle production that is associated with parton energy loss initially decreases when the collisions become less central, it increases again for very peripheral collisions. This non-monotonic behaviour suggests that there is a different mechanism that suppresses high-momentum particle production in very peripheral collisions; one possible explanation is that the individual nucleon-nucleon collisions in the nuclear collision have larger impact parameters and that this reduces the number of parton scatterings, and thus the particle production at high transverse momentum. It is also relevant for the interpretation of collisions of small systems, where the observed azimuthal anisotropy suggests that final state interactions are important, but no suppression of final state particle production is found.
ALICE also presented a first attempt to measure azimuthal anisotropy of direct photons at the LHC, which probes the time evolution of the temperature and pressure in the Quark Gluon Plasma. The measured signal is large, suggesting the importance of late emission of photons. However, the uncertainties are still sizeable and further improvements are needed to firmly establish this conclusion.
The Quark Gluon Plasma is also studied using high momentum particles that traverse the plasma and interact with it. At the conference, ALICE presented new results on the nuclear modification factor for jets, as well as studies of the substructure of jets, which aim to be directly sensitive to the radiation of gluons by fast partons as they go through the plasma. A suppression of large-angle symmetric splittings is found, which suggests that partons in a parton shower interact independently with the Quark Gluon Plasma if the angle between them is large enough.
All in all, the Quark Matter conference was a very interesting meeting, with an unprecedented number of new results from ALICE and the other experiments, as well as discussions of new ideas from theorist colleagues. The topics represented above are only a small selection of what was shown at the conference. The release of a large number of new results at the conference has sparked a lot of new discussions, which are being followed up in several places and we are looking forward to the new insights in strongly interacting matter that this will bring.