The new results shown by the ALICE collaboration at the Quark Matter 2014 conference in Darmstadt focus principally on the most recent collisions at the LHC – those of protons and lead nuclei (pPb) performed towards the end of Run 1 in early 2013. Planned mainly as a control experiment for studies of deconfined matter in lead–lead (PbPb) collisions, the pPb run in fact revealed tantalizing possibilities for studying collective phenomena in small systems, but with exceedingly-high-energy densities. This subject, already tackled in several theoretical papers, featured at the conference. Having already published interesting results on flow-like correlations measured via the azimuthal emission patterns of particles, ALICE presented a host of results on particle correlations, extending out to mesons containing charm quarks (D mesons).
Fig. 1. Transverse momentum dependence of the nuclear modification factor RpPb of charged particles (h±) measured in minimum-bias (NSD) pPb collisions at √sNN = 5.02 TeV in ALICE, in comparison with data on the nuclear modification factor RPbPb in central PbPb collisions at √sNN = 2.76 TeV in CMS (arXiv:1405.2737 [nucl-ex]). The PbPb data are for charged particle, direct photon, Z0 and W± production. All data are for mid-rapidity.
The initial-state effects (gluon shadowing) are studied in pPb collisions via the nuclear modification factor, which quantifies the measurement in nuclear collisions with respect to that in pp collisions scaled by the corresponding number of binary collisions. Its dependence on transverse momentum (pT) for inclusive charged-particle production, already obtained with the data of the pilot pPb run of 2012 has been extended up to pT of 50 GeV/c (figure 1). The data support the binary-collision scaling up to the highest measured pT and confirm the predictions from saturation/shadowing models.
Fig. 2. Transverse momentum dependence of the nuclear modification factor for D-meson production in minimum-bias pPb collisions and two centrality classes in PbPb collisions (arXiv:1405.3452 [nucl-ex]).
ALICE has obtained results for a variety of hadron species, including charmed and beauty hadrons (directly identified or studied via their decays into electrons or muons), as figure 2 shows. An enhancement at pT values around 5 GeV/c – known from measurements at lower energies as the "Cronin effect" – is found for protons and observed to depend on the "centrality" of the collision. For all hadrons, binary-collision scaling is fulfilled at high pT in pPb collisions. By contrast, in PbPb collisions, the measured suppression of hadron production is an indication of strong interactions within the hot and dense medium (or jet quenching, measured also with reconstructed jets).
Studies of Bose–Einstein pion correlations with two and three pions demonstrate that, in pPb collisions, the size of the system at freeze-out is closer to that in pp collisions than to PbPb, for events with the same particle multiplicities.
ALICE has also found that the ψ(2S) charmonium state is more suppressed in pPb, compared with expectations from the scaling of production in pp collisions, than its lower-mass sibling J/ψ. No convincing theoretical explanation has been put forward, but the ALICE results might imply a final-state effect, possibly owing to deconfined matter in pPb collisions.
Another interesting measurement presented in Darmstadt is the dependence of open-charm hadrons (D mesons) and charmonium production on the multiplicity measured in pPb collisions.
The interpretation of the above results is complicated by the observation in ALICE that the connection between experimental observables and collision geometry (the impact parameter) is much more complex in pPb collisions than in PbPb. A comprehensive study performed with data on charged-particle production illustrates this, and is likely to imply further challenges to theory: not the geometry, but the "event activity" in terms of the hadron multiplicity in a given region of rapidity will need to be properly modelled in theoretical descriptions of pPb collisions.
Lastly, the pPb data allowed for a first measurement ever in ALICE – that of W-boson production, performed at forward and backward rapidities with triggered data from the muon spectrometer.
Fig. 3. Measured hadron abundances in comparison with thermal model calculations.
The results for PbPb collisions also span a range of observables. A comprehensive study of hadron production was presented, with comparisons across systems, PbPb, pPb, and pp, as well as with measurements at Brookhaven’s Relativistic Heavy-Ion Collider (RHIC). Comparison of hadron abundances in PbPb collisions with thermal-model calculations lead to a temperature value lower than that extracted from data from RHIC. Some distinct discrepancies between model and data, like that for protons, have been a hot subject of discussion for some time. Those notwithstanding, the measured hadron abundances are, remarkably, predicted by the thermal model at chemical freeze-out (figure 3). The new ALICE data on (hyper)nuclei extend this conclusion to weakly-bound systems, such as the deuteron or the hypertriton.
Fig. 4. Rapidity dependence of the nuclear modification factor for Υ(1S) production in PbPb collisions, as measured by ALICE and CMS.
Insights into collision dynamics are obtained from the analysis of spectra and flow of a variety of hadron species. Among them, the φ meson, containing a strange-antistrange quark pair and with a mass close to that of the proton, plays a prominent role. The data show that it is the mass of the φ meson, and not its quark content, that determines its pT spectrum and flow. The measurement of Υ production in PbPb reveals a rapidity-dependent suppression (figure 4), which finds no explanation in any theoretical model to date. This adds a new facet to the fascinating story of quarkonium in deconfined matter. Here, recent ALICE data on J/ψ agree with the theoretical understanding of charmonium production via (re)generation mechanisms at the chemical freeze-out or during the whole lifetime of the deconfined system.
The new measurement of the nuclear modification factor of electrons from beauty-hadron decays is a first step in ALICE into the sector of b-quark energy loss. ALICE has also performed the first measurement of coherent ψ(2S) photoproduction in ultra-peripheral PbPb collisions. Albeit not directly a "Quark Matter" topic, this is a relevant measurement because it constrains parton shadowing in the colliding nuclei. Another premiere is the first measurement of particle-type jet fragmentation at hadron colliders, achieved in pp collisions in ALICE with pions, kaons, and protons up to pT = 20 GeV/c.
A host of other results in PbPb collisions, covering jets, particle production and flow, and quarkonia – some already submitted for publication – mark the Quark Matter 2014 conference as a milestone in the ALICE physics output.
The article appeared in June's issue of CERN Courier here