by M. Floris, B. Hippollyte - Coordinators of the PWG Light Flavour Spectra . Published: 13 October 2012


Transverse momentum (pT) spectra of light-flavoured hadrons are a crucial observable for the characterization of heavy-ion collisions, with different pT regions being dominated by different physics. Schematically, one can identify 3 pT regions: low (pT < 1.5 GeV/c), intermediate (1.5 < pT < 8 GeV/c) and high (pT > 8 GeV/c) based on the different mechanisms involved in the production of the spectra.

In the low pT region, the shape of the transverse momentum spectra is dominated by bulk effects, and in particular hydrodynamic flow and thermal production. At intermediate pT, an interplay of soft and hard processes gives rise to a rich structure, which is currently very challenging for theoretical modeling and therefore most interesting. Finally, the study of identified spectra at high pT allows to access possible modification of the fragmentation functions induced by parton energy loss in the hot and dense medium.

Moreover, heavy-ion collisions offer a very good test bench to study rare and exotic form of matters such as hyper-matter and heavy anti-nuclei. During the Quark Matter 2012 Conference, many new and exciting results on these studies were presented, including the first LHC results on identified π/k/p at high pT.

(see also M. Ivanov, Results on identified particle spectra from ALICE Plenary)

Low pT and bulk properties

The main physics mechanisms driving the dynamics at low pT are thermal production and hydrodynamics, which determine the bulk properties of the system. At Quark Matter 2012 results were presented for π, K, p and their anti-particles, from ~150 MeV/c up to 20 GeV/c in pT.

The low-pT results for central collisions (arXiv:1208.1974) were compared with expectations from different hydrodynamic and thermal models. Hydrodynamic models were found to give a good description of the transverse momentum distributions, especially for models incorporating annihilation and rescattering in the hadronic phase or non-equilibrium effects.

This result is in agreement with what had been presented in the previous Quark Matter conference in 2011. On the other hand, the p/π ratio is found to deviate from the values predicted by equilibrium thermal models (at least in their simplest implementation).

This observation is now complemented with new results for different particles that were presented for the first time in QM 2012 conference (shown in figure 1). It can be seen that the original thermal model prediction (T = 164 MeV) gives a good description of all stable particles except protons (for a discussion of the K* and φ resonances, see below). If a fit to the data is attempted, this leads to T ≈ 152 MeV, mostly driven by the low proton yield. However, the description of many particle ratios is not very satisfactory in this case.

Figure 1. Yield of different light-flavor particles relative to the π+ yield, compared to thermal model calculations (Thermal model: Phys. Lett. B 673 (2009) 142 [arXiv:0812.1186])

These results indicate that the thermal model, which worked very well at lower energies (at least in the interval ?sNN = 2 – 200 GeV), needs to be critically revised at the LHC. Current speculations indicate that the hadronic phase or non-equilibrium effects could have a significant impact on the observed yields.

A separate discussion is in order for resonances. Resonances have a lifetime comparable to the one of the fireball, which gives a handle on its properties and on effects associated to the hadronic or the partonic phase.

The modification of the mass and/or width of resonances could probe the effects of partial chiral symmetry restoration. Resonances could be dissociated or regenerated in the hadronic phase, or their decay products could suffer rescattering thus rendering the resonance impossible to reconstruct. For these reasons, resonances are not in general expected to fit in the thermal picture (Figure 1). At this conference, ALICE presented the first results on the production of K* and φ resonances in Pb–Pb collisions at the LHC. One of the most striking results is shown in figure 2: the ratio of the K* over the K- is seen to decrease with increasing Npart (and hence centrality) while the φ over K- is constant. Taking into account the different lifetime of the two mesons (the φ lifetime is about 10 times larger than that of the K*) this effect could indicate an increase of the rescattering effects for central collisions.

At QM2012 we also presented an update on our studies of inclusive charged particle production, with an extension of the dNch/d η measurements to cover the range | η | < 5 (as compared to the previously published results in | η | < 0.5). This allowed us to estimate total particle production in Pb–Pb collisions.

(see also M. Broz, Midrapidity antibaryon-to-baryon ratios in pp and Pb-Pb collisions measured by the ALICE experiment
L. Milano, Identified charged hadron production at the LHC with the ALICE experiment
S. Singha, Strange hadrons and resonances in Pb-Pb collisions at sqrt_NN = 2.76 TeV with ALICE experiment at LHC
M. Guilbaud, Pseudorapidity density of charged particles in a wide pseudorapidity range and its centrality dependence in Pb-Pb collisions at 2.76 TeV )

Intermediate and high pT

Quark Matter 2012 was the first conference where results on identified π/K/p out to high pT (~20 GeV/c) were presented. This demonstrated the excellent ALICE PID capabilities and extended existing ALICE results (which were going up to ~3 GeV/c) significantly.

In the intermediate pT region, the so-called “baryon anomaly”, discovered at RHIC and also observed through the Λ/K0S ratio in ALICE, was confirmed with the p/π ratio.

The ratio shows a pronounced maximum, close to unity at pT ~ 3 GeV/c, followed by a rapid decrease. This was interpreted in some models as an evidence of hadronization via recombination, which makes it easier to produce a baryon in heavy-ion collisions as compared to pp collisions. In this region, however, hydrodynamic flow still plays a very important role, and it would naturally explain the rise (but not the fall!) due to the different push received by different particles. The K/p ratio in the same range provides an additional constraint to theoretical models. The intermediate pT region is very rich in physics, and contains an interplay of hard and soft effects, which is still not completely controlled in theoretical models. Therefore, the additional experimental constraints presented by ALICE are very important.

The highest pT was studied in terms of the nuclear modification factor RAA, that is the ratio of the yield in Pb–Pb collisions divided by the yield in pp collisions scaled by the number of binary collisions. The results are shown in figure 3. A strong suppression is observed for all species, as it was the case for charged particles, consistent with the hypothesis of parton energy loss in the medium (aka jet quenching). The differences between different species at low and intermediate pT reflect the hydrodynamic and “baryon anomaly” effects mentioned above. At high pT the results for all particles are consistent: parton energy loss does not seem to modify the fragmentation of of high pT partons, once they hadronize outside the medium. This contradicts the expectations of some theoretical models. However, it is consistent with previous ATLAS and CMS measurements, which showed an agreement between pp and Pb-Pb (unidentified particles) fragmentation functions for pT > 4 GeV/c.

(see also A. Ortiz, Production of Charged Pions, Kaons, and Protons in 2.76 TeV Pb-Pb Collisions at high pT measured with the ALICE Experiment )

Anti, Hyper an exotic matter

Heavy-ion collisions also produce abundantly rare forms of matter (such as heavy anti-nuclei and hyper-matter), and are an ideal test bench for the search for exotic forms of matter (such as penta-quarks or , Λ Λ , Λn bound states). At the conference, an update on these studies was shown. The potential of the Pb–Pb data collected in 2011 for the study of Anti-Alpha and (Anti)-Hypertriton were discussed.

A detailed study for the search of Λ Λ (H-Dibaryon) and Λn were presented. None of these 2 exotic states was observed, and a stringent upper limit was derived. If these particles would exist, their yields would need to be an order of magnitude smaller than expected in thermal models. This potentially contradicts a previous positive observation of Λ n by the HypHI Collaboration.

(see also B. Doenigus, (Anti) matter and hyper-matter production at the LHC with the ALICE experiment)


The Quark Matter 2012 conference saw the presentation on many new and surprising light-flavour results from ALICE, thanks to the dedication and hard work of many of our collaborators. The interpretation of some of these results will require improvements on some widely-used theoretical models, and will surely improve our understanding of heavy-ion collisions.

Figure 2 Ratios of K*/K- and φ/K- as a function of the mean number of participants part>

Figure 3 Nuclear modification factor RAA for identified charged hadrons: π, K, p.