Recent results from the ALICE collaboration

Elena BrunaPaolo Giubellino

Introduction

The world as we know it is composed of quarks and leptons and is governed by four fundamental interactions. One of them is the strong interaction, described by the theory of Quantum ChromoDynamics (QCD), responsible for the confinement of quarks inside nucleons. The only way to experimentally free the quarks from their confined state is to put the nuclear matter under extreme conditions of high temperature and/or energy density with relativistic collisions of heavy ions. This new deconfined medium is called “Quark Gluon Plasma” (QGP) [1]. ALICE (A Large Ion Collider Experiment) [2] is one of the four experiments at the Large Hadron Collider (LHC) at CERN in Geneva. The main objective of ALICE is the study of nucleus-nucleus collisions at a center-of-mass energy (for lead nuclei, Pb208) of 5.5 TeV per nucleon.The ALICE experiment is carrying out the experimental heavy-ion program started about twenty-five years ago at the CERN Super Proton Synchrotron (SPS) at √sNN≤20 GeV and continued at the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven Nation Laboratory in New York at √sNN≤200 GeV. Relativistic heavy-ion collisions provide a unique opportunity to study how the properties of a complex system emerge from the fundamental interactions of QCD.The successful experimental results achieved so far, in parallel with the theoretical developments, indicate that a hot and dense medium, called sQGP (strongly interacting QGP), is created in heavy-ion collisions at RHIC and LHC. The matter created at the LHC has been measured to have a larger lifetime, size and energy density compared to RHIC [3]. One tool to measure the properties of the hot nuclear matter created in heavy-ion collisions is to study the passage of particles through it, in a similar way as the properties of atomic matter are described by the ionization energy loss of charged particles with the Bethe-Bloch formula for the average rate of energy loss, dE/dx. The lifetime of the QGP is far too short [3] to measure its properties with external probes. As an alternative, physicists started to utilize the particles generated in the collision to probe the medium. Particularly powerful tools are the “hard probes”, which are produced via high-Q2 processes in the initial hard scatterings and are therefore exposed to the full evolution of the hot QCD matter. The production rates for these probes are calculable in the standard model and can be compared in proton-proton (pp), proton-nucleus (p-A), nucleus-nucleus (A-A) collisions to disentangle the effects of the cold nuclear matter from those due to the hot QCD medium. At RHIC and even more so in the LHC energy regime, high-Q2 processes are abundant, opening the way to a class of hard probes that behave as calibrated projectiles to investigate the hot nuclear medium.Typical hard probes are high-pT partons which fragment in jets, heavy quarks (charm and beauty) which can be observed as open heavy flavour hadrons, heavy quarkonia (J/ψ, ϒ)

and electroweak probes (l+l-, γ, Z). While there have been significant progresses in both theory and experiments to characterize the medium-induced parton energy loss (the so called “jet quenching” phenomenon) and parton fragmentation, a conclusive picture of such mechanisms is not available yet.

Another way to study the global properties of the Quark Gluon Plasma is via the collective behaviour of the bulk of its particles, the so called “soft probes”. Once the fireball is formed after the collision, its partons start to move chaotically in the QGP. If the medium is strongly interacting, collective behaviours can arise on top of the Brownian motion. Such “flows” of particles are generated by the presence of pressure gradients in the medium and can provide information on the degree of thermalisation of the particles in the medium, on its transport properties and on the size of the fireball.

ALICE is a general-purpose experiment designed to investigate a large array of observables that are relevant for the characterization of the medium. In particular, ALICE is equipped with detectors that provide high precision tracking in a wide transverse momentum range (from 0.1 GeV/c to 100 GeV/c), excellent particle identification capabilities and it is designed to operate in the high-multiplicity environment typical of heavy-ion collisions at the LHC. The challenging goals achieved so far with the heavy-ion program require a clear understanding of the reference systems given by pp and p-Pb collisions. The former provides a necessary test of the theoretical predictions from perturbative QCD and therefore a reference set of measurements needed to compare to the Pb-Pb results to extract the hot nuclear matter effects. The latter provides the control experiment needed to disentangle the effects of the hot and dense medium created in Pb-Pb collisions from the “cold” matter that is created p-Pb collisions.In this article we review some of the most recent results obtained by ALICE in p-Pb and Pb-Pb collisions.

Physics Results

The main focus of ALICE is the investigation of the new state of matter created in Pb-Pb collisions at the LHC. One of the crucial measurements to characterize the fireball created in these collisions is the measurement of the “elliptic flow”. The standard description of the flow is based on hydrodynamics which relies on the assumption that the system is thermalised and therefore it is possible to define a temperature and its thermodynamical variables (entropy, speed of sound in the medium, …). Therefore the measurements of anisotropic particle emissions can provide information on the thermodynamical properties of the matter created in heavy-ion collisions. The azimuthal anisotropy in the particle production is a clear experimental signature of collective flow. It is caused by multiple interactions between the constituents of the created matter and the initial asymmetries in the spatial geometry of a non-central Pb-Pb collision. After the first measurements of the elliptic flow for charged particles in ALICE [4], the analysis continued along on the identified particles and in a larger range of transverse momentum.The elliptic flow is given by the second coefficient v2 in the Fourier expansion of the azimuthal particle distribution on the transverse plane with respect to the symmetry plane of the semi-central Pb-Pb collision.

The v2 as a function of the transverse momentum pT in semi-central Pb-Pb collisions at √sNN=2.76 TeV (the chosen collision centrality corresponds to the 20-40% of the total Pb-Pb cross section) is reported in Fig. 1 for identified particles: pions, kaons, anti-protons, and the multi-strange baryons Ξ and Ω. Compared to the measurements at RHIC, a larger mass splitting is observed in ALICE.The observed mass ordering all the particle species is well reproduced by viscous hydrodynamic model calculations at low pT (also reported in the figure). At high transverse momentum the hydrodynamic predictions start to deviate from the data because other effects like the path-length dependence of the energy loss start to intervene.

Figure 1. Identified particle v2(pT) measured by ALICE for 20-40% centrality class and compared to viscous hydrodynamic model calculations.

Interesting results come from the “hard probes” sector, in particular from the measurements of heavy quarks, which are produced in the early stages of hadronic collisions and are therefore exposed throughout the full evolution of the medium.Therefore, they behave as self-generated probes that carry information of the medium by losing energy via subsequent interactions.Theoretical models of energy loss predict a hierarchical dependence on the colour charge and mass of the projectile parton: ΔEgluon>ΔElight-quark>ΔEcharm>ΔEbottom. The first inequality expresses a larger energy loss for partons with larger colour charge (gluons). The second and the third arise from the so-called “dead-cone effect” [5], which predicts a suppression of gluon radiation at small angles for particles with larger mass. This effect is expected to vanish when the mass of the quark becomes negligible compared to its energy.It is interesting to compare medium effects (i.e. the path-length, colour-charge and mass dependence of the energy loss, as well as the collective motion) on heavy quarks versus light quarks and gluons.The well established observable to quantify the jet quenching on light and heavy quarks is the nuclear modification factor RAA, defined as the particle yield in Pb-Pb collisions

divided by same yield in pp collisions scaled by number of binary nucleon-nucleon collisions expected in a given Pb-Pb centrality range. The RAA can be measured as a function of the particle pT and in a given collisions centrality.The RAA of prompt D mesons was measured with ALICE in the 0-7.5% centrality class using the data sample collected in 2011, which extends the measurements to a wider transverse momentum range (2<pT<36 GeV/c) compared to the published results from the 2010 data [6].As shown in Fig. 2, the RAA values for D0, D+ and D*+ agree within the uncertainties and indicate a strong suppression (factor of 4-5 for 5<pT<16 GeV/c) of the D-meson yields in Pb-Pb collisions relative to pp collisions. The first measurement of the Ds

+ meson in Pb-Pb collisions is also reported. A suppression of the Ds

+ is observed for 8<pT<12 GeV/c, in agreement within the uncertainties with the non-strange charmed mesons RAA in this pT

range. The Ds+-meson yield could be less suppressed at lower pT because of the predicted

c-quark recombination with the enhanced strange quarks in the medium [7], but more statistics is needed to draw a conclusion on the possible enhancement of Ds

+ mesons. The RAA of non-prompt J/ψ coming from decays of B mesons was measured with CMS in the transverse momentum range 6.5 < pT<30 GeV/c and with rapidity |y|<1.2, as shown in Fig. 3 as a function of the number of participants in the Pb-Pb collisions [8]. The results are reported together with the RAA of prompt D mesons measured with ALICE in the transverse momentum range 8<pT<16 GeV/c and rapidity range |y|<0.5. The selected pT

ranges of D mesons and non-prompt J/ψ correspond to similar kinematical ranges for the parent b and c quarks, yet the measurements are performed in different rapidity intervals. An indication of a difference in the suppression of charm and beauty can be observed in the most central collisions, consistent with the mass hierarchy expected from various energy-loss models [9,10,11], like those reported in the figure.

Figure 2. RAA as a function of pT for prompt D0, D+ and D*+ and Ds

+ for the 0-7.5% most central Pb-Pb collisions at √sNN=2.76 TeV. Figure 3. RAA as a function of centrality for prompt D mesons (average of D0, D+ and D*+) in the transverse momentum range 8<pT<16 GeV/c, compared to non-prompt J/ψ measured with CMS with 6.5<pT<30 GeV/c. Results from theoretical calculations are superimposed.

Still on the heavy flavours, a key observable to study the QGP are the bound states of heavy quarks, i.e. quarkonia. The suppression of the charmonium production was for a long time considered as one of the main signatures for a deconfined medium. It was predicted that at large enough temperatures, like those of the QGP, bound states of charm and anti-charm quarks, i.e. charmonia, are dissolved due to the screening effects induced by the high density of colour charges in the medium. The relative production probabilities of charmonium states with different binding energies can provide information on the properties of the medium, in particular on its temperature. The first ALICE measurements of the J/ψ suppression in central Pb-Pb collisions at √sNN=2.76 TeV at forward rapidity showed less suppression compared to the results from PHENIX in central Au-Au collisions at √sNN=200 GeV [12]. Recent measurements on the transverse momentum and rapidity dependence of the J/ψ RAA can add more constraints on the theoretical models based on recombination and suppression. Fig. 4 shows the J/ψ RAA as a function of the transverse momentum measured by ALICE at forward rapidity for the 0-20% most central Pb-Pb collisions [13]. The results are compared to the measurements from PHENIX in 0-20% most central Au-Au collisions. A striking difference is observed, in particular at low pT, where the yield of J/ψ from PHENIX is suppressed by more that a factor four compared to the ALICE measurements. This observation is in qualitative agreement with model calculations predicting a recombination scenario of charm and anti-charm pairs in the medium mainly in the low-pT region (i.e. [14,15]).

Figure 4. Transverse momentum dependence of the measured by ALICE in the 0-20% moste central Pb-Pb collisions at √sNN=2.76 TeV, compared to the results from PHENIX in the 0-20% most central Au-Au collisions at √sNN=200 GeV.

The above results are just a sample of the many striking results related to the formation of hot and dense hadronic state of matter emerging from the collisions of Pb nuclei. However, given the complexity of the Pb-Pb colliding system, an important step in the quest for QGP lies in decoupling the effects of “cold” nuclear matter that arise at the

initial stage of the collisions, as opposed to the “hot” nuclear matter that characterizes the formation of the QGP.The proton–nucleus system represents the perfect benchmark for studying these effects because the nuclear effects of the medium produced in these collisions are either small or even totally absent. Proton–nucleus collisions can therefore provide the data needed to understand better the properties of Pb-Pb collisions at the energy of the LHC. Recent results from the analysis of the ALICE p-Pb data at √sNN=5.02 TeV regard the nuclear modification factor RpPb, shown in Fig. 5 and Fig. 6 for prompt D mesons and reconstructed charged jets.

Figure 5: RpPb as a function of pT for prompt D mesons (average of D0, D+ and D*+) in p-Pb collisions at √sNN=5.02 TeV. Results are compared to model calculations of initial-state effects.

Figure 6: RpPb as a function of pT for charged jets reconstructed with anti-kT in p-Pb collisions at √sNN=5.02 TeV.

The result in Fig. 5 clearly indicates little or no modification of the production of prompt D mesons with transverse momentum greater than 1 GeV/c, and compatible within the uncertainties with theoretical calculations based on initial-state effects in p-Pb collisions [16].Also the results on the charged jets RpPb (Fig. 6) are compatible with unity within the uncertainties, thus confirming that the suppression of high-energy jets in Pb-Pb collisions is not a result of cold nuclear-matter effects.

Interesting results in the p-Pb colliding system also come from two-particle correlations, which have proved to be an interesting tool to measure the mechanisms of particle production in hadrons and nuclei. These analyses are based on correlation in the azimuthal angle φ and pseudorapity η between a “trigger” particle and an associated particle in given ranges of transverse momentum. In ALICE, both particles are reconstructed in the detectors of the central barrel, i.e. at mid-rapidity. The correlations are determined by counting the number of associated particles as a function of their difference in azimuth (Δφ) and pseudorapidity (Δη). Typically, a two-particle correlation measurement shows a jet-like structure coming from the initial hard scattering and peaked on the “near-side” at Δφ~0 and Δη~0. Experimentally, the near-side jet peak shows only a weak evolution with the event multiplicity. So by subtracting the correlations at different event multiplicities from one another, it is possible to remove the jet-like contribution of the correlation. A recent result from ALICE on two-particle correlations in p-Pb collisions at √sNN=5.02 TeV [17] reveals two long-range elongated “ridge-like” structures along the Δη axis, one on the near side (Δφ~0) and one on the away side (Δφ~π), see Fig. 7. These results are observed when the yields (normalized by the number of trigger particles) obtained in low-multiplicity events are subtracted from those measured in high multiplicity events to remove the contributions form jet-like structures.

Figure 7. Δφ and Δη distributions of two-particle correlations measured in p-Pb collisions in the transverse momentum range 2<pT,trig<4 GeV/c for the trigger particles and 1<pT,assoc<2 GeV/c for the associated particles. The z-axis represents the yield of the associated particles normalized to the number of trigger particles. Figure 8. Projection of left panel onto Δφ. Superimposed are fits containing a cos(2Δφ) shape and a combination of cos(2Δφ) and cos(3Δφ). Also shown for comparison the results obtained with the HIJING generator, used to simulate p-Pb collisions [18].

Figure 7 shows an excess in the correlation, which forms two ridges along Δη. The ridge on the near side is accompanied by a second ridge of similar magnitude on the away side, which is observed for the first time.

Such double-ridge structures were found in Pb-Pb collisions at the LHC and have their origins in collective phenomena occurring in the quark–gluon plasma that is created. However, these phenomena are not generally thought to occur in p-Pb collisions, where the size of the collision region is expected to be too small to allow the development of significant collective effects. Figure 8 shows the projection of the left pane of the same figure onto Δφ, along with fits that include a cosine terms, cos(2Δφ) and cos(3Δφ), which allow allows the yield and width of the near-side and away-side ridges to be quantified above a constant baseline. This intriguing and unexpected result still needs to be explained theoretically. Possible explanations of the phenomenon include initial-state effects (colour-glass condensate [19]) and/or final-state effects that assume collective effects, like hydrodynamics, to occur also in p–Pb collisions [20]. Whatever the origin may be, this observation has opened the window on a novel phenomenon. Further analysis of the high-statistics p–Pb data promises to yield exciting results.

Conclusions

The ALICE detector collected so far a large amount of data in different collision systems (about 30 million minimum bias Pb-Pb collisions in 2010, 16 million events with central Pb-Pb trigger in 2011, 130 million p-Pb events in 2013), which yielded to intriguing results that confirmed the main discoveries obtained at RHIC and added more precise measurements toward a quantitative picture of the hot and dense nuclear matter created in

heavy-ion collisions. These results also demonstrated the ALICE excellent capabilities to measure ultra-relativistic heavy-ion collisions. At the end of the current first long shutdown of the LHC, the operation is expected to restart in 2015 with the goal to complete the approved heavy-ion program, consisting in collecting at least 1 nb-1 of Pb-Pb collisions at the top LHC energy, √sNN=5.5 TeV. With the more statistics available after the long shutdown and the usage of different triggers we will able to improve the results obtained so far.However, there are still open frontiers towards a quantitative characterization of the QGP.

An upgrade of the LHC machine is foreseen during the second long shutdown, expected in 2019, which will allow a significant increase of luminosity, i.e. up to 10 nb -1 of Pb-Pb collisions at √sNN=5.5 TeV, corresponding to a net factor of 100 compared to the first LHC Run. In parallel to the LHC upgrade, the LHC experiments are planning an upgrade of their detectors. As part of the upgrade strategy, ALICE plans to have a new inner tracker which will allow measurements of rare probes over a broad range of transverse momenta, together with an upgrade of the trigger and read-out systems to cope with the expected high-rate.

ALICE is entering a new high-luminosity and high-precision era, which will open uncharted territories for a much broader and deeper study of the QGP.

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