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ALICE Physics Summary
Roberto Preghenella , , a for the ALICE Collaboration Centro Studi e Ricerche e Museo Storico della Fisica “Enrico Fermi”, Rome, Italy Sezione INFN, Bologna, Italy
Abstract.
The ALICE experiment at the LHC has collected data in proton-proton (pp) collisions at √ s = √ s NN = √ s NN = ALICE (A Large Ion Collider Experiment) is a general-purpose heavy-ion detector at the CERN LHC (LargeHadron Collider) whose design has been chosen in order tofulfill the requirements to track and identify particles fromvery low ( ∼
100 MeV / c ) up to quite high ( ∼
100 GeV / c )transverse momenta in an environment with large charged-particle multiplicities as in the case of central lead-lead(Pb–Pb) collisions at extremely high energies.The ALICE experiment, shown in Figure 1, consistsof a central-barrel detector system and several forward de-tectors. The central system covers the mid-rapidity re-gion ( | η | ≤ a e-mail: [email protected] ; Figure 1.
Schematic layout of the ALICE detector with its mainsubsystems. hadron identification detectors which exploit transition-radiation (TRD) and time-of-flight (TOF) techniques, re-spectively. Small-area detectors for high- p T particle-identification (HMPID), photon and neutral-meson mea-surements (PHOS) and jet reconstruction (EMCal) com-plement the central barrel. The large-rapidity systems in-clude a single-arm muon spectrometer covering the pseu-dorapidity range -4.0 ≤ η ≤ -2.4 and several smaller detec-tors (VZERO, TZERO, FMD, ZDC, and PMD) for trigger-ing, multiplicity measurements and centrality determina-tion. A detailed description of the ALICE detector layoutand of its subsystems can be found in [1].Since November 2009 when the first LHC collisionsoccured, the ALICE detector collected proton-proton dataat several centre-of-mass energies ( √ s = √ s NN = µ b − and 100 µ b − , respectively.The instant luminosity exceeded 10 cm − s − in the sec-ond run, higher than the design value. Proton-lead (p–Pb)collision data were also recorded with the ALICE detector.This occured during a short run performed in September2012 in preparation for the main p–Pb run scheduled atthe beginning of 2013. Each beam contained 13 bunches;8 pairs of bunches were colliding in the ALICE interactionregion, providing a luminosity of about 8 × cm − s − .Beam 1 consisted of protons at 4 TeV energy circulatingin the negative z-direction in the ALICE laboratory sys-tem, while beam 2 consisted of fully stripped Pb ions at82 × √ s NN = ∆ y NN = a r X i v : . [ h e p - e x ] F e b PJ Web of Conferences lab η -2 0 2 l ab η / d c h N d g = 5.02 TeV NN s p-Pb, ALI−PUB−46124
Figure 2.
Pseudorapidity density of charged particles measuredin NSD p–Pb collisions at √ s NN = Particle production in proton-lead (p–Pb) collisions allowsto study and understand QCD at low parton fractional mo-mentum x and high gluon density. Moreover it is expectedto be sensitive to nuclear e ff ects in the initial state. For thisreason p–Pb measurements provide an essential referencetool to discriminate between initial and final state e ff ectsand they are crucial for the studies and the understandingof deconfined matter created in nucleus-nucleus collisions. The measurement of primary charged-particle pseudora-pidity density performed in non-single di ff ractive (NSD)p–Pb collisions at √ s NN = | η lab | <
2. Aforward-backward asymmetry between the proton and thelead hemispheres is clearly visible. The data is also com-pared to several model predictions of particle productionwhich have been shifted in the laboratory system whenneeded. The comparison shows that the pseudorapiditydependence is best described by the models DPMJET andHIJING 2.1 (where the gluon shadowing parameter s g wastuned on experimental √ s NN =
200 GeV d–Au data atRHIC), whereas the saturation models (KNL, rcBK, IP-Sat) exibit a steeper η lab dependence. The pseudorapid-ity density in the centre-of-mass system at mid-rapidity (GeV/c) T p P b P b , R p P b R ALICE, charged particles | < 0.3 cms η = 5.02 TeV, NSD, | NN sp-Pb | < 0.8 η = 2.76 TeV, 0-5% central, | NN sPb-Pb | < 0.8 η = 2.76 TeV, 70-80% central, | NN sPb-Pb ALI−PUB−44351
Figure 3.
The nuclear modification factor of charged particlesas a function of transverse momentum in NSD p–Pb collisions at √ s NN = √ s NN = | η cms | < . N ch / d η cms = . ± .
71, correspond-ing to 2 . ± .
17 charged particles per unit pseudorapidityper participant when scaled by the number of participatingnucleons, determined using the Glauber model [3].
The measurement of the transverse momentum p T distri-butions of charged particle in p–Pb collisions were also re-ported [4]. It was previously shown that the production ofcharged hadrons in central Pb–Pb collisions at the LHC isstrongly suppressed [5, 6]. The suppression remains sub-stantial up to 100 GeV / c and is also seen in reconstructedjets [7]. Proton-lead collisions provide a control experi-ment to establish whether the initial state of the collidingnuclei plays a role in the observed high- p T hadron produc-tion in Pb–Pb collisions. In order to quantify nuclear ef-fects, the p T -di ff erential yield relative to the proton-protonreference, the so-called nuclear modification factor, is cal-culated. The nuclear modification factor is unity for hardprocesses which are expected to exhibit binary collisionscaling. This has been recently confirmed in Pb–Pb colli-sions at the LHC by the measurements of direct photon, Z and W ± production, observables which are not a ff ected byhot QCD matter. In Figure 3 the measurement of the nu-clear modification factor in p–Pb collisions R pPb is com-pared to that in central (0-5% centrality) and peripheral adron Collider Physics symposium 2012 y -4 -2 0 2 4 / d y ( m b ) σ d RSZ-LTASTARLIGHTGMAB-EPS09AB-MSTW08AB-EPS08AB-HKN07CSS
ALICE PreliminaryReflected = 2.76 TeV NN s ψ Pb+Pb+J/ → Pb+Pb
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Figure 4.
Measured di ff erential cross section of coherent J /ψ photoproduction compared with models. The point at positiverapidity is the reflected result from the measurement at negativerapidities. (70–80%) Pb–Pb collisions R PbPb . R pPb is observed to beconsistent with unity for transverse momenta higher thatabout 2 GeV / c . This demonstrates that the strong suppres-sion observed in central Pb–Pb collisions at the LHC is notdue to an initial-state e ff ect, but it is rather a final state ef-fect related to the hot matter created in high-energy heavy-ion collisions. A limited selection of recent physics results obtained bythe ALICE experiment at the LHC is reported in the fol-lowing section. They include both measurements per-formed in proton-proton (pp) and in lead-lead (Pb–Pb) col-lisions and are meant to give a feeling of the physics capa-bilities of the experiment, though this cannot be done in acomplete way in this report. J /ψ photoproduction Exclusive vector meson production in heavy-ion interac-tions is expected to probe the nuclear gluon distributionfor which there is considerable uncertainty in the low- x region and it has been studied so far in gold-gold (Au–Au)collisions at RHIC. The first LHC results on exclusive pho-toproduction of J /ψ vector mesons measured at forwardrapidities in ultra-peripheral Pb–Pb collisions at √ s NN = (GeV/c) T p ) c - ( G e V d y T dp T p N d e v . N π -7 -6 -5 -4 -3 -2 -1 = 2.76 TeV NN s0-40% Pb-Pb, Direct photons (scaled pp) T = 0.5,1.0,2.0 p µ Direct photon NLO for 51 MeV ± /T), T = 304 T exp(-p × Exponential fit: A
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Figure 5.
Direct-photon invariant yield in 0-40% central Pb–Pbcollisions at √ s NN = ff erential cross section for co-herent J /ψ production is compared with calculations fromvarious models in Figure 4. Best agreement is found withmodels which include nuclear gluon shadowing consistentwith the EPS09 parametrizations of the nuclear gluon dis-tribution functions (AB-EPS09). Direct-photon production has been measured by ALICEin proton-proton and in Pb–Pb collisions [9]. The mea-surements when compared to NLD pQCD predictions arefound to be in good agreement for pp collisions and pe-ripheral Pb–Pb collisions (40-80% centrality). On theother hand, in the case of central (0-40% centrality) Pb–Pb collisions the direct photon signal is well reproducedby NLO pQCD only for photon momenta above 4 GeV / c ,as shown in Figure 5. The low- p T excess, of about 20%at around 2 GeV / c , is attributed to thermal photons, thatis photons produced in the QGP phase by the scattering ofthermalized partons. The thermalized nature of the pro-duction medium should be reflected in the p T distributionof thermal photons. The excess over NLO pQCD is fitin Figure 5 with an exponential in the p T range 0.8–2.2GeV / c . The inverse slope of this exponential is found tobe T = (304 ±
51) MeV. In a similar analysis performed incentral (0-20%) Au–Au collisions at √ s NN =
200 GeV, the
PJ Web of Conferences
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Figure 6.
Transverse momentum distributions of the sum of pos-itive and negative pions, kaons and protons for central Pb–Pbcollisions. The results are compared to RHIC data and hydrody-namic models.
PHENIX experiment at RHIC measures an inverse slopeparameter of T = (221 ± ±
19) MeV. The LHC value isabout 40% higher than the one measured at RHIC.
ALICE has measured the production yields of primarycharged pions, kaon and (anti)protons in a wide momen-tum range. Primary particles are defined as prompt parti-cles produced in the collision and all decay products, ex-cept products from weak decay of strange particles. Themeasurements have been performed both in proton-protoncollisions at several centre-of-mass enegies ( √ s = √ s NN = p T distributions and yields arecompared to previous results at RHIC and expectationsfrom hydrodynamic and thermal models. The results ob-tained for central Pb–Pb collisions are shown in Figure 6and 7 and are also reported in [10]. The spectral shapes areharder than those measured at RHIC, indicating a strongincrease of the radial flow velocity with the centre-of-massenergy. The radial flow at the LHC is found to be about10% higher than at RHIC energy. While the K / π integratedproduction ratio is measured to be in line with lower en-ergy results and predictions from the thermal model, boththe p / π and the Λ /π ratios are lower than at RHIC and Y i e l d r e l a t i v e t o p i on s =2.76 TeV (ALICE, 0-20%, preliminary) NN s =0.2 TeV (STAR) NN s =0.2 TeV (BRAHMS) NN s =0.2 TeV (PHENIX) NN sModel, T=164 MeVModel, T=152 MeV particle+anti-particle K 3 × p 4 ×Λ × - Ξ × - Ω ×φ × K* ALI−PREL−32253
Figure 7.
Mid-rapidity particle ratios compared to RHIC resultsand predictions from thermal models for central Pb–Pb collisionsat the LHC. significantly lower (a factor ∼ p T -dependent production of charged kaons andprotons normalized to charged pions, respectively K / π andp / π , are shown in Figure 8 for pp collisions at √ s = p T . Pion, kaonand (anti)proton production in Pb–Pb collisions were com-pared to that of proton-proton interactions and all showa suppression pattern which is similar to that of inclu-sive charged hadrons at high momenta ( p T above (cid:39) / c ) [14]. This suggests that the dense medium formedin Pb–Pb collisions does not a ff ect the fragmentation. Asimilar conclusion can be drawn by observing the proton-to-pion ratio measured in Pb–Pb collisions: for interme-diate momenta (3–7 GeV / c ) it exibits a relatively strongenhancement, a factor 3 higher than proton-proton colli-sions at p T ≈ / c and gets back to the proton-protonvalue at higher momenta ( p T above (cid:39)
10 GeV / c ) [14]. Asimilar observation is reported also for the Λ / K ratio and adron Collider Physics symposium 2012 ) c (GeV/ T p ) π + + π ) / ( + K + ( K ALICE=7 TeVspp =2.76 TeVspp =2.76 TeVs 7 TeVPythia, Perugia2011NLO (Phys. Rev. D 82, 074011 (2010))
ALI−PREL−34373 ) c (GeV/ T p ) π + + π ) / ( p ( p + ALICE=7 TeVspp =2.76 TeVspp =2.76 TeVs 7 TeVPythia, Perugia2011NLO (Phys. Rev. D 82, 074011 (2010))
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Figure 8. K /π (top) and p /π production ratio in pp collisionscompared with PYTHIA Monte Carlo predictions and NLO cal-culations. possible explanations include among those proposed so farparticle production via quark recombination [15]. ALICE has studied heavy-flavour production in pp andPb–Pb collisions at the LHC. Their prodution in proton-proton collisions is a tool to test pQCD calculations in anew energy domain. The p T -di ff erential production crosssection of prompt charmed mesons (D , D + , D ∗ + , D + s ),heavy-flavour decay electron and muons in pp collisionsat √ s = | y | < .
8) in pp collisions at √ s = ) ) c d y ) ( m b / ( G e V / T p / ( d σ ) d T p π / ( -8 -7 -6 -5 -4 -3 -2
10 = 7 TeVspp, additional 3.5% normalization uncert. b-decay electrons i.p. cut (|y|<0.8), arXiv:1208:1902sec. vtx. (|y|<0.7), PreliminaryFONLL ) c (GeV/ T p D a t a / F O N LL ALI−PREL−44870
Figure 9. p T -di ff erential invariant cross sections of electronsfrom beauty hadron decays. The ratio of the data and the FONLLcalculations are shown in in the bottom panel and the dashedlines indicate the calculation uncertainty. The results are shown in Figure 9 together with the pQCDFONLL prediction.Heavy-flavuor production studies in Pb–Pb collisionsat √ s NN = p T ∼
10 GeV / c in cen-tral Pb–Pb collisions (0-7.5% centrality). The suppressionis observed to be similar for the three studied decay chan-nels, that is heavy-flavour electrons, heavy-flavour muonsand prompt D mesons. The average D meson produc-tion suppression is shown in Figure 10 as a function ofthe number of participating nucleons and it is comparedwith the measurement of non-prompt J /ψ suppression per-formed by CMS [25]. The suppression of non-prompt J /ψ from B meson decays reflects the in-medium energy lossof b quarks. The two measurements provide a first indica-tion of di ff erent suppression for charm and beauty in cen-tral collisions, that is of di ff erent in-medium energy loss. ALICE has measured the inclusive J /ψ production in ppand Pb–Pb collisions at the LHC, down to zero p T [26, 27].A unique study performed by ALICE in pp collisions isthe determination of the inclusive J /ψ production yield asa function of the charged multiplicity at central rapidity. PJ Web of Conferences 〉 part N 〈 AA R <12 GeV/c, |y|<0.5 T , 6
JHEP 09 (2012) 112Read from CMS-PAS-HIN-12-014
ALI−DER−44832
Figure 10.
Average D meson suppression versus number of par-ticipants compared with CMS non prompt J /ψ suppression. These results [28] show a linear increase of the yield withcharged multiplcity which is not explained by theory andmodel predictions yet. A similar increase is observed botha forward and mid-rapidity, as shown in Figure 11.Results on inclusive J /ψ production in Pb–Pb colli-sions clearly indicate a saturation of the suppression atboth central and forward rapidity moving towards centralcollisions, a phenomenon not observed at lower energywhere the suppression increases as shown in Figure 12.There is in fact a clear evidence for a smaller suppressionat LHC with respect to RHIC energy [29]. Di ff erentialstudies of the suppression versus collision centrality forvarious momentum bins seem to favour a scenario where(re)combination processes play a sizeble role [30, 31]. Theobserved hint for a non-zero anisotropic production signal(elliptic flow) are in agreement with such a picture. ALICE has obtained so far a wealth of physics results bothfrom the analysis of proton-proton collision data and fromthe first two LHC heavy-ion runs. First results from a shortpilot run with proton-lead have been already obtained andthe coming dedicated p-Pb run at the beginning of 2013will set the beginning of precision carachterization of thematter formed in heavy-ion collisions at the LHC. A cleardetector upgrade strategy plan for the LHC luminosity up-grade has also been presented at this conference [32]. 〉η /d ch dN 〈 η /d ch dN 〉 / d y ψ J / d N 〈 / d y ψ J / d N (2.5 < y < 4) - µ + µ → ψ J/ (|y| < 0.9) - e + e → ψ J/Normalization uncert.: 1.5% = 7 TeVsALICE pp
ALI−PUB−42097
Figure 11. J /ψ production yield d N J /ψ / d y as a function of thecharged particle multiplicity densities at mid-rapidity d N ch / d η .Both values are normalized by the corresponding pp minimumbias value. References [1] K. Aamodt et al. [ALICE Collaboration], JINST (2008) S08002.[2] B. Abelev et al. [ALICE Collaboration],arXiv:1210.3615 [nucl-ex].[3] B. Alver, M. Baker, C. Loizides and P. Steinberg,arXiv:0805.4411 [nucl-ex].[4] B. Abelev et al. [ALICE Collaboration],arXiv:1210.4520 [nucl-ex].[5] K. Aamodt et al. [ALICE Collaboration], Phys. Lett.B (2011) 30 [arXiv:1012.1004 [nucl-ex]].[6] B. Abelev et al. [ALICE Collaboration],[arXiv:1208.2711 [hep-ex]].[7] G. Aad et al. [ATLAS Collaboration],arXiv:1208.1967 [hep-ex].[8] B. Abelev et al. [ALICE Collaboration], Phys. Lett. B (2013) 1273 [arXiv:1209.3715 [nucl-ex]].[9] M. Wilde [ALICE Collaboration], [arXiv:1210.5958[hep-ex]].[10] B. Abelev et al. [ALICE Collaboration], Phys. Rev.Lett. (2012) 252301 [arXiv:1208.1974 [hep-ex]].[11] J. Steinheimer, J. Aichelin and M. Bleicher,arXiv:1203.5302 [nucl-th].[12] F. Becattini, M. Bleicher, T. Kollegger, M. Mitro-vski, T. Schuster and R. Stock, Phys. Rev. C (2012)044921 [arXiv:1201.6349 [nucl-th]]. adron Collider Physics symposium 2012 〉 part N 〈 AA R -1 b µ ≈ int = 2.76 TeV, L NN s ALICE Preliminary, Pb-Pb 14% ± global sys.= c <8 GeV/ T p <4, 0< y , 2.5< ψ Inclusive J/ = 0.2 TeV NN s PHENIX (PRC 84 (2011) 054912), Au-Au 9.2% ± global sys.= c >0 GeV/ T p <2.2, y , 1.2< ψ Inclusive J/
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Figure 12.
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