QUARKONIA AND HEAVY-FLAVOUR RESULTS FROM ALICE

M. GAGLIARDI for the ALICE Collaboration

Universita degli Studi and Sezione INFN, Torino, Italy

Quarkonia and heavy flavour are important probes of the hot and dense QCD medium formed in high-energy heavy-ion collisions, through the modification of their yields and kinematical distributions. Measurements of their production in proton-nucleus collisions are crucial for the interpretation of heavy-ion results, as they allow one to study cold nuclear matter effects. Quarkonia and heavy-flavour production in Pb-Pb collisions at the LHC is measured in ALICE at both forward and mid-rapidity, by exploiting several experimental techniques. The main results obtained in Pb-Pb and p-Pb collisions are presented.

1 Introduction

The ALICE 1 experiment at the Large Hadron Collider (LHC 2) studies QCD matter in ultra­relativistic heavy-ion (Pb--Pb) collisions at energy densities much larger than that of ordinary nuclear matter. Under these conditions, finite temperature QCD calculations on the lattice (see e.g. 3) predict a transition to a deconfined state of matter known as Quark-Gluon Plasma (QGP). Heavy-flavour particles (open-charm and open-beauty hadrons) are a major tool for probing the properties of the QGP. They are sensitive to the medium density, through the mechanism of in­medium parton energy loss, which causes modifications of the momentum distributions in Pb-Pb collisions with respect to those in pp. The sensitivity of heavy quarks to collective effects in the medium can be studied via the azimuthal angle distribution of heavy-flavour particles in non-central collisions (elliptic flow) . Quarkonium production suppression by colour screening was one of the first signatures proposed for the QGP 4. Charmonium (re)generation due to the recombination of initially uncorrelated c and c quarks may also become relevant at LHC energies 5 . In p-Pb collisions, where no long-lived QGP is expected to be formed, heavy-flavour and quarkonium production can be affected by cold nuclear matter (CNM) effects, in both the initial and the final state. Since these effects are also present in Pb-Pb collisions, their measurement in p-Pb collisions is crucial for the interpretation of the results. Heavy-flavour particles produced at mid-rapidity ( IYI < 0.9) are detected in ALICE by full reconstruction of D-meson decay topologies with displaced vertices, and by measuring the spectra of electrons from decays of heavy-flavour hadrons. At forward rapidity (2.5 < y < 4), their production is studied via their semi-muonic decays. Quarkonium production is measured at mid-rapidity via di-electron decays and at forward rapidity via di-muon decays; for both channels, the acceptance extends down to transverse momentum PT = 0.

,.,..,,

2 Heavy-flavour results

2. 1 Highlights from Pb-Pb collisions

The D-meson nuclear modification factor (RAA) in Pb-Pb collisions 6 is shown as a function of PT in Fig. 1 , left , for two centrality classes. A suppression by a factor up to !"> in cientral collisions is observed for PT > 5 GeV /c. The RAA of electrons and muons 7 from heavy-flavour decays is shown in Fig. 1 , right, where a suppression by about a factor of 3 is observed. The pattern and the magnitude of the suppression are very similar for mid- (electrons) and forward (muons) rapidity. Measurements (not shown) of the elliptic flow of D mesons9 and leptons from heavy-flavour decays show non-zero v2 at intermediate PT in semi-central collisions, pointing to a participation of charm quarks in the collective expansion of the medium. Theoretical models struggle to reproduce simultaneously the heavy-flavour RAA and v2 9 .

1 .6 _ Pb-Pb, \'5tlN "' 2.76 TeV 4 Heavyftavourdecayl'' 0-11l%central, 2 5<Y<4.0

1 .4 - Heavyfiavourdecaye' 0-10%central,JYl<:0 6 • wlthpPref. lromscaledcrosssection at \ S • 7TeV

1 _2 * Wlthpprel lrom FONLLcalcUa:tion at \ S •2.76 TeV

0.8 .1 "' r + I ::l , ��#T++] o 2 4 6 a 10 12 ��(G�6vtc1a

Figure 1 - Left: D-meson nuclear modification factor as a function of PT in p-Pb and Pb-Pb collisions 6 8. Right: nuclear modification factor of muons 7 and electrons in Pb-Pb collisions as a function of PT ·

2.2 Results from p-Pb collisions

fl:.� 2.5

1.5

0.5

p-Pb \''SNN = 5.02 TeV, µ±f- c,b decays

ALICE Preliminary

2 10 12 14 1 6 P, (GeV/c)

00

p-Pb \"SNN = 5.02 TeV, µ±f- c,b decays

ALICE Preliminary

10 12 14 16 P, (GeV/c)

Figure 2 - Nuclear modification factor of muons from heavy-flavour decays in p-Pb collisions as a function of PT at forward (left) and backward (right) rapidity, compared to theoretical models.

The nuclear modification factor RpPb of D mesons in p-Pb collisions 8 is shown in Fig. 1 , left. It is compatible with unity, and reproduced within uncertainties by perturbative QCD (pQCD) calculations including CNM effects such as shadowing and energy loss, or Colour Glass Condensate8 (not shown) . The Rppb of muons from heavy-flavour decays is shown as a function of PT in Fig. 2 for two different rapidity ranges: backward (Pb-going direction) and forward (p-going direction). The RpPb of electrons at mid-rapidity is shown in Fig. 3, left . For both electrons and muons, the results are compatible with no suppression, and are reproduced by

')7')

-+- ALJCEb.c-->(e·+e)l2,TPC-TQF,AUCEfllfere"""

--AUCEb.c-->(e·+e)/2.TPC-EMCaJ, ALICErele<orice +AUCEb.c --> ("• + e)i:l,TPC-BICal.FONU.role""""'

, _..,._ )l FONU.+EPS09stiacl

p-Pb,\S,..=502TeV,mln. blas, ·1.00 < yCl>IS <0.14

Figure 3 - Left: nuclear modification factor of electrons from heavy-flavour decays in p-Pb collisions as a function of PT at mid-rapidity, compared to a theoretical calculation. llight: heavy-flavour electron-hadron correlation function in azimuth and pseudo-rapidity, for 1 < PT < 2 GeV /c and for the 0-20% multiplicity class, after subtraction of the 60-100% class.

pQCD calculations including CNM effects. These results suggest that the nuclear modifications of heavy-flavour production seen in Pb-Pb collisions are due to effects from the hot and dense medium. Figure 3, right, shows the azimuthal correlations between electrons from heavy-flavour decays and charged hadrons, in the trigger-particle (electron) transverse momentum range 1 < PT < 2 GeV /c and for the highest event-activity class, after subtraction of the lowest class to remove jet-like correlations. A double-ridge structure (not seen at higher P1:) appears, similar to what was observed for hadron-hadron correlations 10 . These results point to the presence of collective effects in p-Pb collisions, although a solid theoretical interpretation is not yet available.

3 Quarkonium results

3. 1 Highlights from Pb-Pb collisions

The J/'lj; RAA at forward rapidity in Pb-Pb collisions 11 is shown as a function of PT in Fig. 4, left. A suppression is observed, more pronounced at high PT and relatively small at low PT· Such a PT dependence, not observed in lower-energy experiments at the Relativistic Heavy Ion Collider where a constant RAA was observed 12 , is compatible with models where part of the J/'lj;s are produced via (re)generation in the QGP or at the phase boundary (see references in 11) . Such a hypothesis is corroborated by a hint of non-zero J /'1/J v2 at intermediate PT in semi­central collisions 14 (not shown). Figure 4, right, shows the Y(lS) RAA at forward rapidity as a function of the number of participants in Pb-Pb collisions 15 . A suppression is observed, larger than expected from suppression of feed-down from higher-mass resonances alone. The measured suppression is larger than the one observed at mid-rapidity by the CMS experiment 16.

r:r:.� 1.4

�� 1.2 '&:" ' 1

• f 'li:"o.a 0.6

0.4

0.2

7 8 pT (GeV/c)

r:r:.'!i. 1 .4 . Pb·Pb \' SNN "' 2.76 TeV, inclusive T(1S), Py> 0 1.2 •ALICE: l,m "' 69 µb"1, 2.5 < y < 4 �-" ��S L,., • 15� µb_ ':�I � 2 4 (�RL 1_0� (20_12) �2�3�1)

50 100 150 200 250 300 350 400 <N,.) Figure 4 - Left: J / ,P nuclear modification factor as a function of PT in Pb-Pb collisions 11 , compared to the product of the backward- and forward-rapidity RpPb 13. Right: T(lS) nuclear modification factor in Pb-Pb collisions as a function of the number of participants, compared to the CMS result 16 15 .

3.2 Results from p-Pb collisions

The J / 1/; RpPb 17 in two rapidity ranges is shown in Fig. 5, left. The results are compatible with no suppression at backward rapidity and slight suppression at forward rapidity, in agreement with models including shadowing and energy loss. Assuming factorisation of CNM effects, one can comparP. RAA with the product of RpPb in the forward and backward regions, as a function of PT (Fig. 4, left). It emerges that the magnitude and trend of the suppression in Pb-Pb collisions are not accounted for by CNM effects alone, and can hence be ascribed to the hot and dense medium. The 1/;(28) RpPb 18 in two rapidity ranges is shown in Fig. 5, left. At backward rapidity, the observed suppression is significantly larger than that of J/1/;. An event-activity­dependent analysis (not shown) has shown that the suppression occurs in the most "central" collisions. A possible explanation for such observations is the suppression of the 1/;(28) resonance by interaction with co-moving particles 19 . The Y(lS) Rppb 20 in two rapidity ranges is shown in Fig. 5, right. The suppression is compatible with that of J/1/; in the same ranges. Models including shadowing and energy loss tend to overestimate RpPb at backward rapidity.

r:r:.� 1.8 - -A-UC-,,-�,-, ,-

.s,.;,. 5.02TeV,lncluslveJ/v, ljl(2S)->µ'µ' "• 1.6 • '¥(25)

1.4 ·

�::l .�·�"�· � I 0 2 ELOMWIUJ q :00750.V'llm(AlleOotal) I EPSO&Nl.O+"EL.ouwtlh <1,.0°"GoV'llln(Ark<>olaL) 0.s -� -3 -2 -11 j � '�s

Ycms

0.4 : _____ J CEM+EPS09 NLO{Vogl, arXiv:1301.3395 and priv.comm.)

Eloss(Ar1eo etal.,JHEP 1303{2013)122): 0.2 - ELoss

c::=.=::::J Eloss+ EPS09NLO

-4 -3 -2

Figure 5 - Nuclear modification factor of Jj'lj;, 7f;(2S) (left) and Y(lS) (right) in p-Pb collisions 17 18 20 , compared to theoretical models.

References

1. K. Aamodt et al. [ALICE Collaboration] , JINST 3 808002 (2008). 2. L. Evans et al., JJNST 3 808001 (2008). 3. F. Karstch, BNL-NT-06/2 (hep-lat/0601013). 4. T. Matsui and H. Satz, Phys. Lett. B 178 416 (1986). 5. P. Braun-Munzinger and J. Stachel, Phys. Lett. B 490 196 (2000). 6. B. Abelev et al. [ALICE Collaboration] , JHEP 1209 1 12 (2012). 7. B. Abelev et al. [ALICE Collaboration] , Phys. Rev. Lett. 109 1 12301 (2012). 8. B. Abelev et al. [ALICE Collaboration] , Phys. Rev. Lett. 113 232301 (2014). 9. B. Abelev et al. [ALICE Collaboration] , Phys. Rev. C 90 034904 (2014).

10. B. Abelev et al. [ALICE Collaboration] , Phys. Lett. B 719 29 (2013). 1 1 . B. Abelev et al. [ALICE Collaboration] , Phys. Lett. B 734 314 (2014). 12. A. Adare et al. [PHENIX Collaboration] , Phys. Rev. C 84 054912 (2011) . 13 . B. Abelev et al. [ALICE Collaboration] , arXiv : 1503 . 07179 [nucl-ex] (2015). 14. B. Abelev et al. [ALICE Collaboration] , Phys. Rev. Lett. 111 162301 (2013). 15. B. Abelev et al. [ALICE Collaboration] , Phys. Lett. B 738 361 (2014). 16. S. Chatrchyan et al. [CMS Collaboration] , Phys. Rev. Lett. 109 222301 (2012). 17. B. Abelev et al. [ALICE Collaboration] , JHEP 1402 073 (2014). 18. B. Abelev et al. [ALICE Collaboration] , JHEP 1412 073 (2014). 19. E. Ferreira, arXiv : 1411 . 0549 [hep-ph] (2014). 20. B. Abelev et al. [ALICE Collaboration] , Phys. Lett. B 740 105 (2015).