The dilepton invariant mass spectrum

1. The dilepton invariant mass spectrum “low” s version “high” s version • The study of lepton (e+e-, +-) pairs is one of the most important tools • to extract information on the early stages of the collision • Dileptonsdo not interact strongly, once produced can cross the system • without significant re-interactions (not altered by later stages) • Several resonances can be “easily” accessed through the dilepton spectrum

2. Heavy quarkonium states Quarkonium is a bound state of and q with Several quarkonium states exists, distinguished by their quantum numbers (JPC) q Bottomonium () family Charmonium () family

3. Colour Screening At T=0, the binding of the and quarks can be expressed using the Cornell potential: q q Coulombian contribution, induced by gluonicexchange between and Confinement term What happens to a pair placed in the QGP? The QGP consists of deconfinedcolourcharges  the binding of a pair is subject to the effects of colour screening q q • The “confinement” contribution disappears • The high color density induces a screening of the coulombian term of the potential 3

4. ..and QGP temperature Screening of strong interactionsin a QGP • Screening stronger at high T • D maximum size of a bound • state, decreases when T increases • Different states, different sizes Resonance melting QGP thermometer

5. (3S) (2S) b(2P) c(1P) (2S) b(1P) J/ (1S) J/  Feed-down and suppression pattern • Feed-down process: charmonium (bottomonium) “ground state” • resonances can be produced through decay of larger mass quarkonia • Effect : ~30-40% for J/, ~50% for (1S) • Due to different dissociation temperature for each resonance, one should • observe «steps» in the suppression pattern of measured J/ or (1S) Digal et al., Phys.Rev. D64(2001) 094015 Yield(T)/Yield(T=0) • Ideally, one could vary T • by studying the same system (e.g. Pb-Pb) at various s • by studying the same system for various centrality classes

6. From suppression to (re)generation At sufficiently high energy, the cc pair multiplicity becomes large • Statistical approach: • Charmoniumfully melted in QGP • Charmoniumproduced, together • with all other hadrons, at chemical freeze-out, • according to statistical weights • Kinetic recombination: • Continuous dissociation/regeneration over • QGP lifetime Contrary to the suppression scenarii described before, these approaches may lead to a J/ enhancement

7. How quantifying suppression ? • High temperature should indeed induce a suppression of the • charmonia and bottomonia states • How can we quantify the suppression ? • Low energy (SPS) • Normalize the charmonia yield to another hard process • (Drell-Yan) not sensitive to QGP • At RHIC, LHC Drell-Yan is no more “visible” in the dilepton • mass spectrum  overwhelmed by semi-leptonic decays of • charm/beauty pairs • Solution: directly normalize to elementary collisions (pp), via • nuclear modification factor RAA = RAA<1 suppression RAA>1 enhancement If no nuclear effects  NPAA=Ncoll NPNN (binary scaling)

8. Results:cold nuclear matter also matters…. • pA collisions  no QGP formation. What is observed ? Drell-Yan used as a reference here! NA50, pA 450 GeV • There is suppression of the J/ already in pA! This effect can mask a • genuine QGP signal. Needs to be calibrated and factorized out • Commonly known as Cold Nuclear Matter Effects (CNM) • Effective quantities are used for their parameterization (, abs, …)

9. B. Alessandro et al., EPJC39 (2005) 335 R. Arnaldi et al., Nucl. Phys. A (2009) 345 SPS: the anomalous J/ suppression • Results from NA50 (Pb-Pb) and NA60 (In-In) In-In 158 GeV (NA60) Pb-Pb 158 GeV (NA50) Drell-Yan used as a reference here! Anomalous suppression • In semi-central and central Pb-Pb collisions there is suppression • beyond CNM  anomalous J/ suppression After correction for EKS98 shadowing • Maximum suppression ~ 30%. Could be consistent with suppression • of J/ from c and (2S) decays (sequential suppression)

10. RHIC: first surprises • Let’s simply compare RAA • (i.e. no cold nuclear effects taken into account) • Qualitatively, very similar • behaviour at SPS and • RHIC ! • Do we see (as at SPS) • suppression of (2S) and • c ? • Or does (re)generation • counterbalance a larger • suppression at RHIC ? • RHIC: larger suppression • at forward rapidity: • favours a regeneration • scenario

11. Answer: go to LHC Two main improvements: 1) Evidence for charmonia (re)combination:now or never! (3S) b(2P) (2S) b(1P) (1S) 2) A detailed study (for the first time) of bottomonium suppression  Mass r0 Yes, we can!

12. J/, ALICE vs PHENIX • Even at the LHC, NO rise of J/ yield for central events, but…. • Compare with PHENIX • Stronger centrality dependence at lower energy • Systematically larger RAA values for central events in ALICE First possible evidence for (re)combination

13.  results • (2S), (3S) much less bound than (1S) • Striking suppression effect seen when comparing Pb-Pb and pp !

14. Conclusions on quarkonia • Verystrong sensitivity of quarkonium states to the medium • created in heavy-ion collisions • Two main mechanisms at play in AA collisions • Suppression by color screening/partonic dissociation • Re-generation(for charmonium only!) at high s • can qualitativelyexplainthe main features of the results • Cold nuclear matter effects are an important issue (almost • not covered here and in these lectures): interesting physics in • itself and necessary for precision studies study pA at the LHC

15. High pT particles (and jet!)suppression, open heavy quark particles • Other hard probes • High pThadrons and jets • Mesons and baryons containing heavy quarks (charm+beauty) • Theirproduction cross section can be calculated via • perturbative QCD approaches • Such hard probes come from high pTpartonsproduced on a • short timescale (tform ≈ 1/Q2) • Sensitive to the whole history of the collisions • Can be considered as probes of the medium • But what is the effect of the medium on such hard probes ?

16. q - s /2 Jet xa Q2 xb s /2 q H pp and “normal” AA production • In pp collisions, the following factorized approach holds Parton Distribution Functions xa , xb= momentum fractions of partons a, b in their hadrons Cross section for hadronic collisions (hh) Fragmentation of quark q in the hadron H Partonic cross section In AA collisions, in absence of nuclear and/or QGP effects one should observe binary scaling

17. RAA RAA = 1 RAA < 1 Breaking of binary scaling (1) • Binary scaling for high pTparticles • can be broken by • Initial state effects (active both in pA and AA) • Cronin effect • PDF modifications in nuclei • (shadowing)

18. E - DE Spectrum in pp Quenched spectrum Breaking of binary scaling (3) • Final state effects  change in the fragmentation functions due • to the presence of the medium: energy loss/jet quenching • Parton crossing the medium looses • energy via • scattering with partons in the • medium (collisional energy loss) • gluon radiation (gluonstrahlung) • The net effect is a decrease of the • pT of fast partons (produced on short • timescales) • Quenching of the high-pT spectrum • Radiative mechanism dominant at • high energy

19. Radiative energy loss (BDMPS approach) Energy loss Distance travelled in medium Casimir factor Transport coefficient • aS = QCD coupling constant • (running) • CR = Casimir coupling factor • Equal to 4/3 for • quark-gluon coupling and 3 for • gluon-gluon coupling • q = Transport coefficient • Related to the properties • (opacity) of the medium, • proportional to gluon density and • momenta ^ • L2 dependence related to the fact that • radiated gluons interact with the medium

20. Transport coefficient • The transport coefficient is related • to the gluon density and therefore • to the energy density of the • produced medium QGP • From the measured energy loss • one can therefore obtain an indirect • measurement of the energy density • of the system Pion gas Cold nuclear matter • Typical (RHIC) values • qhat= 5 GeV2/fm • aS= 0.2  value corresponding to a process with Q2= 10 GeV • CR = 4/3 • L = 5 fm Enormous! Only very high-pTpartons can survive (or those produced close to the surface of the fireball)

21. Results for charged hadrons and 0 factor ~5 suppression • Is this striking result due to a final state effect ? • Control experiments • pA collisions • AA collisions, with particles not interacting strongly (e.g., photons)

22. d-Au collisions and photon RAA • Both control experiments confirm that we observe a final state effect • d-Au collisions  observe Cronin enhancement • Direct photons medium-blind probe

23. Angular correlations • qqbar pairs produced inside fireball: both partonshadronize to low pT particles • qqbar pairs produced in the corona: one parton (outward going) gives a high pThadron (jet), the other (inward going) looses energy and hadronizes to low pThadron Near-side peak Away-side peak • Study azimuthal angle • correlations between a “trigger” • particle (the one with largest pT) • and the other high-pTparticles in • the event • At LO, hard particles come from • back-to-back jet fragmentation: • two peaks at 00 and 1800

24. Results on angular correlations • Suppression of back-to-back jet • emission in central Au-Au collisions • Another evidence for parton • energyloss • d-Au results confirm this is a final • state effect

25. High-pTparticles: results from LHC (1) • Comparison RHIC vs LHC • In the common pT region, similar • shape of the suppression • (minimum suppression at • pT~ 2 GeV/c) • Larger suppression at LHC! • Possibly due to higher energy • density (take also into account • that pTspectra are harder at the • LHC and should give a larger • RAAfor the same energy loss)

26. High-pTparticles: results from LHC (2) • Good discriminating power between models at very high pT

27. Dijet imbalance: clear signal at LHC • Significant imbalance of jet energies for central PbPb events! • Jet studies should tell us more about the parton energy loss and • its dynamics (leading hadrons biased towards jets with little interaction)

28. Pushing to very high pT • Strong jet suppression at LHC, extending to pT = 200 GeV! • Radiation is not captured inside the jet cone R • But where does the energy go ?

29. Where does energy go? (1) • Calculate projection of pT on leading jet axis and average over • selected tracks with pT > 0.5 GeV/c and |η| < 2.4 Define missing pT// • Integrating over the whole event final state • the momentum balance is restored Leading jet defines direction 0-30% Central PbPb excess away from leading jet excess towards leading jet balanced jets unbalanced jets

30. in-cone out-of-cone Where does energy go? (2) • Calculate missing pTin ranges • of track pT • The momentum difference in • the leading jet is compensated • by low pT particles at large • angles with respect to the jet • axis

31. Energy loss of (open) heavy quarkmesons/baryons • The study of open heavy quark particles in AA collisions is a crucial • test of our understanding of the energy loss approach • A different energy loss for charmed and beauty hadrons is expected • In particular, at LHC energy • Heavy flavoursmainly come from quark fragmentation, light flavours • from gluons smaller Casimirfactor, smaller energy loss • Dead cone effect: suppression of gluon radiation at small angles, • depending on quark mass • Suppression for • q< MQ/EQ Should lead to a suppression hierarchy Eg > Echarm > Ebeauty RAA (light hadrons) < RAA (D) < RAA (B)

32. gconversion p0 gee h gee, 3p0 w ee, p0ee f ee, hee r ee h’  gee Heavy-flavor measurements: NPE • Non-photonic electrons • (pioneered at RHIC), based • on semi-leptonic decays of • heavy quark mesons • Electron identification • Subtract electrons not coming • from heavy-flavour decays • e+e- (main bckgr. source) • 0, , ’ Dalitz decays • , ,  decays • Sophisticated background • subtraction techniques • Converter method • Vertex detectors… • Indirect measurement, expect non-negligible systematic uncertainties

33. Non-photonic electrons - RHIC • RAA values for non-photonic • electronssimilar to those for • hadrons no dead cone ? • No separation of charm and • beauty, adds difficulty in the • interpretation • Results difficult to explain by • theoretical models, even • including high qvalues and • collisional energy loss ^ • Fair agreement with models including only charm, but clearly not • a realistic description

34. Various techniques forheavy-flavor measurements • Direct reconstruction of hadronicdecay • Pioneered at RHIC, fully exploited at the LHC • Fully combinatorial analysis (build all pairs, triplets,…) prohibitive • Use • Invariant mass analysis of decay topologies separated from • the interaction vertex (need ~100 m resolution) • K identification (time of flight, dE/dx)

35. LHC results – D-mesons • Similar trend vs. pT for D, • charged particles andp± • Good compatibility between various • charmed mesons • Large suppression! (factor~5) • Hint of RAAD > RAAπ at low pT?  Look at beauty

36. Beauty via displaced J/ • Fraction of non-prompt J/yfrom • simultaneous fit to m+m- invariant mass • spectrum and pseudo-proper decay length • distributions(pioneered by CDF) • LHC results from CMS • Background from sideways (sum of 3 exp.) • Signal and prompt from MC template

37. Non-prompt J/ suppression Suppression hierarchy (b vs c) observed, at least for central collisions (note different y range) Larger suppression at high pT ?

38. Heavy quark v2 at the LHC • A non-zero elliptic flow for heavy quark would imply that also heavy • quark thermalize and participate in the collective expansion 38 Indication of non-zero D meson v2(3s effect) in 2<pT<6 GeV/c

39. Data vs models: D-mesons Consistent description of charm RAA and v2 very challenging for models, can bring insight on the medium transport properties, also with more precise data from future LHC runs

40. Heavy quark – where are we ? • Studies pioneered at RHIC • Abundant heavy flavour production at the LHC • Allow for precision measurements • Can separate charm and beauty (vertex detectors!) • Indication for RAAbeauty>RAAcharmand RAAbeauty>RAAlight • More statistics needed to conclude on RAAcharm vs. RAAlight • Indication (3s) for non-zero charm elliptic flow at low pT

41. At the end of the journey….. …let’s try to summarize the main findings • Heavy-ion collisions are our door to the study of the properties of • strong interaction at very high energy densities •  A system close to the first instants of the Universe • Years of experiments at various facilities from a few GeV to a few • TeVcenter-of-mass energies provided a lot of results which shows • a strong sensitivity to the properties of the medium • This medium behaves like a perfect fluid, has spectacular effects • on hard probes (quarkonia, jet,…) and has the characteristics • foreseen for a Quark-Gluon Plasma • Even if many aspects are understood, with the advent of LHC we are • answering long-standing questions but we face new challenges…. • …so QGP physics might be waiting for you! Also because….

42. …sagas never end!

43. Other topics

44. Low-mass resonances anddilepton continuum • Study of low-mass region: • investigate observables related • to QCD chiral symmetry • restoration • Conceptual difference between • study of heavy quarkonia and • low-mass resonances •  (,  to a lesser extent) • Short-lived meson ( = 149 MeV) • Decays to e+e- (+ -) inside the • reaction zone • QGP directly influences spectral • characteristics  may expect • mass, width modifications • J/ • Long-lived meson ( = 93 keV) • Decays outside reaction region • QGP may influence production • cross section but not its spectral • characteristics (mass, width)

45. Chiral symmetry(1) • The QCD lagrangianfor two light massless quarks is where • The quark fields can be decomposed into a left-handed and a • right-handed component • The Lagrangian is unchanged under a rotation of Lby any 2 x 2 unitary • matrix L, and Rby any 2 x 2 unitary matrix R • This symmetry of the lagrangian is called chiral symmetry • It turns out that the non-zero mass for hadrons is generated by a • spontaneous breaking of the chiral symmetry (i.e. the ground state does • not have the symmetry of the lagrangian)

46. Chiral symmetry(2) • In our world, therefore, the QCD vacuum corresponds to a situation • where the scalar field qq (quark condensate) has a non-zero • expectation value • The massless Goldstone bosons • associated with the symmetry • breaking are the pions • Contrary to the expectations m 0, • due to the non-zero (but very small) • bare mass of u,d quarks • Pion mass is anyway much smaller • than that of other hadrons • Lattice QCD calculations predict that , close to the deconfinement • transition, chiral symmetry is (approximately) restored, i.e. qq0 • with consequences on the spectral properties of hadrons

47. Chiral symmetry restoration and QCD phase diagram • Even in cold nuclear matter effects one could observe effects due to • partial restoration of chiral symmetry • Strong sensitivity to baryon density too  study collisions far from • transparency regime • Stronger effect in AA than in pA, but interpretation more difficult • need to understand the fireball evolution, mesons emitted along • the whole history of the collision

48. rB /r0 0 0.1 0.7 2.6 Effects on vector mesons • Dilepton spectrum study vector mesons (JPC=1--) • In the vector meson sector, predictions around TC are model dependent • Some degree of degeneracy between vector and pseudovector states, •  and a1 mesons • Rapp-Wambach broadening scenario • Brown-Rho scaling hypothesis, • hadron masses directly related to • quark condensate

49. f   Results at SPS energy: NA60 In-In collisions, s=17 GeV • Highest-quality data on the market •  ~  ~ 20 MeV • Subtract contributions of • resonance decays, both 2-body • and Dalitz, except  • Investigate the evolution of the • resulting dilepton spectrum, • which includes  meson plus • a continuum possibly due to • thermal production

50. Centrality dependence of  spectral function 12 centrality bins Comparison data vs expected spectrum A clear broadening of the -meson is observed, but without any mass shift Brown-Rho scaling clearly disfavored