Gli adroni ad alto P T sono prodotti da partoni con Hard-scattering iniziale.

1. QGP (II) E. Scapparone 24 Maggio, 2010 • Gli adroni ad alto PT sono prodotti da partoni con Hard-scattering iniziale. • I partoni hanno bisogno di un tempo finito per uscire dalla zone della collisione, • durante il quale si forma il “dense medium”  durante questo tempo subiscono • Interazione forte col mezzo e quindi costituiscono una “sonda” per studiare il • mezzo stesso. Ci sono due variabile che si possono studiare: • Rapporto degli adroni emessi in collisioni tra nuclei rispetto a p-p ( corretto • per effetti geometrici e scalando col numero di collisioni) ; • 2) Disappearance del jet “back to back” nell’angolo azimuthale ( “jet quenching”);

2. PHENIX Nessun effetto in d-Au  effetto associato a interazione tra ioni pesanti. The results are consistent with the effects of parton energy loss in traversing dense medium, predicted before the data were available

3. centralità 90% 50% 0% Per ripdodurre l’intensita’ della soppressione, questi MC devono assumere Una densita’ di gluoni 30 volte maggiore di quella della materia fredda e Confinata ( densita’ di energia 100 volte maggiore).

5. Altra possible normalizzazione: I fotoni

6. As one increases the collision energy in nucleus nucleus collisions, the produced plasma reaches higher energy and particle densities, the system stays longer in the QGP phase, and correspondingly the traversing partons are more quenched.

7. Il meccanismo dominante di perdita di energia dei partoni nel QGP e’ la radiazione di gluoni (“gluonstrahlung”). Near side: dove seleziono la particella “trigger” Away side: lato opposto  Maggiori effetti del mezzo

8. Select a High Pt particle as trigger and measure the ΔF=F−Ftrig angle

9. Evoluzione da low to high pt PHENIX(2008) Medium Fragmentation Shape simile a p-p + soppressione

10. Experimentally, the centrality is evaluated by measuring one or more of these variables: • Nch: number of charged particles produced in a given rapidity interval (near mid-rapidity) • increases (~ linearly) with Npart • ET: transverse energy = SEi sin qi • increases (~ linearly) with Npart • EZDC: energy collected in a “zero degree” calorimeter • increases (~ linearly) with Nspectators

11. dz Consider a thin cylindrical slab of transverse dimension S of expanding matter contained within a thickness dz at timet. dE -dv v=0 dv Bjorken’s formula Bjorken’s formula • To have an estimate of the energy density reached in the initial stages of the collisions, we can project back in time the energy carried by the collision products (“Bjorken’s estimate”) dv = c db = (c/2) dy (non rel.: y = b) dz = 2t dv = c t dy dV = S dz = S c t dy dE = e dV In the center-of-mass frame v=0 at the center of the slab

12. Initial time t0 : usually taken to be ~ 1 fm/c i.e.: equal to the “formation time”: the time it takes for the energy initially stored in the field to materialize into hadrons Bjorken’s formula Transverse dimension S: e ~ (400/160) GeV/fm3 ~ 2.5 GeV/fm3 Enough for deconfinement! Initial energy density Estimate for central (head-on) Pb-Pb collisions at the SPS Published estimate from NA49: e = 3.2  0.3 GeV/fm3 [Phys. Rev. Lett. 75 (1995), 3814] RHICe ~ 5 GeV/fm3

13. K+ s s s s s s s s d d u u d X- u p- u d d d d d d d d d d d s u u u u u u u u s s u d d d d p+ u u s u u u u p u u d d d s W+ u s s u u d d u s d L Strangeness enhancement • restoration of csymmetry -> increased production of s • mass of strange quark in QGP expected to go back to current value • mS ~ 150 MeV ~ Tc • copious production of ss pairs, mostly by gg fusion [Rafelski: Phys. Rep. 88 (1982) 331] [Rafelski-Müller: P. R. Lett. 48 (1982) 1066] • deconfinement  stronger effect for multi-strange • can be built recombining uncorrelated s quarks produced in independent microscopic reactions • strangeness enhancement increasing with strangeness content [Koch, Müller & Rafelski: Phys. Rep. 142 (1986) 167]

14. Charmonium as a Probe of QGP • Matsui and Satz predicted J/y production suppression in Quark Gluon Plasma because of color screening

15. Charmonium suppression • QGP signature proposed by Matsui and Satz, 1986 • In the plasma phase the interaction potential is expected to be screened beyond the Debye length lD (analogous to e.m. Debye screening): • Charmonium (cc) and bottonium (bb) states with r > lD will not bind; their production will be suppressed lD, and therefore which onium states will be suppressed, depends on the temperature

16. with n0 = density of electrons in the plasma getting: n = 28.8 106 MeV3 Debye screening In an electromagnetic plasma, the potential of a charge is screened by the field of the electrons that surround it [see e.g.: Jackson p. 494]: In a QGP, the colour field is likewise going to be screened. In order to have a back-of-envelope estimate the screening length, one can take the aboveformula, and substitute: e2(Gauss system) aQCD ~ 1 n0n = 3.6 T3 (Stefan-Boltzmann law for QGP) kT ~ 200 MeV and, using: 1 MeV-1 = 197.3 fm:lD 0.15 fm

17. NO QGP QGP c c c c c V= kr - a / R V = -a e – r/ld R

18. Come si identifica una risonanza (esempio J/Y ) ? • Identificazione dei leptoni ( canale a 3p molto difficile); • misura del loro momento; • Calcolo della massa invariante e selezione. • minv = sqrt( (E1+E2)2 – (p1+p2)2)

19. NA38

20. Quarkonium production is usually normalised to Drell-Yan production (which is not influenced by strong interactions) Dimuon Spectrum • The measured dimuon spectrum is fitted to a source cocktail in order to extract the J/y, y’ and Drell-Yan contributions NA50 dimuon spectrum (Pb-Pb, 158 A GeV/c) m+ m-

21. Why do we keep using Drell-Yan ? Drell-Yan (muon pairs) is a well known computable process, proportional to the # of elementary nucleon-nucleon collisions, with the following priceless advantages: • identical experimental biases • identical inefficiencies • identical selection criteria • identical cuts as J/ Therefore the correctionscancel out in the ratio  (J/)  (DY) which is insensitiveto normalization factors/uncertainties PUNTI NEGATIVI : 1) statistica DY << statisticaJ/y 2) normalizzazione isospin

22. Nuclear absorption Branching to muons • There is a “normal” suppression of the production of J/y, observed already in pA and lighter ion collisions and attributed to nuclear absorpion • The Pb-Pb point falls below the nuclear absorption curve (“anomalous” suppression)

23. From a fit of experimental p-A data (NA38,NA50): sabs=4.18 +- 0.35 mb (hep-ex/0412036) • In S-U collisions the same suppression is observed • The normal suppression is interpreted as the absorption, in the nuclear environment, of the c-cbar pair before the J/y (or y’ or c) formation : preresonance absorption.

25. The CERN Pb ion programme • Started in 1994 • Pb nuclei accelerated to 158 A GeV/c (40 A GeV/c in 1999) collide on fixed targets (typically 4-6 weeks/year) • 7 experiments: • NA44 (single arm spectrometer: particle spectra, interferometry, particle correlations) • NA45 (e+e- spectrometer: low mass lepton pairs) • NA49 (large acceptance TPC: particle spectra, strangeness production, interferometry, …) • NA50 (dimuon spectrometer: high mass lepton pairs, J/y production) • NA52(focussing spectrometer: strangelet search, particle production) • WA97/NA57 (silicon pixel telescope spectrometer: production of strange and multiply strange particles) • WA98 (photon and hadron spectrometer: photon and hadron production)

26. A Pb-Pb collision at the SPS • “Busy” events! (thousands of produced particles) • High granularity detectors are employed (TPC, Si Pixels,...)

27. NA38 first results • O+U at 200 GeV/c: • Factor 2 suppression… • but… including: • normal nuclear absorption !!! • IMR charm-like excess !!! (fit starts from 1.7 GeV/c2 !!)

28. Experiment NA50 • Aim: study the production of J/y in Pb-Pb collisions • Experimental technique: • absorb all charged particles produced in the collision except muons • detect J/y by reconstructing the decays J/y m+m- (B.R.  5.9 %)

29. Anomalous J/y suppression • J/y normalized to Drell-Yan as a function of the transverse energy (i.e. centrality) • The data points deviate from the solid curve, which indicates the prediction for nuclear absorption • The deviation increases with increasing collision centrality

30. Attempts at describing the NA50 data within purely hadronic models without deconfinement • dissociation of the J/y in final state hadronic interactions with comovers • try harder...

31. Projectile J/y L Target

32. NA60: stesso rivelatore di NA50 con aggiunta di rivelatori al Silicio (tracking). Alcune delle motivazioni dell’esperimento: If the J/psi suppression pattern in Pb-Pb collisions indicates that central Pb-Pb collisions produce a state of matter where colour is no longer confined, we should move on to the detailed understanding of how deconfinement sets in, and what physics variable governs the threshold behaviour of charmonia (cc) suppression: (local) energy density, density of wounded nucleons, density of percolation clusters, etc. This requires collecting data with smaller nuclear systems like In-In. The J/psi data collected in central Pb-Pb collisions indicate that we are already beyond the point where the phase transition takes place, but do not provide any information on the value of the critical temperature. Finite temperature lattice QCD tells us that the strongly bound J/psi ccbar state should be screened when the medium reaches temperatures 30-40 % higher than T_c, while the large and more loosely bound psi' state should melt near T_c. The NA38 experiment has shown that the psi' is significantly suppressed when going from p-U to peripheral S-U collisions. We need to see if this suppression follows a smooth pattern or a sudden transition, within a single collision system rather than comparing p-U to S-U data. If the Y' suppression is due to Debye screening, its suppression pattern could provide a clear measurement of T_c. However, the hadronic "comovers" produced in S-U collisions may "absorb" the psi' mesons, since its binding energy is only around 40 MeV. What mechanism is responsible for the Y' suppression? The presently existing results are not clear in what concerns the onset and pattern of the psi' suppression. A new measurement is needed, with improved mass resolution to have a cleaner separation of the psi‘ peak with respect to the J/Y shoulder, and which scans the energy density region from the p-U to the S-U data. In-In collisions are also well placed for this study.

33. MWPC’s m ~ 1m Muon Spectrometer Iron wall Hadron absorber Toroidal Magnet Target area m beam Trigger Hodoscopes Dipole field2.5 T ZDC TARGET BOX MUON FILTER Matching in coordinate and in momentum space BEAM BEAMTRACKER VERTEX TELESCOPE IC  not to scale • Origin of muons can be accurately determined • Improved dimuon mass resolution allows studies vs. collision centrality  ZDC NA60’s detector concept Idea: place a high granularity and radiation-hard silicon tracking telescope in the vertex regionto measure the muons before they suffer multiple scattering and energy loss in the absorber

34. S-U Pb-Pb L (fm) In-In pure Glauber calculation Npart New and accurate measurements are needed to answer these questions Specific questions that remain open Is the anomalous suppression also present in lighter nuclear systems? Study collisions between other systems, such as Indium-Indium Which is the variable driving the suppression? Study the J/ suppression pattern as a function of different centrality variables, including data from different collision systems What is the normal nuclear absorption cross-section at the energy of the heavy ion data? Study J/ production in p-A collisions at 158 GeV What is the impact of the cc feed-down on the observed J/y suppression pattern? Study the nuclear dependence of cc production in p-A collisions

35. Comparison with previous results An “anomalous suppression” is present already in Indium-Indium The normal absorption curve is based on the NA50 results. Its uncertainty (~ 8%) at 158 GeV is dominated by the (model dependent) extrapolation from the 400 and 450 GeV data

36. Spiegazioni alternativa al QGP 1) Comovers • Comovers model. La J/Y puo’ interagire con gli adroni “comovers”. La sezione • d’urto e’ molto difficile da stimare. Na50 J/y suppression can be reproduced by DPM with absorption by comovers. The number of comovers in Capella model is proportional to number of participants and also to number of collisions. A. Capella, D. Sousa, nucl-th/0303055

37. Suppression by produced hadrons (“comovers”) The model takes into account nuclear absorption and comovers interaction with sco = 0.65 mb (Capella-Ferreiro) In-In @ 158 GeV J/y / NColl nuclear absorption comover + nuclear absorption (E. Ferreiro, private communication) NA60 In-In 158 GeV The smeared form (dashed line) is obtained taking into account the resolution on NPart due to our experimental resolution Pb-Pb @ 158 GeV

38. 2) Percolation [First works: Baym , Physica (Amsterdam) 96A, 131 (1979) Celik et al., Phys. Lett. 97B (1980) 128] Forma di deconfinamento geometrica, di pre-equilibrio. Pre-requisito al deconfinamento Vero e proprio, applicabile ai sistemi finiti. Se il condensato di partoni contiene partoni abbastanza “hard”, puo’ dissociare la J/Y N dischi di raggio r<< R Densita’ n= N/pR2 Superficie di area pR2 Aumentanto la densita’ si trovano cluster di area sempre maggiore Quando N,R  ∞ e n finito , la cluster size diverge a n= 1.13/ pR2. Per N,R finiti si ha Percolazione quando il cluster piu’ largo Riempie tutta la superficie. R 1 Percolation probability r/R=1/100 0.5 n=n(r/R)2 1.5 1

39. A causa dell’overlap, alla soglia di percolation, solo 2/3 dell’area e’ riempita. • Local Percolation: Hard probe, come gli stati c-cbar risentono del mezzo localmente. Si ottiene a 1.72/pr2

40. regeneration suppression • Regeneration models[2,3,4] predict an enhanced production of hidden charm states for sufficiently high charm densities • This would imply thermalization of charm quarks, and by extension, the light quarks that comprise the QGP 3) Regeneration Osservazione sperimentale (Gazdzicki and Gorenstein) Il rapporto di J/Y / p- = cost “a dominant fraction of the Jc mesons produced in hadronic and nuclear collisions at CERN SPS energies is created at hadronization according to the available hadronic phase space” L. Grandchamp and R. Rapp, Phys. Lett. B523 60 (2001)

42. “Two component” model: suppression in hadronic and QGP phase + statistical production at hadronization

43. Satz, Digal, Fortunato Rapp, Grandchamp, Brown Capella, Ferreiro J/y at SPS • J/y in NA60 poorly reproduced by models which fit NA50 data

44. A RHIC tutto piu’ complicato • Lattice Gauge calculations now indicate that the J/ψ remains bound up to • 1.5 to 2 times the deconfinement temperature (the J/ψ is very small in radius, • and would reasonably require a higher density to become unbound due to screening). • The maximum temperature reached in central Au+Au collisions at RHIC is thought to • be ~ 2 times the transition temperature, so it is now not clear if the J/psi is expected • to melt at RHIC. • - some higher states that feed down to the J/ψ are expected to melt just above the • deconfinement temperature; • - the large charm quark production cross section at RHIC leads to predictions that J/ψ • will be formed by random coalescence of unrelated charm pairs in central Au+Au • collisions, even if the initial group of forming J/ψ is destroyed in deconfined matter; • modifications of gluon densities at low momentum-fraction in heavy nuclei are • expected to start to be significant at RHIC and perhaps modify the initial charm • anti-charm production cross sections.

45. RAuAu (y~0) ~ RAuAu (SPS) • Lower rapidity RAA look surprisingly similar, while there are obvious differences: • Cold nuclear matter effects (xBjorken,…) • Energy density • … ±12% global syst ±7% global syst ±11% global syst

46. 60% RAA(y~1.7) RAA(y~0) RAuAu (y~0) > RAuAu (y~1.7) • More suppression at forward rapidity ! ±12% global syst ±7% global syst

47. 44± 23% 25±12% Quick comparison to SPS J/ψ survival beyond CNM • At mid-rapidity, the amount of surviving J/ψ @ RHIC is compatible with SPS (~60%) but depends a lot on CNM (and pp references)… • At forward rapidity, RHIC anomalous suppression is much stronger ! ±11% global systematics ±35% global systematics ±30% global systematics

48. J/ψ RAA over CNM in Cu+Cu and Au+Au Calculations by M.J. Leitch using break-up cross-section and errors estimated from 2008 data Differences between mid and forward rapidity measurement is washed out. Suppression beyond cold nuclear matter effects is observed, consistent with de-confinement 48

49. Conclusions • Two qualitative possible scenarios • Large melting + some regeneration • Initial effects (CGC) + melting (of ψ’, χc ?) • Need better handle of CNM • Need better open charm measurements ! • Smoking gun would have been a J/ψ rise… • v2 could become the smoking gun • (maybe run7 with 4 x run4 and reaction plane detector) • Data is young, new ideas may arise…

50. Johanna Stachel