Signatures of Quark-Gluon-Plasma/2 J/ Ψ suppression

1. Signatures of Quark-Gluon-Plasma/2 J/Ψ suppression

2. J/Ψ : identity card c c bound state (M=3096 MeV, Γ=87 KeV) Decay modes Hadrons 87.7 % e+e- 5.9 % μ+μ- 5.9 %

3. The discovery of J/Ψ: an historical overview In 1974 two experiments observed independently the same resonance. At SLAC the electron-positron storage ring (SPEAR) produced a sharp spike in the number of charged particles emerging from e+e- annihilations.

4. The discovery of J/Ψ: an historical overview At Brookhaven, collisions of high energy protons on a Be target produced the same peak at an energy close to 3.1 GeV

5. The discovery of J/Ψ: an historical overview Such resonance was remarkable because of its very narrowwidth, corresponding to a lifetime of 10-20 s, a factor of 1000 longer than expected for a heavy particle. 10 days later, at SLAC a second spike at slightly higher energy (3.7 GeV) was found: the Ψ ‘ , again with a narrow width and long lifetime. A Ψ ‘ detected in the Mark I detector, as it decays into 2 pions and a J/Ψ, which in turn decays into an electron pair.

6. The discovery of J/Ψ: an historical overview In 1970 Glashow, Iliopoulos and Maiani introduced the charm quark. With the discovery of J/Ψ, its properties were easily explained as a system of charmed quark-antiquark, a system known as charmonium. The J/Ψ corresponds to the lowest energy state of the charmonium system, while the Ψ ‘ to the second lowest. With a mass of 3.1 GeV, the J/Ψ can decay in many ways into lighter particles.

7. Debye screening in the QGP In the QGP, the color charge of a quark is subjected to screening effects, due to quarks, antiquarks and gluons. Similar to the Debye screening in the QED, such effect will weaken the interaction between the charm quark and antiquark. Moreover, in the QGP, quarks are deconfined and the string tension between the pair vanishes. Because of the two effects, the J/Ψ placed in the QGP may become dissociated, and its production will be suppressed in high-energy nucleus-nucleus collisions. Such phenomenon was predicted by Matsui and Satz(1986)

8. In standard conditions (without the QGP), the interaction between the two charm quark-antiquark may be represented by the phenomenological color potential and by the confining potential linearly increasing with their separation The Hamiltonian for the quark-antiquark system is μ= mc/2= reduced mass

9. Charmonium states are well described by the previous equation, with realistic set of parameters • When the system is placed in the QGP, the presence of quarks, antiquarks and gluons will produce two effects: • Since the string tension depends on the temperature, quark matter at finite temperature will result in a modification of the string tension. • Due to the screening of the color charge, the interaction between quark and antiquark will be modified from a Coulomb-type to a Yukawa-type interaction. • The two effects may alter so much the interaction between the two charm quark-antiquark that they cannot form a bound state.

10. For the ideal case of massless quarks and antiquarks, the long-range potential is modified into a short-range Yukawa potential with a Debye screening length gq=6 Nf = degeneracy of quark gas

11. J/Ψ suppression in the QGP The Debye screening length is inversely proportional to the temperature T. At high T, the range of attractive interaction becomes so small, to make it impossible to form a bound state, and the J/Ψ is suppressed. When such system dissociates, charm quarks hadronize, by combining with light quarks, and emerge as open charm D, Ds mesons. Then, if QGP is formed, the J/Ψ particle will be suppressed in comparison to the case when no QGP is formed. Such effect may be used as a QGP signature. How to estimate such critical temperature?

12. Estimation of critical temperature The Hamiltonian for the quark-antiquark system in the QGP is and the energy of the system The condition for a bound state is dE(r)/dr=0, which gives

13. Defining x=r/λD : The function f(x) has a maximum f(x)=0.84 near x=1.62. Then, to have solutions: The Debye screening length depends on the temperature T. For a QGP with a flavour number=3, perturbative QCD provides

14. With an effective coupling constant of 0.52, the Debye screening length at a temperature T=200 MeV is 0.36 fm. The critical dissociation temperature above which a charm quark-antiquark system cannot be a bound system is given by Using a charm quark mass 1.84 GeV and a coupling constant of 0.52 , then Tc=209 MeV. However, the coupling constant depends on the temperature, so the estimates for the critical temperature range in a large interval (100-200 MeV). In summary, it is expected that a QGP with a temperature exceeding the critical value will result in the suppression of J/Ψ.

15. J/Ψ suppression in a hadron environment J/Ψparticles are produced by hard scattering processes in NN collisions These may subsequently interact with hadrons, leading to the break-up of such particles. For instance via the process The J/Ψ production cross section in a NN collision is of the order of 10-30 cm2 (10-4 fm2)

16. The total probability for producing a J/Ψ in the collision of nuclei A and B at impact parameter b is where the first term (n=1) is dominant, since T(b)σNNJ/Ψ is small (terms with n>1 represent multiple J/Ψ production and can be neglected) So, to a first approximation,

17. In absence of J/Ψ – hadron interaction wich breaks up the produced J/Ψ , the cross section ratio (nucleus-nucleus to NN) is (rapidity-integrated): To study the effects of J/Ψ - nucleon interaction, it is important to know the momentum distribution of the produced J/Ψ particles. The threshold energy of the process is E=6.34 GeV in the nucleon rest-frame. J/Ψ particles produced in the central rapidity region may have enough energy to exceed such threeshold, so they can be broken up as they interact with target nucleons.

18. For the case of proton-nucleus collisions (p+A) it can be shown that (*) where σabs is the J/Ψ - nucleon cross section leading to the break-up of J/Ψ . Since experimental data show that with α≈1, the degree of absorption is small.

19. Expanding the equation (*) in powers of σabs, one gets with approximated value where L is the effective path length and ρ0 = 0.17/fm3 (standard density)

20. The thickness function TA(bA) may be parametrized in two ways 1) Heavy nucleus with uniform density 2) Light nucleus with Gaussian density distribution

21. Comparing and and expanding Aα in powers of (1- α): For a heavy target nucleus

22. For a nucleus-nucleus collision (A+B) by analogy: (exponential shape)

23. Collection of J/Ψ -> 2 μ data for different systems. Data are represented by σabs = 5.2 mb ρ0 = 0.14/fm3

24. Experimental data follow approximately such exponential shape with L. This supports the interpretation that to a first approximation the J/Ψabsorption is dependent on their path length after they are produced. Such suppression of J/Ψ arising from interactions with hadrons is important and must be taken into account before searching for additional (originating from QGP?) suppression.

25. The experimental situation

26. Experimental overview on J/Ψ suppression NA38 and NA50 at SPS have studied J/Ψ production with p, O and S beams on several targets. The analysis of such data showed that the yield of J/Ψis suppressed with respect to the yield of Drell-Yann dimuons. Such result is interpreted in terms of the nuclear absorption of the charm quark-antiquark pair before it forms the J/Ψparticle. This “normal” suppression is the reference point to check the behaviour in Pb-Pb central collisions. A first clear departure from such smooth trend was seen in 1995, with a factor 0.77 below the expected yield.

27. The study of the process as a function of the centrality (as measured by the transverse energy) in the next years allowed to establish that in peripheral Pb-Pb collisions the trend is compatible with that obtained for light systems. However, for central Pb-Pb collisions, the yield is considerably reduced and this sudden decrease may be interpreted as due to QGP formation.

28. The NA50 set-up

29. The NA50 set-up

30. Main detector components: A dimuon spectrometer An electromagnetic calorimeter, to measure the neutral transverse energy ET released in the interaction A zero-degree calorimeter, to measure the energy of the projectile (not-participating) nucleons EZDC. A silicon microstrip detectors, to measure the multiplicity

31. Data selection • Muon pairs measured between y=2.92 and y=3.92 • Centrality selection according to neutral transverse energy ET • J/Ψ suppression also studied as a function of EZDC. • 2 different analysis: • Use Drell-Yann events as a reference • Use minimum-bias events as a reference

32. Neutral transverse energy distribution

33. Zero-degree energy distribution

34. Ratio between the J/Ψ and the Drell-Yann cross sections, as a function of ET.

35. Solid line = Suppression due to standard nuclear absorption Second drop observed near ET=90 GeV Ratio between the J/Ψ and the Drell-Yann cross sections, performed with a different analysis (minimum-bias events) as a function of ET.

36. Solid line = Suppression due to standard nuclear absorption Ratio between the J/Ψ and the Drell-Yann cross sections, performed with a different analysis (minimum-bias events) as a function of EZDC. Correlation between the two centrality variables is not perfect, so only qualitative comparison possible

37. The strong decrease of the J/Ψ yield in central Pb-Pb collisions is in disagreement with conventional hadronic models. Such models predict a smooth decrease from pp to central Pb-Pb collisions, with a saturation in the most central collisions.

38. Measurements from NA38/NA50 (1996-1998) cannot be explained by any existing description based on conventional hadronic processes

39. Energy density from Bjorken’s estimate Ratio between observed and expected suppression, as a function of the energy density.

40. A first drop is observed near 2.3 GeV/fm3. An even stronger suppression is observed near 3.1 GeV/fm3. Conclusions from NA50 are that the observed J/Ψ suppression pattern provides enough evidence for deconfinement of quarks and gluons.

41. Additional measurements have been carried out by NA50 at SPS in the year 2000, improving the rejection of contaminating events (measurements under vacuum) and Measuring more peripheral collisions Latest results show that the (J/Ψ)/(Drell-Yann) cross section ratio follows that expected from standard nuclear absorption only for the very peripheral collisions, whereas the anomalous suppression is confirmed for Pb-Pb central collisions.

42. The main improvement in the experimental set-up and data analysis procedures was a new target system under vacuum. This allowed a better rejection of out-of-target interactions (in particular Pb-air interactions). Improved methods of analysis and event selections were also used. 3 centrality detectors used: ZDC, ET, multiplicity silicon detector

43. Correlation between transverse and zero-degree deposited energy (improved)

47. Method of analysis Particle yields extracted from a fit of the dimuon mass spectrum. Combinatorial background estimated from like-sign muon pairs Opposite-sign invariant mass spectrum fitted to extract the 4 relevant contributions J/Ψ Ψ ‘ Drell-Yann Open charm After acceptance corrections, extract ratio of cross sections (J/Ψ over DY)

49. To establish the normal nuclear absorption pattern, data were measured for p-nucleus collisions at 400-450 GeV and parametrized (in the following figures as a solid line). Results for Pb-Pb at 160 A GeV were analyzed as a function of the centrality using the 3 independent variables

50. For peripheral Pb-Pb collisions the ratio is consistent with the normal nuclear absorption pattern, as deduced from p-nucleus data.