Signatures of Quark-gluon-plasma/4 Strangeness production

1. Signatures of Quark-gluon-plasma/4 Strangeness production

2. Strangeness content: different in hadron matter and in QGP In hadron matter, valence quarks consist of u,d quarks with a negligible contribution from s quarks. In NN collisions, quark-antiquark pairs are produced (including strange quarks, which may combine to form strange particles) How many strange quarks may be produced?

3. A qualitative description of the particle production mechanism is the Schwinger description, where the field between quark and antiquark is represented as the analogous of a constant electric field in a capacitor in QED. A particle-antiparticle pair is produced when a particle tunnels from the negative energy continuum to the positive energy continuum.

4. Only quarks with energy E between these limits may tunnel So, the production of a quark-antiquark pair will not occur if the separation L is not enough large to satisfy previous equation. This is of the order of 0.7 fm for pT=0, and increases up to 1 fm for pT=0.35 GeV.

5. The tunneling probability decreases with increasing particle mass Rate of particle production per unit time and unit volume = (no. of states in the phase space volume element) x (penetrability P)

6. Assuming z=-E/k (particles produced in the midpoint) and integrating over pT

7. Using a set of constituent quark masses mu=md=0.325 GeV and ms=0.45 GeV we get

8. Simple relation between ratio (strange quarks)/(up and down quarks) and ratio (kaons/pions) Taking the quark composition of mesons

9. The number of u,d,s quarks and antiquarks may be related to that of the mesons by

10. From that one can estimate the ratio kaons/pions: with an estimate of 0.18

11. What is the experimental value of the ratio (Kaons/pions)? In e+e- annihilation 0.157 ± 0.024 at √s=10 GeV 0.144 ± 0.009 at √s=29 GeV In p-Be collisions 0.08 at 14.5 GeV In the last case (or, similarly in NN collisions) the ratio (kaon/pion) is lower, since a non-negligible fraction of the energy is taken by the leading particles (less energy available for particle production). The value 0.08 corresponds to a ratio of 0.05 for the ratio (strange quarks)/(non-strange quarks)

12. Some experimental data

13. What happens in a nucleus-nucleus collision? A large number of hadrons are produced. If the hadron gas reaches thermal and chemical equilibrium, what is the ratio (strange particles)/(non-strange particles)? The density of particle i with rest mass mi at temperature T is given by which leads to

14. Then the ratio between kaons and pions is

15. At a temperature T=200 MeV which means that these ratios are enhanced with respect to pp collisions. It is not clear whether there is time enough in a heavy-ion collision to reach chemical equilibrium even with strange particles, because the production of strange hadron pair requires longer times. Therefore additional mechanisms have been proposed to explain the strangeness enhancement.

16. Question: What are the densities of the different types of quarks if the plasma has a life long enough to reach thermal and chemical equilibrium? At thermal and chemical equilibrium the occupation probabilities of the quarks are given by Fermi-Dirac distributions:

17. When μ=0 the density of all quarks and antiquarks is nearly the same. In such situation, the number of strange quarks and antiquarks is much greater than in an equilibrated gas of hadrons without phase transition. Hence, an enhancement of the number of strange quarks and antiquarks is considered as a QGP signal.

18. The enhancement of the number of strange quarks and antiquarks leads to an enhancement of the production of mesons with s quarks (Kaons and φ-mesons) and hyperons

19. At a temperature T=0 a critical baryon density of 0.72/fm3 is expected for the phase transition (μ=434 MeV). At temperature different from 0, lower values of the chemical potential are expected. In such case the density of u,d quarks is higher than the strange quark density, which in turn is higher than the antiquark density. According to such quark abundances , it will be more likely to form K+ or K0 than K- or antiK0. Similarly, it will be more likely to form Λ, Σ+, Σ0, Σ- than antiK0 and K-.

20. Experiments often measure the relative abundance of the different particles (particle ratios). Such values help to understand the thermodynamical state of the QGP.

21. Question: at which rate the system approaches thermal and chemical equilibrium? What is the time scale for the strangeness content? Consider as a starting point a plasma composed only of u,d quarks (and antiquarks) and gluons, without s quarks. The plasma will then evolve toward chemical equilibrium through reactions between its constituents. Strange quarks and antiquarks may be produced by the process

22. Another process is The two cross sections as a function of the c.m. energy M of the quark-antiquark (or gluon-gluon) pair are comparable

23. It can be shown that the number of strange-antistrange s pairs per unit time, due to the quark-antiquark process is The rate due to the other process through gluons is

24. The rate for the strange pair process is much higher from gluons rather than from quarks. Therefore, the dynamics of the change of strangeness content is dominated by the gluon interactions.

25. The equilibration time constant is given by For T=200 MeV it is of the order of 10 fm/c, while it decreases to a few fm/c at higher temperature. Since the collision process between heavy ions takes 5-10 fm/c, the equilibration of strangeness may be incomplete at T=200 MeV, but it is complete at larger T.

26. The experimental situation

27. Early experiments to measure the strangeness production in high-energy heavy-ion collisions have been carried out by several groups: At BNL: E802, E810 At CERN: NA34, NA35, NA38, WA85 The main results of such experiments are concerned with kaon and pion production. Main finding: the number of positive kaons increases more rapidly than the number of positive pions with the size of the system, whereas the number of negative kaons increases only slightly faster than negative pions

28. The enhancement of the ratio Kaon/pion is higher in the low rapidity region, and increases with the size of the system. For instance at y=1 the ratio for Si+Au increases a factor 3 with respect to p+Be.

29. Such preliminary results are in agreement with the expected QGP scenario: In the QGP the plasma is rich in u,d quarks, depleted in anti (u,d) quarks and with an intermediate density of strange quarks. Then it is likely for the anti s quark to find an u or d quark to form K+ or K0, and difficult to find anti-u or anti-d to form K- or anti-K0. Then the number of K+ is much enhanced than K-.

30. However, such simple results may also have explanations based on conventional hadron scenarios. According to the production from hadron environment one should consider the following processes The K- and anti-K0 are more easily absorbed, which leads to an enhancement of K+, Λ and K0 and a depletion of K- and anti-K0. Quantitative estimation of such processes do no fully explain the results. In more recent years, new – more sensitive - results concerning strangeness enhancement have been however obtained at SPS from NA49 and WA97/NA57.

31. Early results from NA49 at SPS were concerned with the kaon/pion ratios at different bombarding energies. After having established a sharp maximum in the K+/π+ ratio near 30 A GeV lab energy (7.6 A GeV c.m. energy), the energy scan has been completed in recent years with additional measurements. Moreover, different strangeness carriers have been recently analyzed after the findings on strangeness enhancement from WA97/NA57.

32. NA49 set-up

33. NA49 is a wide acceptance hadron detector with • 2 superconducting dipole magnets • 4 TPC for tracking and momentum measurement • A time-of-flight system for particle identification • A zero-degree calorimeter for event centrality selection • Data were taken at the following energies per nucleon • Lab c.m. • 6.3 • 30 7.6 • 40 8.8 • 80 12.3 • 158 17.3

34. PID (Particle Identification) is achieved by a combined TOF and dE/dxanalysis Hyperons are identified by their invariant mass, according to the decay topology Yield of strange particles is normalized to the pion yield

35. NA49 results on kaon/pion ratios A sharp structure is observed in the positive kaon/pion ratio, which is not reproduced by current hadronic models (for instance HGM, Hadron Gas Model), although they predict a broad maximum. No such behaviour is seen for negative pions. Only SHM (Statistical Hadronisation Model), which includes a transition to a deconfined state around 30 A GeV, predicts a sharp peak.

36. NA49 results on Lambda and Xi ratios The same sharp peak is seen also for Lambdas, while results for Xi are not clear. HGM is able to reproduce results for Lambdas

37. NA49 results on Omega and Phi ratios Results unclear, with large statistical errors so far

38. NA57: a dedicated experiment to strangeness production@SPS

39. The NA57 set-up

40. The NA57 set-up Trigger

41. The NA57 set-up Centrality

42. The NA57 set-up Tracking

44. Multiplicity evaluation

45. Centrality selection

46. Centrality selection

47. Mid-rapidity Acceptance windows

48. Reconstruction of hyperon decays

49. Examples of reconstruction of invariant masses

50. Evaluating and subtracting the background by mixed event analysis Before cuts After cuts