Challenges for transport theories

1. Challenges for transport theories to describe dense matter J. Aichelin 1) CBM does not study unknown land Is this energy region under control? Can we still expect surprises? Example: phase transition 2) A lessons we have learnt: a AA program without a NN and pA program will not reveal any new physics Example: HADES dileptons 3) What can we do if CBM explores the plasma phase and all standard transport theories fail? Symposium on the Physics of High Baryon Density

2. Unknown Land ? No! (I) Even worse: Energy density or number of charged particles at midrapdity seem to indicate that this is a dull region. Is this really the case? and, if not, what do we have to look for? Symposium on the Physics of High Baryon Density

3. AGS NA49 BRAHMS Unknown Land ? No! (II) Multiplicity data available (and momentum distributions in a limited region of phase space) In detail very complicated Especially for Phi Strange/multistrange baryons the excitation function is rather exotic and not understood at all (models: sudden transition from hadron to string dynamics) Excitation fct of baryonic resonances even more difficult to understand  already multiplicities are not understood Symposium on the Physics of High Baryon Density

4. Unknown land? No! (III theory) What’s about momentum distributions? Weber, Bratkovskaya nucl-th/0209079 • Theory (URQMD and HSD) • reproduces quite well • - Excitation fct of K multiplicity • at y = 0 and for 4π (if avail.) • - Rapidity distribution • at least for the K’s • although we known that in URQMD • the production mechanism is wrong • whereas it is right in HSD (at least • at low energy) • These data are not very sensitive to the production mechanism and hence to the physics Y=0 4π HSD URQMD Symposium on the Physics of High Baryon Density

5. Unknown land? No! (IV theory) Χ • But: • The rarer the particles the less well they are described • and the more sensitive they are to the reaction mech. • π are always a problem from 2 AGeV to • 20 AGeV ( horn of Marek) and we do not know yet why. SQM04 Unpublished Due to the lack of pp data AA data cannot be interpreted Mechanism for production very different Y=0 4π Ω Symposium on the Physics of High Baryon Density Weber, Bratkovskaya nucl-th/0209079

6. The present situation for the 10 AGeV physics is comparable to that for 1 AGeV at the shutdown of the Beverlac (and hence before SIS): some exploratory experiments ( some of them wrong) some premature conclusions (d~entropy, π~temperature, system in equilibrium …….) Nobody talks anymore about these results because the interesting physics has been studied subsequently at SIS The same will be true for CBM as compared to AGS because the lessons from AGS are the same: Light charged particles are created at the end of the reaction. If we want to study the high density phase we have to use -electromagnetic probes -particles which are only (predominantly) produced if the system are dense (like the K,phi,Xi at SIS) These data do not exist yet ! Before even thinking on simulations of heavy ion reactions experiment has to provide the elementary cross sections Symposium on the Physics of High Baryon Density

7. Interesting physics hidden by dull excitation fcts. An example of the SIS days – how to observe a phase transition? Standard EOS for hadronic matter One of the possible signals of a phase transition is a sudden change in density without an increase of the energy So we build such an equation of state put it in the simulation program and look at the influence on the observables EOS with a phase transition like hadrons  plasma Symposium on the Physics of High Baryon Density

8. Consequences for the observables (studied around 1 AGeV) with help of a density isomer) Collision number increases smoothly if the system passes a phase transition ( because the system is denser) but excitation fct is smooth The number of K+increase by a factor of 10 in a small interval of the beam energy ( because here K are produced only at high density) The number of π increases only little. They are created at the very end of the reaction. The excitation fct is smooth. It is impossible to use for nailing down a phase transition Log! For CBM we need J/psi instead of K Symposium on the Physics of High Baryon Density

9. Lesson to learn: Even if light charged particle excitation functions • looks dull, there may be interesting physics. • If we want to uncover this physics we have to look for • particles which are produced predominantly at high density • J/Ψ mesons • D mesons • charmed baryons • Φ • antibaryons and multistrange baryons • Charm is for CBS what was strangeness for SIS: a tool to study • the heavy ion reaction in detail • low invariant mass electromagnetic probes which may give (after a lot • of efforts on experimental as well as on theoretical side) as well information • on the high density region Symposium on the Physics of High Baryon Density

10. Neither experiment nor theory is prepared for this energy !! • Why? • Theory ( lets assume that we do not create quark matter): • All present transport theoriesare 15 years old • are designed to work on computers of that time • used the experimental and theoretical knowledge of that time • Fortunately there was progress in theory and in experiment: • - Experiments told us that 2 to 2 collisions are not sufficient (as in URQMD) •  we need 2 to N collisions • Detailed balance requires then to have then also N to two collisions •  enhanced production of heavy hadrons at high density ( Greiner) • The propagation of resonances has been studied ( Bratkovskaya) • theory as well as detailed comparison with experiments have taught us that • hadrons change their properties in the medium (up to now this is included • in a very rudimentary way only. • To construct a transport theory in which all this is built in consistently is a • tremendous work and I do not see even the possibility to finance such an • Adventure. Symposium on the Physics of High Baryon Density

11. Hadrons change in matter Mass change is more important for increasing ρ than for increasing T and more for baryons than for (pseudoscalar) mesons. ρ=0 Gastineau et al. to be publ. (NJL) T=0 NJL T=0 Symposium on the Physics of High Baryon Density agrees with Brückner

12. But even if we find the theorists who will do this : Transport theories need inputs: cross sections interactions In the past it was very often the different input which causes difference of the results: And if we want to study the high density zone we have first to understand what is going on at low densities. This is a tremendous work for experimentalists Symposium on the Physics of High Baryon Density

13. Without experimental NN data no predictive power for AA Example: HADES or what can we learn from e+e- spectra in AA collisions? First observation: Simulation give the right trend But why seemingly identical theories give a different mass yield above M = 0,2? Because the input cross sections differ due to lack of experimental results lack of theory Symposium on the Physics of High Baryon Density nucl-exp/0608031

14. η production σ(np η) = σ(pp η) • No data σ(np η) in the region • of interest • No theory available • Different parametrization in the different transport theories Different results Last data point for np σ(np η) = 2 σ(pp η) Sensitive region World data Symposium on the Physics of High Baryon Density

15. In momentum space the situation is even more complex The η is produced by 30% in pp  ppη according to 3 body phase space 70% in pp N*(1535)+p collision in the decay of the N*(1535) This is clearly visible in the momentum spectrum of the η Phase space and N*(1535) decay Phase space None of the present transport theories includes this Symposium on the Physics of High Baryon Density

16. No data for σ( np  ω) Theory predicts: σ(np  ω) = 5 σ(pp  ω) if vector meson exchange dominates This is incompatible with σ(np dω) which gives an enhancement factor of 1-2.5 In addition: Coupling to N(1535) (gallmeister et al.) nucl-th/0608025 or N*(1700) (tsushima et al) nucl-th/0304017 induce strong in medium dependence. Change of the ω mass Δm  50 MeV at ρ/ρ0 = 0.6 reported in γA coll. (nucl-ex/0405010) Consequences of all this on AA coll is unknown ω production Situation worse for ω  simulation programs may differ more sensitive region World data Symposium on the Physics of High Baryon Density

17. No wonder that predictions in the ω regionare up to a factor of 4 off the data and quite different in the different simulations. • We cannot distinguish whether NN input is wrong or whether we have an interesting physics phenomenon A bit more provocative: If we cannot do this why spent the time for simulations? Symposium on the Physics of High Baryon Density

18. What is the lesson? • even the best transport theory looses its predictive power • if the elementary cross sections are not known • One cannot learn new physics from the HADES C+C data without having detailed pn and pA data. One can only conclude that with some reasonable assumptions data agree with theory. • Seemingly identical theories give different results • Very bad image to people outside the community Consequence: The heavy ion program at CBM has to be accompanied by a NN and by a pA program if one want to enable theory to predict something or in other words if one wants to understand the physics. Symposium on the Physics of High Baryon Density

19. And if we have created a plasma? As shown a dull looking excitation function may hide very interesting physics (like a phase transition) which reveals itself in more sophisticated observables. It is not excluded that we produce a QGP here or here Symposium on the Physics of High Baryon Density

20. Then we have to find a transport theory which can simulateneously describe the quark and the hadron phase Candidate for exploratory studies: Nambu-Jona-lasinio Same phase diagram as QCD, hadrons bound quark states Symposium on the Physics of High Baryon Density

21. Mesons are obtained as pole of the qqbar propagator qqbar scatt. by meson exchangeqqbar scattering by qqbar loops (RPA) • Both scatterings are formally identical  determination the meson mass as • the pole of the propagator • It depends on μ and T because the quark propagator depends on these quantities • The few parameters of the theory are adjusted to reproduce free meson properties: • Both limits (T=μ=0) and T and μ  reproduced for the q sector Symposium on the Physics of High Baryon Density

22. From quarks to mesons Meson lighter than its constituents  stable s u,d s+u Via hadronisation cross sect. 2u K π Symposium on the Physics of High Baryon Density

23. First surprise: Cross sections become very large For each sqrt(s) there is a temperature for which the cross section becomes very large ( s-channel resonance) Much larger than pQCD cross sections  system becomes liquid hadronisation very effective most of history before becomes hidden Calculated for μ =0 Symposium on the Physics of High Baryon Density Gastineau et al., PRL

24. For a CERN initial condition all quarks hadronize Due to the large cross sect. Initial distribution of q and π Finite size effect Mass at the surface is high Symposium on the Physics of High Baryon Density

25. If one makes a transport • theory based on the • Nambu Jona-Lasinio • one can simulate • (qualitatively) how the • system passes the phase • transition • First results: • Finite size effects are • very important: • In a fast expanding system • hadronization takes not place • at one given temperature • but the temperature varies by • 20% • Masse of particles has as well a distribution Temp. at creation π mass when created Symposium on the Physics of High Baryon Density

26. In this model information from the plasma phase is carried to the hadronic phase. Example: average momentum of K and π reflect the momentum of s and u,d quarks in the plasma phase Symposium on the Physics of High Baryon Density

27. The dull looking spectra in the CBM energy region may • hide very interesting physics. • To explore this physics we need • - pp, pn and pA data for the elementary processes • (without them we will learn nothing) • improved transport theories which include the state • of the art many body results. This requires manpower • It is by far not sure whether we stay in the hadronic • If we enter the plasma regime interesting physics awaits • us but the challenge for transport theories are tougher. • First ideas are around how to model this but it will be a • long way until they can give quantitative reliable results. Symposium on the Physics of High Baryon Density