1. Spectator response to participants blast - experimental evidence and possible implications New tool for investigating the momentum- dependent properties of nuclear matter. Vladimir Henzl GSI-Darmstadt, Germany

2. Outline Introduction & motivation: properties of nuclear matter: and relevance for astrophysics static vs. dynamic properties: peculiarities of investigated systems standard tools of investigation: collective flows, kaon production,... Spectator response: original idea and theoretical predictions: first observation: reacceleration of fragmentation residues Results of dedicated experiments: Summary & Outlook spectator response in Au+Au @ 1 A GeV spectator response in Au+Au,Al @ 500 A MeV: comparison with BUU calculations Experimental approach: possibilities of FRS

3. Astrophysical interest: evolution of the early universe supernovae explosions formation and stability of neutron stars influenced by properties of the nuclear matter (NM) under extreme conditions (high T, P, ρ, …) Properties of the NM: static – (in)compressibility, phase transitions, excitation… dynamic – viscosity, momentum dependence of the mean field … General motivation

4. Static vs. dynamic properties Problem: most of the experimental observables are not selective; Static properties are studied in dynamical processes !!! Aichelin et al. PRL 58(1987)1926 => the interpretation is influenced by competing phenomena, !!! The results are very often ambiguous !!!

5. Present knowledge Recent analysis: (Danielewicz et al.) Science 298 (2002) 1592 Attempt to constrain nuclear matter equation of state by results of performed experiments. Only the most extreme models could be excluded by the experiment

6. Tools to investigate the nuclear matter BUU calculations : 124 Sn + 124 Sn T lab = 800 MeV/u b = 5 fm L. Shi, P. Danielewicz, R. Lacey, PRC 64 (2001) the spectator is not a passive witness, but rather a victim of violent participants ! Standard tools: Spectator response: eliptic flow, radial flow, transverse momentum, (anti)kaon production, … L. Shi, P. Danielewicz, R. Lacey, PRC 64 (2001) M.V.Ricciardi et al. PRL 90(2003)212302 ! New !

7. Spectator response to the participant blast

8. Spectator response or „what can we learn from victims“ Theoretical prediction: (Shi, Danielewicz, Lacey) 1)Net momentum change depends on nonlocality of nuclear mean field. 2)Net momentum change is almost insensitive to the stiffness of the EoS. Spectator response is selectively sensitive to nonlocal properties of nuclear mean field !!! P CMS /A = 682 MeV/c

9. Observation 3) to study possible dependence of spectator response on incident energy and/or size of the colliding system 2) to establish correlation of A res and impact parameter b 1) to improve experimental signature of spec. response Idea BUU calculations : 124 Sn + 124 Sn T lab = 800 MeV/u b = 5 fm L. Shi, P. Danielewicz, R. Lacey, PRC 64 (2001) T.Enqvist et al. NPA658(1999)47BUU by V.H. Motivation for new experiments: Proposal of new experimental program: 197 Au+ 197 Au @ 0.5 and 1.0 A GeV... aproved and carried out in 2004

10. Experimental approach

11. Experiment at the Fragment Separator Once mass and charge are identified (A, Z are integer numbers) the velocity is calculated from B  : A/∆A ≈ 400 Inverse kinematics: From ToF:  /∆  ≈ 400  /∆  = B  / ∆ B  ≈ 2000 (36m) => very precise determination!

12. Characteristics of the data unambiguous identification & precise longitudinal momenta only one fragment in one reaction measured Au+Au@1 A GeVPb+p@1 A GeV limited acceptance: ±15mrad, ±1.5% in momentum

13. Velocity distributions with limited acceptance 136 Xe+ nat Pb @ 1 A GeV (D.Henzlova) limited momentum acceptance: Several magnetic field settings need to be combined to get complete velocity distribution (each color = 1 magnetic setting)

14. fission Information from full acceptance experiments fission 197 Au+ 197 Au @ 1 A GeV - ALADIN 238 U+Cu @ 1 A GeV - ALADIN  single, unambiguously identified fragment at FRS predominantly largest residue per collision  single, unambiguously identified fragment at FRS is predominantly largest residue per collision fragmentation

15.  the largest fragment is well correlated with Z bound (for Z max >30 ~ A max >65) Information from the Z max Aladin data: Au+Au @ 400, 600, 800, 1000 A MeV  peripheral collisions investigated Aladin data: Au+Au @ 600 A MeV  fragments with Z>20 are produced in reactions with b ≥ 9fm (in Au+Au system) Z max carries information on impact parameter  Z bound is a measure of the impact parameter

16. Different reaction processes fission How to distinguish fragmentation and fission? 238 U (1 A GeV) + Pb Fragmentation: heaviest residues fully accepted (A>90) Fission: Only forward and backward component accepted 197 Au+ 197 Au @ 1 A GeV (V.H.)

17. Results of dedicated experiments

18. First dedicated experiment most peripheral collisions yield deceleration residues with A res ≤ 85 on average faster than the beam mean velocities in agreement with Morrissey systematics. with decreasing mass loss, velocities level off and increase V.H.: PhD thesis (in preparation)

19. More dedicated experiments reacceleration smaller with smaller incident energy reacceleration smaller in smaller reaction systems all systems yield clearly visible net reacceleration Reacceleration and its behavior is an experimental fact ! Interpretation possible only with help of calculations V.H.: PhD thesis (in preparation)

20. New experimental data => new simulations BUU calculations with MD mean fields qualitatively agree with experiment (no sensitivity to stiffness of EoS) BUU calculations with static MI field are not able to describe trend of the data BUT: Correlation of A res with impact parameter b nontrivial !!!

21. How to relate A res and an impact parameter Z max2 has different (and worse) correlation with Z bound (impact par.) Aladin: Region of mixing Z max & Z max2 can‘t be used to deduce b !

22. Only data in blue area suitable for extraction of impact parameter !!! New experimental data => new simulations Calculations with MD MF induce gain of the momenta, but for too low impact parameters with respect to the experiment !!! Open question: which parameters of BUU can influence the spectator response?... one possible idea...

23. Summary & outlook

24. Summary & Outlook More BUU calculations on the way, many more needed Outlook : further experiments & simulations needed to constrain MD properties of nuclear mean field Summary : reacceleration phenomena seen in all systems, its strength depends on incident energy and size of the colliding system only BUU with MD MF induce recovery of the fragment velocities with decreasing impact parameter, in qualitative agreement with the experiment longitudinal velocities of fragmentation residues measured in Au+Au @ 0.5, 1 A GeV, Au+Al @ 0.5 A MeV Au+Au/Al @ 1/0.5 A GeV: data analysis finished, interpretation in progress 112,124 Sn+ 112,124 Sn @ 1 A GeV: experiment in preparation - 2005/2006

25. Comparison with ALADIN

26. Information from full acceptance experiments fission 197 Au+ 197 Au 1AGeV - ALADIN  single, unambiguously identified fragment at FRS predominantly largest residue per collision  single, unambiguously identified fragment at FRS is predominantly largest residue per collision fragmentation