What transport theories do Problems with the input of transport

1. Dilepton production in pp and AA a challenge for transport and experiment J. Aichelin, E. Bratkovskaya, M. Thomère , S. Vogel and M.Bleicher What transport theories do Problemswith the input of transport Hadesdilepton data - can transport reproduce the HI data? - does a medium modify the spectra? Whatcanwelearnfrom the present data (and whatremainsunknown)

2. What transport theoriescan do and whattheycannot do Transport theories study the time evolution of heavy ion reactions by following the (curved) trajectories of nucleons created by their mutual potential interactions and including their Fermi motion and collisions They can model: - when and where a collisions takes place ( ) for given σ tot - whether the collisions are allowed (Pauli blocking) - the angular distribution (if dσ/dΩ is known) - the density and temperature at which a collision occurs They can predict all observables Whattransport theoriescan do and whattheycannot do BUT THEY CANNOT PREDICT THE ELEMENTARY CROSS SECTIONS These are input quantities: either theory or experiment

3. They are used to investigate -Reactionswhichexistonly in a medium (ΔN -> K+NΛ) - Medium properties of particles (ρ , K- , K+ ) and their cross sections - Nuclear matter properties (EOS, momentum dependence of NN potential) - Collective phenomena like in plane and elliptic flow, (hyper)nuclei prod. In the past it turned out that different results from transport theories are usually a consequence of different input quantities (different parametrizations of unknown cross sections etc). The complicated transport itself is well under control. As far as dileptons as concerned: beautiful data + established transport (which reproduce the whole strangeness sector of HADES) So why it is challenging to calculate dilepton production?

4. Dilepton predictions in transport pose a couple of problems already in pp the dilepton spectra is a superposition channel separation is experimentally difficult most of the channels little known for energies of interest (and each channels translates differently to HI) for npchannel very few data pd data only of limited use but HI have neutrons (bremsstrahlung) So the challenge is to explain a verycomplicated exit channel withouthavingsufficientknowledge about the simple ones.

5. Input of the transport theories: from the energyunder control ( pp @ 1.25 GeV) to the realm of speculations (pp @ 3.5 GeV)

6. ppreactionsat 1.25 GeV For pp at 1.25 GeV the situation is under control: single πproduction dominates σineliswellknown πdata compatible with isobar model (all π’s produced via Δ) NN ->Δ ->NNπ This energyis the cleanest for for studying theΔ channel. But phase spacelimits the production of high mass Δ Thusneither sensitive to ΓΔnor to the electromagneticdecaywidth dΓ/dM IQMD HSD

7. pnreactionsat 1.25 GeV πyield in pnisknown but Bremsstrahlung more important than Δ Dalitzabove M >0.15 Little guidance from data More essential: Tagged pdis NOT the same as pn Easy to verify: HSD HSD IQMD is not equal to pn pd Diff. pn and pn(d) not exploredneithertheor. norexp. Kinematiclimit

8. pp reactionsat 2.2 GeV Going up in energy the complications increase severalchannelscontribute (M<0.6 GeV:Δ,η, bremsstrahlung) for most of themonlylimitedexperimental information available Here I discuss the 2 dominant channels : Δ and η

9. Between 1.5-2GeV:twoπ production starts to dominate origin of π’s and henceΔ production ratherunknown mostrecent data: Celsius/WASA, theory: Oset group • PLB679 (09) 30 • PLB695 (11) 115 • NPA633(98) 519 • Below T= 1.5 • dominantlyΔΔ • but also contributions from • N* • and • higher mass Δ • above T=1.5 GeV • unknown land

10. η production I: Excessenergydistr . in CC Excess energy in CC No data for np data pnη non trivial (N* and direct) and not known (Using CC η TAPS data is of limiteduse:Fermi, absorpt.)

11. In momentum space the situation is even more complex (and more informative) At T=2.85 GeVη 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 η’s Atotherenergiesrepartitionunknown Phase space and N*(1535) decay Phase space PRC69,064003 Presently only IQMD includes this. Very important for HI: Resonance contribution differs from pp due to finite lifetime (reabsorption).

12. η production III: • No quantitative theory available (coupling to N*’s) • Every transport theory has a different parameterization (2 or 3 body , different pn extrapolations) Different results (but in the error bars for the yield ) σ(np η) = 2 σ(pp η) σ(np η) = σ(pp η) BR=BR/10 World data Hades Collaboration Meeting Cyprus, Nov 2007

13. pp reactions for T > 2.2 GeV realm of speculations: - No theoryavailable - Nomeasurements of exclusive channelsavailable Not even right degrees of freedom are known Stillhadronic (n-dim phase space) or already string (longitudinal phase space)? Only 2 possibilities: either - Fit pp - extrapolate to pn - includingyour imagination about resonance (string) production - thenpredictpA or Wait for better (Hades) data whichmaylimit the almost absolutefreedom. HSD No solid information -> input of transport modelscandifferwildly and so do the results for pA and HI reactions.

14. Heavy Ion reactions seen by the three transport approaches

15. To understand heavy ion reactions we have to explore the uncertainties imposed by the elementary reaction input We can profite from the fact that in ratios of cross sections for different systems most of the uncertainties drop out (determination of the EOS) Problem: elementary data and HI data are not taken at the same energy -> we have first to assure to reproduce the data and then extract the physics from calculations at the same energy.

16. Heavy Ions around 1 AGeV HSD IQMD Samepnbremsstrahlung parametrization HSD & IQMD: similar CC spectraat 1 AGeV (dileptonspectrawasevenpredicted) - Input based on experiments and - HI dynamics (not trivial) controlled by many HI data analyzed by HSD and IQMD HSD IQMD

17. Heavy Ions around 2 AGeV UrQMD HSD No bremsstrahlung All 3 welltested transport models HSD, IQMD, UrQMD agree on first glancewith the data But a detailed look revealsdifferences: UrQMD:too few η, toomanyρ, no bremsstrahlung IQMD: toomanyω (σ(np->ω)=5σ(pp->ω) IQMD

18. At 2 GeV C+C same observation bothapproachesagreewellwith data however channeldecomposition not identical Sum over differentchannels washes out the differences IQMD HSD

19. Whatreveal the data about the medium?

20. Best access: RAA : HI resultsdivided by scaled NN Complextask: wefollowexactly the expanalysis Fermi motion difference p(d) and pn Ratio compatible with 1 for M < .45 HSD ratio around 2 For .12 < M < .325

21. Ratio AA/NN >1 if E/N the same even for CC 2AGeV/ NN 1.25 GeV Onlywhenapplying (exp) 1D –transformation transport results compatible with 1 ratio ArKCl/NN > CC/NN HSD Resultsof differenttheoriesin betweenerrorbars IQMD HSD

22. In medium enhancementsurprising? Not really !! Bremsstrahlung ~ number of pn collisions -> ratio >1 final πmultiplicity ~ number of participants but not ~ number of producedΔ and eachΔcanemitdileptons enhancementincreaseswith mass for Au+Aureactions≈ 4!! but littlewithenergy. Bremsstrahlung Δ - Dalitz

23. Bass PhDthesis1997: long N -> Δ -> π -> Δ -> π -> Δ ->…cycle Only 20 % of the producedΔ create a final state π but all producedileptons Strongenhancement of the dileptonyield in AA Au+Au 1 AGeV

24. Is this observation robust? against modifications of ΓΔ against modification of dΓ/dM The final dileptonspectraisgiven by: Spectral fctelectrom.decaywidth identical

25. Δ spectral function HSD: Monitz UrQMD: Bass But phase spacesuppresses the differences in HI reactionsat SIS energies

26. The differentdecaywidthsgivedifferentdileptonspectra for MΔ≠ MΔ Pole HSD,IQMD,URQMD: Wolf param. differentΓΔgivesimilarspectra different dΓ/dMgivedifferent spectra ΓΔof spectral fct of decaywidthcancel

27. … but ΔDalitzisonly one of the decaychannels HSD changes of the electromagndecaywidth are almost invisible in the total yield. What’s about ratios?

28. HSD with Wolf and Krivoruchenko electromag. decaywidth yieldssimilarresults for ratio HSD and IQMD use different Δwidths. In medium enhancementalso not verydifferent In medium enhancementdoes doeslittledepend on the explicit form of ΓΔ and dΓ/dM Present data do not allow for fixing the electrom. form factor HSD IQMD

29. What HI tell us about ΓΔand dΓ/dM ? Atlowenergy DileptonsfromΔ are a prominantchannel but phase spacelimits the contribution of high mass Δ ->insensitive to Wolf/ Krivoruchenko, insensitive to ΓΔ Athigherenergies: High mass Δ -> yielddiffersfor Wolf/ Krivoruchenko but dileptonsfromΔ are not a prominantchannel ->Influence of electrom. FF on the total yieldissmall. So itwillbedifficult to use dileptons to nail down the Δproperties in detail

30. Conclusions HADES dilepton data in AA reveal for the first time the the existence of the N -> Δ -> π -> Δ -> π -> Δ ->.. chain Results on dileptons of transport modelsonlymodestly sensitive to input quantitieslikeΓΔ and dΓ/dM. To discover more from the data weneedelementary cross sections for np -> η, ω, Δ, bremsstrahlung We are in a veryinterestingenergydomain: - transition fromhadrons to quarks as degrees of freedom. - controlledstudy of vectormesons in matter