1. Ralf Averbeck Department of Physics & Astronomy High Energy Dilepton Experiments Experiments @ RHIC Future Outlook

2. Ralf Averbeck, 2 l RHIC = Relativistic Heavy Ion Collider l located at Brookhaven National Laboratory RHIC

3. Ralf Averbeck, 3 RHIC and its experiments l what’s so special about RHIC? l it’s a collider –no thick targets! –detector systematics do not depend on E CM ! l p+p: √s ≤ 500 GeV (polarized beams!) l A+A: √s NN ≤ 200 GeV (per NN pair) STAR l experiments with specific focus l BRAHMS (until Run-6) l PHOBOS (until Run-5) l multi purpose experiments l PHENIX l STAR l all experiments are crucial!!

4. Ralf Averbeck, 4 PHENIX in practice

5. Ralf Averbeck, 5 PHENIX in principle l 3 detectors for global event characterization two forward muon spectrometers l forward spectrometers l muon measurement in range: 1.2 < |  | < 2.4 p  2 GeV/c l central spectrometers l measurement in range:  0.35 p  0.2 GeV/c two central electron/photon/hadron spectrometers

6. Ralf Averbeck, 6 102110 Total baryon density 8.6 21.4 33.5 85 p – p participants nucleons (p – p )A/Z 20.1 80.4 6.2 24.8 dN( p ) / dy produced baryons (p, p, n, n ) RHIC (Au-Au) SPS (Pb-Pb) Low mass e + e - : prospects @ RHIC l 2 scenarios @ SPS profit from high baryon density –dropping  mass –broadening of  l what to expect at RHIC? l baryon density: almost the same at SPS & RHIC (although the NET baryon density is not!)

7. Ralf Averbeck, 7 e- e+ e + e - : theoretical guidance at RHIC R. Rapp: nucl-th/0204003 l in-medium modifications of vector mesons persists l open charm contribution becomes significant

8. Ralf Averbeck, 8 The founding fathers’ view l before 1991 l proposals for various experiments at RHIC –STAR, TALES, SPARC, OASIS, DIMUON … –except for STAR everything else is burned down l from the ashes rises PHENIX –Pioneering High Energy Nuclear Interaction eXperiment l 1991: PHENIX “conceptual design report” l philosophy –measure simultaneously as many observables relevant for QCD phase transitions as you can imagine –all but one: low-mass dielectrons l why no dielectrons? –included in first TALES proposal –considered to be “too difficult” for PHENIX l a lot of work can make impossible things happen

9. Ralf Averbeck, 9 Au-Au collision as seen in PHENIX

10. Ralf Averbeck, 10 PC1 PC3 DC ee e+e+   PHENIX: tracking & particle ID

11. Ralf Averbeck, 11 l first attempt from 2002 Au-Au Run l S/B ~ 1/500 (!) for minimum bias events l not enough statistics l Au-Au data taken in 2004 l ~ 100x statistics l photon conversions reduced by factor 2-3 l expect background reduction by ~ 2 PHENIX measures dielectrons Real and Mixed e + e - Distribution Real - Mixed

12. Ralf Averbeck, 12  Signal to Background: S/B = 1 / 250 Low-mass e + e - pairs: the problem l electrons/event in PHENIX l N e = (dN/d  )  0 * (BR+CONV) * acc * f(p T >0.2GeV) 350 (0.012+0.02) 0.5*0.7 0.32 = 1.3 l combinatorial background pairs/event l B = ½ * ½N e 2 e -N = 0.1 l expected signal pairs/event (m>0.2GeV, p T >0.2 GeV) l S = 4.2*10 -4  signal/background l as small as 1/ few hundred l depends on mass l what can we do to reduce the background?

13. Ralf Averbeck, 13 Conversion/Dalitz rejection? l typically only one “leg” of the pair is in the acceptance l acceptance holes l “soft” tracks curl up in the magnetic field l only (!) solution l catch electrons before they are lost l need new detector and modification of magnetic field

14. Ralf Averbeck, 14 Consequences of poor S/B l how is the signal obtained? l unlike-sign pairs: F l combinatorial background: B (like-sign pairs or event mixing) l  S = F – B l statistical error of S l depends on magnitude of B, not S l  S ≈ √2B (for S<<B) l “background free equivalent” signal S eq l signal with same relative error in a situation with zero background l S eq = S * S/2B l example: S = 10 4 pairs with S/B = 1/250  S eq = 20 l systematic uncertainty of S l dominated by systematic uncertainty of B l example: event mixing with 0.25% precision (fantastic!)  ~60% systematic uncertainty of S (for S/B = 1/250)

15. Ralf Averbeck, 15 Combinatorial background l ingredients for the battle plan l PHENIX: 2 arm spectrometer –dN like ≠ dN unlike  different shape  need event mixing l analyze pairs –unlike sign (N +- ) and like sign (N ++ and N -- ) l produce mixed events –unlike-sign pairs (B +- ) and like-sign pairs (B ++ and B -- ) l normalize mixed events properly (2√N ++ N -- ) l and be careful to: –do the pair analysis identically in real and mixed events –mix only events with the same topology (centrality, vertex) –remove detector artifacts –remove unphysical correlations –use like sign pairs as cross check for the normalization l two years later …..

16. Ralf Averbeck, 16 Background shape: like sign --- Foreground: same evt N++ --- Background: mixed evt B++ RATIO l small signal in like sign pairs at low mass l from double conversion or Dalitz+conversion l normalize B ++ and B – to N ++ and N – for m > 0.7 GeV l  very good agreement in shape

17. Ralf Averbeck, 17 Background normalization: 2√N ++ N - - --- Foreground: same evt --- Background: mixed evt TOTAL SYSTEMATIC ERROR = 0.25% l uncertainties l statistics of N ++ and N -- : 0.12 % l different pair cuts in like and unlike sign: 0.2 %

18. Ralf Averbeck, 18 inclusive conversions conversions removed Conversion rejection l artifact of PHENIX tracking l assume that all tracks originate from the vertex l off vertex tracks  wrong momentum vector  conversions are reconstructed with m≠0 (m~r)  need to be removed since affect low-mass region  how? z y x e+e+ e-e- B conversion pair z y x e+e+ e-e- B Dalitz decay l conversions “open” in a plane perpendicular to the magnetic field

19. Ralf Averbeck, 19 Data: like Monte Carlo: Cross Like Cross Unlike l  0   *  e + e - e + e - X unlike cross like cross unlike 4-body yield in 4  yield in acceptance Subtraction of “cross” pairs

20. Ralf Averbeck, 20 submitted to Phys. Rev. Lett arXiv:0706.3034 Raw unlike-sign mass spectrum l put it all together l a powerful cross check: l additional converter  2.5 times more combinatorial background

21. Ralf Averbeck, 21 submitted to Phys. Rev. Lett arXiv:0706.3034 Cocktail comparison l low-mass continuum: enhancement l intermediate mass continuum: PYTHIA agrees with data?

22. Ralf Averbeck, 22 submitted to Phys. Rev. Lett arXiv:0706.3034 Comparison with theory l calculations for minimum bias collisions l our “favorite” scenarios l thermal radiation from QGP is included in addition l clear enhancement above cocktail l large uncertainties  not conclusive regarding in-medium  modification R.Rapp, Phys.Lett. B 473 (2000) R.Rapp, Phys.Rev.C 63 (2001) R.Rapp, nucl/th/0204003

23. Ralf Averbeck, 23 Reference: dielectrons in p-p l very good agreement of data and cocktail l PYTHIA does NOT describe the charm contribution (was seen for single electrons as well)

24. Ralf Averbeck, 24 Comparison: p-p vs. Au-Au l binary scaling of p-p data to compare with Au-Au l suppressed: charmonia, charm, , ,  0 l enhanced: low-mass continuum

25. Ralf Averbeck, 25 Yield in different mass ranges 0-100 MeV:  0 dominated; scales approximately with N part 150-750 MeV: continuum; scaling? 1.2-2.8 GeV: charm dominated; scales with N coll l study centrality dependence of yields in these regions

26. Ralf Averbeck, 26 Centrality dependence l  0 production scales approximately with N part l expectation for low-mass continuum l if in-medium enhancement is related to  or qq annihilation  yield should scale faster than N part (and it does) l charm is a hard probe l total yield follows binary scaling (known from single e ± ) l intermediate mass yield shows the same scaling

27. Ralf Averbeck, 27 Summary l sorry, no conclusion yet! l PHENIX at RHIC l first dielectron measurements in HI collisions at a collider –despite low signal/background ratio –reasonably good statistics –unprecedented accuracy of combinatorial background calculation l observations at low dielectron mass –enhancement relative to the cocktail and to p-p –not enough precision to distinguish between models –enhancement increases faster than N part with centrality l observations at intermediate dielectron mass –PYTHIA doesn’t describe data in p-p collisions –PYTHIA does a reasonable job in min. bias Au-Au collisions –just a coincidence? –room for thermal radiation? l can these measurements be improved by collecting (much) more statistics with the existing apparatus?

28. Ralf Averbeck, 28 (Near) future precision e + e - measurement Top view of PHENIX magnet region Outer coil Inner coil Magnet yoke +/- field cancellation HBD l identification of dielectrons with small opening angle BEFORE one of the “legs” is lost l electron ID before the magnetic field l “full” acceptance electron detector  new field configuration  HadronBlindDetector (HBD)

29. Ralf Averbeck, 29 Hadron Blind Detector (HBD) signal electron Cherenkov blobs partner positron needed for rejection e+e+ e-e-  pair opening angle l Dalitz rejection via opening angle l identify e ± in field free region l veto signal e ± with partner l HBD concept l windowless CF4 Cherenkov detector l 50 cm radiator length l CsI reflective photocathode l triple GEM with pad readout l construction/installation 2005/2006 (repair 2007)

30. Ralf Averbeck, 30 Future l dielectron measurements in high energy HI collisions l go to even higher energy, i.e. maximum temperature  LHC l go back to lower energy, i.e. maximum baryon density  FAIR l stay at RHIC –HBD (and silicon vertex upgrades) for improved experiments at maximum RHIC energy –“low energy” program, i.e. use RHIC as a storage ring instead of an accelerator

31. Ralf Averbeck, 31 Projections for RHIC: high energy l impact of the HBD & modified B field at top energy l recorded collisions l 10 9 l 10 10

32. Ralf Averbeck, 32 Projections for RHIC: low energy l collision rates decrease with decreasing beam energy l ~40 Hz @ 8.6 GeV/u l 2 weeks run time gives ~50M events l HBD ‘eliminates’ sys. uncertainty l electron cooling in RHIC can increase the collision rate by a factor 10  ~500M events in 2 weeks  very promising!!!