1. 417 th WE-Heraeus-Seminar Characterization of the Quark Gluon Plasma with Heavy Quarks Physikzentrum Bad Honnef June 25-28, 2008 Ralf Averbeck, Heavy-Flavor Cross Sections at RHIC

2. R. Averbeck, 2 June 26, 2008 l charm and bottom from hadronic collisions l m c ~1.3 GeV, m b ~4.5 GeV hard process (m q >>  QCD ), even at low p T open heavy flavor (D,  c, B,  b ) quarkonia (J ,  l heavy-ion collisions l heavy quarks are produced before the medium is formed Introduction D mesons,  ’,  l investigating QCD matter with hard probes l well calibrated in pp collisions l slightly affected and well understood in hadronic matter l strongly affected in a partonic medium l today's focus: calibration at RHIC vacuum hadronic matter QGP

3. R. Averbeck, 3 June 26, 2008 l hadronic decay channels D 0  K  (BR: ~4%) D 0  K  0 (BR: ~14%) D ±  K  (BR: ~10%) l  c  pK  (BR: ~5%) How to measure open heavy flavor K+K+ -- l advantage l unambiguous identification, i.e. a peak in invariant mass l disadvantages l difficult to trigger l huge combinatorial background l improvement? –resolve decay vertices –charm: c  ~ 100-200  m –bottom: c  ~ 400-500  m  silicon vertex detectors

4. R. Averbeck, 4 June 26, 2008 l semileptonic decay channels l D 0  lX (BR: ~7%) D ±  lX (BR: ~17%) l  c  lX (BR: ~5%) l B 0,±  lX (BR: ~11%) How to measure open heavy flavor K+K+ -- l advantages l 'straight forward' trigger l no combinatorial BG l disadvantages l need to control/subtract background from other lepton sources l loss of kinematic information l continuum  can NOT disentangle c & b with single leptons only

5. R. Averbeck, 5 June 26, 2008 2 central electron/photon/hadron spectrometer arms:  0.35 p  0.2 GeV/c PHENIX & STAR at RHIC 2 forward muon spectrometers: 1.2 < |  | < 2.4 p  2 GeV/c l muons in forward arms l tracking l muon ID: l “absorber” l electrons in central arms l tracking l electron ID: l RICH + EMC large acceptance  tracking detector: TPC l hadrons: l TPC (dE/dx) l Time-of-Flight detector l electron ID: l EMC in addition PHENIX optimized for leptons but can do hadrons STAR optimized for hadrons but can do leptons

6. R. Averbeck, 6 June 26, 2008 l MANY electrons sources l Dalitz decay of light neutral mesons –most important    →  e + e - –but also:  ’  l conversion of photons –main photon source:    →  –in material:  → e + e - l weak kaon decays –K e3, e.g.: K ± →   e ± e l dielectron decays of vector mesons –  → e + e - l direct/thermal radiation –conversion of direct photons in material –virtual photons:  * → e + e - l heavy flavor decays  need excellent BG subtraction! e ± from heavy flavor: difficulties PHOTONIC e ± NON-PHOTONIC e ± electrons are rare: e ± /  ± ~ 10 -2  need excellent PID!

7. R. Averbeck, 7 June 26, 2008 Cocktail subtraction l ALL relevant background sources are measured l calculate e ± BG l BG subtraction  e ± from heavy-flavor decays l performance limited by signal/background ratio l works well towards high p T –good for measurement of e ± spectra l difficult towards low p T –limited use for measurement of total cross sections PRL 96(2006)032001 p+p @ √s = 200 GeV

8. R. Averbeck, 8 June 26, 2008 PRL 97, 252002 (2006) p+p @ √s = 200 GeV Converter subtraction l converter (known X/X 0 ) added for part of the run l converter multiplies photonic BG by KNOWN factor  difference between converter in & out runs MEASURES photonic BG l performance limited by statistics in converter run l works well towards low p T –good for total cross section measurement l difficult towards high p T l excellent agreement between methods!

9. R. Averbeck, 9 June 26, 2008 PRL 97, 252002 (2006) l total cross section l  cc = 567  57(stat)±224(sys)  b e ± from heavy flavor in p+p ( √s=200 GeV ) l non-photonic e ± from c  e ± and b  e ± l comparison with FONLL calculation –Fixed Order Next-to-Leading Log perturbative QCD (M. Cacciari, P. Nason, R. Vogt PRL95,122001 (2005)) –data ~ 2 x FONLL –seen also in charm yields at »DESY (photoproduction) »FNAL (hadroproduction) –consistent within large uncertainties l high p T : b is important!

10. R. Averbeck, 10 June 26, 2008 Background subtraction in STAR l photonic e ± BG in STAR l dominant source –photon conversions –mainly in Si detectors near vertex –conv. / Dalitz ~ 5 –compare with PHENIX: conv. / Dalitz ~ 0.5 l subtraction –large acceptance TPC –reconstruction and subtraction of conversion and Dalitz pairs (efficiency: ~ 70-80% for p T > 4 GeV/c) –remaining BG: cocktail

11. R. Averbeck, 11 June 26, 2008 l ratio of heavy-flavor e ± spectra to FONLL l PHENIX –spectral shape of e ± agrees with FONLL –total cross section above FONLL by a factor ~2 l STAR –shape consistent with PHENIX and FONLL –total cross section above FONLL by a factor ~4 l systematic uncertainties in pQCD are large, i.e. a factor ~2 (or even ~4: R. Vogt hep-ph/0709.2531 ) PHENIX vs. STAR vs. FONLL

12. R. Averbeck, 12 June 26, 2008 PRL 98, 172301 (2007 ) Hot matter: Au+Au at √s NN =200 GeV PRL 98, 172301 (2007) l binary scaling of total e ± yield from heavy-flavor decays  hard process production and no destruction (as expected) l high p T e ± suppression increasing with centrality l footprint of medium effects; similar to  0 (a big surprise)

13. R. Averbeck, 13 June 26, 2008 Hot matter: Au+Au at √s NN =200 GeV l STAR & PHENIX: consistent in nucl. modification factor R AA l normalization discrepancy does NOT depend on system size! l high p T e ± suppression - a challenge for models l what about bottom?  need additional observables to address these issues!

14. R. Averbeck, 14 June 26, 2008 l D 0  K  invariant mass analysis l main problem: S/B ratio << 1/100  need huge stat. (yield uncertainty ~ 40-50%) l currently limited to p T ≤ ~3 GeV/c –reasonable for total cross section –insufficient to address high p T suppression D-meson reconstruction in STAR PRL 94(2005)062301 A. Shabetai, QM'08 arXiv:0805.0364

15. R. Averbeck, 15 June 26, 2008 l muon identification at low p T (~0.2 GeV/c) l Time-of-Flight and dE/dx in the TPC Low p T muons in STAR l subtraction of BG from  and K decay l distance of closest approach of tracks to primary vertex l low p T muon yield l sensitive to total charm cross section l insensitive to spectral shape

16. R. Averbeck, 16 June 26, 2008 l combined fit to e ±,  ±, D 0 l data are consistent Total charm cross section in STAR l binary scaling of charm yield l total charm cross section ~ 1 mb l ~ 4x pQCD value (still within huge uncertainties) l ~ 2x PHENIX value

17. R. Averbeck, 17 June 26, 2008 Charm and bottom from e + e - pairs l e + e - inv. mass after background subtraction compared to cocktail l absolutely normalized l excellent agreement l charm & bottom accessible after subtracting the cocktail charm: integration after cocktail subtraction  cc = 544 ± 39 (stat) ± 142 (sys) ± 200 (model)  b from single e ± :  cc = 567  57(stat)±224(sys)  b simultaneous fit of charm and bottom:  cc = 518 ± 47 (stat) ± 135 (sys) ± 190 (model)  b  bb = 3.9 ± 2.4 (stat) +3/-2 (sys)  b l bottom irrelevant for total e ± yield, but crucial at high p T ! arXiv: 0802.0050

18. R. Averbeck, 18 June 26, 2008 l electron – kaon charge correlation l D decay  unlike-sign eK pairs l B decay  mostly like sign eK pairs (with small (1/6) admixture of unlike-sign pairs) l approach –eh (for higher statistics) invariant mass –subtract like-sign pairs from unlike-sign pairs –disentangle charm and remaining bottom contribution via (PYTHIA) simulation of charm and bottom decay kinematics Separating c  e from b  e (I) l the key: electron-hadron correlations l charm and bottom are different

19. R. Averbeck, 19 June 26, 2008 l electron-hadron azimuthal angle correlations l small angle (near side)  electron and hadron are from the same decay l width of near side correlation: largely due to decay kinematics l B decay has larger "Q value" than D decay l approach –eh azimuthal angle correlation for B and D decays from PYTHIA –fit measured correlation with B/(B+D) as parameter Separating c  e from b  e (II) l the key: electron-hadron correlations l charm and bottom are different

20. R. Averbeck, 20 June 26, 2008 l electron-D 0 correlations l trigger on e from heavy-flavor decay l use D meson (reconstructed in hadronic decay) as a probe l investigate eD correlation in azimuth Separating c  e from b  e (III) l the key: electron-hadron correlations l charm and bottom are different

21. R. Averbeck, 21 June 26, 2008 l e from b / e from c ≥ 1 for p T ≥ 6 GeV/c l PHENIX & STAR: consistent with FONLL B contribution to e ± spectra l not precise enough to extract b suppression l need vertex detectors to measure charm and bottom hadrons!

22. R. Averbeck, 22 June 26, 2008 l high p T muons in PHENIX: 1.2<|  |<2.2 l again, background subtraction is difficult Rapidity dependence of charm production

23. R. Averbeck, 23 June 26, 2008 l charm yield similar at mid and forward rapidity l large uncertainties everywhere l better data are needed  measurement of displaced vertices Rapidity dependence of charm production

24. R. Averbeck, 24 June 26, 2008 l charm (& bottom) are crucial probes for the medium produced in HI collisions @ RHIC l even calibration measurements are difficult  large uncertainties l charm cross section / binary collision l binary scaling is observed in STAR & PHENIX l but the cross sections differ by a factor ~2 Summary from e, , D from e, e + e -

25. R. Averbeck, 25 June 26, 2008 l complete systematics of existing observables l PHENIX –e ± from d+Au & Cu+Cu –D reconstruction in p+p (D 0  K +  -  0 ) –heavy flavor from e-  pairs Outlook: near future photonic background reduced by factor ~10 l STAR –improved e ± data from running without inner silicon detectors X. Dong, Hard Probes '08

26. R. Averbeck, 26 June 26, 2008 l silicon vertex trackers for unambiguous resolution of displaced vertices  direct D- and B-meson measurements Outlook: longer term future PHENIX STAR