Investigating Open Heavy Flavor Production and Background Subtraction in Heavy-Ion Collisions at RHIC

1. Heavy-Flavor Cross Sections at RHIC

2. D mesons vacuum , Y’, c hadronicmatter QGP Introduction • charm and bottom from hadronic collisions • mc~1.3 GeV, mb~4.5 GeV • hard process (mq >> LQCD), even at low pT • open heavy flavor (D, Lc, B, Lb) • quarkonia (J/y, U) • heavy-ion collisions • heavy quarks are produced before the medium is formed • investigating QCD matter with hard probes • well calibrated in pp collisions • slightly affected and well understood in hadronic matter • strongly affected in a partonic medium • today's focus: calibration at RHIC

3. K+ p- How to measure open heavy flavor • hadronic decay channels • D0 Kp (BR: ~4%) • D0  Kpp0 (BR: ~14%) • D±  Kpp (BR: ~10%) • Lc  pKp (BR: ~5%) • disadvantages • difficult to trigger • huge combinatorial background • improvement? • resolve decay vertices • charm: ct ~ 100-200 mm • bottom: ct ~ 400-500 mm •  silicon vertex detectors • advantage • unambiguous identification, i.e. a peak in invariant mass

4. K+ p- How to measure open heavy flavor • semileptonic decay channels • D0 lX (BR: ~7%) • D±  lX (BR: ~17%) • Lc  lX (BR: ~5%) • B0,±  lX (BR: ~11%) • disadvantages • need to control/subtract background from other lepton sources • loss of kinematic information • continuum  can NOT disentangle c & b with single leptons only • advantages • 'straight forward' trigger • no combinatorial BG

5. 2 central electron/photon/hadron spectrometer arms: |h|  0.35 p  0.2 GeV/c PHENIX optimized for leptons but can do hadrons STAR optimized for hadrons but can do leptons 2 forward muon spectrometers: 1.2 < |h| < 2.4 p  2 GeV/c PHENIX & STAR at RHIC • large acceptance (|h| < 1) tracking detector: TPC • hadrons: • TPC (dE/dx) • Time-of-Flight detector • electron ID: • EMC in addition • muons in forward arms • tracking • muon ID: • “absorber” • electrons in central arms • tracking • electron ID: • RICH + EMC

6. PHOTONIC e± NON-PHOTONIC e± e± from heavy flavor: difficulties • electrons are rare: e±/p± ~ 10-2  need excellent PID! • MANY electrons sources • Dalitz decay of light neutral mesons • most important p0→ g e+e- • but also: h, w, h’, f • conversion of photons • main photon source: p0→ gg • in material: g → e+e- • weak kaon decays • Ke3, e.g.: K± → p0 e±ne • dielectron decays of vector mesons • r, w, f → e+e- • direct/thermal radiation • conversion of direct photons in material • virtual photons: g* → e+e- • heavy flavor decays  need excellent BG subtraction!

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

8. PRL 97, 252002 (2006) p+p @ √s = 200 GeV Converter subtraction • converter (known X/X0) added for part of the run • converter multiplies photonic BG by KNOWN factor  difference between converter in & out runs MEASURES photonic BG • performance limited by statistics in converter run • works well towards low pT • good for total cross section measurement • difficult towards high pT • excellent agreement between methods!

9. PRL 97, 252002 (2006) • total cross section • scc= 567±57(stat)±224(sys) mb e± from heavy flavor in p+p (√s=200 GeV) • non-photonic e± from c  e± and b  e± • 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 • high pT: b is important!

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

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

12. PRL 98, 172301 (2007) PRL 98, 172301 (2007) Hot matter: Au+Au at √sNN=200 GeV • binary scaling of total e± yield from heavy-flavor decays  hard process production and no destruction (as expected) • high pT e± suppression increasing with centrality • footprint of medium effects; similar to p0 (a big surprise)

13. Hot matter: Au+Au at √sNN=200 GeV • STAR & PHENIX: consistent in nucl. modification factor RAA • normalization discrepancy does NOT depend on system size! • high pT e± suppression - a challenge for models • what about bottom?  need additional observables to address these issues!

14. A. Shabetai, QM'08 arXiv:0805.0364 PRL 94(2005)062301 D-meson reconstruction in STAR • D0 Kp invariant mass analysis • main problem: S/B ratio << 1/100  need huge stat. (yield uncertainty ~ 40-50%) • currently limited to pT ≤ ~3 GeV/c • reasonable for total cross section • insufficient to address high pT suppression

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

16. Total charm cross section in STAR • combined fit to e±, m±, D0 • data are consistent • binary scaling of charm yield • total charm cross section ~ 1 mb • ~ 4x pQCD value (still within huge uncertainties) • ~ 2x PHENIX value

17. charm: integration after cocktail subtraction • scc= 544 ± 39 (stat) ± 142 (sys) ± 200 (model) mb • from single e±: scc= 567±57(stat)±224(sys) mb simultaneous fit of charm and bottom: • scc= 518 ± 47 (stat) ± 135 (sys) ± 190 (model) mb • sbb= 3.9 ± 2.4 (stat) +3/-2 (sys) mb Charm and bottom from e+e- pairs • e+e- inv. mass after background subtraction compared to cocktail • absolutely normalized • excellent agreement • charm & bottom accessible after subtracting the cocktail arXiv: 0802.0050 • bottom irrelevant for total e± yield, but crucial at high pT!

18. Separating ce from be (I) • the key: electron-hadron correlations • charm and bottom are different • electron – kaon charge correlation • D decay  unlike-sign eK pairs • B decay  mostly like sign eK pairs (with small (1/6) admixture of unlike-sign pairs) • 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

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

20. Separating ce from be (III) • the key: electron-hadron correlations • charm and bottom are different • electron-D0 correlations • trigger on e from heavy-flavor decay • use D meson (reconstructed in hadronic decay) as a probe • investigate eD correlation in azimuth

21. B contribution to e± spectra • e from b / e from c ≥ 1 for pT ≥ 6 GeV/c • PHENIX & STAR: consistent with FONLL • not precise enough to extract b suppression • need vertex detectors to measure charm and bottom hadrons!

22. Rapidity dependence of charm production • high pT muons in PHENIX: 1.2<|h|<2.2 • again, background subtraction is difficult

23. Rapidity dependence of charm production • charm yield similar at mid and forward rapidity • large uncertainties everywhere • better data are needed  measurement of displaced vertices

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

25. Outlook: near future • complete systematics of existing observables • PHENIX • e± from d+Au & Cu+Cu • D reconstruction in p+p (D0 K+p-p0) • heavy flavor from e-m pairs X. Dong, Hard Probes '08 • STAR • improved e± data from running without inner silicon detectors photonic background reduced by factor ~10

26. Outlook: longer term future • silicon vertex trackers for unambiguous resolution of displaced vertices  direct D- and B-meson measurements STAR PHENIX