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Heavy-Flavor Cross Sections at RHIC. D mesons. vacuum. , Y ’, c. hadronic matter. QGP. Introduction. charm and bottom from hadronic collisions m c ~1.3 GeV, m b ~4.5 GeV hard process (m q >> L QCD ), even at low p T

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introduction

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
how to measure open heavy flavor

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
how to measure open heavy flavor1

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
phenix star at rhic

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
e from heavy flavor difficulties

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!

cocktail subtraction
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

converter subtraction

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!
e from heavy flavor in p p s 200 gev

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!
background subtraction in star
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
phenix vs star vs fonll
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)
hot matter au au at s nn 200 gev

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)
hot matter au au at s nn 200 gev1
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!

d meson reconstruction in star

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
low p t muons in star
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
total charm cross section in star
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
slide17

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!
separating c e from b e i
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
separating c e from b e ii
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
separating c e from b e iii
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
b contribution to e spectra
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!
rapidity dependence of charm production
Rapidity dependence of charm production
  • high pT muons in PHENIX: 1.2<|h|<2.2
  • again, background subtraction is difficult
rapidity dependence of charm production1
Rapidity dependence of charm production
  • charm yield similar at mid and forward rapidity
  • large uncertainties everywhere
  • better data are needed  measurement of displaced vertices
summary
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-

outlook near future
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

outlook longer term future
Outlook: longer term future
  • silicon vertex trackers for unambiguous resolution of displaced vertices

 direct D- and B-meson measurements

STAR

PHENIX