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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


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


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