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
Jan 16, 2016
417th 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
R. Averbeck,2 June 26, 2008
charm and bottom from hadronic collisions mc~1.3 GeV, mb~4.5 GeV hard process (mq >> QCD),
even at low pT
open heavy flavor (D, c, B, b) quarkonia (J,
heavy-ion collisions heavy quarks are produced before the medium is formed
Introduction
D mesons
, ’,
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
vacuum
hadronicmatter
QGP
R. Averbeck,3 June 26, 2008
hadronic decay channels D0 K (BR: ~4%) D0 K0 (BR: ~14%) D± K (BR: ~10%) c pK (BR: ~5%)
How to measure open heavy flavor
c c
0DK
0D
K+
-
advantage unambiguous identification,
i.e. a peak in invariant mass
disadvantagesdifficult to triggerhuge combinatorial
background improvement?
–resolve decay vertices–charm: c ~ 100-200 m–bottom: c ~ 400-500 m
silicon vertex detectors
R. Averbeck,4 June 26, 2008
semileptonic decay channels D0 lX (BR: ~7%) D± lX (BR: ~17%) c lX (BR: ~5%) B0,± lX (BR: ~11%)
How to measure open heavy flavor
c c
0DK
0D
K+
-
advantages 'straight forward' trigger no combinatorial BG
disadvantagesneed to control/subtract
background from other lepton sources
loss of kinematic information
continuum can NOT disentangle c & b with single leptons only
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
muons in forward arms tracking muon ID:“absorber”
electrons in central arms tracking electron ID:
RICH + EMC
large acceptance tracking detector: TPC hadrons:
TPC (dE/dx) Time-of-Flight detector
electron ID: EMC in addition
PHENIX
optimized fo
r leptons
but can do hadrons STAR
optimized fo
r hadrons
but can do leptons
R. Averbeck,6 June 26, 2008
MANY electrons sources Dalitz decay of light neutral mesons
– most important → e+e-
– but also: ’ conversion of photons
– main photon source: → – in material: → e+e-
weak kaon decays– Ke3, e.g.: K± → e± e
dielectron decays of vector mesons– → e+e-
direct/thermal radiation– conversion of direct photons in material– virtual photons: * → e+e-
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!
R. Averbeck,7 June 26, 2008
Cocktail subtractionALL relevant background
sources are measured calculate e± BGBG 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
R. Averbeck,8 June 26, 2008
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!
R. Averbeck,9 June 26, 2008
PRL 97, 252002 (2006)
total cross section cc= 56757(stat)±224(sys) b
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!
R. Averbeck,10 June 26, 2008
Background subtraction in STARphotonic 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
R. Averbeck,11 June 26, 2008
ratio of heavy-flavor e± spectra to FONLLPHENIX
– 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)
PHENIX vs. STAR vs. FONLL
R. Averbeck,12 June 26, 2008
PRL 98, 172301 (2007)
Hot matter: Au+Au at √sNN=200 GeV
ppinYieldN
AuAuinYieldR
binaryAA
PRL 98, 172301 (2007)
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 0 (a big surprise)
R. Averbeck,13 June 26, 2008
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!
R. Averbeck,14 June 26, 2008
D0 K invariant mass analysismain 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
D-meson reconstruction in STAR
PRL 94(2005)062301A. Shabetai, QM'08 arXiv:0805.0364
R. Averbeck,15 June 26, 2008
muon identification at low pT (~0.2 GeV/c)Time-of-Flight and
dE/dx in the TPC
Low pT muons in STAR
subtraction of BG from and K decaydistance of closest
approach of tracks to primary vertex
low pT muon yield sensitive to total charm
cross section insensitive to spectral shape
R. Averbeck,16 June 26, 2008
combined fit to e±, ±, D0
data are consistent
Total charm cross section in STAR
binary scaling of charm yield total charm cross section ~ 1 mb
~ 4x pQCD value (still within huge uncertainties)
~ 2x PHENIX value
R. Averbeck,17 June 26, 2008
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
charm: integration after cocktail subtraction cc= 544 ± 39 (stat) ± 142 (sys) ± 200 (model) b from single e±: cc= 56757(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
bottom irrelevant for total e± yield, but crucial at high pT!
arXiv: 0802.0050
R. Averbeck,18 June 26, 2008
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 ce from be (I) the key: electron-hadron correlations
charm and bottom are differentc c
0DK
R. Averbeck,19 June 26, 2008
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 ce from be (II) the key: electron-hadron correlations
charm and bottom are differentc c
0DK
R. Averbeck,20 June 26, 2008
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
Separating ce from be (III) the key: electron-hadron correlations
charm and bottom are different
R. Averbeck,21 June 26, 2008
e from b / e from c ≥ 1 for pT ≥ 6 GeV/cPHENIX & STAR: consistent with FONLL
B contribution to e± spectra
not precise enough to extract b suppressionneed vertex detectors to measure charm and bottom hadrons!
R. Averbeck,22 June 26, 2008
high pT muons in PHENIX: 1.2<||<2.2 again, background subtraction is difficult
Rapidity dependence of charm production
R. Averbeck,23 June 26, 2008
charm yield similar at mid and forward rapidity large uncertainties everywhere
better data are needed measurement of displaced vertices
Rapidity dependence of charm production
R. Averbeck,24 June 26, 2008
charm (& bottom) are crucial probes for the medium produced in HI collisions @ RHIC
even calibration measurements are difficult large uncertainties
charm cross section / binary collisionbinary scaling is observed in STAR
& PHENIXbut the cross sections differ
by a factor ~2
Summary
from e, , D
from e, e+e-
R. Averbeck,25 June 26, 2008
complete systematics of existing observablesPHENIX
– e± from d+Au & Cu+Cu
– D reconstruction in p+p (D0 K+-0)
– heavy flavor from e- pairs
Outlook: near future
photonic backgroundreduced by factor ~10
STAR– improved e± data from
running without inner silicon detectors
X. Dong, Hard Probes '08
R. Averbeck,26 June 26, 2008
silicon vertex trackers for unambiguous resolution of displaced vertices direct D- and B-meson measurements
Outlook: longer term future
PHENIX STAR