417 th WE-Heraeus-Seminar Characterization of the Quark Gluon Plasma with Heavy Quarks Physikzentrum...

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


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


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


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


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!


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


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!


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!


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


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


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)


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!


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


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


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


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


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


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


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


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!


high pT muons in PHENIX: 1.2<||<2.2 again, background subtraction is difficult

Rapidity dependence of charm production


charm yield similar at mid and forward rapidity large uncertainties everywhere

better data are needed measurement of displaced vertices

Rapidity dependence of charm production


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-


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


silicon vertex trackers for unambiguous resolution of displaced vertices direct D- and B-meson measurements

Outlook: longer term future

PHENIX STAR

417 th WE-Heraeus-Seminar Characterization of the Quark Gluon Plasma with Heavy Quarks Physikzentrum...

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