Transcript Slide 1

Non-Photonic Electron Angular Correlations with
Charged Hadrons from the STAR Experiment:
First Measurement of Bottom Contribution to
Non-Photonic Electrons at RHIC
Xiaoyan Lin
(for the STAR Collaboration)
Institute of Particle Physics
Wuhan, P.R. China
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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Outline
Introduction
Data Analysis
Electron Identification
Photonic Background Removal
Electron-Hadron Azimuthal Angular Correlations
Results and Discussion
Summary
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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Non-Photonic Electron Measurement at RHIC
Non-photonic electron energy loss
The high pT region nonphotonic electron RAA is
surprising: Heavy quark RAA
has similar magnitude as
light quark RAA!
Describing the suppression
is difficult for theoretical
models.
Curve-I: M. Djordjevic et.al. PLB632 (2001) 199
Curve-II,V: N. Armesto et.al. PLB637 (2006) 362
Curve-III: S. Wicks et.al. nucl-th/0512076
Curve-VI: H Van Hees et.al. PRC73(2006)034913
Xiaoyan Lin
Where is the bottom
contribution?
SQM 2007, Levoca, Slovakia, June 26, 2007
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Non-Photonic Electron Measurement at RHIC
Non-photonic electron elliptic flow
QM2006
Reduction of v2 at
pT > 2 GeV/c.
Bottom contribution??
Y. Zhang, Nucl.Phys.A783:489-492,2007
The decay kinematics of D
and B mesons are different!
The same D and B v2 can
lead to very different nonphotonic electron v2 !
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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B Versus D Contributions to Electrons
Quantitative understanding of features in heavy quark
measurements requires experimental measurement of B and
D contributions to non-photonic electrons!
Such information should be best obtained from direct
measurement of hadronic decays of charm and bottom
mesons. This motivates the STAR vertex detector upgrade!
We have proposed an experimental method which uses the
azimuthal angular correlations between non-photonic
electrons and charged hadrons to measure the relative
contributions to non-photonic electrons from D and B meson
decays.
Our method is based on the different decay kinematics of D
and B mesons.
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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PYTHIA Simulation: e pT VS. parent pT
Charm quark needs to have larger momentum than bottom
quark to boost the decayed electron to high pT.
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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PYTHIA Simulation of e-h Correlations
Associated pT >
0.3 GeV/c.
Significant
difference in the
near-side
correlations.
Width of nearside correlations
largely due to
decay kinematics.
B
D
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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Major STAR Detectors Used
EMC’s
Partner Detector:
Shower
Maximum Detector (SMD)
Time Projection
Electro-Magnetic
Chamber
Calorimeter
(TPC)
(EMC)
Coverage:
<
2π,
-1.0
< the
η<<ηinner
1.0
Coverage:
0length
<Φ
Φ<
<depth
2π, ~
-1.25
< 1.25
5
radiation 0
from
surface of the EMC
120 calorimeter modules, 40 towers for each module
Uniform
electrical
and resolution
magnetic field along the beam direction
Providing
highbarrel
spatial
¾ of the total
was instrumented during RUN V
Tracking
charged
particles
and particle identification
Measuring
the position
and size
of
the shower
Providingmid-rapidity
energy
information
for
electrons/positions
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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Data Set, Electron Signal and Background
Data Set:
--- p+p collisions at sNN = 200 GeV in year 5 run.
--- 2.37 million EMC HT1 triggered events with threshold 2.6 GeV.
--- 1.68 million EMC HT2 triggered events with threshold 3.5 GeV.
Electron Signal:
Non-photonic electrons: electrons from semi-leptonic decays of
heavy quarks (charm and beauty).
Background
--- Hadron contamination
--- Photonic electron background
Photon conversion
Dalitz decays of π0, η
Kaon decays
ρ, ω, Φ decays
Other possible contributions
Xiaoyan Lin
dominant
negligible
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Electron ID Using TPC, EMC and SMD
<BSMD
z<distance
<
3.38
< dE/dx
< 4.45
keV/cm
#-3σ
of
0.3
p/E
hits
< 1.5
> 3σ
1
-3σ < Φ distance < 3σ
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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Purity of Inclusive Electron Sample
The purity is above 98% up to pT ~ 6.5 GeV/c.
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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Photonic Background Removal
A pair of photonic electrons are correlated. Their
invariant mass should be small.
Use invariant mass calculation to reconstruct the
photonic background.
--- For each tagged e+(e-), we select
partner e-(e+) identified only with the
TPC and calculate the invariant mass of
the pair. (Opposite-sign)
--- Combinatorial background:
non-photonic electrons may be
falsely identified as photonic electrons;
reconstructed by Same-sign
technique.
γ
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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Photonic Background Removal
m<100 MeV/c2
STAR Preliminary
STAR Preliminary
The combinatorial background is small in p+p collisions.
Reconstructed photonic = OppSign – SameSign.
Photonic electron = (reconstructed-photonic)/ε.
ε is the background reconstruction efficiency, ~70% from simulation.
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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Procedure to Extract the Signal of e-h Correlations
All Tracks
Pass EID cuts
Inclusive electron
Non-photonic electron
Photonic electron
Reco-photonic electron
=OppSign - SameSign
Not-reco-photonic electron
Semi-inclusive electron
Signal:
non-photonic = (semi-inclusive) + SameSign – (not-reco-photonic)
Advantage: Smaller overall uncertainties.
Each item has its own corresponding Δφ histogram.
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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Technique to Deal with Not-Reco-Photonic Part
In non-photonic electron yield or v2 analyses,
However, efficiency correction alone is not enough in e-h correlation
analysis.
Tagged e
Partner e found
h h
h
h
Tagged e
Partner e missing
h h
h
h
Reco-Photonic Part
Not- Reco-Photonic Part
Final equation to extract the signal of e-h correlations:
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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e-h Angular Correlations after Bkgd. Subtraction
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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Use PYTHIA Curves to Fit Data Points
B
D
Fit function:
R is B contribution, i.e. B/(B+D), as a parameter in fit function.
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SQM 2007, Levoca, Slovakia, June 26, 2007
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B/(B+D) Consistent Varying Fit Range
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SQM 2007, Levoca, Slovakia, June 26, 2007
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Chi-square Sensitive to B/(B+D) Ratio
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SQM 2007, Levoca, Slovakia, June 26, 2007
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Results: B Contribution VS. pT
Error bars are statistical only!
Data uncertainty includes
statistic errors and systematic
uncertainties from:
---- photonic background
reconstruction efficiency
(dominant).
---- difference introduced by
different fit functions.
A finite B contribution in the pT
region of 2.5-6.5 GeV/c has
been observed.
The FONLL theoretical
calculations are consistent with
the measured data.
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SQM 2007, Levoca, Slovakia, June 26, 2007
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Discussion: Bottom Suppression
M. Djordjevic, Phys. Lett. B632:81-86 (2006)
Radiative energy loss theory:
Bottom significantly less quenched than charm quark and light quarks.
The measured B/(B+D) ratio together with the large suppression of nonphotonic electrons and a tendency for the non-photonic v2 to decrease at
high pT implies that bottom quark may be suppressed in central Au+Au
collisions at RHIC in contrast to the theory prediction!
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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Summary
The method to estimate D and B contributions is
developed in PYTHIA and implemented in real data.
We have measured e-h correlations in 200 GeV p+p
collisions.
The first measured B/(B+D) ratios at RHIC indicate at pT
~ 4-6 GeV/c the measured B contribution to non-photonic
electrons is comparable to D contribution based on
PYTHIA model.
The result of measured B/(B+D) ratios is consistent with
the FONLL prediction.
The measured B/(B+D) ratios imply that the bottom
quarks may suffer considerable amount of energy loss in
the dense QCD medium.
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SQM 2007, Levoca, Slovakia, June 26, 2007
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Extra Slides
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PYTHIA Simulation: e pT VS. hadron pT
The efficiency of associated pT cut is different between D decay and
B decay. Therefore, it is better to use lower pT cut on the associated
particles in order to avoid analysis bias!
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SQM 2007, Levoca, Slovakia, June 26, 2007
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PYTHIA Simulation: e pT VS. hadron pT
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SQM 2007, Levoca, Slovakia, June 26, 2007
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PYTHIA parameters used in this analysis
PYTHIA version: v6.22
δ fragmentation function used for both charm and bottom.
Parameters for charm:
PARP(67) = 4 (factor multiplied to Q2)
<kt> = 1.5 GeV/c
mc = 1.25 GeV/c2
Kfactor = 3.5
MSTP(33) =1 (inclusion of K factor)
MSTP(32) = 4 (Q2 scale)
CTEQ5L PDF
Parameters for bottom are the same as for charm except
mb = 4.8 GeV/c2.
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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Near-side width due to decay kinematics
With δ fragmentation function
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SQM 2007, Levoca, Slovakia, June 26, 2007
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Near-side width does not strongly depend on FF
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SQM 2007, Levoca, Slovakia, June 26, 2007
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Near-side width does not strongly depend on FF
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SQM 2007, Levoca, Slovakia, June 26, 2007
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Check on Systematics I
Allow an overall
normalization factor in the fit
function to float:
A reflects the uncertainties
in the normalization which
possibly arises from the
counting of the number of
non-photonic triggers and
tracking efficiency for the
associated tracks.
The fit results gives A close
to unity and consistent
B/(B+D) ratios.
Xiaoyan Lin
SQM 2007, Levoca, Slovakia, June 26, 2007
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Check on Systematics II
Add an adjustable constant
to the fit function:
C freely adjusts the overall
background level and it
contains soft particle
production.
The fit results gives a value
for the constant C close to
zero and consistent B/(B+D)
ratios.
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SQM 2007, Levoca, Slovakia, June 26, 2007
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