Xiaoyan Lin (for the STAR Collaboration) Institute of Particle Physics Wuhan, P.R. China

Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007 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


Outline

Introduction

Data Analysis Electron Identification

Photonic Background Removal

Electron-Hadron Azimuthal Angular Correlations

Results and Discussion

Summary


Non-Photonic Electron Measurement at RHIC

Non-photonic electron energy loss

The high pT region non-photonic electron RAA is surprising: Heavy quark RAA has similar magnitude as light quark RAA!

Describing the suppression is difficult for theoretical models.

Where is the bottom contribution?

Curve-I: M. Djordjevic et.al. PLB632 (2001) 199Curve-II,V: N. Armesto et.al. PLB637 (2006) 362Curve-III: S. Wicks et.al. nucl-th/0512076Curve-VI: H Van Hees et.al. PRC73(2006)034913


QM2006

Non-Photonic Electron Measurement at RHIC

Non-photonic electron elliptic flow

Y. Zhang, Nucl.Phys.A783:489-492,2007

Reduction of v2 at pT > 2 GeV/c.

Bottom contribution??

The decay kinematics of D and B mesons are different!

The same D and B v2 can lead to very different non-photonic electron v2 !


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.


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.


PYTHIA Simulation of e-h Correlations

B

D

Associated pT > 0.3 GeV/c. Significant

difference in the near-side correlations. Width of near-

side correlations largely due to decay kinematics.


Time Projection Chamber (TPC)

Coverage: 0 < Φ < 2π, ~ -1.25 < η < 1.25

Uniform electrical and magnetic field along the beam direction

Tracking mid-rapidity charged particles and particle identification

Major STAR Detectors Used

Electro-Magnetic Calorimeter (EMC) Coverage: 0 < Φ < 2π, -1.0 < η < 1.0 120 calorimeter modules, 40 towers for each module ¾ of the total barrel was instrumented during RUN V Providing energy information for electrons/positions

EMC’s Partner Detector: Shower Maximum Detector (SMD)

5 radiation length depth from the inner surface of the EMC

Providing high spatial resolution

Measuring the position and size of the shower


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

Data Set, Electron Signal and Background

dominant

negligible


Electron ID Using TPC, EMC and SMD

0.3 < p/E < 1.5# of BSMD hits > 1-3σ < z distance < 3σ

-3σ < Φ distance < 3σ

3.38 < dE/dx < 4.45 keV/cm


Purity of Inclusive Electron Sample

The purity is above 98% up to pT ~ 6.5 GeV/c.



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

e+ (e

- )

e -(e +)

Tagged

Partner



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.

m<100 MeV/c2

STAR Preliminary

STAR Preliminary


All Tracks

Inclusive electron

Pass EID cuts

Non-photonic electron Photonic electron

Reco-photonic electron=OppSign - SameSign

Not-reco-photonic electron

Procedure to Extract the Signal of e-h

Correlations

Semi-inclusive electron

Signal:non-photonic = (semi-inclusive) + SameSign – (not-reco-photonic) Advantage: Smaller overall uncertainties. Each item has its own corresponding Δφ histogram.


Technique to Deal with Not-Reco-Photonic Part

Final equation to extract the signal of e-h correlations:

In non-photonic electron yield or v2 analyses,

However, efficiency correction alone is not enough in e-h correlation analysis.

Reco-Photonic Part

h

Tagged ePartner e found

h

h

h

Not- Reco-Photonic Part

h

Tagged ePartner e missing

h

h

h


e-h Angular Correlations after Bkgd. Subtraction


Fit function: R is B contribution, i.e. B/(B+D), as a parameter in fit function.

Use PYTHIA Curves to Fit Data Points

D

B


B/(B+D) Consistent Varying Fit Range


Chi-square Sensitive to B/(B+D) Ratio


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.


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 non-photonic 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!


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.


Extra Slides


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!


PYTHIA Simulation: e pT VS. hadron pT


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.


Near-side width due to decay kinematics

With δ fragmentation function


Near-side width does not strongly depend on FF


Near-side width does not strongly depend on FF


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.


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.

Check on Systematics II

Documents

Xiaoyan Lin (for the STAR Collaboration) Institute of Particle Physics Wuhan, P.R. China