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The Study of D and B Meson Semi-leptonic Decay Contributions to the Non-p

hotonic Electrons

Xiaoyan Lin

CCNU, China/UCLA

for the STAR Collaboration

22nd Winter Workshop on Nuclear Dynamics 11-19 Mar. 2006

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Outline Motivation

Why study this? Simulation

PYTHIA p+p Collisions at ssNN NN = 200 GeV= 200 GeV

Real Data Analysis

Electron Identification

Photonic Electron Removal

Electron-Hadron Correlation Outlook

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Motivation

• The present non-photonic electron RAA data is a challenge to the existing theoretical calculations of heavy quark energy loss in the medium produced at RHIC.

• The separation of B and D meson decay contributions to the non-photonic electrons will help to fully understand the energy loss mechanism for heavy quarks.

Armesto et al. hep-th/0511257, van Hess et al. nucl-th/0508055DVGL theory from nucl-th/0507019 nucl-th/0512076

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PYTHIA Simulation: Parameter SetupCharm quark pt distributions

The existent measurements have no constraint on the D meson production at high pt. The c-quark spectra from both of these two sets of parameters match the measured data.

NLO pQCD predictions from R. Vogt, Int. J. Mod. Phys. E12 (2003) 211

Start with the parameter set ((refer it as refer it as set Iset I) --- <kt> = 1.5, mc = 1.25, K factor = 3.5, PDF = CTEQ5L, PARP(67) = 1. ( from PHENIX Collaboration, Phys. Rev. Lett. 88, 192303(2002))

Further tune PARP(67) to 4 (refer it as refer it as set IIset II). In PYTHIA it is to account for higher order pQCD effect. It allows for gluon splitting and can effectively reproduce NLO calculation.

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PYTHIA Simulation: Parameter SetupCharmed hadron pt distributions

• Modified Peterson function to make the fragmentation function harder. ε = 10-5 is used.

• PYTHIA calculation with modified Peterson function using both of these sets of parameters can reasonably depict the measured open charm data.

The default Peterson fragmentation function (ε = 0.05) is too soft to reproduce the measured spectrum.

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PYTHIA Simulation: Parameter SetupNon-photonic electron pt distribution • Electron spectra from PYTHIA

calculation with modified fragmentation using both of these two parameter sets are consistent with the STAR measured data.

• In PYTHIA calculation with parameter set II, b quark decays are not dominant for pt up to 8 GeV/c while in the calculation with parameter set I b contribution is not dominant for pt up to 6 GeV/c.

• The STAR open charm data and non-photonic electron data are consistent within our PYTHIA calculation.

(STAR Prelim.)

(STAR Prelim.)

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PYTHIA Simulation: e-h correlation

Δφ Distribution • Trigger: electron Association: charged hadron

• The Δφ distributions change slightly from PYTHIA calculations with parameter set II to parameter set I.

• When the electron pt range goes higher, the peaks centered at zero become narrower and higher.

• The peaks centered at zero from D decay are much narrower than those from B decay.

D B

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PYTHIA Simulation: e-h correlationSummed pt of charged hadrons distribution

• The summed pt is the sum of charged hadron pt within a cone of triggered high pt electrons.

• The cone is defined by |ηh - ηe| < 0.35 and |φh - φe| < 0.35.

• The summed pt distributions do not change significantly between the parameter set II and the parameter set I.

• The distributions from D decay are much wider than those from B decay.

• When electron pt range goes higher, the distributions become broader.

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PYTHIA Simulation: B contribution

Electron pt (GeV/c) 2.5-3.5 3.5-4.5 4.5-5.5

parameter set II 8.33% 15.07% 23.23%

parameter set I 9.23% 16.58% 24.17%

The B contribution to the non-photonic electrons can be determined directly from the electron spectra.

• B contribution = (B decay electron yield)/(B+D decay electron yield)

• Remove those electrons which have no hadrons in the cone around them.

• This method is not experimentally feasible.

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PYTHIA Simulation: B contributionAn experimental way to determine the B contribution fraction.

• Use the summed pt histograms from B decay and D decay to fit the summed pt histogram from PYTHIA inclusive , and let B contribution fraction as a parameter.

• The contribution fraction is determined by the minimum value of χ2.

• The fitting error is determined by one σ shift of χ2/ndf from the minimum value.

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PYTHIA Simulation: B contributionElectron pt (GeV/c) 2.5-3.5 3.5-4.5 4.5-5.5

parameter set II 8.33±2.17% 15.07±3.72% 23.23±5.89%

parameter set I 14.49±2.32% 17.74±4.02% 28.46±6.49%

• Comparing the results of PYTHIA calculation with parameter set I with those from electron spectra, the results are consistent for the high pt ranges within fitting errors, while for the pt range 2.5-3.5 GeV/c, the results are inconsistent.

• The inconsistency is due to the window size of Δφ. The low pt range is more sensitive than the high pt range to the choice of window size of Δφ.

8.33 % 15.07 % 23.23 %

9.23 % 16.58 % 24.17 %

• The results for PYTHIA calculation with parameter set II are consistent with those directly from the electron spectra. This indicates the method we propose is self-consistent.• Use summed pt histograms of B and D decays from PYTHIA calculation with parameter set II to fit the summed pt histogram of the inclusive case from PYTHIA calculation with parameter set I.

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Summary for Simulation

We find that the charm quark fragmentation function has to be harder than default Peterson function. A delta fragmentation function scheme yields consistent result with the STAR measurements.

An experimental method using e-h correlation is proposed based on PYTHIA calculation to quantitatively determine the contributions from D and B semi-leptonic decays.

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• Signal: non-photonic electrons

• Background: hadrons and photonic electrons (photon conversion and π0 Dalitz decays, etc.)

Real Data Analysis: Solenoidal Tracker At RHIC

Three detectors used in this analysis:

• Time Projection Charmber (TPC)

• Barrel Electro-Magnetic Calorimeter (BEMC)

• Barrel Shower Maximum Detector (BSMD)

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• 0 <φ< 2π• -1<η<1• 120 calorimeter modules• 40 towers for each module• Energy resolution~16%/EE

Real Data Analysis: BEMC

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Real Data Analysis: BSMD

• Provide high spatial resolution• Measure the position of the shower• Measure the size of the shower

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Real Data Analysis: Data Set

200 GeV pp year 5 data

Event cut: |Z| < 30 cm

About 9M events were used

1.3M HT One events

0.87M HT Two events

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Electron IdentificationIn this part of analysis, about 2.9M pp events were analyzed.

dEdx

dEdx cut: (0σ, 3σ)

NFitPts [20,50) NdEdxPts [15,100) Chi square [0,3.0) NFitPts/NMaxPts [0.52, 1.2) Eta [-0.7, 0.7)

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Electron IdentificationP/E

• P is measured by TPC. E is the sum of the associated BEMC points’ energy measured by BEMC.

• Electrons will deposit almost all of their energy in the BEMC towers. 0.3 < P/E <1.5 was used to keep electrons and reject hadrons.

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Electron IdentificationDistance between Projection point and EMC Point

• -3σ < ZDist < 3σ and -3σ < PhiDist < 3σ were set to remove lots of random associations between TPC tracks and BEMC points.

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Electron IdentificationNumber of BSMD hits

• Electrons have larger number of BSMD hits than those for hadrons.

• Electron candidates have to satisfy Number of BSMD hits > 1.

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The purity of Electron sample

The electron purity > 99%!

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Photonic Background

• Global info. is used.

• The cut for the partner only is dEdx within 3σ on electron band.

• Dca between two tracks less than 1cm.

• Angle between two tracks less than 0.1. Angle in eta plane less than 0.02 and angle in phi plane less than 0.1

• The invariant mass for a pair of photonic electrons is small.

• A cut less than 100MeV is chosen.

• No worry about the combinatorial background!

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Electron-hadron Correlation

Cuts for hadron selection

NFitPts: [15,50)

NFitPts/NMaxPts: [0.52,1.2)

Pt > 0.1 GeV/c

-0.75 < η < 0.75

Track Id different from electron track Id

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Electron-hadron CorrelationΔφ Distribution

Inc. E Inc. EInc. E2.5-3.5 GeV/c

4.5-5.5 GeV/c

3.5-4.5 GeV/c

4.5-5.5 GeV/c

3.5-4.5 GeV/c

2.5-3.5 GeV/c

Pho. EPho. EPho. E

Very preliminary!Very preliminary!

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Electron-hadron CorrelationSummed pt of charged hadron Distribution

2.5-3.5 GeV/c Inc. E

3.5-4.5 GeV/c Pho. E

2.5-3.5 GeV/c Pho. E

4.5-5.5 GeV/c Pho. E

Very preliminary!Very preliminary!

3.5-4.5 GeV/c Inc. E

4.5-5.5 GeV/c Inc. E

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Outlook

The largest uncertainty in STAR is the background from photon conversion in materials before the TPC.

To calculate the photonic background removal efficiency.

To get the results for the non-photonic electrons

Compare to PYTHIA simulation.