combinatorial pair background in the e + e - mass spectra in p+p collisions at √s = 14tev

Yonsei University

Yonsei University

Combinatorial pair background in the e+e- mass spectra

in p+p collisions at √s = 14TeV

Yonsei Univ.

M. G. Song, D. H. Lee, B. K. Kim, B. S. Chang, Y. Kwon, J. H. Kang

2Yonsei University

CONTENTS

Physics motivation : Low pT production Measurement scheme : e+e- pairs Combinatorial background & Its estimators (Pairs from

Mixed event, Like-sign pairs) Simulated event sample : PYTHIA Background estimation for the given events

Difference between two estimators Subtracted signal level

Consideration for Detector effect Efficiency for Single track Efficiency for close track pairs

Summary

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Physics motivation Low pT production

Direct production in p+p One of the best known QCD process…

Leading order diagram in perturbation theory

qgq gqq

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Measurement scheme: e+e- pairs

* e+e- , R H Dalitz 1951 Proc. Phys. Soc. A 64 667-669

AND we can avoid 0 background! e-

e+

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Combinatorial background

Real Pair -> e1

+e1-, e2

+e2-

Reconstructed Pair-> e1

+e1-, e2

+e2-, e1

+e2-, e2

+e1-

e1+e2

-, e2+e1

-

-> Combinatorial Background

Background estimate-> mixed event, like-sign pairs

e1+

e2+

e1-

e2-

e2+

e1-

e2-

e1+

Case 1

Case 2

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Two estimatorsMixed event pairs

Estimate fake track pairs with each track and other similar events

Current work : Previous 99 events in mixed event pool, normalization factor = 1/99

Good statistics Need independent estimate for

Cross pairs (Double Dalitz decay), and Jet pairs (correlated pairs from all 0 correlation)

--- Details can be found in the recent PHENIX publication.

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Two estimatorsLike-sign pairs

Major sources of single electrons : 0, Dalitz, Vector mesons, heavy flavor

Assume fake number of unlike sign (e+e-) pairs is related with like-sign(e+e+, e-e-) pairs Charge symmetric sources N++~N-+ ~N+- ~N--

Difference in acceptance ΝΝ2ΝΝ

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Simulated event sample : PYTHIA

300M p+p events @ Default setting with MSEL=1

TeVs 14

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e+e- pairs distribution

blue box : raw pairssolid line : hadron decaydotted line : correlated heavy flavor decayspink line : fictitious virtual

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Difference between two estimators Like-sign pairs vs Mixed-event pairs

combinatorial pair mass distribution

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Subtracted signal level Like-sign pairs vs Mixed-event pairs•Distribution of ‘raw pairs’ with substraction

•Red box : Subtracted counts by Mixed event, •Blue box : Subtracted counts by Like-sign•Black solid line : From light hadrons•Black dotted line : From heavy flavor pairs

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Consideration for Detector effect Efficiency for Single track

|| <1 pT > 0.3 GeV/c,

Two case in acceptance 1. Complete acceptance

without inefficiencies 2. substantial inefficiencies

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Efficiency for Single track with & without acceptance

Uniform reduction over the mass range Need more statistics

e+e- mass distribution (like-sign pair subtraction) with & without acceptance.

Ratio of two distributions

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Consideration for Detector effect Track Pairs

Too close track pairs can be lost because of cluster sharing

Remove tracks Closest distance < 2cm in

TPC-like volume.

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Track Pairswith vs. without track pair filter

No loss but the 1st mass bin.

e+e- mass distribution (like-sign pair subtraction) with (red) & without (blue) track pair filter.

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Summary

1. Estimation by like-sign pairs yield better representation of the combinatorial background for the e+e- pair analysis in p+p collisions.

2. Strong appearance of direct * is expected for Mee ~ 300 MeV.

3. Semi-leptonic decays of heavy flavor pairs appear as animportant source of e+e- pair at large mass.

4. Single track or track pair acceptance will not affect e+e- pair mass distribution (in the kinematic/mass region of interest).

5. We note experimental background tracks as from conversion in the detector material are not included in the current work.

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Backup

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Like-sign pairs Major sources of single electrons : 0, Dalitz,

Vector mesons, heavy flavor ? Charge symmetric sources N++~N-+~N+-~N–

Difference in acceptance NNNN 2

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Two estimators Like-sign pairs

Background Real Pair → e1

+e1-, e2

+e2-

Reconstructed Pair → e1

+e1-, e2

+e2-, e1

+e2-, e1

-e2+

e1+e2

-, e2+e1

-

→ Combinatorial Background Like-sign pairs → e1

+e2+, e1

-e2-

Background estimate Charge symmerty : N+-~ N++~ N-+~ N—

Difference in acceptance →

e1+

e2+

e1-

e2-

e2+

e1-

e2-

e1+

NNNN 2

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?

e+e- pair mass distribution from PYTHIA eventsA = blue box : raw pairsB = solid line : hadron decayC = dotted line : correlated heavy flavor decaysD = pink line : fictitious virtual A-(B+C+D) = Combinatorial pair background

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Lepton pairs from light hadron decay, PYTHIA

From 0 and

1. e+e- from Light hadron decay

2. Heavy flavors( 3. Virtual )