combinatorial pair background in the e + e - mass spectra in p+p collisions at √s = 14tev
DESCRIPTION
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. CONTENTS. Physics motivation : Low p T production Measurement scheme : e + e - pairs - PowerPoint PPT PresentationTRANSCRIPT
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
3Yonsei University
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+
5Yonsei University
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.
7Yonsei University
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 )