Download - Combinatorial pair background in the e + e - mass spectra in p+p collisions at √s = 14TeV
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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
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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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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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4Yonsei University
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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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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6Yonsei University
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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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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8Yonsei University
Simulated event sample : PYTHIA
300M p+p events @ Default setting with MSEL=1
TeVs 14
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9Yonsei University
e+e- pairs distribution
blue box : raw pairssolid line : hadron decaydotted line : correlated heavy flavor decayspink line : fictitious virtual
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10Yonsei University
Difference between two estimators Like-sign pairs vs Mixed-event pairs
combinatorial pair mass distribution
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11Yonsei University
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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12Yonsei University
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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13Yonsei University
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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14Yonsei University
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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15Yonsei University
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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16Yonsei University
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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17Yonsei University
Backup
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18Yonsei University
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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19Yonsei University
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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20Yonsei University
?
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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21Yonsei University
Lepton pairs from light hadron decay, PYTHIA
From 0 and
1. e+e- from Light hadron decay
2. Heavy flavors( 3. Virtual )