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

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

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

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  1. 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

  2. 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

  3. Physics motivation Low pT production • Direct  production in p+p •  One of the best known QCD process… • Leading order diagram in perturbation theory

  4. e- e+ Measurement scheme: e+e- pairs •   * e+e- , R H Dalitz 1951 Proc. Phys. Soc. A64 667-669 • AND we can avoid 0 background!

  5. Combinatorial background e1+ e2+ e1- • 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 e2- Case 1 e1+ e1- e2+ Case 2 e2-

  6. 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.

  7. Two estimators Like-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 

  8. Simulated event sample : PYTHIA • 300M p+p events @ • Default setting with MSEL=1

  9. e+e- pairs distribution • blue box : raw pairs • solid line : hadron decay • dotted line : correlated heavy flavor decays • pink line : fictitious virtual 

  10. Difference between two estimatorsLike-sign pairsvs Mixed-event pairs combinatorial pair mass distribution

  11. Subtracted signal levelLike-sign pairsvs 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

  12. 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

  13. Efficiency for Single trackwith & without  acceptance • Uniform reduction over the mass range • Need more statistics Ratio of two distributions e+e- mass distribution (like-sign pair subtraction) with & without  acceptance.

  14. 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.

  15. 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.

  16. Summary • Estimation by like-sign pairs yield better representation of the combinatorial background for the e+e- pair analysis in p+p collisions. • Strong appearance of direct * is expected for Mee ~ 300 MeV. • Semi-leptonic decays of heavy flavor pairs appear as animportant source of e+e- pair at large mass. • Single track or track pair acceptance will not affect e+e- pair mass distribution (in the kinematic/mass region of interest). • We note experimental background tracks as from  conversion in the detector material are not included in the current work.

  17. Backup

  18. Like-sign pairs • Major sources of single electrons : 0,  Dalitz, Vector mesons, heavy flavor ? • Charge symmetric sources  N++~N-+~N+-~N– • Difference in acceptance 

  19. Two estimators Like-sign pairs e1+ e1- • 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 → e2+ e2- e1+ e2+ e1- e2-

  20. ? • e+e- pair mass distribution from PYTHIA events • A = blue box : raw pairs • B = solid line : hadron decay • C = dotted line : correlated heavy flavor decays • D = pink line : fictitious virtual  • A-(B+C+D) = Combinatorial pair background

  21. Lepton pairs from light hadron decay, PYTHIA 1. e+e- from Light hadron decay From 0 and  2. Heavy flavors ( 3. Virtual  )

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