1. 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

2. 2 Yonsei University CONTENTS Physics motivation : Low p T  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. 3 Yonsei University Physics motivation Low p T  production Direct  production in p+p   One of the best known QCD process…  Leading order diagram in perturbation theory

4. 4 Yonsei 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- e+e+

5. 5 Yonsei University Combinatorial background Real Pair -> e 1 + e 1 -, e 2 + e 2 - Reconstructed Pair -> e 1 + e 1 -, e 2 + e 2 -, e 1 + e 2 -, e 2 + e 1 - e 1 + e 2 -, e 2 + e 1 - -> Combinatorial Background Background estimate -> mixed event, like-sign pairs e1+e1+ e2+e2+ e1-e1- e2-e2- e2+e2+ e1-e1- e2-e2- e1+e1+ Case 1 Case 2

6. 6 Yonsei University Two estimators Mixed 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. 7 Yonsei University 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. 8 Yonsei University Simulated event sample : PYTHIA 300M p+p events @ Default setting with MSEL=1

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

10. 10 Yonsei University Difference between two estimators Like-sign pairs vs Mixed-event pairs combinatorial pair mass distribution

11. 11 Yonsei 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

12. 12 Yonsei 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

13. 13 Yonsei 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

14. 14 Yonsei 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.

15. 15 Yonsei University Track Pairs with vs. without track pair filter No loss but the 1 st mass bin. e + e - mass distribution (like-sign pair subtraction) with (red) & without (blue) track pair filter.

16. 16 Yonsei 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 M ee ~ 300 MeV. 3. Semi-leptonic decays of heavy flavor pairs appear as an important 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.

17. 17 Yonsei University Backup

18. 18 Yonsei 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 

19. 19 Yonsei University Two estimators Like-sign pairs Background Real Pair → e 1 + e 1 -, e 2 + e 2 - Reconstructed Pair → e 1 + e 1 -, e 2 + e 2 -, e 1 + e 2 -, e 1 - e 2 + e 1 + e 2 -, e 2 + e 1 - → Combinatorial Background Like-sign pairs → e 1 + e 2 +, e 1 - e 2 - Background estimate Charge symmerty : N +- ~ N ++ ~ N -+ ~ N — Difference in acceptance → e1+e1+ e2+e2+ e1-e1- e2-e2- e2+e2+ e1-e1- e2-e2- e1+e1+

20. 20 Yonsei University ? 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. 21 Yonsei University Lepton pairs from light hadron decay, PYTHIA From  0 and  1. e + e - from Light hadron decay 2. Heavy flavors ( 3. Virtual  )