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Search for η -meson production in γγ interactions with e + e - colliding beams at √s = 1 GeV

23-09-2009. Search for η -meson production in γγ interactions with e + e - colliding beams at √s = 1 GeV. e + e -  η e + e - process. Integrated luminosity L= 240 pb -1. η  π + π – π 0. BR( h  p + p - p 0 )=22.73%.

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Search for η -meson production in γγ interactions with e + e - colliding beams at √s = 1 GeV

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  1. 23-09-2009 Search for η-meson production in γγ interactions with e+e- colliding beams at √s = 1 GeV

  2. e+ e- η e+ e- process Integrated luminosity L= 240 pb-1 η  π+π–π0 BR(hp+p-p0)=22.73% N. expected events = L σ (e+ e- η e+ e-) BR (η  π+π–π0) ε

  3. SND e+e- → p+p-p0 SND e+e- hg, h p+p- g (4.6%) e+e- hg, h  e+e- g (0.68%) e+e- hg,h 3p0, with gN  e+e-N e+e- gg, with gN  e+e-N don’t survive the analysis cuts

  4. The measured cross section s(K+K-) has a very large error. We prefer to evaluate it using the formula

  5. Phase space distribution EPA The MC generator used accounts for non negligible high pT values for e+e- and so for the h, that is different from EPA predictions

  6. signal e+e- hg p+p-p0g kinematics Signal Unreducible background Φ ηγ Mmiss2 vs π+ π- π0 longitudinal momentum from MC distributions

  7. Mmiss2and π+ π- π0 longitudinal momentum from MC distributions Φ ηγ Signal (GeV2) (MeV) kinematics

  8. Analysis criteria Event selection • at least 2 neutral prompt clusters with Eclu> 15 MeV • 100 MeV < Σ Eγ < 900 MeV • Eγ1 > 50 MeV • Only 2 neutral prompt clusters • Xγγ pairing < 8 • Cut on Xη • “Electron likelihood” cut • Cuts to reduce “pathological” background: • “Split track” cut • Eγ1< 225 MeV • sinθγ1> 0.462 • a > 30° • Kinematic cut • TRIGGER, FILFO • γγ filter (see KLOE Memo n.346), in detail: • 2 tracks with opposite charge from a cylinder with ρPCA < 8 cm, |zPCA|< 8 cm, ρfirst-hit < 50 cm

  9. Energy resolution Xγγ pairing: Xη: Kinematic fit using Lagrange multipliers method • 10 g variables • constraints

  10. c2h signal c2h hg c2h data c2gg signal X2pair distribution for data and MC signal c2gg data X2η distribution for low values of X2η:MC signal (red) and MC ηγ (blue) X2η distribution for data, MC signal and MC ηγ Very good agreement between official ALLRAD and private e+e-  e+e- eta MC productions

  11. Mη ≈ 550 MeV Mω ≈ 780 MeV + c2h< 400 e+e-g p+p-p0 wp0 h peak not visible • π+π- γγinvariant mass distribution (MeV): A) data events with npro=2 (black); B) npro=2 and X2pair < 8 (red) • π+π- γγinvariant mass distribution (MeV) after the cut Xη < 400 11

  12. Pairing with the monochromatic g! Eg1< 225 MeV and ch2<~20 to further reduce this contribution Energy of the most energetic photon (MeV) vs X2η for MC ηγ before and after the requirement npro=2

  13. QED background gN  e+e-N θπ+ vs θπ– (deg), data e+e- e+e-g Likelihood cut

  14. “Pathological” background After the electron likelihood cut SPLIT TRACKS

  15. Split tracks Event display |p(π+)|-|p(π-)| (MeV) vs θπ+π- (deg) • Large values of the angle between the two tracks • Opposite values of the momenta

  16. Split tracks Correlation between the tracks opening angle and the distance between the first hit of the positive track and the first hit of the negative track for data (top) and for signal simulated events (bottom) The cut applied is indicated by the straight line + a > 30° to further reduce background due to g N e+e-N

  17. QED background (2) data • Eγ1< 225 MeV • 27.5° < θγ1< 152.5° MC signal Cut out with the cuts on Eγ1 and θγ1 Eγ1 (MeV) vs θγ1 (deg)

  18. Kinematic cut Mmiss2 vs π+ π- π0 longitudinal momentum for data (top) and MC signal (bottom)

  19. Efficiencies and reduction factors

  20. A Trigger, filfo, data filter and track requirements efficiencies (GeV2) B (MeV) • Squared missing mass • Longitudinal momentum of π+π- π0 A (GeV2) Analysis cuts efficiencies B (MeV)

  21. signal pL distributions 21

  22. signal Mmiss2 distributions

  23. N(data) after cuts = 1576 Integrated luminosity L= 240 pb-1 Fractions Intervals for the fractions

  24. e+e- e+e-g Fit e+e-+- c2fit /d.o.f.= 75.9/83 c2fit /d.o.f.= 59.8/43 signal ch2 < 16

  25. e+e- e+e-g Fit e+e-+- c2fit /d.o.f.= 91.4/83 c2fit /d.o.f.= 38.4/43 signal ch2 < 8

  26. Results BR(hp+p-p0)=22.73%

  27. e1 (no pT cut) = 0.209 ± 0.003 e2 (pT cut) = 0.196 ± 0.003 Systematics • Output s(hg) value always 15% lower than expected value • Spread among results from different ch2 cuts ~ 11% • For a fixed ch2 cut, the difference between pL and Mmiss2 fits ~ 2% • With or without pT<100 MeV cut, related to the generated pT distribution for MC signal ~ 3% s(e+e- e+e-h, s=1 GeV) = (50±2stat±9syst) pb

  28. Conclusions • ggh events have been observed in the same mass range of interest for other studies (see ggp0p0) • The cross section e+e-e+e-hp+p-p0e+e- has been measured and found consistent with the EPA prediction even at small √s value compared with existing literatures

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