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Study of η → π0γγ Decay and Background Reduction Strategies

This study focuses on the η → π0γγ decay with theoretical predictions and experimental data analysis. Various background reduction methods are explored, such as cluster merging rejection and signal extraction techniques. Plans for further improvements in signal extraction are discussed.

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Study of η → π0γγ Decay and Background Reduction Strategies

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  1. Study of 0 B.Di Micco, P.Gauzzi KLOE General Meeting – Roma 13/11/2003

  2. 0 • For PT it is a unique test of O(p6) terms: • the leading term O(p2) is absent for massless quarks • the tree-level amplitude O(p4) is also zero • loop contributions O(p4)  (4)(0) = 4 – 7  10-3 eV • the chiral expansion starts from O(p6) • Theoretical predictions: (0) [eV] • VDM 0.300.16 (Ng-Peters) • Vector+axial res. 0.470.20 (Ko) • Quark-box diagram 0.70 – 0.92 (Ng-Peters, Nemoto et al.) • PT 0.42  0.20 (Ametller et al.) • PT 0.58 0.30 (Bellucci-Bruno) • The  invariant mass spectrum is sensitive to the model

  3. Previous measurements (Br = (0)/tot()) : • GAMS-2000 (1981): (-pn) 6  105 produced ; 38 evts. • Br(0) = (9.52.3)10-4 • GAMS-2000 reanalysis (1984): Br(0) = (7.11.4)10-4 • SND (2001):  ; 2.6  105 produced; 7 signal evts/170 found •  Br(0) < 8.4 10-4@90% C.L. • Crystal Ball (preliminary) : 2  107 produced • 12040 evts. Br(0) = (2.70.9)10-4 0 • KLOE: with 2001 + 2002 statistics  ~ 2  107 produced • (same as Crystal Ball) • expected 3 -- 6  103 0 evts. with  = 1

  4. (4) M2 (MeV) Old analysis • Standard 5 analysis: • 1st kin.fit + pairing • + 2nd kin.fit • cut on M4 (combinatorial) • 00 and 0 rejection: • Signal (MC) • Residual 00 (MC) • 000 (MC) • Data • Main background: • 000 •    •  •   only one 0 (1) (2) (3)

  5. Etot (MeV) Old analysis • After cut on the 0 peak: • 3900 events selected (  15 %) • S/B  0.3 – 0.5 • Background: 000 • 1) with lost photons  asymmetric • total energy • 2) with merged clusters • No clear signal of 0 • 000 background simulates signal

  6. A different analysis strategy • no recover-splitting procedure • Emin> 30 MeV • 5 prompt photons selection and f0, a0 and • 0 rejection similar to old analysis • no photon pairing and rad. photon assignment in the • hypothesis 0

  7. DATA MC – phi all signal (GAMS Br) MC no sig. MC no sig -DATA DATA – MC comparison Data: 2002, L  250 pb-1 — MC: phi all (with acc.), L  80 pb-1 After kinematic fit E (MeV) E (MeV)

  8. DATA MC – phi all signal (GAMS Br)  0 f0 a0g  sig MC no sig. MC no sig -DATA E (MeV) E (MeV) DATA – MC comparison • After f0, a0 and 0 rejection

  9. Diff.: Data – MC no signal signal MC – phi all DATA MC no sig -DATA MC no sig. 363 MeV Emax (MeV) DATA – MC comparison • Max energy photon Emax (MeV) Emax (MeV)

  10. Cluster merging • Main problem for 0 : background from 000 • lost photons • merged clusters • Exploit shower shape variables:  Barrel : Yrms, Zrms (local coordinates) • End cap: Xrms, Yrms • use also the 3rd moments (X3, Y3, Z3) • Discriminant analysis: sample 1 = good  sample 2 = merged  S = cov. matrix

  11. Barrel – rms (MC) good  merged 

  12. merged  good  merged  good  good  merged  Barrel – 3rd moments (MC) X3 Y3 Z3

  13. Cut: D(barr) < 11.6 • D(ec) < 12.4 Cluster merging D(barr) D(ec) • Reduction factor : ~ 2 for signal • (old analysis) 4 — 5 for 000

  14. good  good  D(ec) D(barr) ptot (MeV) merged merged E (MeV) D(barr) D(ec) Data-MC comparison • Sample A:     7; 7 prompt clusters • Sample B:     7; 6 prompt clusters with a cut on E vs ptot • Sample B: • 5/6 good  + 1/6 merged  • Xi(B)-5/6 Xi(A)= Xi(merg)

  15. Before cut on D After cut on D • with signal • without signal Emax (MeV) Emax (MeV) Background reduction (MC) • MC sample: ~ 55 pb-1 • Reduction factor: ~ 2 for signal • ~ 3 for 000

  16. L L  Data Data -2 120  40 evts. L3 L3 Data - 3 Data - 2 - 3 Crystal ball (from G.Lolos seminar at LNF) Signal: -pnn0 Background: pn20n pn n30n6

  17. Conclusions • Difficult to extract the 0 signal • We are running the new analysis (with the merging rejection) • on the whole 2001+2002 statistics • Plans: • improve the merging rejection • use the QCAL information to reject 000 with lost photons

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