Study of  0 

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