Study of Study of 00

B.Di Micco, P.Gauzzi B.Di Micco, P.Gauzzi

KLOE General Meeting – Roma 13/11/2003

00• 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 0.300.16 (Ng-Peters) Vector+axial res. 0.470.20 (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

00

• Previous measurements (Br = (0)/tot()) :– GAMS-2000 (1981): (-pn) 6 105 produced ; 38 evts.

Br(0) = (9.52.3)10-4

– GAMS-2000 reanalysis (1984): Br(0) = (7.11.4)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 12040 evts. Br(0) = (2.70.9)10-4

43.81.9

0 10)(2.1γγ)πBr(η

• KLOE: with 2001 + 2002 statistics ~ 2 107 produced (same as Crystal Ball) expected 3 -- 6 103 0 evts. with = 1

Old analysisOld analysis(1) (2)

(3)

Standard 5 analysis:1st kin.fit + pairing + 2nd kin.fit• cut on M4 (combinatorial) 00 and 0 rejection:(1) Signal (MC)(2) Residual 00 (MC)(3) 000 (MC)(4) DataMain background: 000 only one 0

M2 (MeV)

(4)

Old analysisOld analysis

Etot (MeV)

• After cut on the 0 peak: 3900 events selected ( 15 %) S/B 0.3 – 0.5 • Background: 000 1) with lost photons asymmetric total energy 2) with merged clusters

• No clear signal of 0 000 background simulates signal

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

DATA – MC comparisonDATA – MC comparison

DATA MC – phi all

MC no sig. MC no sig -DATA

signal(GAMS Br)

After kinematic fit

E (MeV)E (MeV)

Data: 2002, L 250 pb-1 — MC: phi all (with acc.), L 80 pb-1

0

f0 a0sig

DATA – MC comparisonDATA – MC comparison

DATA MC – phi all

MC no sig. MC no sig -DATA

signal(GAMS Br)

E (MeV)E (MeV)

•After f0, a0 and 0 rejection

DATA MC – phi all

MC no sig. MC no sig -DATA

signal

DATA – MC comparisonDATA – MC comparison

Emax (MeV)Emax (MeV)Emax (MeV)

Diff.: Data – MC no signal• Max energy photon

363 MeV

Cluster merging Cluster merging

• Main problem for 0 : background from 000 − lost photons− merged clusters

• Exploit shower shape variables: Barrel : Yrms, Zrms (local coordinates) End cap: Xrms, Yrmsuse also the 3rd moments (X3, Y3, Z3)

• Discriminant analysis:

3XX X3

i

5

1iixαD

21

1 xxSα

S = cov. matrix

sample 1 = good sample 2 = merged

Barrel – rms (MC)Barrel – rms (MC)

good merged

Barrel – 3Barrel – 3rdrd moments (MC) moments (MC)

... Z3...,Y3

XX X33

X3

Y3 Z3

good merged

merged good

merged good

D(barr) D(ec)

• Cut: D(barr) < 11.6 D(ec) < 12.4

• Reduction factor : ~ 2 for signal (old analysis) 4 — 5 for 000

Cluster merging Cluster merging

E (MeV)

pto

t (M

eV)

Data-MC comparisonData-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)

D(barr)

D(barr)

good

merged

D(ec)

D(ec)

good

merged

Background reduction (MC)Background reduction (MC)• MC sample: ~ 55 pb-1

Before cut on D

Emax (MeV)

with signal without signal

After cut on D

Emax (MeV)

• Reduction factor: ~ 2 for signal ~ 3 for 000

L

L3

Crystal ballCrystal ball (from G.Lolosseminar at LNF)

Signal: -pnn0 Background: pn20n pn n30n6

L

Data Data -2

Data - 3L3Data - 2 - 3

120 40 evts.

ConclusionsConclusions

• 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 000 with lost photons