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 – 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.