Measuring the branching ratio

of the K 00 decay

KLOE Memo n° 279 – December 2002

E. Gorini, M. Primavera, A. Ventura

A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

’ : World data vs. KLOE data

PDG (units 10–2)

1.73±0.04 (fit)

1.77±0.07 (average)

A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

’ K±±00

Analyzed DATA sample

• 112 pb–1 (Aug-Sep 2002)

Monte Carlo samples

• 1.5107 all events• 7.5 105 ’ decays

Datarec DBV-14 reconstruction

A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

’ event topology

0 0

– K–

K++0

Two tagging strategies: K0 “K’’ K “K’’

Self-triggering required for the tags

EMC trigger kpmfilt “tag” algo Cluster splitting recovery Track to cluster optimized

A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

Measurement of BR(’)

FILFOCV

FILFOCV

selbckg

bckg

tag

tag

BRN

NBR 20 )(

11

1

1)(

Ntag = number of tagged events

bckg = background fractions

’sel = efficiency to select ’ given the tag

BR(0) = (98.7980.032)% CV = trigger cosmic veto efficiency

FILFO = FILFO algorithm efficiency

(1 – /’) = “tag bias”

Event selection

A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

K / K self-tag

A K track according to any kpmfilt algo A 2-track vertex with K @ r>25cm

Daughter track momentum: p* < 135

MeV 4 clusters on-time with vertex:

Ei > 15 MeV , |(t–r/c)| < 4

Etot < 450 MeV

}K

vtx

clu

Etot

EtotcluvtxKsel '

}

A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

Efficiency evaluation (I)

Tracking and vertexing• Only EMC variables used

• K separately estimated for K+ and K– ( nuclear interaction)

K = 0.466 0.001stat 0.003syst

All the efficiencies have been extracted on data by means of variousSamples of Normalization. Systematic errors take into account thedifferences between the two tags used. MC has been used only for estimating background fractions.

v

p

• vtx dependence on p studied

vtx = 0.539 0.001stat 0.003syst

Efficiency evaluation (II)

Clustering

A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

E (MeV)

Aclu

• 4onT depends on the tag

onTcluclu A 44

Aclu4 = 0.799 0.001stat 0.003syst

{ 4onT

= 0.695 0.004

4onT

= 0.744 0.004

• Studied systematics on the |(t–r/c)|<4 cut• Checked wrong on-time cluster probability on data

• Etot = 0.9942 0.0014

Background evaluation

A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

Tags: bckg = 0.37%

bckg = 0.21%

Signal contaminations from: - Main K decays 0.46% - Kl4

00 decays 0.17% - Nuclear interactions 0.10% - Other rare K decays 0.03% - Other decays ~10–5

- Bhabha, “monotracks” ~10–5

bckg’ = (0.750.11)%

Estimations on MC corrected by comparing with data

Systematics and corrections

A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

Cosmic Veto : CV /CV

’ = 0.99860.0008

FILFO algorithm: |FILFO /FILFO

’ – 1| < 10–3

Dependence of BR on the neutral cluster definition

Splitting recovery tuning

Minimum energy cut Emin>15 MeV

Differences in K and K tags for track/vertex efficiencies

4on

T

Emin (MeV)1513 17

0.7

0.72

Nta

g’ /N

tag

0.74

Results and perspectives

A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

BR(’) = (1.8070.008stat0.018syst)%

K , clu and many systematics coincide for Kee

Ntag statistics 4.7 Total energy cut 1.4

Ntag statistics 0.3 Background subtraction 1.0

K

reconstruction/identification

6.1 Residual cluster systematics

5.4

Vertex reconstruction 7.1 Cosmic veto 0.8

Splitting recovery 2.3 FILFO algorithm <1

Four cluster acceptance 3.6 BR(0)2 0.7

On-time requirements 5.2 TOTAL UNCERTAINTY 11.7

Contributions to the total error (10–3 units)