Measuring the branching ratio of the K      0  0 decay

1. Measuring the branching ratioof the K  00 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

2. ’ : World data vs. KLOE data ’ K±±00 • AnalyzedDATA sample • 112 pb–1(Aug-Sep 2002) • Monte Carlo samples • 1.5107all events • 7.5 105’ decays PDG(units 10–2) 1.73±0.04 (fit) 1.77±0.07 (average) Datarec DBV-14 reconstruction A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

3. ’ event topology • Two tagging strategies: • K0“K’’ • K “K’’   0 Self-triggering required for the tags + K+  – K– • EMC trigger • kpmfilt “tag” algo • Cluster splitting recovery • Track to cluster optimized 0 0    A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

4. Measurement of BR(’)    Ntag = number of tagged events bckg = background fractions ’sel = efficiency to select ’ given the tag BR(0) = (98.7980.032)% CV = trigger cosmic veto efficiency FILFO = FILFO algorithm efficiency (1 –  /’) = “tag bias” A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

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

6. Efficiency evaluation (I) All the efficiencies have been extracted on data by means of various Samples of Normalization. Systematic errors take into account the differences between the two tags used. MC has been used only for estimating background fractions. Tracking and vertexing • Only EMC variables used • Kseparately estimated for K+ and K–( nuclear interaction) v K = 0.466  0.001stat 0.003syst • vtxdependence onpstudied vtx = 0.539  0.001stat 0.003syst p A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

7. Efficiency evaluation (II) Clustering Aclu Aclu4 = 0.799  0.001stat 0.003syst • 4onTdepends on the tag { 4onT= 0.695  0.004 4onT= 0.744  0.004 E (MeV) • Studied systematics on the |(t–r/c)|<4cut • Checked wrong on-time cluster probability on data • Etot = 0.9942  0.0014 A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

8. Background evaluation Estimations on MC corrected by comparing with data • Tags: bckg = 0.37% bckg = 0.21% • Signal contaminations from: - Main K decays 0.46% - Kl400 decays 0.17% - Nuclear interactions 0.10% - Other rare K decays 0.03% - Other  decays ~10–5 - Bhabha, “monotracks” ~10–5 bckg’ = (0.750.11)% A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

9. Systematics and corrections 4onT • 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 Ntag’/Ntag 0.74 0.72 0.7 13 15 17 Emin (MeV) Cosmic Veto : CV /CV’ = 0.99860.0008 FILFO algorithm: |FILFO /FILFO’ – 1| < 10–3 A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002

10. Results and perspectives BR(’) = (1.8070.008stat0.018syst)% Contributions to the total error (10–3 units) K,cluand many systematics coincide for Kee A. Ventura – 61st KLOE General Meeting – Tor Vergata – 19-20/12/2002