Few days before submitting the BR(’) paper to Physics Letters B (December 2003), an objection was raised to our evaluation of the ratio of tagging efficiencies in ’ (called “tag bias”). A dedicated meeting of K group + F.Bossi was held in Lecce (January 2004) in order to understand the discrepancies between different evaluations of “tag bias”. Misalignments in MC versions used in the analysis (2002) and later (2003) were found to be partially responsible for this. It was decided to wait for the last MC campaign (2004) to re-compute “tag bias” and re-check all the steps in the analysis which could be affected by such misalignments.

Since then, we have used all the MC releases produced in the meantime to:

1)estimate the “tag bias”,2)check the consistency of our efficiency evaluation method and 3)understand the effect of the different MC scenario on the definition of the samples of normalization.

• Monte Carlo versions used : “Old” MC 2002 production (analysis) “New” MC 2003 production “SuperNew” MC Feb 2004 (bugs on radiatives) “HyperNew” MC Mar 2004 (bugs fixed)

All ingredients in BR(’) have been recomputed one by one.No relevant differences wrt KLOE Note #187 found, except for:

Factor Rb’ introduced in BR formula (tag bias)

Ontime-clusters efficiency 4onT re-computed on data following the hints given by last MC’s BR(’)

=

N’tag

Ntag

Rb’

K vtx Aclu4 4onT Etot

1BR(0)2

1 – ’

1 – tag

Bias induced on signal selection

• The so-called “Tag Bias” Rb includes effects from: *) Event Trigger, *) Cosmic Veto, *) Event Classification, *) Tag selection, *) Self-triggering conditions• It has been computed on Monte Carlo, as it was discussed and decided last January in Lecce• Good agreement among various releases of last MC’s• Systematics on Rb evaluated from MC as fluctuations among different MC releases

Rb(K++00)2 = 1.1016 0.0082 0.0069

Rb(K++00)2 = 1.0587 0.0059 0.0047

Rb(K––00)2 = 1.0983 0.0081 0.0083

Rb(K––00)2 = 1.0544 0.0059 0.0080

Efficiency of finding 4 ’s ontime

Previously efficiencieswere found to be :

by imposing “Base”-cuts for K+d :

4 clusterssuch that:

K+d contamination (OLD MC):

3% (2 tag), 2% (2 tag)

K+d contamination (LAST MC’s):

>12% (2 tag), >7% (2 tag)

but…

Emin

Emax E’’’+Emax

Emin+E’’

Data – MC comparison : Energies (I)

Data – MC comparison : Energies (II)

Etot

E’’’ M12

E’’

Data – MC comparison : Energies (III)

E3 + E4

E4 = | E3+E4 – E1– E2| t’12

E1 + E2

Data – MC comparison : t2

Triangular cut :360 – E4(MeV) < 40 t2

t2 = min{[(t’1k2+t’2k

2)/2]1/2}k=3,4

Data – MC comparison : E4 vs. t2

Data

MC

t2

t2

E4

E4

Triangular cut :360 – E4(MeV) < 40 t2

Triangular cut :360 – E4(MeV) < 40 t2

Efficiency of finding 4 ’s ontime (now)

Then, applying this additional set of cuts, efficiencies are found to be

Besides “Base”-cuts, further “strict” cuts are imposed for K+d

The 4 clustersare such that:

• Cuts have been tuned on LAST MC’s in order to let the efficiency on new K+d reproduce the true 4onT

• Systematic error evaluated by studying the sensitivity to cuts’ definitions

Conclusions

• KLOE Note #187 & Paper draft (= KLOE Note #190) updated

BR(K++00)2 = ( 1.768 0.024 0.023 ) %BR(K++00)2 = ( 1.759 0.021 0.021 ) %BR(K––00)2 = ( 1.729 0.024 0.027 ) %BR(K––00)2 = ( 1.788 0.021 0.030 ) %

BR(K++00) = ( 1.764 0.019 0.021 ) %BR(K––00) = ( 1.759 0.019 0.023 ) %

BR(K00) = ( 1.765 0.013 0.022 ) %It was BR(K00) = ( 1.810 0.013 0.017 ) % on previous

paper draft

BR(K00)2 = ( 1.748 0.017 0.021 ) %BR(K00)2 = ( 1.774 0.015 0.019 ) %

Evaluation of 4onT through MC

bckgbckgonTtrue

onT PR

4

4 1

1

4onT = Measured efficiency in K+d SoN

P = Purity of K+d SoN after “strict” cuts (9596%)

bckg = Efficiency of background of K+d

= Fraction of signal lost after “strict” cuts (7%)

4onT (rejected signal)

4onT (total signal)true

true

R =

Cuts have been tuned on last MC’s so that 4onT and 4onT coincide true

Strategy

(65%)

About K and vtx

By using OLD-MC parameters: K = 0.46630.0032

By using HYPERNEW-MC parameters: K = 0.46320.0032

where Pbckg goes from 1.6% (OLD) to 12% (HYPERNEW)

On last MC’s v does not change even if background is increased

v(MC)= 0.60060.0062computed on signal only computed on signal+ 12%bckg

v(MC)= 0.60580.0058