Abstract
A sample of 65210 K ± → π 0 π 0 e ± ν (K e4 00 ) decay candidates with 1% background contamination has been collected in 2003-2004 by the NA48/2 collaboration at the CERN SPS. A study of the differential rate provides the first measurement of the hadronic form factor variation in the plane (M 2 , M 2 ) and brings evidence for a cusp-like structure in the distribution of the squared π 0 π 0 invariant mass around $$ 4{m}_{\pi^{+}}^2 $$ . Exploiting a model independent description of this form factor, the branching ratio, inclusive of radiative decays, is obtained using the K ± → π 0 π 0 π ± decay mode as normalization. It is measured to be BR(K e4 00 ) = (2.552 ± 0.010stat ± 0.010syst ± 0.032ext) × 10−5, which improves the current world average precision by an order of magnitude while the 1.4% relative precision is dominated by the external uncertainty from the normalization mode. A comparison with the properties of the corresponding mode involving a π + π − pair (K e4 + − ) is also presented.
Highlights
Background estimateThe K030π decay is the most significant background source contributing to the K0e40 signal
Where the error is dominated by the external uncertainty from the normalization mode branching ratios (BR)(K030π) = (1.761 ± 0.022)% [10]
The BR(K0e40) values obtained for the ten statistically independent subsamples are shown in figure 12, in agreement with the values measured separately for K+ and K−: BR(K+e4) = (2.548 ± 0.013) × 10−5, BR(K−e4) = (2.558 ± 0.018) × 10−5, where the quoted uncertainties include statistical and time-dependent systematic contributions
Summary
Background estimateThe K030π decay is the most significant background source contributing to the K0e40 signal. The shape of the background can be modified by extending further out the ellipse cut in the (M3π, pt) plane (section 4): due to the location of the fake-electron background close to the ellipse cut boundary, its fraction varies rapidly from 0.65% to 0.50%, 0.39% and 0.31% when increasing the ellipse main axes by 10%, 20% and 30% of their nominal values while the signal loss (estimated from simulation) is 1.2%, 2.7% and and 4.4%, respectively This changes both the rate and the shape of the fake-electron background while the relative fraction of background from π± → e±ν decays (0.12%) and accidentals (0.22%) are unaffected. The half difference between the evaluations based on two control subregions (restricted to E/p ranges from 0.2 to 0.45 and from 0.45 to 0.7, respectively, see figure 3) is assigned as an additional systematic error of ±5 events and added in quadrature This background contributes δBR/BR = 1 × 10−4.
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