Precise measurement of the decay KL→π0γγ

Abstract

The decay rate of K L → π 0 γγ has been measured with the NA48 detector at the CERN SPS. A total of 2558 K L → π 0 γγ candidates have been observed with a residual background of 3.2%. The branching ratio is determined to be (1.36 ± 0.03 (stat) ± 0.03 (syst) ± 0.03 (norm) ) × 10 −6 and the vector coupling constant a v = −0.46 ± 0.03 (stat) ± 0.04 (syst) . This result suggests that the CP-violation effects are dominating in the K L → π 0 e + e − decay. An upper limit for the K L → π 0 γγ decay rate in the two photon mass region m γγ < m π 0 is also given.

Kaon Decays and the Flavour Problem

“…This amplitude does not interfere with the CP-violating one, and recent NA48 data on K L → π 0 γγ (at small dilepton invariant mass) indicate that it is very suppressed, with an impact on B(K L → π 0 e + e − ) below the 10 −12 level [32].…”

Recent Developments in Rare Kaon Decays

“…Two experiments have measured these decays in terms of one parameter, a V : KTeV [45] and NA48 [46]. Their results and spectrum are shown in from Fig.…”

On the electron–positron energy asymmetry in KL→π0e+e−

“…Very recently, the authors of Ref. [5], based on two new experimental results from NA48 Collaboration: the first observation of K S → π 0 e + e − [6] and the precise measurement of the K L → π 0 γγ spectrum [7], have argued that the CP-conserving part is essentially negligible for the rate of K L → π 0 e + e − , and the standard model (SM) contribution to its branching ratio has been predicted as BR(K L → π 0 e + e − ) SM ≃ 3 × 10 −11 , which is dominated by CP-violating components with about 40% from the direct CP-violating amplitude, through the interference with the indirect CPviolating one. Recalling the new experimental upper bound BR(K L → π 0 e + e − ) < 2.8×10…”

“…where c 2 = 1.1 is the coefficient ruling the strength of the unitarity corrections from K → 3π, β = −0.13, which is originated from the O(p 6 ) local terms except the vector resonance contribution, and the a V part characterizes the O(p 6 ) local contributions generated by the vector resonance exchange with a V = −0.46 ± 0.05 fixed in the recent NA48 experiment [7]. The functions F (z/r 2 π ) and R(z/r 2 π ) are generated by π-loop diagrams, which can be defined as…”

“…We would like to give some remarks on this point here. First, we use a V = −0.46 fixed in the new NA48 experiment [7] instead of a V = −0.96 in [4]; the vector form factor of K S → π 0 e + e − in Eq. (10) has been evaluated up to O(p 6 ) in chiral perturbation theory while only the leading 2 As pointed out in Ref.…”