Measurement of the CP-Violation Parameterη+−γin Neutral Kaon Decays

Abstract

Interference between Ks and KL decays into the final state n*'n~ y has been observed in experiment 731 at Fermilab. By fitting the distribution of decays downstream of a regenerator, a new CP-violation parameter n+-y, analogous to //+-, has been measured, with its magnitude equal to (2.15 ±0.26 ±0.20)xl0 -3 , and 0+y , the associated phase angle, equal to (72±23±17)°. Assuming that any difference between this fit value for 77+y and the previously measured value for n+-can be attributed to direct CP violation in this decay, a limit of |6+_ y |/6 < 0.3 at the 90% confidence level has been made.Decays of the neutral kaons, KL and K §, have been studied in great detail because they represent the only known example of a system that exhibits violation of CP symmetry. A demonstration of this symmetry violation is the fact that the KL can decay into both CP-odd and CP-even final states [ll. Because mixing can occur in the neutral kaon system, it is possible to see an interference pattern in the proper time distribution of its decays [2]. We report here the first observation of K L -Ks interference for the decay into the final state K + K~Y, and our measurement of the level of CP violation in this decay. By comparing the magnitude of the observed CP violation to that of the K*n~ decay mode, a limit for the amount of direct CP violation in the decay has been obtained.The two electroweak eigenstates of neutral kaons can be described aswhere e is the complex parameter describing the amount of CP violation.Because the magnitude of e is small (~2.3xlO~~3), KL decays primarily into CP-odd states and K § into CP-dN oc F (E K )e-x [\p\ 2 e-rsr + I 12 ~ r z. T + 2|p| |r7|cos(Amr + 0 p -<t> v )e even states. In addition, CP violation may also be present in the decay amplitudes of Ks and KL. Such, so-called, "direct" CP violation is characterized for the two-pion decay by the complex parameter e and is predicted to occur within the standard model due to second order weak transitions. However, the two most recent experimental searches for such an effect have not unambiguously established the presence of direct CP violation [3,4].To search for evidence of direct CP violation in neutral kaon decay, Fermilab experiment 731 collected data on both KL and K § decaying simultaneously into the experimental apparatus. Two KL beams entered the active decay volume, whereupon one encountered a regenerator made of B4C. The regenerator consisted of four blocks of this material, totaling two interaction lengths, with a scintillator following each block to veto decays and interactions occurring inside the regenerator. The regenerator alternated between the two KL beams during the course of data taking to reduce systematic errors. (A more detailed description of the apparatus may be found elsewhere [3,5,6].) Because K° and A: 0 interact differently in matter, K § were regenerated from the KL beam. Downstream of the regenerator, the number of decays into a final state can be described as a function of proper time r as (r s +r L )T/2j(2)

Reanalysis of CP violation in KL→π+π-γ

“…The presence of CP violation in K L → π + π − γ has been observed in recent experiments [8,9], and this is characterized by the parameter η +−γ , defined in analogy to the parameter η +− in the K → π + π − case. However, the result has not yet reached the level of sensitivity needed for detecting direct CP violation in K L → π + π − γ.…”

“…Such an experiment typically [8,9] employs two K L beams, one of which is passed through a "regenerator", which coherently converts some K L to K S . It follows that the initial kaon state can be expressed as a coherent mixture…”

Kaon decay interferometry as meson dynamics probes

“…Concerning CP violation in this radiative nonleptonic process, and direct CP violation in particular, we recall that in the limit of CP conservation, and to the lowest contributing multipoles, for the K S decay h S M = 0 and the direct emission amplitude is determined by h S E1 (electric dipole moment E1), whereas K L decay proceeds only through h L M 1 (magnetic dipole moment M1) [18]. However, reflecting the bremsstrahlung enhancement for E * γ → 0, the CP violating A L IB is experimentally found to compete with the CP conserving DE amplitude h L M [20,21]. According to (18) A L IB is proportional to A(K L → π + π − ), where the direct CP violation parameter ǫ ′ ππ is very small, being suppressed by the ∆I = 1/2 rule.…”

Results on Direct CP Violation from NA48

“…CP violation was first observed in the decay K L → π + π − [1]. Effects have subsequently been found in K L → π 0 π 0 [2], the charge asymmetry of K L → e ± π ∓ ν (K e3 ) [3] and K L → µ ± π ∓ ν (K µ3 ) [4], K L → π + π − γ [5] and most recently in K L → ππee [6]. All of these effects can be explained by applying a single CP violating effect in the mixing between K 0 and K 0 which proceeds through the box Feynman diagram and are characterised by the parameter ε.…”