Recent results and prospects for precision tests of the Standard Model in kaon decay-in-flight experiments at CERN are presented. A measurement of the ratio of leptonic decay rates of the charged ...kaon at the level of 0.4% precision constrains the parameter space of new physics models with extended Higgs sector, a fourth generation of quarks and leptons or sterile neutrinos. Searches for heavy neutrino mass states and the dark photon in the ∼ 100 MeV c2 mass range based on samples collected in 2003-2007 are in progress and prospects will be discussed. The NA62 experiment, starting in 2014, will search for a range of lepton number and lepton flavour violating decays of the charged kaon and the neutral pion at improved sensitivities down to ∼ 10-12, which will probe new physics scenarios involving heavy Majorana neutrinos or R-parity violating SUSY.
The KS→π+π−e+e− decay mode was investigated using the data collected in 2002 by the NA48/1 Collaboration. With about 23 k KS→π+π−e+e− events and 59 k KL→π+π−πD0 normalization decays, the KS→π+π−e+e− ...branching ratio relative to the KL→π+π−πD0 one was determined to be BR(KS→π+π−e+e−)/BR(KL→π+π−πD0)=(3.28±0.06stat±0.04syst)×10−2. This result was used to set the upper limit |gE1/gBR|<3.0 at 90% CL on the presence, in the decay amplitude, of an E1 direct emission (gE1) term relative to the dominant inner bremsstrahlung (gBR) term. The CP-violating asymmetry Aϕ in the sinϕcosϕ distribution of KS→π+π−e+e− events, where ϕ is the angle between the π+π− and the e+e− decay planes in the kaon centre of mass, was found to be Aϕ=(−0.4±0.8)%, consistent with zero. These results are in good agreement with a description of the KS→π+π−e+e− decay amplitude dominated by the CP-even inner bremsstrahlung process.
The K sub(S--pisuper+pisuper-esuper+esuper- decay mode was investigated using the data collected in 2002 by the NA48/1 Collaboration. With about 23 k K) sub(S)--pisuper+pisuper-esuper+esuper- events ...and 59 k inline imageinline image normalization decays, the K sub(S--pisuper+pisuper-esuper+esuper- branching ratio relative to the inline imageinline image one was determined to be inline imageinline image. This result was used to set the upper limit)
A sample of more than one million K±→π+π−e±ν (Ke4) decay candidates with less than one percent background contamination has been collected by the NA48/2 experiment at the CERN SPS in 2003–2004, ...allowing a detailed study of the decay properties. The branching ratio, inclusive of Ke4γ decays, is measured to be BR(Ke4)=(4.257±0.016exp±0.031ext)×10−5 with a total relative error of 0.8%. This measurement complements the study of S- and P-wave hadronic form factors by assigning absolute values to the relative hadronic form factors obtained earlier in a simultaneous analysis of the ππ scattering lengths conducted on the same data sample. The overall form factor normalization fs=5.705±0.017exp±0.031ext is obtained with a total relative precision of 0.6%.
Using the full data set of the NA48/2 experiment, the decay K±→π±e+e−γ is observed for the first time, selecting 120 candidates with 7.3±1.7 estimated background events. With K±→π±πD0 as ...normalisation channel, the branching ratio is determined in a model-independent way to be Br(K±→π±e+e−γ,meeγ>260 MeV/c2)=(1.19±0.12stat±0.04syst)×10−8. This measured value and the spectrum of the e+e−γ invariant mass allow a comparison with predictions of chiral perturbation theory.
The distribution of the K±→π±π+π− decays in the Dalitz plot has been measured by the NA48/2 experiment at the CERN SPS with a sample of 4.71×108 fully reconstructed events. With the standard Particle ...Data Group parameterization the following values of the slope parameters were obtained: g=(−21.134±0.017)%, h=(1.848±0.040)%, k=(−0.463±0.014)%. The quality and statistical accuracy of the data have allowed an improvement in precision by more than an order of magnitude, and are such as to warrant a more elaborate theoretical treatment, including pion–pion rescattering, which is in preparation.
A total of 368 415 Ξ0→Λπ0 and 31 171 Ξ0¯→Λ¯π0 were selected from data recorded in the NA48/1 experiment during 2002 data taking. From this sample, the polarization of Ξ0 and Ξ0¯ hyperons was measured ...to be PΞ0=−0.102±0.012(stat)±0.008(syst) and PΞ0¯=−0.01±0.04(stat)±0.008(syst). The dependence of PΞ0 on the Ξ0 transverse momentum with respect to the primary proton beam is also presented. With the same data sample, the ratio of Ξ0¯ and Ξ0 fluxes in proton collisions at 400 GeV/c on a beryllium target was measured.
A precision test of lepton flavour universality has been performed by measuring the ratio R sub(K of kaon leptonic decay rates Ksuper+-->esuper+nu and Ksuper+-->microsuper+nu in a sample of 59 813 ...reconstructed Ksuper+-->esuper+nu candidates with (8.71+/-0.24)% background contamination. The result R) sub(K)=(2.487+/-0.013)x10super-5 is in agreement with the Standard Model expectation.
We report a measurement of the direct CP violation asymmetry parameter Ag in charged kaon decays K±→π±π+π− by the NA48/2 experiment at the CERN SPS. The experiment has been designed not to be limited ...by systematics in the asymmetry measurement. Using 1.67×109 such decays collected during the 2003 run, the charge asymmetry in the Dalitz plot linear slope parameter g has been measured to be Ag=(1.7±2.9)×10−4. The precision of the result is limited by the statistics used.
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