Sterile neutrinos and RKR_K

Peer reviewe

Search for the dark photon in π0π^0 decays

A sample of $1.69 imes 10^7$ fully reconstructed $pi^0 ogamma e^+e^-$ decay candidates collected by the NA48/2 experiment at CERN in 2003--2004 is analysed to search for the dark photon ($A'$) production in the $pi^0 ogamma A'$ decay followed by the prompt $A' o e^+e^-$ decay. No signal is observed, and an exclusion region in the plane of the dark photon mass $m_A'$ and mixing parameter $arepsilon^2$ is established. The obtained upper limits on $arepsilon^2$ are more stringent than the previous limits in the mass range 9~ m MeV/c^2<70~ m MeV/c^2. The NA48/2 sensitivity to the dark photon production in the $K^pm opi^pm A'$ decay is also evaluated

Order p^6 chiral couplings from the scalar K Pi form factor

Employing results from a recent determination of the scalar KPi form factor F_0^KPi within a coupled channel dispersion relation analysis \cite{JOP01}, in this work we calculate the slope and curvature of F_0^KPi(t) at zero momentum transfer. Knowledge of the slope and curvature of the scalar KPi form factor, together with a recently calculated expression for F_0^KPi(t) in chiral perturbation theory at order p^6, enable to estimate the O(p^6) chiral constants C_12^r=(0.3 +- 5.4)10^-7 and (C_12^r+C_34^r)=(3.2 +- 1.5)10^-6. Our findings also allow to estimate the contribution coming from the C_i to the vector form factor F_+^KPi(0) which is crucial for a precise determination of |V_us| from K_l3 decays. Our result F_+^KPi(0)|_C_i^r=-0.018 +- 0.009, though inflicted with large uncertainties, is in perfect agreement with a previous estimate by Leutwyler and Roos already made twenty years ago.Comment: 19 pages, discussion of scale dependence of the chiral couplings added; version to appear in JHE

ChPT tests at the NA48 and NA62 experiments at CERN

The NA48/2 Collaboration at CERN has accumulated unprecedented statistics of rare kaon decays in the Ke4 modes: Ke4(+-) ($K^\pm \to \pi^+ \pi^- e^\pm \nu$) and Ke4(00) ($K^\pm \to \pi^0 \pi^0 e^\pm \nu$) with nearly one percent background contamination. The detailed study of form factors and branching rates, based on these data, has been completed recently. The results brings new inputs to low energy strong interactions description and tests of Chiral Perturbation Theory (ChPT) and lattice QCD calculations. In particular, new data support the ChPT prediction for a cusp in the $\pi^0\pi^0$ invariant mass spectrum at the two charged pions threshold for Ke4(00) decay. New final results from an analysis of about 400 $K^\pm \to \pi^\pm \gamma \gamma$ rare decay candidates collected by the NA48/2 and NA62 experiments at CERN during low intensity runs with minimum bias trigger configurations are presented. The results include a model-independent decay rate measurement and fits to ChPT description.Comment: XIIth International Conference on Heavy Quarks and Leptons 2014, Mainz, German

First observation and study of the K±→π0π0μ±νK^{\pm} \rightarrow \pi^{0} \pi^{0} \mu^{\pm} \nu decay

The NA48/2 experiment at CERN reports the first observation of the $K^{\pm} \rightarrow \pi^{0} \pi^{0} \mu^{\pm} \nu$ decay based on a sample of 2437 candidates with 15% background contamination collected in 2003--2004. The decay branching ratio in the kinematic region of the squared dilepton mass above $0.03$~GeV$^2/c^4$ is measured to be $(0.65 \pm 0.03) \times 10^{-6}$. The extrapolation to the full kinematic space, using a specific model, is found to be $(3.45 \pm 0.16) \times 10^{-6}$, in agreement with chiral perturbation theory predictions.Comment: 16 pages, 9 figure

Search for the dark photon in π0π^0 decays

A sample of $1.69\times 10^7$ fully reconstructed $\pi^0\to\gamma e^+e^-$ decay candidates collected by the NA48/2 experiment at CERN in 2003--2004 is analysed to search for the dark photon ($A'$) production in the $\pi^0\to\gamma A'$ decay followed by the prompt $A'\to e^+e^-$ decay. No signal is observed, and an exclusion region in the plane of the dark photon mass $m_{A'}$ and mixing parameter $\varepsilon^2$ is established. The obtained upper limits on $\varepsilon^2$ are more stringent than the previous limits in the mass range $9~{\rm MeV}/c^2m_{A

Searches for lepton number violation and resonances in K± → πμμ decays

The NA48/2 experiment at CERN collected a large sample of charged kaon decays to final states with multiple charged particles in 2003--2004. A new upper limit on the rate of the lepton number violating decay $K^{\pm}\to\pi^{\mp}\mu^{\pm}\mu^{\pm}$ is reported: $\mathcal{B}(K^{\pm}\to\pi^{\mp}\mu^{\pm}\mu^{\pm}

Measurement of the form factors of charged kaon semileptonic decays

A measurement of the form factors of charged kaon semileptonic decays is presented, based on 4.4 × 106K± → π0e±νe (Ke3 ±) and 2.3 × 106K± → π0μ±νμ (Kμ3 ±) decays collected in 2004 by the NA48/2 experiment. The results are obtained with improved precision as compared to earlier measurements. The combination of measurements in the Ke3 ± and Kμ3 ± modes is also presented. © 2018, The Author(s)