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

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)

Detailed study of the Ke4K_{e4} decay mode properties from the NA48/2 experiment at CERN

The NA48/2 Collaboration at CERN has accumulated an unprecedented statistics of rare semileptonic four-body $K_{e4}$ decay modes $K^{\pm}\rightarrow\pi^+\pi^-e\nu$ and $K^{\pm}\rightarrow\pi^0\pi^0e\nu$, with nearly 1$\%$ background contamination. Final results from the analyses of the full $K_{e4}$ data samples are described. The most accurate measurements to date of form factors and branching ratios have been achieved by NA48/2, bringing new inputs to low energy QCD description and stringent tests of predictions from Chiral Perturbation Theory and Lattice QCD

Detailed study of the K ± → π 0 π 0 e ± ν (K e4 00 ) decay properties

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 ππ 2 , M eν 2 ) and brings evidence for a cusp-like structure in the distribution of the squared π 0 π 0 invariant mass around 4m2π+ . 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