Radiative corrections in Dalitz decays of π0, η and η′ mesons
We briefly summarize current experimental and theoretical results on the two important processes of the low-energy hadron physics involving neutral pions: the Dalitz decay of π0 and the rare decay π0 → e+e−. As novel results we present the complete set of radiative corrections to the Dalitz decays η(′) → l+l−γ beyond the soft-photon approximation, i.e. over the whole range of the Dalitz plot and with no restrictions on the energy of a radiative photon. The corrections inevitably depend on the η(′) → γ*γ(*) transition form factors.
Lepton-flavour violation in hadronic tau decays and μ-τ conversion in nuclei
Within the Standard Model Effective Field Theory framework, with operators up to dimension 6, we perform a model-independent analysis of the lepton-flavour-violating processes involving tau leptons. Namely, we study hadronic tau decays and $\ell$--$\tau$ conversion in nuclei, with $\ell = e,\mu$. Based on available experimental limits, we establish constraints on the Wilson coefficients of the operators contributing to these processes. Our work paves the way to extract the related information from Belle II and foreseen future experiments.
Radiative corrections for Dalitz decays of π0, η(′) and Σ0
Abstract We briefly summarize current experimental and theoretical results on the π 0 Dalitz decay, and provide a new value for the ratio R = Γ ( π 0 → e + e − γ ( γ ) ) / Γ ( π 0 → γ γ ) = 11.978 ( 6 ) × 10 − 3 , which is by two orders of magnitude more precise than the current PDG average. This value is obtained using the complete set of the next-to-leading-order radiative corrections in the QED sector, and incorporates up-to-date values of the π 0 -transition-form-factor slope. Furthermore, we present the complete set of radiative corrections for the Dalitz decays η ( ′ ) → l + l − γ beyond the soft-photon approximation. The corrections inevitably depend on the η ( ′ ) → γ ⁎ γ ( ⁎ ) tran…
Searches for lepton number violating $K^+$ decays
The NA62 experiment at CERN reports a search for the lepton number violating decays K+ -> pi(-)e(+)e(+) and K+ -> pi(-)mu(+)mu(+) using a data sample collected in 2017. No signals are observed, and upper limits on the branching fractions of these decays of 2.2 x 10(-10) and 4.2 x 10(-11) are obtained, respectively, at 90% confidence level. These upper limits improve on previously reported measurements by factors of 3 and 2, respectively.
Search for heavy neutral lepton production in K+ decays to positrons
A search for heavy neutral lepton ($N$) production in $K^+\to e^+N$ decays using the data sample collected by the NA62 experiment at CERN in 2017--2018 is reported. Upper limits of the extended neutrino mixing matrix element $|U_{e4}|^2$ are established at the level of $10^{-9}$ over most of the accessible heavy neutral lepton mass range 144--462 MeV/$c^2$, with the assumption that the lifetime exceeds 50 ns. These limits improve significantly upon those of previous production and decay searches. The $|U_{e4}|^2$ range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/$c^2$.
NA48/62 latest results
The NA62 experiment at the CERN SPS recorded in 2007 a large sample of K+ ? µ+?µ decays. A peak search in the missing mass spectrum of this decay is performed. In the absence of observed signal, the limits obtained on B(K+ ? µ+?h) and on the mixing matrix element |Uµ 4| are reported. The upgraded NA62 experiment started data taking in 2015. About 5×1011K+ decays have been recorded so far to measure the branching ratio of the K+ ? ?+?? decay. Preliminary results from the K+ ? ?+?? analysis based on about 5% of the 2016 statistics are reported.
Precise determination of the branching ratio of the neutral-pion Dalitz decay
We provide a new value for the ratio $R={\Gamma(\pi^0\to e^+e^-\gamma(\gamma))}/{\Gamma(\pi^0\to\gamma\gamma)}=11.978(6)\times10^{-3}$, which is by two orders of magnitude more precise than the current Particle Data Group average. It is obtained using the complete set of the next-to-leading-order radiative corrections in the QED sector, and incorporates up-to-date values of the $\pi^0$-transition-form-factor slope. The ratio $R$ translates into the branching ratios of the two main $\pi^0$ decay modes: $\mathcal{B}(\pi^0\to\gamma\gamma)=98.8131(6)\,\%$ and $\mathcal{B}(\pi^0\to e^+e^-\gamma(\gamma))=1.1836(6)\,\%$.
Search for heavy neutral lepton production in K+ decays
A search for heavy neutral lepton production in $K^+$ decays using a data sample collected with a minimum bias trigger by the NA62 experiment at CERN in 2015 is reported. Upper limits at the $10^{-7}$ to $10^{-6}$ level are established on the elements of the extended neutrino mixing matrix $|U_{\ell 4}|^2$ ($\ell=e,\mu$) for heavy neutral lepton mass in the range $170-448~{\rm MeV}/c^2$. This improves on the results from previous production searches in $K^+$ decays, setting more stringent limits and extending the mass range.
Search for Lepton Number and Flavor Violation in K+ and π0 Decays
Searches for the lepton number violating $K^{+} \rightarrow \pi^{-} \mu^{+} e^{+}$ decay and the lepton flavour violating $K^{+} \rightarrow \pi^{+} \mu^{-} e^{+}$ and $\pi^{0} \rightarrow \mu^{-} e^{+}$ decays are reported using data collected by the NA62 experiment at CERN in $2017$-$2018$. No evidence for these decays is found and upper limits of the branching ratios are obtained at 90% confidence level: $\mathcal{B}(K^{+}\rightarrow\pi^{-}\mu^{+}e^{+})<4.2\times 10^{-11}$, $\mathcal{B}(K^{+}\rightarrow\pi^{+}\mu^{-}e^{+})<6.6\times10^{-11}$ and $\mathcal{B}(\pi^{0}\rightarrow\mu^{-}e^{+})<3.2\times 10^{-10}$. These results improve by one order of magnitude over previous results for thes…