CERN Accelerating science

{\bf Left}: Total acceptance $A_{\pi\nu\nu}$ in bins of \pic momentum, and in Regions 1, 2 separately with their respective uncertainties. {\bf Right}: signal efficiency $\varepsilon_{RV}$ in bins of instantaneous beam intensity after photon and multiplicity rejection with total uncertainty, after photon rejection, after IRC and SAC veto only, after LAV veto only, after LKr veto only. Lines are for eye guidance only. The instantaneous intensity (estimated from out-of-time activity in GTK) can vary up to a factor of two within a spill with respect to the average intensity.

{\bf Left}: reconstructed $\mmis$ distribution of the~\mng control events selected by assigning the $\pi^+$ mass to the $\mu^+$ for data (dots), and for two MC~\mng samples superimposed: one selected as in data (red line), the other selected as~\pnnc without particle identification (blue line, referred as MC~\mng in the legend). Signal regions 1 and 2 are also shown. {\bf Right}: expected~\mng background in bins of \pic momentum compared to the expected number of SM~\pnnc events.

Schematic top view of the NA62 beam line and detector. The trajectory is shown of an un-decayed beam particle in vacuum, crossing the detector apertures which avoid interactions with material. A dipole magnet between MUV3 and SAC deflects the charged particles of the beam out of the SAC acceptance.

{\bf Left}: true $\mmis$ distribution of the \pnnc decay and the main \kp decays, computed under \pic mass hypothesis for the charged particle in the final state. Signal (red) is multiplied by $10^{10}$ for visibility and the dashed areas show the signal search regions. {\bf {\bf Right}}: reconstructed $\mmis$ as a function of \pic momentum for {\it control events} selected without applying \pic identification and photon rejection. Signal regions 1 and 2, as well as the $3\pi$, $\pi^+\pi^0$ and $\mu^+\nu$ background regions are also shown. The control regions located between the signal and background regions are indicated by dashed lines.

Schematic top view of the NA62 beam line and detector. Dipole magnets are displayed as boxes with superimposed crosses. The trajectory is shown of an un-decayed beam particle in vacuum, crossing the detector apertures which avoid interactions with material. A dipole magnet between MUV3 and SAC deflects the charged particles of the beam out of the SAC acceptance.

{\bf Left}: true $\mmis$ distribution of the \pnnc decay and the main \kp decays, computed under \pic mass hypothesis for the charged particle in the final state. Signal (red) is multiplied by $10^{10}$ for visibility and the dashed areas show the signal search regions. {\bf {\bf Right}}: reconstructed $\mmis$ as a function of \pic momentum for {\it control events} selected without applying \pic identification and photon rejection. Signal regions 1 and 2, as well as the $3\pi$, $\pi^+\pi^0$ and $\mu^+\nu$ background regions are also shown. The control regions located between the signal and background regions are indicated by dashed lines.

{\bf Left}: reconstructed $\mmis$ distribution of the~\mng control events selected by assigning the $\pi^+$ mass to the $\mu^+$ for data (dots), and for two MC~\mng samples superimposed: one selected as in data (red line), the other selected as~\pnnc without particle identification (blue line, referred as MC~\mng in the legend). Signal regions 1 and 2 are also shown. {\bf Right}: expected~\mng background in bins of \pic momentum compared to the expected number of SM~\pnnc events.

{\bf Left}: reconstructed $\mmis$ distribution of the~\pp {\it control events} selected from data by tagging the $\pi^0$ (dots, see text for details). Two~\pp MC samples are superimposed: one selected as in data (red line), the other selected as~\pnnc (blue line, referred to as MC $K^+\rightarrow\pi^+\pi^0(\gamma)$ in the legend). Signal regions 1 and 2 are also shown. The MC distributions are normalised to the data in the $\pi^+\pi^0$ region. {\bf Right}: expected~\ppg background in bins of \pic momentum compared to the expected number of SM~\pnnc events.

{\bf Left}: Total acceptance $A_{\pi\nu\nu}$ in bins of \pic momentum, and in Regions 1, 2 separately with their respective uncertainties. {\bf Right}: signal efficiency $\varepsilon_{RV}$ in bins of instantaneous beam intensity after photon and multiplicity rejection with total uncertainty, after photon rejection, after IRC and SAC veto only, after LAV veto only, after LKr veto only. Lines are for eye guidance only. The instantaneous intensity (estimated from out-of-time activity in GTK) can vary up to a factor of two within a spill with respect to the average intensity.

{\bf Left}: reconstructed $\mmis$ as a function of \pic momentum for {\it PNN-triggered events} (markers) satisfying the~\pnnc selection, except the $\mmis$ and \pic momentum criteria. The grey area corresponds to the expected distribution of~\pnnc MC events. Red contours define the signal regions. The event observed in Region~2 is shown together with the events found in the control regions. {\bf Right}: Signal in RICH (open circles) detected in the \pnnc candidate event with rings superimposed as built under different particle mass hypotheses.

{\bf Left}: reconstructed $\mmis$ as a function of \pic momentum for {\it PNN-triggered events} (markers) satisfying the~\pnnc selection, except the $\mmis$ and \pic momentum criteria. The grey area corresponds to the expected distribution of~\pnnc MC events. Red contours define the signal regions. The event observed in Region~2 is shown together with the events found in the control regions. {\bf Right}: Signal in RICH (open circles) detected in the \pnnc candidate event with rings superimposed as built under different particle mass hypotheses.

{\bf Left}: transverse position at the FV entrance of pions from a data sample enriched with upstream events. Blue lines correspond to the contour of the last dipole of the second achromat; red lines show the contour of the final collimator; the black line indicates the acceptance region covered by CHANTI. {\bf Right}: time difference between RICH and KTAG versus that of GTK and KTAG for the same sample of pions.

{\bf Left}: transverse position at the FV entrance of pions from a data sample enriched with upstream events. Blue lines correspond to the contour of the last dipole of the second achromat; red lines show the contour of the final collimator; the black line indicates the acceptance region covered by CHANTI. {\bf Right}: time difference between RICH and KTAG versus that of GTK and KTAG for the same sample of pions.

{\bf Left}: reconstructed $\mmis$ distribution of the~\pp {\it control events} selected from data by tagging the $\pi^0$ (dots, see text for details). Two~\pp MC samples are superimposed: one selected as in data (red line), the other selected as~\pnnc (blue line, referred to as MC $K^+\rightarrow\pi^+\pi^0(\gamma)$ in the legend). Signal regions 1 and 2 are also shown. The MC distributions are normalised to the data in the $\pi^+\pi^0$ region. {\bf Right}: expected~\ppg background in bins of \pic momentum compared to the expected number of SM~\pnnc events.