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\title{The NA62 experiment at CERN}

\author{Antonino Sergi, on behalf of the NA62 collaboration}

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\ead{antonino.sergi@cern.ch}

\begin{abstract}
The  rare  decays
are  theoretically  clean  processes  excellent  to  make  tests  of  new
physics at the highest scale complementary to LHC. The NA62 experiment at CERN SPS aims
to collect of the order of 100
events in two years of data taking, keeping the back-
ground less than 20\% of the signal. 
\end{abstract}

\section{Introduction}
Among the flavour changing neutral current $K$ and $B$ decays, the  $K\to\pi\nu\bar\nu$ decays play a key role in the search for new physics through the underlying mechanisms of flavour mixing. These decays are strongly suppressed in the SM (the highest CKM suppression), and are dominated by top-quark loop contributions. The SM branching ratios have been computed to high
precision with respect to other loop-induced meson decays: ${\rm
BR}(K^+\to\pi^+\nu\bar\nu)=8.22(75)\times 10^{-11}$ and ${\rm
BR}(K_L\to\pi^0\nu\bar\nu)=2.57(37)\times 10^{-11}$; the uncertainties are dominated by parametric ones, and the irreducible theoretical uncertainties are at a $\sim 1\%$ level~\cite{br11}. The theoretical cleanness of these decays remains also in certain new physics scenarios. Experimentally, the $K^+\to\pi^+\nu\bar\nu$ decay has been observed by the BNL E787/E949 experiments, and the measured branching ratio is
$\left(1.73^{+1.15}_{-1.05}\right)\times 10^{-10}$~\cite{ar09}. The
achieved precision is inferior to that of the SM expectation.

The main goal of the NA62 experiment at CERN is the measurement of the $K^+\to\pi^+\nu\bar\nu$ decay rate at the 10\% precision level, which would constitute a significant test of the SM. The experiment is expected to collect about 100 signal events in two years of data taking, keeping the systematic uncertainties and backgrounds low. Assuming a 10\% signal acceptance and the SM decay rate, the kaon flux should correspond to at least $10^{13}$ $K^+$ decays in the fiducial volume. In order to achieve a small systematic uncertainty, a rejection factor for generic kaon decays of the order of $10^{12}$ is required, and the background suppression factors need to be measured directly from the data. In order to achieve the required kaon intensity, signal acceptance and
background suppression, most of the NA48/NA62 apparatus used until 2008
was replaced with new detectors. The CERN SPS extraction line used by the NA48 experiment is capable of delivering beam intensity sufficient for the NA62. Consequently the new setup is housed at the CERN North Area High Intensity Facility where the NA48 was located. The decay in flight technique will be used; optimisation of the signal acceptance drives the
choice of a 75 GeV/$c$ charged kaon beam with 1\% momentum bite. The
experimental setup includes
a $\sim 100$~m long beam line to form the appropriate secondary
beam, a $\sim 80$~m long evacuated decay volume, and a series of
downstream detectors measuring the secondary particles from the
$K^+$ decays in the fiducial decay volume.

The signal signature is one track in the final state matched to one $K^+$ track in the beam. The integrated rate upstream is about 800 MHz (only 6\% of the beam particles are kaons, the others being mostly $\pi^+$ and protons). The rate seen by the detector downstream is about 10 MHz, mainly due to $K^+$ decays. Timing and
spatial information are required to match the upstream and downstream tracks. Backgrounds come from kaon decays with a single reconstructed track in the final state, including accidentally matched upstream and downstream tracks. The background suppression profits from the high kaon beam momentum. A variety of techniques are employed in combination in order to reach the required level of background rejection. They can be schematically divided into kinematic rejection, precise timing, highly efficient photon and muon veto systems, and precise particle identification systems to distinguish $\pi^+$, $K^+$ and positrons. The above requirements drove the design and the construction of the subdetector systems.

The main NA62 subdetectors are: a differential Cherenkov counter (CEDAR) on the beam line to identify the $K^+$ in the beam; a silicon pixel beam tracker; guard-ring counters surrounding the beam tracker to veto catastrophic interactions of particles; a downstream spectrometer composed of 4 straw chambers operating in vacuum; a RICH detector to identify pions and muons; a scintillator hodoscope; a muon veto detector. The photon veto detectors include a series of annular lead glass calorimeters surrounding the decay and detector volume, the NA48 LKr calorimeter, and two small angle calorimeters to provide hermetic coverage for photons emitted at close to zero angle to the beam. The design of the experimental apparatus and the R\&D of the new subdetectors have been completed. The experiment started collecting physics data in 2015, and since 2016 is fully commissioned and in its production phase.


\section{NA62 computing model and the role of CNAF}
NA62 raw data consist in custom binary files, collecting data packets directly from the DAQ electronics, after a minimal overall formatting; there is a one to one correspondence between files and spills from the SPS. Data contains up to 16 different level-0 trigger streams, for a total maximum bandwidth of 1 MHz, which are filtered by software algorithms to reduce the output rate to less than 50kHz.
Raw data is stored on CASTOR and promptly calibrated and reconstructed, on a scale of few hours, for data quality monitoring using the batch system at CERN and EOS. Near-line fast physics selection for data quality, off-line data processing and analysis is currently performed using only CERN computing facilities.

Currently NA62 exploits the GRID only for Monte Carlo productions, under the management of the UK GRID-PP collaboration members; in 2018 CNAF resources have been used as one of the GRID sites that serve NA62VO. 

\section*{References}
\begin{thebibliography}{99} % Use for 10-99 references
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\bibitem{br11}
J. Brod, M. Gorbahn and E. Stamou, Phys. Rev. {\bf D83}, 034030
(2011).
%
\bibitem{ar09}
A.V. Artamonov {\it et al.}, Phys. Rev. Lett. {\bf 101} (2008) 191802.
%
\end{thebibliography}

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