The NA62 experiment (known as P-326 at the stage of the proposal) is a fixed-target particle physics experiment in the North Area of the SPS accelerator at CERN. The experiment was approved in February 2007. Data taking began in 2015, and the experiment is expected to become the first in the world to probe the decays of the charged kaon with probabilities down to 10−12. The experiment's spokesperson is Giuseppe Ruggiero (since October 2022; the former spokespersons were Cristina Lazzeroni 2019–2022 and Augusto Ceccucci before 2019). The collaboration involves 308 participants from 33 institutions and 16 countries around the world.
Goals
The experiment is designed to conduct precision tests of the Standard Model by studying rare decays of charged kaons. The principal goal, for which the design has been optimized, is the measurement of the rate of the ultra-rare decay K+ → π+ + ν + ν with a precision of 10%, by detecting about 100 decay candidates with low background. This will lead to the determination of the CKM matrix element |Vtd| with a precision better than 10%. This element relates very accurately to the likelihood that top quarks decay to down quarks. The Particle Data Group's 2008 Review of Particle Physics lists |Vtd| = 0.00874+0.00026−0.00037. A broad program of studies of kaon physics is run in parallel including studies of other rare decays, searches for forbidden decays, and for new exotic particles not predicted by the standard model (for example Dark Photons).
Experimental Apparatus In order to achieve the desired precision, the NA62 experiment requires a certain level of signal sensitivity and background rejection. Namely, high-resolution timing (to support a high-rate environment), kinematic rejection (involving the cutting on the square of the missing mass of the observed charged particle from the decay with respect to the incident kaon vector), particle identification (especially distinguishing between pions and muons), hermetic vetoing of photons out to large angles, and redundancy of information. Due to these necessities, the NA62 experiment has constructed a detector which is approximately 270 m in length. The components of the experiment are explained briefly below, for full details see.
Beam Line The foundation of the NA62 experiment is observing the decays of kaons. A primary beam of protons from the SPS is delivered to a target at which a secondary hadron beam is produced which contains the kaons. The Primary Beam, called P42, is used for the production of the K + {\displaystyle K^{+}} beam. The 400 GeV/c SPS proton beam is split into three branches and strikes three targets (T2, T4, and T6). This produces beams of secondary particles which are directed through the underground target tunnel (TCC2). At the exit of T4, the beam of transmitted protons passes through apertures in two vertically-motorized beam-dump/collimator modules, TAX 1 and TAX 2 for P42, in which holes of different apertures define the angular acceptance of the beam and hence allow the flux of protons to be selected over a wide range. In order to protect the components of the apparatus, a computer surveillance program allows the currents in the principle magnets along the P42 beam line to be monitored and to close TAX2 in case of error. A secondary beam line, K12HIKA+, is the kaon beam line. The target/beam tunnel, TCC8, and the cavern, ECN3, where the detectors of the NA62 experiment have been installed, have a combined length of 270m. The T10 target (located 15m from the beginning of TCC8) is used to produce the final secondary hadron beam (K12). This K12 beam-line has a length of 102m, ending at the exit of the final collimator which marks the beginning of the decay fiducial region and points to the NA62 detectors (notably the liquid krypton electro-magnetic calorimeter, LKr). The K12 beam has an instantaneous rate of about 600 MHz and is composed primarily of pions (70%) and protons (23%) with only 6% being kaons. Approximately 10% of the kaons decay in the fiducial decay volume, corresponding to a rate of 4 MHz of kaon decays in the fiducial region.
Cedar/KTAG The KTAG is the 'kaon tagger', designed to identify K + {\displaystyle K^{+}} particles within the unseparated hadron beam. This detector is a differential Cherenkov counter (CERN west-area Cedar until 2022, replaced by a new Cedar-H in 2023), instrumented with a bespoke detector consisting of 8 arrays of photodetectors (KTAG).
GigaTracker (GTK) Placed immediately upstream of the fiducial decay region, the GTK is designed to measure the time, direction, and momentum of all the beam tracks. The GTK is a spectrometer and provides the measurements of the incoming 75 GeV/c kaon beam. The GTK measurements are used for decay selections and background reduction. The GTK is composed of four hybrid silicon pixel tracker stations labeled GTK0 (installed in 2021) GTK1, GTK2, and GTK3 based on the order in which they are found relative to the beam path. They are mounted around four achromat magnets (which are used to deflect the beam). The entire system is placed along the beam line and is inside the vacuum tank.
CHANTI This charged anti-counter detector (CHANTI) is primarily designed to veto events with inelastic interactions between beam particles and the GTK3. The detector is constructed from six planes of scintillator detectors surrounding the beam.
Straw Tracker The kaon beam passes through the upstream region and into the decay region, a roughly 60 m long region inside a large vacuum vessel, after which the decay products are detected in the straw tracker stations. The system measures the direction and the momentum of secondary charged particles which come from the decay region. This spectrometer is made with four chambers intersected with a high aperture dipole magnet. Each of the chambers consists of multiple straw tubes positioned to offer four views to give four coordinates. Out of 7168 straws in the whole system, only one was flawed. The leaking straw was sealed and the detector operated normally during the 2015 run.
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