Search for π⁰ decays to invisible particles

The NA62 experiment at the CERN SPS reports a study of a sample of 4 × 109 tagged π0 mesons from K+ → π+π0(γ), searching for the decay of the π0 to invisible particles. No signal is observed in excess of the expected background fluctuations. An upper limit of 4.4 × 10−9 is set on the branching ratio at 90% confidence level, improving on previous results by a factor of 60. This result can also be interpreted as a model- independent upper limit on the branching ratio for the decay K+ → π+X, where X is a particle escaping detection with mass in the range 0.110–0.155 GeV/c2 and rest lifetime greater than 100 ps. Model-dependent upper limits are obtained assuming X to be an axion-like particle with dominant fermion couplings or a dark scalar mixing with the Standard Model Higgs boson

Measurement of the very rare K + → π+νν¯ decay

The NA62 experiment reports the branching ratio measurement BR(K+→π+νν¯)=(10.6−3.4+4.0|stat±0.9syst)×10−11 at 68% CL, based on the observation of 20 signal candidates with an expected background of 7.0 events from the total data sample collected at the CERN SPS during 2016–2018. This provides evidence for the very rare K+→π+νν¯ decay, observed with a significance of 3.4σ. The experiment achieves a single event sensitivity of (0.839 ± 0.054) × 10−11, corresponding to 10.0 events assuming the Standard Model branching ratio of (8.4 ± 1.0) × 10−11. This measurement is also used to set limits on BR(K+→ π+X), where X is a scalar or pseudo-scalar particle. Details are given of the analysis of the 2018 data sample, which corresponds to about 80% of the total data sample

Recent results in kaon physics

A review of the present experimental status of the K → πνν (Kπνν) and other kaon decay analyses at experiments NA62 (CERN) and KOTO (J-PARC) is given. The Kπνν decay is one of the best candidates among the rare meson decays for indirect searches for new physics in the mass ranges complementary to those accessible by current accelerators. The Standard Model (SM) prediction of the branching fraction (B) of the Kπνν decay is lower than 10−10 in both neutral and charged modes. The NA62 experiment aims to measure the B of the charged mode with better than 10% precision. Three candidate events, compatible with the SM prediction, have been observed from a sample of 2.12×1012 K+ decays collected in 2016 and 2017 by NA62. More than twice the statistics is available in the 2018 dataset currently being analysed. The KOTO experiment in Japan aims to measure B(KL → π0νν) using a technique similar to NA62, but with much lower momentum. In the first dataset taken in 2015 zero signal candidate events were observed. The current status of the analysis of the 2016-2018 dataset with 1.4 times more data is presented. Finally, the most recent results of other physics analyses at the NA62 experiment are summarised

The beam and detector of the NA62 experiment at CERN

NA62 is a fixed-target experiment at the CERN SPS dedicated to measurements of rare kaon decays. Such measurements, like the branching fraction of the K+ → π+ ν bar nu decay, have the potential to bring significant insights into new physics processes when comparison is made with precise theoretical predictions. For this purpose, innovative techniques have been developed, in particular, in the domain of low-mass tracking devices. Detector construction spanned several years from 2009 to 2014. The collaboration started detector commissioning in 2014 and will collect data until the end of 2018. The beam line and detector components are described together with their early performance obtained from 2014 and 2015 data

An unshielded radio-frequency atomic magnetometer with sub-femtoTesla sensitivity

We demonstrate a radio-frequency potassium-vapor magnetometer operating with sensitivities of 0.3 fT/ Hz at 0.5 MHz and 0.9 fT/ Hz at 1.31 MHz in the absence of radio-frequency and mu-metal or magnetic shielding. The use of spatially separated magnetometers, two voxels within the same cell, permits for the subtraction of common mode noise and the retention of a gradient signal, as from a local source. At 0.5 MHz the common mode noise was white and measured to be 3.4 fT/ Hz ; upon subtraction the noise returned to the values observed when the magnetometer was shielded. At 1.31 MHz, the common mode noise was from a nearby radio station and was reduced by a factor of 33 upon subtraction, limited only by the radio signal picked up by receiver electronics. Potential applications include in-the-field low-field magnetic resonance, such as the use of nuclear quadrupole resonance for the detection of explosives

Search for a feebly interacting particle X in the decay K⁺ → π⁺X

A search for the K+ → π+X decay, where X is a long-lived feebly interacting particle, is performed through an interpretation of the K+ → π+νν¯ analysis of data collected in 2017 by the NA62 experiment at CERN. Two ranges of X masses, 0–110 MeV/c2 and 154–260 MeV/c2, and lifetimes above 100 ps are considered. The limits set on the branching ratio, BR(K+ → π+X), are competitive with previously reported searches in the first mass range, and improve on current limits in the second mass range by more than an order of magnitude

Improved calorimetric particle identification in NA62 using machine learning techniques

Abstract Measurement of the ultra-rare K + → π + ν ν ¯ $${K}^{+}\to {\pi}^{+}\nu \overline{\nu}$$ decay at the NA62 experiment at CERN requires high-performance particle identification to distinguish muons from pions. Calorimetric identification currently in use, based on a boosted decision tree algorithm, achieves a muon misidentification probability of 1.2 × 10 −5 for a pion identification efficiency of 75% in the momentum range of 15–40 GeV/c. In this work, calorimetric identification performance is improved by developing an algorithm based on a convolutional neural network classifier augmented by a filter. Muon misidentification probability is reduced by a factor of six with respect to the current value for a fixed pion-identification efficiency of 75%. Alternatively, pion identification efficiency is improved from 72% to 91% for a fixed muon misidentification probability of 10 −5

A search for the K+ → μ−νe+e+ decay

A search for the K+→μ−νe+e+ decay, forbidden within the Standard Model by either lepton number or lepton flavour conservation depending on the flavour of the emitted neutrino, has been performed using the dataset collected by the NA62 experiment at CERN in 2016–2018. An upper limit of 8.1×10−11 is obtained for the decay branching fraction at 90% CL, improving by a factor of 250 over the previous search

Searches for lepton number violating K+ → π−(π0)e+e+ decays

none209siINFN è stato l'ente italiano che ha finanziato l'esperimento NA62 al CERNSearches for lepton number violating K+ decays have been performed using the complete dataset collected by the NA62 experiment at CERN in 2016–2018. Upper limits of 5.3 10^-11 and 8.5 10^-10 are obtained on the decay branching fractions at 90% confidence level. The former result improves by a factor of four over the previous best limit, while the latter result represents the first limit on the decay rate k+ > pi- pi0 e+ e+.openCortina Gil, E.; Kleimenova, A.; Minucci, E.; Padolski, S.; Petrov, P.; Shaikhiev, A.; Volpe, R.; Numao, T.; Petrov, Y.; Velghe, B.; Wong, V.W.S.; Bryman, D.; Fu, J.; Husek, T.; Jerhot, J.; Kampf, K.; Zamkovsky, M.; Aliberti, R.; Khoriauli, G.; Kunze, J.; Lomidze, D.; Peruzzo, L.; Vormstein, M.; Wanke, R.; Dalpiaz, P.; Fiorini, M.; Neri, I.; Norton, A.; Petrucci, F.; Wahl, H.; Cotta Ramusino, A.; Gianoli, A.; Iacopini, E.; Latino, G.; Lenti, M.; Parenti, A.; Bizzeti, A.; Bucci, F.; Antonelli, A.; Georgiev, G.; Kozhuharov, V.; Lanfranchi, G.; Martellotti, S.; Moulson, M.; Spadaro, T.; Tinti, G.; Ambrosino, F.; Capussela, T.; Corvino, M.; Di Filippo, D.; Fiorenza, R.; Massarotti, P.; Mirra, M.; Napolitano, M.; Saracino, G.; Anzivino, G.; Brizioli, F.; Imbergamo, E.; Lollini, R.; Piandani, R.; Santoni, C.; Barbanera, M.; Cenci, P.; Checcucci, B.; Lubrano, P.; Lupi, M.; Pepe, M.; Piccini, M.; Costantini, F.; Di Lella, L.; Doble, N.; Giorgi, M.; Giudici, S.; Lamanna, G.; Lari, E.; Pedreschi, E.; Sozzi, M.; Cerri, C.; Fantechi, R.; Pontisso, L.; Spinella, F.; Mannelli, I.; D'Agostini, G.; Raggi, M.; Biagioni, A.; Cretaro, P.; Frezza, O.; Leonardi, E.; Lonardo, A.; Turisini, M.; Valente, P.; Vicini, P.; Ammendola, R.; Bonaiuto, V.; Fucci, A.; Salamon, A.; Sargeni, F.; Arcidiacono, R.; Bloch-Devaux, B.; Boretto, M.; Menichetti, E.; Migliore, E.; Soldi, D.; Biino, C.; Filippi, A.; Marchetto, F.; Engelfried, J.; Estrada-Tristan, N.; Bragadireanu, A.M.; Ghinescu, S.A.; Hutanu, O.E.; Baeva, A.; Baigarashev, D.; Emelyanov, D.; Enik, T.; Falaleev, V.; Kekelidze, V.; Korotkova, A.; Litov, L.; Madigozhin, D.; Misheva, M.; Molokanova, N.; Movchan, S.; Polenkevich, I.; Potrebenikov, Yu.; Shkarovskiy, S.; Zinchenko, A.; Fedotov, S.; Gushchin, E.; Khotyantsev, A.; Kudenko, Y.; Kurochka, V.; Medvedeva, M.; Mefodev, A.; Kholodenko, S.; Kurshetsov, V.; Obraztsov, V.; Ostankov, A.; Semenov, V.; Sugonyaev, V.; Yushchenko, O.; Bician, L.; Blazek, T.; Cerny, V.; Kucerova, Z.; Bernhard, J.; Ceccucci, A.; Danielsson, H.; De Simone, N.; Duval, F.; Döbrich, B.; Federici, L.; Gamberini, E.; Gatignon, L.; Guida, R.; Hahn, F.; Holzer, E.B.; Jenninger, B.; Koval, M.; Laycock, P.; Lehmann Miotto, G.; Lichard, P.; Mapelli, A.; Marchevski, R.; Massri, K.; Noy, M.; Palladino, V.; Perrin-Terrin, M.; Pinzino, J.; Ryjov, V.; Schuchmann, S.; Venditti, S.; Bache, T.; Brunetti, M.B.; Duk, V.; Fascianelli, V.; Fry, J.R.; Gonnella, F.; Goudzovski, E.; Henshaw, J.; Iacobuzio, L.; Lazzeroni, C.; Lurkin, N.; Newson, F.; Parkinson, C.; Romano, A.; Sergi, A.; Sturgess, A.; Swallow, J.; Tomczak, A.; Heath, H.; Page, R.; Trilov, S.; Angelucci, B.; Britton, D.; Graham, C.; Protopopescu, D.; Carmignani, J.; Dainton, J.B.; Jones, R.W.L.; Ruggiero, G.; Fulton, L.; Hutchcroft, D.; Maurice, E.; Wrona, B.; Conovaloff, A.; Cooper, P.; Coward, D.; Rubin, P.Cortina Gil, E.; Kleimenova, A.; Minucci, E.; Padolski, S.; Petrov, P.; Shaikhiev, A.; Volpe, R.; Numao, T.; Petrov, Y.; Velghe, B.; Wong, V. W. S.; Bryman, D.; Fu, J.; Husek, T.; Jerhot, J.; Kampf, K.; Zamkovsky, M.; Aliberti, R.; Khoriauli, G.; Kunze, J.; Lomidze, D.; Peruzzo, L.; Vormstein, M.; Wanke, R.; Dalpiaz, P.; Fiorini, M.; Neri, I.; Norton, A.; Petrucci, F.; Wahl, H.; Cotta Ramusino, A.; Gianoli, A.; Iacopini, E.; Latino, G.; Lenti, M.; Parenti, A.; Bizzeti, A.; Bucci, F.; Antonelli, A.; Georgiev, G.; Kozhuharov, V.; Lanfranchi, G.; Martellotti, S.; Moulson, M.; Spadaro, T.; Tinti, G.; Ambrosino, F.; Capussela, T.; Corvino, M.; Di Filippo, D.; Fiorenza, R.; Massarotti, P.; Mirra, M.; Napolitano, M.; Saracino, G.; Anzivino, G.; Brizioli, F.; Imbergamo, E.; Lollini, R.; Piandani, R.; Santoni, C.; Barbanera, M.; Cenci, P.; Checcucci, B.; Lubrano, P.; Lupi, M.; Pepe, M.; Piccini, M.; Costantini, F.; Di Lella, L.; Doble, N.; Giorgi, M.; Giudici, S.; Lamanna, G.; Lari, E.; Pedreschi, E.; Sozzi, M.; Cerri, C.; Fantechi, R.; Pontisso, L.; Spinella, F.; Mannelli, I.; D'Agostini, G.; Raggi, M.; Biagioni, A.; Cretaro, P.; Frezza, O.; Leonardi, E.; Lonardo, A.; Turisini, M.; Valente, P.; Vicini, P.; Ammendola, R.; Bonaiuto, V.; Fucci, A.; Salamon, A.; Sargeni, F.; Arcidiacono, R.; Bloch-Devaux, B.; Boretto, M.; Menichetti, E.; Migliore, E.; Soldi, D.; Biino, C.; Filippi, A.; Marchetto, F.; Engelfried, J.; Estrada-Tristan, N.; Bragadireanu, A. M.; Ghinescu, S. A.; Hutanu, O. E.; Baeva, A.; Baigarashev, D.; Emelyanov, D.; Enik, T.; Falaleev, V.; Kekelidze, V.; Korotkova, A.; Litov, L.; Madigozhin, D.; Misheva, M.; Molokanova, N.; Movchan, S.; Polenkevich, I.; Potrebenikov, Yu.; Shkarovskiy, S.; Zinchenko, A.; Fedotov, S.; Gushchin, E.; Khotyantsev, A.; Kudenko, Y.; Kurochka, V.; Medvedeva, M.; Mefodev, A.; Kholodenko, S.; Kurshetsov, V.; Obraztsov, V.; Ostankov, A.; Semenov, V.; Sugonyaev, V.; Yushchenko, O.; Bician, L.; Blazek, T.; Cerny, V.; Kucerova, Z.; Bernhard, J.; Ceccucci, A.; Danielsson, H.; De Simone, N.; Duval, F.; Döbrich, B.; Federici, L.; Gamberini, E.; Gatignon, L.; Guida, R.; Hahn, F.; Holzer, E. B.; Jenninger, B.; Koval, M.; Laycock, P.; Lehmann Miotto, G.; Lichard, P.; Mapelli, A.; Marchevski, R.; Massri, K.; Noy, M.; Palladino, V.; Perrin-Terrin, M.; Pinzino, J.; Ryjov, V.; Schuchmann, S.; Venditti, S.; Bache, T.; Brunetti, M. B.; Duk, V.; Fascianelli, V.; Fry, J. R.; Gonnella, F.; Goudzovski, E.; Henshaw, J.; Iacobuzio, L.; Lazzeroni, C.; Lurkin, N.; Newson, F.; Parkinson, C.; Romano, A.; Sergi, A.; Sturgess, A.; Swallow, J.; Tomczak, A.; Heath, H.; Page, R.; Trilov, S.; Angelucci, B.; Britton, D.; Graham, C.; Protopopescu, D.; Carmignani, J.; Dainton, J. B.; Jones, R. W. L.; Ruggiero, G.; Fulton, L.; Hutchcroft, D.; Maurice, E.; Wrona, B.; Conovaloff, A.; Cooper, P.; Coward, D.; Rubin, P