102 research outputs found
Search for Ļā° decays to invisible particles
The NA62 experiment at the CERN SPS reports a study of a sample of 4 Ć 109 tagged Ļ0 mesons from K+ ā Ļ+Ļ0(Ī³), searching for the decay of the Ļ0 to invisible particles. No signal is observed in excess of the expected background fluctuations. An upper limit of 4.4 Ć 10ā9 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+ ā Ļ+X, where X is a particle escaping detection with mass in the range 0.110ā0.155 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
Search for heavy neutral lepton production in K+ decays to positrons
A search for heavy neutral lepton (N) production in K+āe+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 |Ue4|^2 are established at the level of 10^ā9 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 |Ue4|^2 range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/c^2
Measurement of the very rare K + ā Ļ+Ī½Ī½ĀÆ decay
The NA62 experiment reports the branching ratio measurement BR(K+āĻ+Ī½Ī½ĀÆ)=(10.6ā3.4+4.0|statĀ±0.9syst)Ć10ā11 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 2016ā2018. This provides evidence for the very rare K+āĻ+Ī½Ī½ĀÆ decay, observed with a significance of 3.4Ļ. The experiment achieves a single event sensitivity of (0.839 Ā± 0.054) Ć 10ā11, corresponding to 10.0 events assuming the Standard Model branching ratio of (8.4 Ā± 1.0) Ć 10ā11. This measurement is also used to set limits on BR(K+ā Ļ+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
An investigation of the very rare K+ ā Ļ+ vvĀÆ decay
The NA62 experiment reports an investigation of the K+āĻ+Ī½ ĢĪ½ mode from a sample of K+ decays collected in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity of (0.389Ā±0.024)Ć10ā10, corresponding to 2.2 events assuming the Standard Model branching ratio of (8.4Ā±1.0)Ć10ā11. Two signal candidates are observed with an expected background of 1.5 events. Combined with the result of a similar analysis conducted by NA62 on a smaller data set recorded in 2016, the collaboration now reports an upper limit of 1.78Ć10ā10 for the K+āĻ+Ī½ ĢĪ½ branching ratio at 90% CL. This, together with the corresponding 68% CL measurement of (0.48+0.72ā0.48)Ć10ā10, are currently the most precise results worldwide, and are able to constrain some New Physics models that predict large enhancements still allowed by previous measurements
Development of a new CEDAR for kaon identification at the NA62 experiment at CERN
The NA62 experiment at CERN utilises a differential Cherenkov counter with achromatic ring focus (CEDAR) for tagging kaons within an unseparated monochromatic beam of charged hadrons. The CEDAR-H detector was developed to minimise the amount of material in the path of the beam by using hydrogen gas as the radiator medium. The detector was shown to satisfy the kaon tagging requirements in a test-beam before installation and commissioning at the experiment. The CEDAR-H performance was measured using NA62 data collected in 2023
Search for K+ decays to a muon and invisible particles
The NA62 experiment at CERN reports searches for K+ ā Ī¼+N and K+ ā Ī¼+Ī½X decays, where N and X are massive invisible particles, using the 2016ā2018 data set. The N particle is assumed to be a heavy neutral lepton, and the results are expressed as upper limits of O(10ā8) of the neutrino mixing parameter |UĪ¼4|2 for N masses in the range 200ā384 MeV/c2 and lifetime exceeding 50 ns. The X particle is considered a scalar or vector hidden sector mediator decaying to an invisible final state, and upper limits of the decay branching fraction for X masses in the range 10ā370 MeV/c2 are reported for the first time, ranging from O(10ā5) to O(10ā7). An improved upper limit of 1.0Ć10ā6 is established at 90% CL on the K+ ā Ī¼+Ī½Ī½Ī½ĀÆ branching fraction
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Performance of the NA62 trigger system
The NA62 experiment at CERN targets the measurement of the ultra-rare K
+
ā
Ļ
+
Ī½
Ī½
ĀÆ
decay, and carries out a broad physics programme that includes probes for symmetry violations and searches for exotic particles. Data were collected in 2016ā2018 using a multi-level trigger system, which is described highlighting performance studies based on 2018 data
New measurement of the radiative decay Ke3g at the NA62 experiment at CERN
The NA62 experiment at CERN reports new results from the study of the radiative kaon decay K+āĻ0e+Ī½Ī³
(Ke3Ī³
), using a data sample recorded in 2017 and 2018. Preliminary results with the most precise measurement of the Ke3Ī³
branching ratio, and a T-asymmetry measurement in the Ke3Ī³
decay, are presented
Search for lepton number and flavour violation in K+ and pi0 decays
The NA62 experiment at CERN collected a large sample of charged kaon decays into ļ¬nal states with multiple charged particles in 2016-2018. This sample provides sensitivities to rare decays with branching ratios as low as 10 ā11 . Searches for the lepton number violating K + ā Ļ ā Āµ + e+ decay and the lepton ļ¬avour violating K + ā Ļ + Āµ ā e + and Ļ 0 ā Āµ ā e + decays are reported. No evidence for these decays is found and upper limits of the branching ratios are obtained at 90% conļ¬dence level. These results improve by one order of magnitude over previous results for these decay modes
Measurement of the very rare decay
The decay K+āĻ+Ī½Ī½ĀÆ
, with a very precisely predicted branching ratio of less than 10ā10
,
is among the best processes to reveal indirect effects of new physics.
The NA62 experiment at CERN SPS is designed to study the K+āĻ+Ī½Ī½ĀÆ
decay and to measure its branching ratio using a decay-in-flight technique.
NA62 took data in 2016, 2017 and 2018, reaching the sensitivity of the Standard Model
for the K+āĻ+Ī½Ī½ĀÆ
decay by the analysis of the 2016 and 2017 data,
and providing the most precise measurement of the branching ratio to date
by the analysis of the 2018 data.
This measurement is also used to set limits on BR(K+āĻ+X
), where X
is a scalar
or pseudo-scalar particle.
The final result of the BR(K+āĻ+Ī½Ī½ĀÆ
) measurement and its interpretation in terms
of the K+āĻ+X
decay from the analysis of the full 2016-2018 data set is presented, and future plans and prospects are reviewed
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