Measurement of the very rare K+→π+ννˉK^+ \to \pi^+ \nu \bar\nu decay

The decay K+→π+νν¯ , with a very precisely predicted branching ratio of less than 10−10 , is among the best processes to reveal indirect effects of new physics. The NA62 experiment at CERN SPS is designed to study the K+→π+νν¯ 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+→π+νν¯ 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+→π+X ), where X is a scalar or pseudo-scalar particle. The final result of the BR(K+→π+νν¯ ) measurement and its interpretation in terms of the K+→π+X decay from the analysis of the full 2016-2018 data set is presented, and future plans and prospects are reviewed

Study of GigaTracker performance and a search for Axion-Like Particles at the NA62 experiment

The Standard Model (SM) is the most rigorous theory of particle physics; however, as indicated by various experimental observations, it is far from being complete. For that reason, searches for New Physics effects became the central focus of modern experimental particle physics. The work described in this thesis was carried out using data collected by the fixed-target experiment NA62. The primary goal of NA62 is to test the SM by accurately measuring the branching ratio of the ultra-rare flavour-changing neutral current kaon decay into a pion and a neutrino-antineutrino pair. Given the chosen decay-in-flight technique, such a measurement would be impossible without a high-precision tracking system. One of the projects reported in this thesis is dedicated to the silicon hybrid beam spectrometer GigaTracKer (GTK). This work describes the time calibration procedure, data quality monitoring tools and detector performance in 2017-2018 data taking periods. In addition, since the GTK operates in a harsh radiation environment, a preliminary study of detector ageing, including the bulk current measurement and the fluence estimation, is also provided. Lastly, the thesis presents searches for Axion-like particles (ALPs) produced in b-quark to s-quark transitions and decaying into a pair of muons. The thesis describes the developed Monte Carlo simulation, the selection criteria, and the analysis of data collected by NA62 in 2017 and 2018. The expected sensitivity in the 0-background assumption is discussed, along with the preliminary background evaluations and recommendations for future data taking.(SC - Sciences) -- UCL, 202

Study of GigaTracker performance and a search for Axion-Like Particles at the NA62 experiment

This thesis discusses the results of two projects carried out within the NA62 collaboration operating a fixed-target kaon experiment located in the North area of the CERN SPS. The NA62 experiment aims to measure the ultra-rare kaon decay $K^+ \rightarrow \pi^+ \nu \bar\nu$ with 10% precision. This task is impossible without using the beam spectrometer, an hybrid silicon detector called GigaTracKer (GTK). The development of the dedicated time calibration procedure, crucial for achieving the best timing performance of the GTK, is discussed. The timing performance of the GTK and the effects of radiation damage on the detectors installed during the 2016-2018 data taking periods are also investigated. The results of this work can be directly used in any NA62 physics analysis, which relies on the GTK performance. The second project focuses on the searches for New Physics processes. A sensitivity study to the production of a light scalar or pseudoscalar particle coupling to fermions produced in the rare decays of B-mesons is performed. A model-independent approach, the analysis strategy, the signal Monte Carlo, and the analysis of the 2017-2018 data are discussed. Dedicated trigger conditions are set up to be used in future data taking at improved sensitivity

Latest results from NA62

The ultra rare kaon decay $K\rightarrow \pi \nu \bar{\nu}$, being one of the theoretically cleanest meson decays, is very sensitive to the effects of New Physics at high mass scales. NA62 is a fixed-target experiment at the CERN SPS designed to measure the branching ratio of the $K^{+}\rightarrow \pi^{+} \nu \bar{\nu}$ decay with 10% precision using a novel decay-in-flight technique. NA62 took data during run periods in 2016, 2017 and 2018. The results from the analysis of the data collected by NA62 in 2016, corresponding to $(1.21 \pm 0.04_{syst}) \times 10^{11}$ $K^{+}$ decays (2% of full available statistics), will be presented and future prospects will be reviewed. Although NA62 was designed to measure $K\rightarrow \pi \nu \bar{\nu}$, it's also sensitive to other rare and forbidden kaon decays, especially those with two leptons in the final state. The status and prospects of searches for lepton flavour and lepton number violation in kaon decays at the NA62 experiment will also be discussed

The NA62 GigaTracKer: a low mass high intensity beam 4D tracker with 65 ps time resolution on tracks

The GigaTracKer (GTK) is the beam spectrometer of the CERN NA62 experiment. The detector features challenging design specifications, in particular a peak particle flux reaching up to 2.0 MHz/mm^2, a single hit time resolution smaller than 200 ps and, a material budget of 0.5% X_0 per tracking plane. To fulfill these specifications, novel technologies were especially employed in the domain of silicon hybrid time-stamping pixel technology and micro-channel cooling. This article describes the detector design and reports on the achieved performance

Search for \pi^0π 0 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 invisibleparticles. 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 modelindependent 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 axionlike particle with dominant fermion couplings or a dark scalar mixing with the Standard Model Higgs boson

Measurement of the very rare K^{+} + →{\pi}^{+}\nu \overline{\nu}π + ν ν decay

The NA62 experiment reports the branching ratio measurement BR(K+ →π+νν¯) = (10.6+4.0−3.4|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

Search for heavy neutral lepton production in K^+K + decays to positrons

A search for heavy neutral lepton ( N ) production in K+→e+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 10−9 over most of the accessible heavy neutral lepton mass range 144–462 MeV/ c2 , 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/ c2

Search for K^+K + decays to a muon and invisible particles

The NA62 experiment at CERN reports searches for K + → μ+N and K + → μ+ν X decays, where N and X are massive invisible particles, using the 2016–2018 data set. The N particle is assumed to be a heavy neutral lepton, and the results are expressed as upper limits of O(10−8) of the neutrino mixing parameter |Uμ4|2 for N masses in the range 200–384 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 10–370 MeV/c2 are reported for the first time, ranging from O(10−5) to O(10−7). An improved upper limit of 1.0 × 10−6 is established at 90% CL on the K + → μ+ννν¯ branching fraction

Search for a feebly interacting particle XX in the decay K^{+}\rightarrow\pi^{+}XK + →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