Searches for rare and forbidden kaon decays at the NA62 experiment at CERN

Studies of rare and forbidden $K^{+}$ decays are presented based on analysis of data taken by the NA62 experiment between 2016 and 2018 (Run 1). The primary physics objective of the NA62 experiment is the study of the ultra-rare decay $K^{+} \rightarrow \pi^{+}\nu\bar{\nu}$. Studies of backgrounds are presented and the results of the Run 1 $K^{+} \rightarrow \pi^{+}\nu\bar{\nu}$ analysis are summarised. A search for the $K^{+} \rightarrow \pi^{+}X$ decay, where $X$ is a long-lived feebly interacting particle, is performed through an interpretation of the $K^{+} \rightarrow \pi^{+}\nu\bar{\nu}$ analysis of 2017 data. Upper limits are established on the branching ratio, $\mathcal{B}(K^{+} \rightarrow \pi^{+}X)$ for ranges of $X$ masses, 0--110 MeV/$c^{2}$ and 154--260 MeV/$c^{2}$, and lifetimes above 100 ps. Searches for the lepton number violating $K^{+} \rightarrow \pi^{-}\mu^{+}e^{+}$ decay and lepton flavour violating $K^{+} \rightarrow \pi^{+}\mu^{-}e^{+}$ and $\pi^{0} \rightarrow \mu^{+}e^{+}$ decays are described. No signals are observed and upper limits on the branching ratios of these decays at 90% confidence level are obtained: $\mathcal{B}(K^{+} \rightarrow \pi^{-}\mu^{+}e^{+}) < 4.2 \times 10^{-11}$, $\mathcal{B}(K^{+} \rightarrow \pi^{+}\mu^{-}e^{+}) < 6.6 \times 10^{-11}$, and $\mathcal{B}(\pi^{0} \rightarrow \mu^{-}e^{+}) < 3.2 \times 10^{-10}$, improving by an order of magnitude on previous results

Transition from electron accumulation to depletion at β-Ga2O3 surfaces: The role of hydrogen and the charge neutrality level

The surface electronic properties of bulk-grown β-Ga2O3 (2⎯⎯01) single crystals are investigated. The band gap is found using optical transmission to be 4.68 eV. High-resolution x-ray photoemission coupled with hybrid density functional theory calculation of the valence band density of states provides insights into the surface band bending. Importantly, the standard linear extrapolation method for determining the surface valence band maximum (VBM) binding energy is found to underestimate the separation from the Fermi level by ∼0.5 eV. According to our interpretation, most reports of surface electron depletion and upward band bending based on photoemission spectroscopy actually provide evidence of surface electron accumulation. For uncleaned surfaces, the surface VBM to Fermi level separation is found to be 4.95 ± 0.10 eV, corresponding to downward band bending of ∼0.24 eV and an electron accumulation layer with a sheet density of ∼5 × 1012 cm−2. Uncleaned surfaces possess hydrogen termination which acts as surface donors, creating electron accumulation and downward band bending at the surface. In situ cleaning by thermal annealing removes H from the surface, resulting in a ∼0.5 eV shift of the surface VBM and formation of a surface electron depletion layer with upward band bending of ∼0.26 eV due to native acceptor surface states. These results are discussed in the context of the charge neutrality level, calculated bulk interstitial hydrogen transition levels, and related previous experimental findings

Indium Gallium Oxide Alloys: Electronic Structure, Optical Gap, Surface Space Charge, and Chemical Trends within Common-Cation Semiconductors

The electronic and optical properties of (InxGa{1–x})_{2}O_{3} alloys are highly tunable, giving rise to a myriad of applications including transparent conductors, transparent electronics, and solar-blind ultraviolet photodetectors. Here, we investigate these properties for a high quality pulsed laser deposited film which possesses a lateral cation composition gradient (0.01 ≤ x ≤ 0.82) and three crystallographic phases (monoclinic, hexagonal, and bixbyite). The optical gaps over this composition range are determined, and only a weak optical gap bowing is found (b = 0.36 eV). The valence band edge evolution along with the change in the fundamental band gap over the composition gradient enables the surface space-charge properties to be probed. This is an important property when considering metal contact formation and heterojunctions for devices. A transition from surface electron accumulation to depletion occurs at x ∼ 0.35 as the film goes from the bixbyite In_{2}O_{3} phase to the monoclinic β-Ga_{2}O_{3} phase. The electronic structure of the different phases is investigated by using density functional theory calculations and compared to the valence band X-ray photoemission spectra. Finally, the properties of these alloys, such as the n-type dopability of In_{2}O_{3} and use of Ga_{2}O_{3} as a solar-blind UV detector, are understood with respect to other common-cation compound semiconductors in terms of simple chemical trends of the band edge positions and the hydrostatic volume deformation potential

Influence of Polymorphism on the Electronic Structure of Ga2O3

The search for new wide band gap materials is intensifying to satisfy the need for more advanced and energy efficient power electronic devices. Ga$_2$O$_3$ has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main $\beta$-phase. Here, three polymorphs of Ga$_2$O$_3$, $\alpha$, $\beta$ and $\varepsilon$, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure - electronic structure relationships. Valence and conduction electronic structure as well as semi-core and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provide detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This work provides a strong basis for the exploration of the Ga$_2$O$_3$ polymorphs as materials at the heart of future electronic device generations.Comment: Updated manuscript version after peer revie

Feebly-Interacting Particles:FIPs 2020 Workshop Report

With the establishment and maturation of the experimental programs searching for new physics with sizeable couplings at the LHC, there is an increasing interest in the broader particle and astrophysics community for exploring the physics of light and feebly-interacting particles as a paradigm complementary to a New Physics sector at the TeV scale and beyond. FIPs 2020 has been the first workshop fully dedicated to the physics of feebly-interacting particles and was held virtually from 31 August to 4 September 2020. The workshop has gathered together experts from collider, beam dump, fixed target experiments, as well as from astrophysics, axions/ALPs searches, current/future neutrino experiments, and dark matter direct detection communities to discuss progress in experimental searches and underlying theory models for FIPs physics, and to enhance the cross-fertilisation across different fields. FIPs 2020 has been complemented by the topical workshop "Physics Beyond Colliders meets theory", held at CERN from 7 June to 9 June 2020. This document presents the summary of the talks presented at the workshops and the outcome of the subsequent discussions held immediately after. It aims to provide a clear picture of this blooming field and proposes a few recommendations for the next round of experimental results.Comment: 240 pages, 71 figure

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

Searches for lepton flavour and lepton number violation in K+K^{+} decays at NA62

A search for the lepton number violating $K^{+}\rightarrow\pi^{-}\mu^{+}e^{+}$ and lepton flavour violating $K^{+}\rightarrow\pi^{+}\mu^{-}e^{+}$ decays is reported using NA62 data collected in 2017 and 2018. No significant excess is observed above the background expectation and therefore upper limits are established on the branching ratios at $90\%$ confidence level: $\mathcal{B}(K^{+}\rightarrow\pi^{-}\mu^{+}e^{+})<4.2\times10^{-11}$ and $\mathcal{B}(K^{+}\rightarrow\pi^{+}\mu^{-}e^{+})<6.6\times10^{-11}$. These results improve over the previous limits by factors of $12$ and $8$ respectively

Searches for lepton flavour/number violation in K+K^{+} and π0\pi^{0} decays at the NA62 experiment

Searches for the lepton number violating $K^{+}\rightarrow\pi^{-}\mu^{+}e^{+}$ decay and lepton flavour violating decays $K^{+}\rightarrow\pi^{+}\mu^{-}e^{+}$ and $\pi^{0}\rightarrow\mu^{-}e^{+}$ are reported, using NA62 data collected in 2017 and 2018. In the absence of a significant excess above the background in each channel, upper limits on the branching ratios are established at $90\%$ confidence level: $\mathcal{B}(K^{+}\rightarrow\pi^{-}\mu^{+}e^{+})<4.2\times10^{-11}$, $\mathcal{B}(K^{+}\rightarrow\pi^{+}\mu^{-}e^{+})<6.6\times10^{-11}$, and $\mathcal{B}(\pi^{0}\rightarrow\mu^{-}e^{+})<3.2\times10^{-10}$