Search

We demonstrate an injection-seeded thin-disk Yb:YAG laser at 1030 nm, stabilized by the Pound-Drever-Hall (PDH) method. We modified the PDH scheme to obtain an error signal free from Trojan locking points, which allowed robust re-locking of the laser and reliable long-term operation. The single-frequency pulses have 50 mJ energy (limited to avoid laser-induced damage) with a beam quality of $\text{M}^2$ < 1.1 and an adjustable length of 55-110 ns. Heterodyne measurements confirmed a spectral linewidth of 3.7 MHz. The short pulse build-up time (850 ns) makes this laser suitable for laser spectroscopy of muonic hydrogen, pursued by the CREMA collaboration.

Hydrogen-like light muonic ions, in which one negative muon replaces all the electrons, are extremely sensitive probes of nuclear structure, because the large muon mass increases tremendously the wave function overlap with the nucleus. Using pulsed laser spectroscopy we have measured three 2S-2P transitions in the muonic helium-3 ion ($\mu^3$He$^+$), an ion formed by a negative muon and bare helium-3 nucleus. This allowed us to extract the Lamb shift $E(2P_{1/2}-2S_{1/2})= 1258.598(48)^{\rm exp}(3)^{\rm theo}$ meV, the 2P fine structure splitting $E_{\rm FS}^{\rm exp} = 144.958(114)$ meV, and the 2S-hyperfine splitting (HFS) $E_{\rm HFS}^{\rm exp} = -166.495(104)^{\rm exp}(3)^{\rm theo}$ meV in $\mu^3$He$^+$. Comparing these measurements to theory we determine the rms charge radius of the helion ($^3$He nucleus) to be $r_h$ = 1.97007(94) fm. This radius represents a benchmark for few nucleon theories and opens the way for precision tests in $^3$He atoms and $^3$He-ions. This radius is in good agreement with the value from elastic electron scattering, but a factor 15 more accurate. Combining our Lamb shift measurement with our earlier one in $\mu^4$He$^+$ we obtain $r_h^2-r_\alpha^2 = 1.0636(6)^{\rm exp}(30)^{\rm theo}$ fm$^2$ to be compared to results from the isotope shift measurements in regular He atoms, which are however affected by long-standing tensions. By comparing $E_{\rm HFS}^{\rm exp}$ with theory we also obtain the two-photon-exchange contribution (including higher orders) which is another important benchmark for ab-initio few-nucleon theories aiming at understanding the magnetic and current structure of light nuclei., Comment: 8 pages, 5 figures

The toolbox for material characterization has never been richer than today. Great progress with all kinds of particles and interaction methods provide access to nearly all properties of an object under study. However, a tomographic analysis of the subsurface region remains still a challenge today. In this regard, the Muon-Induced X-ray Emission (MIXE) technique has seen rebirth fueled by the availability of high intensity muon beams. We report here a study conducted at the Paul Scherrer Institute (PSI). It demonstrates that the absence of any beam time-structure leads to low pile-up events and a high signal-to-noise ratio (SNR) with less than one hour acquisition time per sample or data point. This performance creates the perspective to open this technique to a wider audience for the routine investigation of non-destructive and depth-sensitive elemental compositions, for example in rare and precious samples. Using a hetero-structured sample of known elements and thicknesses, we successfully detected the characteristic muonic X-rays, emitted during the capture of a negative muon by an atom, and the gamma-rays resulting from the nuclear capture of the muon, characterizing the capabilities of MIXE at PSI. This sample emphasizes the quality of a continuous beam, and the exceptional SNR at high rates. Such sensitivity will enable totally new statistically intense aspects in the field of MIXE, e.g. elemental 3D-tomography and chemical analysis. Therefore, we are currently advancing our proof-of-concept experiments with the goal of creating a full fledged permanently operated user station to make MIXE available to the wider scientific community as well as industry.

The CREMA collaboration is pursuing a measurement of the ground-state hyperfine splitting (HFS) in muonic hydrogen (μp) with 1 ppm accuracy by means of pulsed laser spectroscopy to determine the two-photon-exchange contribution with 2×10-4 relative accuracy. In the proposed experiment, the μp atom undergoes a laser excitation from the singlet hyperfine state to the triplet hyperfine state, then is quenched back to the singlet state by an inelastic collision with a H2 molecule. The resulting increase of kinetic energy after the collisional deexcitation is used as a signature of a successful laser transition between hyperfine states. In this paper, we calculate the combined probability that a μp atom initially in the singlet hyperfine state undergoes a laser excitation to the triplet state followed by a collisional-induced deexcitation back to the singlet state. This combined probability has been computed using the optical Bloch equations including the inelastic and elastic collisions. Omitting the decoherence effects caused by the laser bandwidth and collisions would overestimate the transition probability by more than a factor of two in the experimental conditions. Moreover, we also account for Doppler effects and provide the matrix element, the saturation fluence, the elastic and inelastic collision rates for the singlet and triplet states, and the resonance linewidth. This calculation thus quantifies one of the key unknowns of the HFS experiment, leading to a precise definition of the requirements for the laser system and to an optimization of the hydrogen gas target where μp is formed and the laser spectroscopy will occur., SciPost Physics, 13 (2), ISSN:2542-4653

It has been proposed that there could be a mirror copy of the standard model particles, restoring the parity symmetry in the weak interaction on the global level. Oscillations between a neutral standard model particle, such as the neutron, and its mirror counterpart could potentially answer various standing issues in physics today. Astrophysical studies and terrestrial experiments led by ultracold neutron storage measurements have investigated neutron to mirror-neutron oscillations and imposed constraints on the theoretical parameters. Recently, further analysis of these ultracold neutron storage experiments has yielded statistically significant anomalous signals that may be interpreted as neutron to mirror-neutron oscillations, assuming nonzero mirror magnetic fields. The neutron electric dipole moment collaboration performed a dedicated search at the Paul Scherrer Institute and found no evidence of neutron to mirror-neutron oscillations. Thereby, the following new lower limits on the oscillation time were obtained: Tnn´ > 352 s at B~= 0 (95% C.L.), Tnn~> 6s for 0.4 µT < B~> 25.7 µT (95% C.L)., and Tnn~/ √cosβ > 9s for 5.0 µT < β´< 25.4 µT (95% C.L.),where β is the fixed angle between the applied magnetic field and the local mirror magnetic field, which is assumed to be bound to the Earth. These new constraints are the best measured so far around B`~10 µT and B`~20 µT. ISSN:0370-2693 ISSN:0031-9163 ISSN:1873-2445

Ultracold neutrons (UCN) are free neutrons that can be stored in experimental setups for several minutes. Some of the most important properties of the neutron, such as its tiny permanent electric dipole moment and its beta decay lifetime, are best measured with UCN. Also searches for well motivated but yet unknown, hypothetical additional interactions are being pursued with UCN. Such measurements in the field of low-energy, precision physics may have far reaching implications from particle physics to cosmology. Most experiments are statistics limited and need high-intensity UCN sources. The UCN source at PSI is at the forefront of the field and home to the international nEDM collaboration and its world-leading search for the neutron electric dipole moment. This article aims at giving an overview of the fascinating research using ultracold neutrons emphasizing on activities at PSI including various physics side-analyses which were pioneered by the nEDM collaboration., 20 pages, 10 figures; invited article in 'Swiss Neutron News', the journal of the Swiss Neutron Science Society https://sgn.web.psi.ch/

Ultracold neutrons (UCN) are free neutrons that can be stored in experimental setups for several minutes. Some of the most important properties of the neutron, such as its tiny permanent electric dipole moment and its beta decay lifetime, are best measured with UCN. Also searches for well motivated but yet unknown, hypothetical additional interactions are being pursued with UCN. Such measurements in the field of low-energy, precision physics may have far reaching implications from particle physics to cosmology. Most experiments are statistics limited and need high-intensity UCN sources. The UCN source at PSI is at the forefront of the field and home to the international nEDM collaboration and its world-leading search for the neutron electric dipole moment. This article aims at giving an overview of the fascinating research using ultracold neutrons emphasizing on activities at PSI including various physics side-analyses which were pioneered by the nEDM collaboration., Swiss Neutron News, 55, ISSN:1022-2758

SiPM operation at very low temperatures has the potential to improve detector systems for experiments at cryogenic temperatures. We characterized electrical and optical properties of a commercially available cryogenic SiPM over a temperature range of 6.5–286 K, such as breakdown voltage, quenching resistance, gain, pulse waveform, photon detection efficiency and dark count rate. We observed a non-linear temperature dependence of the breakdown voltage and a small change of the pulse waveform at low temperatures. The SiPM gain and maximum allowed overvoltage decrease at low temperatures, however, stable operation down to 6.5 K has been demonstrated. Furthermore, the feasibility of assembling a detector with a plastic scintillator was studied., JPS Conference Proceedings, 27, ISBN:978-4-89027-137-5

Il Nuovo Cimento C, 42 (2-3), ISSN:1124-1896, ISSN:1826-9885, ISSN:2037-4909

The European Particle Physics Strategy Update (EPPSU) process takes a bottom-up approach, whereby the community is first invited to submit proposals (also called inputs) for projects that it would like to see realised in the near-term, mid-term and longer-term future. National inputs as well as inputs from National Laboratories are also an important element of the process. All these inputs are then reviewed by the Physics Preparatory Group (PPG), whose role is to organize a Symposium around the submitted ideas and to prepare a community discussion on the importance and merits of the various proposals. The results of these discussions are then concisely summarised in this Briefing Book, prepared by the Conveners, assisted by Scientific Secretaries, and with further contributions provided by the Contributors listed on the title page. This constitutes the basis for the considerations of the European Strategy Group (ESG), consisting of scientific delegates from CERN Member States, Associate Member States, directors of major European laboratories, representatives of various European organizations as well as invitees from outside the European Community. The ESG has the mission to formulate the European Strategy Update for the consideration and approval of the CERN Council. The European Particle Physics Strategy Update (EPPSU) process takes a bottom-up approach, whereby the community is first invited to submit proposals (also called inputs) for projects that it would like to see realised in the near-term, mid-term and longer-term future. National inputs as well as inputs from National Laboratories are also an important element of the process. All these inputs are then reviewed by the Physics Preparatory Group (PPG), whose role is to organize a Symposium around the submitted ideas and to prepare a community discussion on the importance and merits of the various proposals. The results of these discussions are then concisely summarised in this Briefing Book, prepared by the Conveners, assisted by Scientific Secretaries, and with further contributions provided by the Contributors listed on the title page. This constitutes the basis for the considerations of the European Strategy Group (ESG), consisting of scientific delegates from CERN Member States, Associate Member States, directors of major European laboratories, representatives of various European organizations as well as invitees from outside the European Community. The ESG has the mission to formulate the European Strategy Update for the consideration and approval of the CERN Council.

Laser spectroscopy of the Lamb shift (2S-2P energy difference) in light muonic atoms or ions, in which one negative muon μ− is bound to a nucleus, has been performed. The measurements yield significantly improved values of the root-mean-square charge radii of the nuclei, owing to the large muon mass, which results in a vastly increased muon wave function overlap with the nucleus. The values of the proton and deuteron radii are 10 and 3 times more accurate than the respective CODATA values, but 7 standard deviations smaller. Data on muonic helium-3 and -4 ions is being analyzed and will give new insights. In future, the (magnetic) Zemach radii of the proton and the helium-3 nuclei will be determined from laser spectroscopy of the 1S hyperfine splittings, and the Lamb shifts of muonic Li, Be, and B can be used to improve the respective charge radii., JPS Conference Proceedings, 18, Proceedings of the 12th International Conference on Low Energy Antiproton Physics (LEAP2016), ISBN:978-4-89027-125-2

Der Titel mag erstaunlich klingen, sind doch elektrische Dipolmomente in vielen Systemen wie dem Wassermolekül wohlbekannt. In (sub-)atomaren Teilchen sind sie jedoch mit der Verletzung grund­legender Symmetrien verbunden und eng verknüpft mit einer der großen Fragen der Teilchen­physik und Kosmologie: Warum leben wir in einer Welt, in der Materie dominiert, obwohl nach der gängigen Urknalltheorie am Anfang des Universums Materie und Antimaterie im Gleichgewicht standen?Elektrische Dipolmomente entstehen in der makroskopischen Welt, wenn positive und negative Ladungsträger unterschiedliche räumliche Verteilungen haben. Für zwei entgegengesetzt geladene Punktladungen Q beträgt das elektrische Dipolmoment der Verbindungsvektor von der negativen zur posi­tiven Ladung ist. Naiv erwartet man demnach bei einem Wassermolekül ein Dipolmoment, das dem Abstand der Wasserstoffatome zum Sauerstoffatom (etwa 10–8 cm) multipliziert mit der Elementarladung e entspricht, also 10–8 e · cm. Dieses Ergebnis kommt dem experimentellen Wert von 3,8 · 10–9 e · cm recht nahe. Analog folgt für ein Neutron, das aus zwei d-Quarks und einem u-Quark aufgebaut ist und einen Durchmesser von 10–13 cm besitzt, ein Dipolmoment von 10–13 e · cm. Experimente haben bislang jedoch nur einen mit Null verträglichen Wert ergeben. Die experimentelle Messunsicherheit lässt auf eine obere Grenze von etwa 3 · 10–26 e · cm schließen – ein Wert, der um 13 Größenordnungen unter der naiven Abschätzung liegt! Die elektroschwache Wechselwirkung des Standardmodells der Teilchenphysik sagt sogar einen Wert kleiner als 10–31 e · cm voraus.Daher stellt sich die Frage, warum die elektrischen Dipolmomente (EDM) subatomarer bzw. atomarer Teilchen so klein sind, obwohl in klassischen Systemen wie Plattenkondensatoren, Dioden oder Batterien und in gewissen Molekülen die geo­metrisch zu erwartenden Resultate auftreten. Die Ursache hierfür liegt darin, dass die Ausrichtung und Existenz des EDM-Vektors dieser Teilchen mit der expliziten Brechung diskreter Symmetrien verknüpft sind. Konkret sind das die Parität (P) und die Zeitumkehrinvarianz (T), die empirisch nur sehr schwach verletzt sind (Infokasten Parität, Zeitumkehr und Ladungs­konjugation). Wie aber führen diese Symmetriebrechungen zu einem EDM?...

The gravitational acceleration of antimatter, gbar, has yet to be directly measured; an unexpected outcome of its measurement could change our understanding of gravity, the universe, and the possibility of a fifth force. Three avenues are apparent for such a measurement: antihydrogen, positronium, and muonium, the last requiring a precision atom interferometer and novel muonium beam under development. The interferometer and its few-picometer alignment and calibration systems appear feasible. With 100 nm grating pitch, measurements of gbar to 10%, 1%, or better can be envisioned. These could constitute the first gravitational measurements of leptonic matter, of 2nd-generation matter, and possibly, of antimatter., Comment: 14 pages, 8 figures, submitted to Atoms

International audience; We plan to measure several 2S-2P transition frequencies in mu He-4(+) and mu He-3(+) by means of laser spectroscopy with an accuracy of 50 ppm. This will lead to a determination of the corresponding nuclear rms charge radii with a relative accuracy of 3 x 10(-4), limited by the uncertainty of the nuclear polarization contribution. First, these measurements will help to solve the proton radius puzzle. Second, these very precise nuclear radii are benchmarks for ab initio few-nucleon theories and potentials. Finally when combined with an ongoing measurement of the 1S-2S transition in He+, these measurements will lead to an enhanced bound-state QED test of the 1S Lamb shift in He+.

Proposals exist and efforts are under way to construct pulsed spallation ultra-cold neutron (UCN) sources at accelerator laboratories around the world. At the Paul Scherrer Institut (PSI), Switzerland, and at the Los Alamos National Laboratory (LANL), U.S.A., it is planned to use solid deuterium (SD_2) for the UCN production from cold neutrons. The philosophies about how the cold neutrons are obtained are quite different, though. The present proposal describes a third approach which applies a temperature optimized graphite moderator in combination with the SD_2 and qualitatively combines advantages of the different schemes. The scheme described here allows to build a powerful UCN source. Assuming a pulsed 2 mA, 590 MeV proton beam with an average current of 10 microA, one obtains UCN densities in excess of 2000 cm^{-3}, UCN fluxes of about 10^6 cm^{-2} s^{-1}, and total numbers of UCN in excess of 2*10^9 every 800 s., Comment: 13 pages, including 2 tables and 1 figure. Submitted to NIM A

We present the new spectrometer for the neutron electric dipole moment (nEDM) search at the Paul Scherrer Institute (PSI), called n2EDM. The setup is at room temperature in vacuum using ultracold neutrons. n2EDM features a large UCN double storage chamber design with neutron transport adapted to the PSI UCN source. The design builds on experience gained from the previous apparatus operated at PSI until 2017. An order of magnitude increase in sensitivity is calculated for the new baseline setup based on scalable results from the previous apparatus, and the UCN source performance achieved in 2016., EPJ Web of Conferences, 219, ISSN:2100-014X, ISSN:2101-6275

It has been proposed that there could be a mirror copy of the standard model particles, restoring the parity symmetry in the weak interaction on the global level. Oscillations between a neutral standard model particle, such as the neutron, and its mirror counterpart could potentially answer various standing issues in physics today. Astrophysical studies and terrestrial experiments led by ultracold neutron storage measurements have investigated neutron to mirror-neutron oscillations and imposed constraints on the theoretical parameters. Recently, further analysis of these ultracold neutron storage experiments has yielded statistically significant anomalous signals that may be interpreted as neutron to mirror-neutron oscillations, assuming nonzero mirror magnetic fields. The neutron electric dipole moment collaboration performed a dedicated search at the Paul Scherrer Institute and found no evidence of neutron to mirror-neutron oscillations. Thereby, the following new lower limits on the oscillation time were obtained: Tnn´ > 352 s at B~= 0 (95% C.L.), Tnn~> 6s for 0.4 µT < B~> 25.7 µT (95% C.L)., and Tnn~/ √cosβ > 9s for 5.0 µT < β´< 25.4 µT (95% C.L.),where β is the fixed angle between the applied magnetic field and the local mirror magnetic field, which is assumed to be bound to the Earth. These new constraints are the best measured so far around B`~10 µT and B`~20 µT., Physics Letters B, 812, ISSN:0370-2693, ISSN:0031-9163, ISSN:1873-2445

The European Particle Physics Strategy Update (EPPSU) process takes a bottom-up approach, whereby the community is first invited to submit proposals (also called inputs) for projects that it would like to see realised in the near-term, mid-term and longer-term future. National inputs as well as inputs from National Laboratories are also an important element of the process. All these inputs are then reviewed by the Physics Preparatory Group (PPG), whose role is to organize a Symposium around the submitted ideas and to prepare a community discussion on the importance and merits of the various proposals. The results of these discussions are then concisely summarised in this Briefing Book, prepared by the Conveners, assisted by Scientific Secretaries, and with further contributions provided by the Contributors listed on the title page. This constitutes the basis for the considerations of the European Strategy Group (ESG), consisting of scientific delegates from CERN Member States, Associate Member States, directors of major European laboratories, representatives of various European organizations as well as invitees from outside the European Community. The ESG has the mission to formulate the European Strategy Update for the consideration and approval of the CERN Council., 254 p. Revised version (10 January 2020): Update of the Physics Briefing Book for the European Strategy for Particle Physics, to take account of comments received from the community, in time for the strategy drafting session that will take place in Bad Honnef (Germany) on 20-24 January 2020

We report on a search for ultralow-mass axionlike dark matter by analyzing the ratio of the spin-precession frequencies of stored ultracold neutrons and 199Hg atoms for an axion-induced oscillating electric dipole moment of the neutron and an axion-wind spin-precession effect. No signal consistent with dark matter is observed for the axion mass range 10−24≤ma≤10−17 eV. Our null result sets the first laboratory constraints on the coupling of axion dark matter to gluons, which improve on astrophysical limits by up to 3 orders of magnitude, and also improves on previous laboratory constraints on the axion coupling to nucleons by up to a factor of 40., Physical Review X, 7 (4), ISSN:2160-3308

Instruments, 2 (3)

The search for a permanent electric dipole moment (EDM) of the muon is a unique probe to explore additional charge-parity (CP) violation sources beyond the CP violating phase of the Cabibbo–Kobayashi–Maskawa matrix in the Standard Model (SM) of particle physics. Any detection of a non-zero EDM at the current experimental sensitivity would be a strong signal of new CP violation sources that can be modelled in many theories Beyond the Standard Model (BSM) physics. Particularly, BSM models that allow for a large muon EDM without a tight constraint from an electron EDM are often intrinsically linked to the reported 4.2σ deviation from the SM prediction in the anomalous magnetic dipole moment of a muon. The observed tensions between measurements and SM predictions in B-decays also support similar models. This makes the muon EDM search pivotal to further push EDM searches beyond the first generation of fundamental particles with the potential of disentangling a possible violation of the lepton flavour universality. At the Paul Scherrer Institute (PSI), the first dedicated muon EDM search employing the frozen-spin technique is on its way. The muon EDM experiment aims for a sensitivity of better than 6 × 10⁻²³ e·cm, more than three orders of magnitude improvement over the current best upper limit placed by the E821 experiment at Brookhaven National Laboratory. In preparation for a dedicated high precision muon EDM measurement, this thesis covers three main areas. Firstly, the definition of a muon EDM was implemented in Geant4 to allow for simulating the experimental scheme. Secondly, the multiple Coulomb scattering of positrons in potential detector materials was measured to validate the simulations and to guide the design of the experiment. A reasonable agreement between measurements and simulation models has been confirmed. Finally, the beam characterisation of the potential beamlines to host the muon EDM experiment has been performed at the PSI µE1 and πE1 beamlines. The µ⁺ beam rate, transverse phase space (emittance) and beam polarisation level have been measured. This set the ground for beam injection studies and detailed simulation of the experiment. Moreover, as future prospects for the muon EDM experiment, the quality of the beam which will be delivered by a combination of the High-Intensity Muon Beams (HIMB), providing 10¹⁰surface µ⁺/s, and the muCool, a novel muon beam compression scheme, has been investigated with Geant4 simulations. The total compression efficiency and the beam quality of the HIMB-muCool beam have been updated.

This thesis was conducted in the context of the muX collaboration, which aims to determine for the first time the absolute nuclear charge radius of 226Ra employing the muonic atom spectroscopy method. Radium constitutes a good candidate for measurements of atomic parity violation (APV) which serves as a sensitive probe to search for new physics. The interpretation of such an APV experiment in radium and the extraction of the fundamental underlying parameters requires the knowledge of its nuclear charge radius with a precision of 0.2 %. Having developed the analysis and experimental techniques that enable the measurement of radium, 248Cm was studied in a first step as the heaviest and largest nucleus ever probed with muons. Nuclear charge radii can be precisely measured using muonic atom spectroscopy. With the muons being 207 times heavier than electrons, the low-lying muonic wave functions extensively overlap with the nuclear charge distribution. As a result, the muonic levels exhibit high sensitivity to the nuclear properties, which in turn can be determined by measuring the energies of emitted muonic x rays with High Purity Germanium (HPGe) detectors. The traditional muonic atom spectroscopy method has been heavily used in the past decades to determine the nuclear charge radii of almost all stable and a few unstable atoms. The formation of muonic atoms is traditionally realized by stopping a negative muon beam in macroscopic targets of O(100 mg). However, only microgram quantities are nowadays typically permitted in experimental facilities for open radioactive targets such as radium and curium, for which the direct muon stops cannot be realized. The muX collaboration has developed a technique that employs subsequent transfer reactions of the muon inside a vessel filled with 100 bar of hydrogen and per mill admixtures of deuterium gas, enabling the measurement of radioactive targets with microgram masses. As the last stable element with a yet unmeasured nuclear charge radius, the measurement of isotopically pure 185Re and 187Re was pursued prior to the radioactive targets. First, the spectroscopic quadrupole moments of rhenium are extracted from the analysis of the 5 → 4 muonic transitions as Q(185Re) = 2.07(5) b and Q(187Re) = 1.94(5) b. In a next step, the determination of the nuclear charge radius from the analysis of the 2p → 1s muonic transitions was undertaken. The preliminary charge radii are R(185Re) = 5.297(2)stat(6)sys fm and R(187Re) = 5.288(2)stat(4)sys fm. Improved and more complete theoretical input outside of the scope of this thesis is still needed to conclude the analysis and provide final absolute charge radii for these two isotopes. The measurement of the 226Ra and 248Cm isotopes was conducted in 2019 with a 1.35 μg and 15.46 μg target, respectively. Due to the low efficiency of the HPGe detectors at the 2p → 1s muonic x-ray energies, various methods are employed to improve the signal-to-background ratio. Despite the efforts, no muonic transitions are observed in radium and an additional attempt needs to be undertaken in the near future. On the other hand, the 2p → 1s transitions are clearly observed in curium. The nuclear charge radius and quadrupole moment determined from the analysis of the curium hyperfine transitions are R = 5.9455(1)stat(117)sys fm and Q = 12.003(8)stat(361)sys b. With a further revision of the theory and a combined analysis with higher muonic x-ray transitions, the current dominant sources of systematic effects are expected to be significantly reduced in the future.

Muonium (M = µ⁺ + e⁻) and muonic hydrogen (µp p⁺ + µ⁻) are exotic versions of the hydrogen atom, in which the proton or electron are replaced with an antimuon or a muon respectively. In muonium, the antimuon serves as a leptonic nucleus without any measurable size or electro-magnetic sub-structure, which makes it an excellent atomic system for precision tests of quantum electrodynamics and indirect searches for new physics. Muonic hydrogen, on the other hand, is eminently suited for precision studies of the proton’s internal structure, to which its atomic states are more sensitive due to the larger mass of the muon compared to the electron. Currently, a number of fundamental physics experiments with M and µp atoms are in preparation at the Paul Scherrer Institute (PSI). Although these experiments may have implications for physics at high energy scales, their measurement principles all involve low-energy processes, such as exotic atoms diffusing through the target after formation or travelling through vacuum at thermal energies. This thesis is mainly concerned with such low-energy processes and presents studies of the emission of M atoms from target samples at room temperature, as well as simulations of the diffusion of µp atoms through hydrogen gas. The LEMING Collaboration is currently preparing an experiment at PSI with the aim of measuring the gravitational interaction of muonium atoms with the Earth. While much effort in recent years has been dedicated to the development of a cryogenic muonium source using superfluid helium, additional measurements of muonium emission into vacuum from conventional target samples were carried out at room temperature during test beams in 2018 and 2020. The room-temperature measurement in 2018 was carried out before the start of this PhD thesis. Detection of M decays in vacuum was performed using small scintillator bars to detect decay positrons in combination with a system to accelerate and detect the atomic electrons in coincidence. In 2020, two different roomtemperature setups were used, which had both been developed within this PhD project. One of these setups featured an optimized arrangement of scintillator bars to detect M decays with increased efficiency and without the need for an atomic electron detection system. The other setup contained two Micromegas tracking detectors used in a telescope configuration to obtain an image of the cloud of M decays in vacuum. In this thesis, the three room-temperature studies are discussed and analyses of the measured data are presented. Comparing measured data with simulations, emission characteristics of the tested laser-ablated aerogel samples and zeolite samples are examined and muon-tovacuum-muonium conversion efficiencies are extracted. With that, this thesis contributes to the research field investigating muonium emission from novel target materials. For the development of a cryogenic muonium source with superfluid helium, elastic scattering of M atoms with ⁴He atoms in the gas phase must be taken into account. A measurement of the scattering strength was pursued during the test beam in 2018 by observing M emission into low-density helium gas. Within this thesis, calculations of the elastic cross sections for M–⁴He scattering are carried out assuming an empirical interaction potential. The cross sections were used for the implementation of M–⁴He scattering simulations in G4BEAMLINE. Within uncertainties, good agreement between the simulations and measured data provide validation of the calculated cross sections and the underlying scattering potential. Another experiment which is currently being prepared at PSI is a measurement of the ground-state hyperfine splitting (HFS) in muonic hydrogen. This is part of a comprehensive effort by the CREMA Collaboration to further the understanding of the proton structure with laser spectroscopy experiments using muonic atoms. In this particular experiment, negative muons are stopped in a cryogenic hydrogen gas target and form µp atoms, which then diffuse through the target. µp atoms reaching one of the gold-coated target walls lead to muonic gold x rays which can be measured outside the target cell. Exciting the diffusing µp atoms with a laser on the resonance frequency of the HFS transition increases their probability of reaching one of the walls. A peak in the x-ray yield consequently occurs at the resonance frequency and thus allows for precise determination of the desired HFS transition frequency. In the scope of this thesis, simulations of the µp diffusion process through the hydrogen gas were implemented and carried out in G4BEAMLINE. These simulations use differential cross sections for the scattering of µp atoms with hydrogen molecules which were calculated by a collaborator. In combination with inputs of theoretical and experimental origin, the diffusion simulations are used to predict the event rates of signal and background to be expected in the measurement. Based on these rates, the measurement time needed to find the resonance and the precision which can be reached in the measurement are estimated. With that, the simulations provide important input for the planning of the measurement campaign and for the further development of the experimental system.

Surgery, chemotherapy and radiotherapy are the most commonly employed treatment options in the fight against cancer. Out of these, radiotherapy presents a non-invasive strategy that makes use of ionizing radiation for selective tumor cell sterilization, being administered to about 50$\%$ of all cancer patients in Europe. The concept of precision medicine consists in moving away from a one-fits-all approach and towards personalized treatment schemes that account for the expected individual patient response to a given therapy. As such, radiotherapy is a highly personalized form of treatment, where a dedicated treatment plan is designed based on individual patient data in the form of biomedical images and clinical characteristics. In order to determine the optimal treatment approach, radiobiological modeling is an essential tool in this context, translating patient properties and treatment parameters into estimations of the probability for tumor control and radiation-induced side effects. Not only does this form of modeling allow for a treatment design suited to the patient-specific type of cancer and consequent risk for specific side effects, but it can also be employed to assess the potential benefit of new and alternative treatment strategies. The use of charged particles for external beam therapy is a modality rapidly gaining in popularity thanks to their peculiar physical interactions with matter, characterized by a sharp dose peak at the end of their track. This allows for the sparing of healthy tissue distal to the target, reducing the probability of side effects compared to conventional photon therapy at equal tumor control. Especially in the field of proton therapy, developments in beamline technology and biomedical imaging have enabled a wide range of highly specialized treatment options that emphasize the possibility to tailor treatments to the patient-specific situation. In order to estimate the effectiveness of these different treatment strategies, the development and continuous validation of radiobiological models for outcome estimation, based on the history of previously treated patients, is a prerequisite. The first part of this thesis is therefore dedicated to the collection and analysis of treatment outcome data from a large number of patients treated with proton therapy. Follow-up data about the incidence of late side effects, such as radiation-induced optic neuropathy and temporal lobe radionecrosis, has been collected to model the patient-specific risk for these side effects. It was found that a combination of risk factors, such as an older age, arterial hypertension or higher dose are the driving factors behind the incidence of the studied healthy tissue toxicities. These risk factors as well as normal tissue complication probability estimations can be transferred into the treatment planning process and, by placing patients into specific risk groups, adjustments be made to personalize each patient treatment to their maximum benefit. Further treatment personalization possibilities are becoming available by the usage of newly developed biomarkers that, based on modern medical imaging, allow functional mapping of the tumor microenvironment. Tumor response to the same therapy can vary greatly across the patient population and the investigation of the metabolic or oxygenation status can add essential information to the planning procedure with the option to tailor the treatment accordingly. Hypoxia refers to a state in the tumor microenvironment, where the cells are exposed to insufficient amounts of oxygen, which has been shown to correlate with worse treatment outcomes. For example, proton therapy with a capacity for precise dose painting facilitated by pencil-beam scanning, could be a suitable treatment modality for cancers that are subject to particularly high radioresistance in the form of hypoxia. The second part of this thesis is dedicated to the evaluation of the clinical potential for hypoxia-guided proton therapy in advanced-stage lung cancer treatments, making use of dedicated radiobiological models concerning radiosensitivity and treatment outcome estimations. A strategy with proton therapy is proposed that makes use of novel functional oxygenation imaging with positron emission tomography, to determine the patient-specific level of hypoxia which are taken into account in the design of dose-escalated treatments. Not only is it possible to, by calibrating the dose escalation, mitigate the hypoxia-induced loss in tumor control probability, but also reduce normal tissue complication probabilities for lungs, heart and esophagus significantly compared to the state-of-the-art approach with conventional photon therapy. Because of the well-known sensitivity of proton therapy to respiratory organ motion, time-resolved anatomical computed tomography imaging is taken into account to simulate these proton treatments under realistic motion conditions. While motion-induced dose degradation reduces the chances to control the disease, benefits of this hypoxia-targeted dose escalation strategy remain. Considering that dose escalation with photon therapy is challenging due to excess dose to healthy tissue, this makes proton therapy a valid option for the treatment of advanced-stage lung cancer affected by hypoxia which should be explored in clinical trials once the availability of reliable functional oxygenation imaging is given. Overall, these results showcase how patient data and emerging biomedical imaging technology in combination with radiobiological modeling can contribute to finding the best radiotherapy treatment for each patient. While requiring further validation, clinical trials and improved accessibility to modern imaging technology, the presented models and treatment strategies lay a foundation for a personalized proton treatment design through which the quality of care in radiation oncology can be improved.

Due to its lack of internal structure, muonium is an excellent candidate to provide stringent tests for bound state QED. This work presents the design and the measurement of the muonium Lamb shift performed at the PSI low energy muon (LEM) beamline in the Mu-MASS (MUoniuM lASer Spectroscopy) experiment. The analysis of the data showed the $n=2$ muonium Lamb shift to be 1047.2(25) MHz, which agrees well with theory and is an improvement of an order of magnitude upon the last measurements. Additionally, the $2S_{1/2,F=0}\rightarrow2P_{1/2,F=1}$ transition was measured for the first time in muonium at 580.6(6.8) MHz. From the two measurements, the $2S$ hyperfine splitting was extracted to be 559.6(7.2) MHz. Furthermore, muonium in the excited $n=3$ state was detected for the first time. In preparation of these measurements, this thesis includes as well the characterization of the intensity of the metastable flux of muonium at LEM by commissioning the antihydrogen Lamb shift setup designed for GBAR. Physics with antihydrogen is a blossoming field of research which studies aim to shed light on the observed matter-antimatter asymmetry in the Universe. The GBAR project at CERN primarily aims to measure the free fall acceleration of ultracold neutral antihydrogen atoms in the terrestrial gravitational field. The work performed within this dissertation on the Micromegas GBAR tracker is required for the efficient reconstruction of the antihydrogen annihilation vertex and thus essential for the measurement of its free fall. As a parasitic measurement, the Lamb shift of antihydrogen will be measured with the setup presented in this work with a targeted precision of 100 ppm, which would lead to its first direct determination and allow the extraction of the antiproton charge radius to a level of 10%.

The CREMA collaboration currently prepares a laser spectroscopy experiment at Paul Scherrer Institute in Switzerland to measure the ground-state hyperfine splitting in muonic hydrogen with a 1 ppm relative precision. This thesis is devoted to one of elements of the apparatus, a multipass cell employed to concentrate laser radiation on the volume of interest. Two design types are considered: a closed toroidal mirror and a cell made of two opposing toric mirrors. Within the scope of this work, I developed several analytical tools and experimental methods to evaluate the performance of the cell. These include an approximate description of the light distribution in the cell derived from paraxial phase-space optics of multipass systems, a volumetric definition of light fluence consistent with the usual formulation as energy per unit area, and a ray-trace model based on that definition to compute three-dimensional fluence distributions. A light injection system allowing for precise control of laser beam parameters was designed and built. Three mirror samples were investigated with this system in ring-down experiments with a pulsed laser source. The analysis algorithm is backed by a statistical model of evolution of light energy circulating in the cell and emerging from it as the measured signal. A circular cell made of uncoated copper shows a lifetime of around 35 ns, corresponding to a mirror reflectivity of 99.2 %, whereas a two-mirror glass cell with a commercial-grade dielectric coating has a lifetime of up to 80 ns, implying a reflectivity of 99.9 % or better. The third sample, a circular cell with an experimental dielectric coating, appears to be nonreflective at the desired wavelength of 6.8 μm and has a reflectivity similar to the bare copper cell at wavelengths below 6.0 μm. Basing on ray-trace simulations, for an injected pulse energy of 1 mJ, we expect an average fluence in a disk-shaped volume of a thickness of 1 mm and a diameter of 10 mm at the level of 1.4 J/cm² in the circular bare copper cell and 2.4–2.9 J/cm² in the two-mirror dielectric cell, which may be sufficient for our experiment. An improvement in coating reflectivity to 99.9 % could boost the average fluence in the circular cell to 5 J/cm². Methods established in this work will continue to be employed in tests of future samples and will guide the development of the experimental coating. The spatial fluence distributions computed with the ray-trace models are used by other members of the collaboration in calculations of the signal rate in the experiment.

This thesis was written in the context of the HyperMu experiment being performed at the Paul Scherrer Institut (PSI), Switzerland, aiming at laser spectroscopy of muonic hydrogen (μp), the hydrogen-like bound state of a negative muon and a proton. The goal of the experiment is to measure the ground-state hyperfine splitting (HFS) of μp at a relative precision of 1 parts-per-million (ppm). Since the muon is ~200 times heavier than the electron, it orbits the proton much closer than the electron of a regular hydrogen atom. This makes μp an exquisite system to investigate the proton structure. In particular, the HFS is sensitive to the two-photon exchange contribution that can be related to the polarized structure functions of the proton and the Zemach radius, which depends on the electric and magnetic form factors of the proton. The precise result of the HyperMu measurement, will thus serve as a benchmark for future precision calculations of the proton structure. The experiment is based on the measurement of the \left(1S,\,F=0\right)\rightarrow\left(1S,\,F=1\right) transition in μp using a pulsed, tuneable laser, at a wavelength around 6.8 μm. Negative, low momentum muons, arriving at a rate of ca. 500\,\text{s}^{-1}, are stopped in a short (~1 mm), \text{H}_{2} gas-target at cryogenic temperature (~22 K), where they form μp in a highly excited state. After de-exciting to the ground state \left(1S,\,F=0\right) and thermalizing in 1 μs to the \text{H}_{2} gas temperature, a laser pulse illuminates the μp atom. If the laser is on resonance, the μp atom is excited to the \left(1S,\,F=1\right). A subsequent inelastic collision with an \text{H}_{2} molecule from the surrounding gas, de-excites the μp atom back to F=0. In this process the μp atom gains kinetic energy. This makes the μp atom quickly diffuse to the gold coated target wall, where the muon is transferred from the proton to a gold atom, forming μAu in an excited state. The subsequent de-excitation of μAu that results in the emission of several MeV X-rays heralds a successful laser excitation of the hyperfine transition. The resonance is obtained by scanning the laser frequency and counting the number of X-rays versus laser frequency. The laser system for the HyperMu experiment is based on the creation of high energy pulses in the near IR which are converted to the target wavelength of 6.8 μm in the mid IR, by a series of non-linear frequency conversion stages. The near-IR pulses are created by a single-frequency thin-disk oscillator/amplifier system running at 1030 nm wavelength. The pulses are split into two independent branches where optical parametric oscillators and amplifiers create ~2 μm and ~3 μm radiation, respectively. A single-pass difference frequency generation stage is then used to obtain the radiation at 6.8 μm with a bandwidth 300 mJ) needed because of the small hyperfine transition matrix-element, and single-frequency operation at bandwidth

Positronium is the lightest known atom and comprises a structureless point-like electron and its anti-particle, the positron. As such, it is purely leptonic and can be described to very high precision by bound-state QED without the inherent complications given by the finite size and quark substructure of protonic atoms. Furthermore, being a true onium atom, recoil effects are strongly enhanced and its quantum numbers sum to zero, making it an ideal candidate to test fundamental symmetries like CPT-invariance. However, positronium is also a very challenging system for precision measurements due to its ephemeral nature. Being a bound state of anti-particles it tends to self-annihilate quickly and due to its lightness it exhibits much larger velocities than other atoms. Nevertheless, being a precision test-bench for QED with many unique features, it represents a particularly interesting system for spectroscopic measurements. This thesis presents the efforts towards precise determinations of the $\text{1}^\text{3}\text{S}_\text{1} \to \text{2}^\text{3}\text{S}_\text{1}$ transition and both the hyperfine and fine structure of positronium in the $n=2$ state. The pulsed slow positron beamline at ETH Zurich, being a key prerequisite for these measurements, was optimized and a new beam transport and bunching system was developed. It was found that the efficiencies were comparable to similar beamlines and several further possible improvements were identified. This thesis includes a measurement of the $\text{1}^\text{3}\text{S}_\text{1} \to \text{2}^\text{3}\text{S}_\text{1}$ transition by Doppler-free two-photon laser spectroscopy of $1\,233\,607\,210.5\:\mathrm{MHz} \pm 49.6\:\mathrm{MHz}$, which paves the way towards a new precision determination of this interval. Improvements by roughly two orders of magnitude on this result are suggested and discussed. The design of an experiment to measure the hyperfine transition in the first excited state is presented. A microwave system supporting more than $100\,\mathrm{kW}$ of circulating power was developed and tested. Extensive simulations of the excitation and detection show the expected sensitivity to be $9.9 \, \mathrm{ppm}$ (statistical) and $2.8 \, \mathrm{ppm}$ (systematic) and further envisioned improvements are presented. This measurement will constitute the first measurement of this transition and the first determination of a hyperfine interval of positronium in vacuum altogether. It was found that the experimental setup directly supports a determination of the $\text{2}^\text{3}\text{S}_\text{1} \to \text{2}^\text{3}\text{P}_\text{0}$ interval. Minor changes allow also for the measurement of the $\text{2}^\text{3}\text{S}_\text{1} \to \text{2}^\text{3}\text{P}_\text{1}$ and $\text{2}^\text{3}\text{S}_\text{1} \to \text{2}^\text{3}\text{P}_\text{2}$ transitions. Simulations show the expected sensitivities to be approximately $7 \, \mathrm{ppm}$ (statistical) and $0.5 \, \mathrm{ppm}$ (systematic), which will allow to shed light on a $4.5\sigma$ discrepancy of the most precise fine structure measurement in positronium with theory calculations.

Lee and Yang had noted, in their Nobel prize winning paper, that observation of apparent parity violation in the weak interaction of particles could be mitigated with the introduction of a parity conjugated copy of the same particles. The existence of a parity conjugated copy in terms of the weak interaction of normal matter, called mirror matter, that does not interact with normal matter through known forces, has long been theorized. Baryon number violation is required for baryogenesis in order to explain the observed asymmetry between matter and antimatter in the universe. Neutron to mirror-neutron oscillations could be an observable baryon number violating process. In late 2000s, it was pointed out that the oscillation time of such neutron to mirror-neutron oscillation could be of the order of a few seconds, and that a magnetic field dependent ultracold neutron storage measurement could be sensitive to such oscillations. Furthermore, it was also shown that the ambient mirror magnetic field on the surface of the Earth could be as high as the Earth's magnetic field. Subsequently, two separate groups performed experiments at the Institut Laue-Langevin in search of neutron to mirror-neutron oscillations and reported having found no evidence. The limit set on the oscillation time was $\tau_{nn'} > 414~$s (90\% C.L.) for the case where the mirror magnetic field, $B'=0$, and $\tau_{nn'}>12~$s (95\% C.L.) for the case where $B'\in(0,12.5)~\mu$T. These constraints have since been improved to $\tau_{nn'} > 448~$s (90\% C.L., $B'=0$), $\tau_{nn'} > 17~\text{s}~\forall~B'\in(8,17)~\mu\text{T (95\% C.L.)}$, and $\tau_{nn'} > 27~\text{s}~\forall~B'\in(6,25)~\mu\text{T (95\% C.L.)}$. Soon after, when the results of these experiments were further analyzed by Berezhiani et al., $5\sigma$, $3\sigma$, and $2.5\sigma$ statistically significant signals for mirror-neutron oscillation in the presence of a non zero mirror magnetic field were reported. The current leading constraints upon $\tau_{nn'}$ do not exclude these signals. Thus a new experiment was required to test these claimed signals. The neutron electric dipole moment experiment based at the Paul Scherrer Institute performed a dedicated search to investigate these signals and found no evidence of neutron mirror-neutron oscillations. We thereby impose the following new lower limits on the oscillation time: $\tau_{nn'} > 388~$s (90\% C.L., $B'=0$), $\tau_{nn'} > 6~\text{s}~\forall~B'\in(0.38,25.66)~\mu\text{T (95\% C.L.)}$, and $\left(\tau_{nn'}/\sqrt{\cos(\beta)}\right) > 9~\text{s}~\forall~B'\in(5.04,25.39)~\mu\text{T (95\% C.L.)}$, where $\beta$ is the fixed angle between the applied magnetic field and the ambient mirror magnetic field, which is bound to the reference frame of the Earth. For the case when the mirror magnetic field is fixed in the cosmos, we have also for the first time placed the following constraints: $\tau^{\Omega_{\bigoplus}}_{nn'} > 7~\text{s (95\% C.L.)}~\forall~B'\in(5.20,24.43)~\mu\text{T}$, $\tau^{\Omega_{2\bigoplus}}_{nn'}~>~6~\text{s}~\forall~B'\in(5.51,25.78)~\mu\text{T}~(95\%~\text{C.L.})$, $\tau^{\Omega_{\bigodot}}_{nn'}~>~4~\text{s}~\forall~B'\in(6.77,27.18)~\mu\text{T}~(95\%~\text{C.L.})$, and $\tau^{\Omega_{2\bigodot}}_{nn'}~>~5~\text{s}~\forall~B'\in(4.20,24.17)~\mu\text{T}~(95\%~\text{C.L.})$, for modulation frequencies associated with one sidereal day, half a sidereal day, an annual year, and half an annual year, respectively. Our new constraints in the assumption of a mirror magnetic field bound to Earth are the best constraints around $B'\sim10~\mu$T, and also in the region where the mirror magnet field falls in the range of $B'>37~\mu$T. While this result excludes large portions of the three statistically significant signals indicated by Berezhiani et al., especially where at least two signal regions overlap, it does not exclude all the signals. We therefore need further tests of these signals in the vicinity of $B'\in(4,37)~\mu$T.