Search

We present a novel Active Magnetic Shield (AMS), designed and implemented for the n2EDM experiment at the Paul Scherrer Institute. The experiment will perform a high-sensitivity search for the electric dipole moment of the neutron. Magnetic-field stability and control is of key importance for n2EDM. A large, cubic, 5m side length, magnetically shielded room (MSR) provides a passive, quasi-static shielding-factor of about 10^5 for its inner sensitive volume. The AMS consists of a system of eight complex, feedback-controlled compensation coils constructed on an irregular grid spanned on a volume of less than 1000m^3 around the MSR. The AMS is designed to provide a stable and uniform magnetic-field environment around the MSR, while being reasonably compact. The system can compensate static and variable magnetic fields up to +-50muT (homogeneous components) and +-5muT (first-order gradients), suppressing them to a few muT in the sub-Hertz frequency range. The presented design concept and implementation of the AMS fulfills the requirements of the n2EDM experiment and can be useful for other applications, where magnetically silent environments are important and spatial constraints inhibit simpler geometrical solutions.

Muonic atom spectroscopy -- the measurement of the x rays emitted during the formation process of a muonic atom -- has a long standing history in probing the shape and size of nuclei. In fact, almost all stable elements have been subject to muonic atom spectroscopy measurements and the absolute charge radii extracted from these measurements typically offer the highest accuracy available. However, so far only targets of at least a few hundred milligram could be used as it required to stop a muon beam directly in the target to form the muonic atom. We have developed a new method relying on repeated transfer reactions taking place inside a 100-bar hydrogen gas cell with an admixture of 0.25% deuterium that allows us to drastically reduce the amount of target material needed while still offering an adequate efficiency. Detailed simulations of the transfer reactions match the measured data, suggesting good understanding of the processes taking place inside the gas mixture. As a proof of principle we demonstrate the method with a measurement of the 2p-1s muonic x rays from a 5-{\mu}g gold target., Comment: 16 pages, 14 figures

We report on a search for a new, short-range, spin-dependent interaction using a modified version of the experimental apparatus used to measure the permanent neutron electric dipole moment at the Paul Scherrer Institute. This interaction, which could be mediated by axion-like particles, concerned the unpolarized nucleons (protons and neutrons) near the material surfaces of the apparatus and polarized ultracold neutrons stored in vacuum. The dominant systematic uncertainty resulting from magnetic-field gradients was controlled to an unprecedented level of approximately 4 pT/cm using an array of optically-pumped cesium vapor magnetometers and magnetic-field maps independently recorded using a dedicated measurement device. No signature of a theoretically predicted new interaction was found, and we set a new limit on the product of the scalar and the pseudoscalar couplings $g_sg_p\lambda^2 < 8.3 \times 10^{-28}\,\text{m}^2$ (95% C.L.) in a range of $5\,\mu\text{m} < \lambda < 25\,\text{mm}$ for the monopole-dipole interaction. This new result confirms and improves our previous limit by a factor of 2.7 and provides the current tightest limit obtained with free neutrons.

Ultra-low-mass axions are a viable dark matter candidate and may form a coherently oscillating classical field. Nuclear spins in experiments on Earth might couple to this oscillating axion dark-matter field, when propagating on Earth's trajectory through our Galaxy. This spin coupling resembles an oscillating pseudo-magnetic field which modulates the spin precession of nuclear spins. Here we report on the null result of a demonstration experiment searching for a frequency modulation of the free spin-precession signal of \magHg in a \SI{1}{\micro\tesla} magnetic field. Our search covers the axion mass range $10^{-16}~\textrm{eV} \lesssim m_a \lesssim 10^{-13}~\textrm{eV}$ and achieves a peak sensitivity to the axion-nucleon coupling of $g_{aNN} \approx 3.5 \times 10^{-6}~\textrm{GeV}^{-1}$., Comment: 18 pages, 4 images, submitted to SciPost Physics

For identification and 3D-tracking of low-energy electrons a new type of gas-based detector was designed that minimizes scattering and energy loss. The current version of the detector is a combination of a plastic scintillator, serving as a trigger source and energy detector, and a hexagonally structured multi-wire drift chamber (MWDC), filled with a mixture of helium and isobutane gas. The drift time information is used to track particles in the plane perpendicular to the wires, while a charge division technique provides spatial information along the wires. The gas tracker was successfully used in the miniBETA project as a beta spectrometer for a measurement of the weak magnetism form factor in nuclear beta decay. The precision of the three-dimensional electron tracking, in combination with low-mass, low-Z materials and identification of backscattering from scintillator, facilitated a reduction of the main systematics effects. At certain conditions, a spatial resolution better than 0.5 mm was obtained in the plane perpendicular to the wires, while resolutions of about 6 mm were achieved along wires. Thanks to precise tracking information, it is possible to eliminate electrons and other particles not originating from the desired decay with high efficiency. Additionally, using the coincidence between MWDC and scintillator, background from gamma emission typically accompanying radioactive decays, was highly suppressed. An overview of different event topologies is presented together with the tracker's ability to correctly recognize them. The analysis is supported by Monte Carlo simulations using Geant4 and Garfield++ packages. Finally, the preliminary results from the 114In spectrum study are presented., 5 pages, 3 figures, accepted for publication in JINST - PSD12(2021)

Experiments dedicated to the measurement of the electric dipole moment of the neutron require outstanding control of the magnetic-field uniformity. The neutron electric dipole moment (nEDM) experiment at the Paul Scherrer Institute uses a Hg199 co-magnetometer to precisely monitor temporal magnetic-field variations. This co-magnetometer, in the presence of field nonuniformity, is, however, responsible for the largest systematic effect of this measurement. To evaluate and correct that effect, offline measurements of the field nonuniformity were performed during mapping campaigns in 2013, 2014, and 2017. We present the results of these campaigns, and the improvement the correction of this effect brings to the neutron electric dipole moment measurement. ISSN:1094-1622 ISSN:0556-2791 ISSN:1050-2947

In recent years a number of correlation measurements in nuclear $��$ decay have been performed reaching a precision of the order of 1\% and below and it is expected that even higher precision will be reached in the near future. At these levels of precision higher-order corrections due to e.g. recoil terms induced by the strong interaction and radiative corrections cannot necessarily be neglected anymore when interpreting these results in terms of new physics or extracting a value for the $V_{ud}$ quark-mixing matrix element. We provide here an update of the $\mathcal{F} t$ values of the $T=1/2$ mirror $��$ decays as well as an overview of current experimental and theoretical knowledge of the most important recoil term, weak magnetism, for both the $T=1/2$ mirror $��$ transitions and a large set of $��$ decays in higher isospin multiplets. The matrix elements determining weak magnetism were calculated in the nuclear shell model and cross-checked against experimental data, showing overall good agreement. Additionally, we show that further insight can be obtained from properly deformed nuclear potentials, in particular for mirror $T=1/2$ decays.. The results provide new insights in the size of weak magnetism, extending the available information to $��$ transitions of nuclei with masses up to $A =$ 75. This provides important guidance for the planning and interpretation of ongoing and new precise correlation measurements in nuclear $��$ decay searching for new physics or to extract the $V_{ud}$ quark-mixing matrix element in mirror $��$ decays. This more detailed knowledge of weak magnetism can also be of interest for further theoretical work related to the reactor neutrino problem., 64 pages, 21 figures

We present the design of a next-generation experiment, n2EDM, currently under construction at the ultracold neutron source at the Paul Scherrer Institute (PSI) with the aim of carrying out a high-precision search for an electric dipole moment of the neutron. The project builds on experience gained with the previous apparatus operated at PSI until 2017, and is expected to deliver an order of magnitude better sensitivity with provision for further substantial improvements. An overview is given of the experimental method and setup, the sensitivity requirements for the apparatus are derived, and its technical design is described.

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: $\tau_{nn'} > 352~$s at $B'=0$ (95% C.L.), $\tau_{nn'} > 6~\text{s}$ for all $0.4~\mu\text{T}9~\text{s}$ for all $5.0~\mu\text{T}, Comment: Submitted to Physics Letters B, 9 pages, 3 figures

The hyperfine splitting of the 5g -> 4f transitions in muonic 185,187-Re has been measured using high resolution HPGe detectors and compared to state-of-the-art atomic theoretical predictions. The spectroscopic quadrupole moment has been extracted using modern fitting procedures and compared to the values available in literature obtained from muonic X rays of natural rhenium. The extracted values of the nuclear spectroscopic quadrupole moment are 2.07(5) barn and 1.94(5) barn, respectively for 185-Re and 187-Re. This work is part of a larger effort at the Paul Scherrer Institut towards the measurement of the nuclear charge radii of radioactive elements., 15 pages, 8 figures, 8 tables; accepted for publication in Phys. Rev. C

We report first time measurements of the beta-neutrino angular correlation based on the kinetic energy shift of protons emitted in parallel or anti-parallel directions with respect to the positron in the beta decay of $^{32}$Ar. This proof of principle experiment provided simultaneous measurements for the superallowed 0$^+$~$\rightarrow$~0$^+$ transition followed by a 3356~keV proton emission and for a Gamow-Teller transition followed by a 2123~keV proton emission. The results, respectively ${\tilde a_{\beta\nu}}=1.01(3)_{(stat)}(2)_{(syst)}$ and ${\tilde a_{\beta\nu}}=-0.22(9)_{(stat)}(2)_{(syst)}$, are found in agreement with the Standard Model. A careful analysis of the data shows that future measurements can reach a precision level of 10$^{-3}$ for both pure Fermi and pure Gamow-Teller decay channels, providing new constraints on both scalar and tensor weak interactions., Comment: 5 pages, 5 figures

The electron shakeoff of 19F and 35Cl atoms resulting from the \b{eta}+ decay of 19Ne+ and 35Ar+ ions has been investigated using a Paul trap coupled to a time of flight recoil-ion spectrometer. The charge-state distributions of the recoiling daughter nuclei were compared to theoretical calculations based on the sudden approximation and accounting for subsequent Auger processes. The excellent agreement obtained for 35Cl is not reproduced in 19F. The shortcoming is attributed to the inaccuracy of the independent particle model employed to calculate the primary shakeoff probabilities in systems with rather low atomic numbers. This calls for more elaborate calculations, including explicitly the electron-electron correlations., published in Phys. Rev. A (2018)

The neutron and its hypothetical mirror counterpart, a sterile state degenerate in mass, could spontaneously mix in a process much faster than the neutron $\beta$-decay. Two groups have performed a series of experiments in search of neutron - mirror-neutron ($n-n'$) oscillations. They reported no evidence, thereby setting stringent limits on the oscillation time $\tau_{nn'}$. Later, these data sets have been further analyzed by Berezhiani et al.(2009-2017), and signals, compatible with $n-n'$ oscillations in the presence of mirror magnetic fields, have been reported. The Neutron Electric Dipole Moment Collaboration based at the Paul Scherrer Institute performed a new series of experiments to further test these signals. In this paper, we describe and motivate our choice of run configurations with an optimal filling time of $29~$s, storage times of $180~$s and $380~$s, and applied magnetic fields of $10~\mu$T and $20~\mu$T. The choice of these run configurations ensures a reliable overlap in settings with the previous efforts and also improves the sensitivity to test the signals. We also elaborate on the technique of normalizing the neutron counts, making such a counting experiment at the ultra-cold neutron source at the Paul Scherrer Institute possible. Furthermore, the magnetic field characterization to meet the requirements of this $n-n'$ oscillation search is demonstrated. Finally, we show that this effort has a statistical sensitivity comparable to the current leading constraints for $n-n'$ oscillations., Comment: Proceedings of International Workshop on Particle Physics at Neutron Sources, PPNS-2018; May 24-26, 2018, LPSC, Grenoble, France; Abs. ID: 28 and 60. 10 Pages, 9 Figures

We report on a search for ultralow-mass axionlike dark matter by analyzing the ratio of the spinprecession 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. ispartof: Physical Review X vol:7 issue:4 status: published

We present for the first time a detailed and comprehensive analysis of the experimental results that set the current world sensitivity limit on the magnitude of the electric dipole moment (EDM) of the neutron. We have extended and enhanced our earlier analysis to include recent developments in the understanding of the effects of gravity in depolarizing ultracold neutrons (UCN); an improved calculation of the spectrum of the neutrons; and conservative estimates of other possible systematic errors, which are also shown to be consistent with more recent measurements undertaken with the apparatus. We obtain a net result of $d_\mathrm{n} = -0.21 \pm 1.82 \times10^{-26}$ $e$cm, which may be interpreted as a slightly revised upper limit on the magnitude of the EDM of $3.0 \times10^{-26}$ $e$cm (90% CL) or $ 3.6 \times10^{-26}$ $e$cm (95% CL). This paper is dedicated by the remaining authors to the memory of Prof. J. Michael Pendlebury., 23 pages, 19 figures. Authorship list changed to put J.M. Pendlebury first. Other very minor edits

Precision measurements at low energy search for physics beyond the Standard Model in a way complementary to searches for new particles at colliders. In the weak sector the most general $��$ decay Hamiltonian contains, besides vector and axial-vector terms, also scalar, tensor and pseudoscalar terms. Current limits on the scalar and tensor coupling constants from neutron and nuclear $��$ decay are on the level of several percent. The goal of this paper is extracting new information on tensor coupling constants by measuring the $��$-asymmetry parameter in the pure Gamow-Teller decay of $^{67}$Cu, thereby testing the V-A structure of the weak interaction. An iron sample foil into which the radioactive nuclei were implanted was cooled down to milliKelvin temperatures in a $^3$He-$^4$He dilution refrigerator. An external magnetic field of 0.1 T, in combination with the internal hyperfine magnetic field, oriented the nuclei. The anisotropic $��$ radiation was observed with planar high purity germanium detectors operating at a temperature of about 10\,K. An on-line measurement of the $��$ asymmetry of $^{68}$Cu was performed as well for normalization purposes. Systematic effects were investigated using Geant4 simulations. The experimental value, $\tilde{A}$ = 0.587(14), is in agreement with the Standard Model value of 0.5991(2) and is interpreted in terms of physics beyond the Standard Model. The limits obtained on possible tensor type charged currents in the weak interaction hamiltonian are -0.045 $< (C_T+C'_T)/C_A

The electron shakeoff of $^{35}$Cl atoms resulting from the $\beta$$^+$ decay of $^{35}$Ar$^+$ ions has been investigated using a Paul trap coupled to a recoil-ion spectrometer. The charge-state distribution of the recoiling daughter nuclei is compared to theoretical calculations accounting for shakeoff and Auger processes. The calculations are in excellent agreement with the experimental results and enable to identify the ionization reaction routes leading to the formation of all charge states., Comment: Physical Review A In press (2013)

A permanent electric dipole moment of fundamental spin-1/2 particles violates both parity (P) and time re- versal (T) symmetry, and hence, also charge-parity (CP) symmetry since there is no sign of CPT-violation. The search for a neutron electric dipole moment (nEDM) probes CP violation within and beyond the Stan- dard Model. The experiment, set up at the Paul Scherrer Institute (PSI), an improved, upgraded version of the apparatus which provided the current best experimental limit, dn < 2.9E-26 ecm (90% C.L.), by the RAL/Sussex/ILL collaboration: Baker et al., Phys. Rev. Lett. 97, 131801 (2006). In the next two years we aim to improve the sensitivity of the apparatus to sigma(dn) = 2.6E-27 ecm corresponding to an upper limit of dn < 5E-27 ecm (95% C.L.), in case for a null result. In parallel the collaboration works on the design of a new apparatus to further increase the sensitivity to sigma(dn) = 2.6E-28 ecm., APS Division for particles and fields, Conference Proceedings, Two figures

Nuclear magnetic resonance (NMR/ON) measurements with beta- and gamma-ray detection have been performed on oriented Ag-104(g,m) nuclei with the NICOLE He-3-He-4 dilution refrigerator setup at ISOLDE/CERN. For Ag-104(g) (I-pi = 5(+)) the gamma-NMR/ON resonance signal was found at nu = 266.70(5) MHz. Combining this result with the known magnetic moment for this isotope, the magnetic hyperfine field of Ag impurities in an Fe host at low temperature (< 1 K) is found to be vertical bar B-hf(AgFe)vertical bar = 44.709(35) T. A detailed analysis of other relevant data available in the literature yields three more values for this hyperfine field. Averaging all four values yields a new and precise value for the hyperfine field of Ag in Fe; that is, vertical bar B-hf(AgFe)vertical bar = 44.692(30) T. For Ag-104(m) (I-pi = 2(+)), the anisotropy of the beta particles provided the NMR/ON resonance signal at nu = 627.7(4) MHz. Using the new value for the hyperfine field of Ag in Fe, this frequency corresponds to the magnetic moment mu(Ag-104m) = +3.691(3) mu(N), which is significantly more precise than previous results. The magnetic moments of the even-A Ag102 -110 isotopes are discussed in view of the competition between the (pi g(9/2))(7/2+)(-3)(nu d(5/2)nu g(7/2))(5/2+) and the (pi g(9/2))(9/2+)(-3)(nu d(5/2)nu g(7/2))(5/2+) configurations. The magnetic moments of the ground and isomeric states of Ag-104 can be explained by an almost complete mixing of these two configurations., 12 pages, 7 figures

The $��$-asymmetry parameter $\widetilde{A}$ for the Gamow-Teller decay of $^{60}$Co was measured by polarizing the radioactive nuclei with the brute force low-temperature nuclear-orientation method. The $^{60}$Co activity was cooled down to milliKelvin temperatures in a $^3$He-$^4$He dilution refrigerator in an external 13 T magnetic field. The $��$ particles were observed by a 500 $��m$ thick Si PIN diode operating at a temperature of about 10 K in a magnetic field of 0.6 T. Extensive GEANT4 Monte-Carlo simulations were performed to gain control over the systematic effects. Our result, $\widetilde{A} = -1.014(12)_{stat}(16)_{syst}$, is in agreement with the Standard-Model value of $-0.987(9)$, which includes recoil-order corrections that were addressed for the first time for this isotope. Further, it enables limits to be placed on possible tensor-type charged weak currents as well as other physics beyond the Standard Model.

The half-life of the $\alpha$ decaying nucleus $^{221}$Fr was determined in different environments, i.e. embedded in Si at 4 K, and embedded in Au at 4 K and about 20 mK. No differences in half-life for these different conditions were observed within 0.1%. Furthermore, we quote a new value for the absolute half-life of $^{221}$Fr of t$_{1/2}$ = 286.1(10) s, which is of comparable precision to the most precise value available in literature. The half-life of the $\alpha$ decaying nucleus $^{221}$Fr was determined in different environments, i.e. embedded in Si at 4 K, and embedded in Au at 4 K and about 20 mK. No differences in half-life for these different conditions were observed within 0.1%. Furthermore, we quote a new value for the absolute half-life of $^{221}$Fr of t$_{1/2}$ = 286.1(10) s, which is of comparable precision to the most precise value available in literature.

The beta asymmetry parameter A for the pure Gamow-Teller decay of 114In is reported. The low temperature nuclear orientation method was combined with a GEANT4 based simulation code allowing for the first time to address in detail the effects of scattering and of the magnetic field. The result, A = -0.994 +/- 0.010stat +/- 0.010syst, constitutes the most accurate value for the asymmetry parameter of a nuclear beta transition to date. The value is in agreement with the Standard Model prediction of A = -1 and provides new limits on tensor type charged weak currents., 11 pages, 2 figures; additional information was added on systematic effects, the magnetic field map and the calculation of the Qcos(theta) values

The Vud element of the Cabibbo-Kobayashi-Maskawa quark mixing matrix has traditionally been determined from the analysis of data in nuclear superallowed 0+ to 0+ transitions, neutron decay and pion beta decay. We show here that this element can independently be determined from nuclear mirror transitions. The extracted value, |Vud| = 0.9719 +- 0.0017, is at 1.2 combined standard deviations from the value obtained in superallowed 0+ to 0+ transitions and has a similar precision than the value obtained from neutron decay experiments. We discuss some prospects to improve its precision through experiments in nuclear mirror transitions., 4 pages, 2 figures

A complete survey is presented of all half-life and branching-ratio measurements related to the isospin T = 1/2 mirror beta transitions ranging from 3He to 83Mo. No measurements are ignored, although some are rejected for cause. Using the decay energies obtained in the 2003 Mass Evaluation experimental ft values are then determined for the transitions up to 45V. For the first time also all associated theoretical corrections needed to convert these results into "corrected" Ft values, similar to the superallowed 0+ -> 0+ pure Fermi beta transitions, were calculated. Precisions of the resulting values are in most cases between 0.1 % and 0.4 %. These Ft(mirror) values can now be used to extract precise weak interaction information from past and ongoing correlation measurements in the beta decay of the T = 1/2 mirror beta transitions.

A new measurement of the neutron EDM, using Ramsey's method of separated oscillatory fields, is in preparation at the new high intensity source of ultra-cold neutrons (UCN) at the Paul Scherrer Institute, Villigen, Switzerland (PSI). The existence of a non-zero nEDM would violate both parity and time reversal symmetry and, given the CPT theorem, might lead to a discovery of new CP violating mechanisms. Already the current upper limit for the nEDM (|d_n, Flavor Physics & CP Violation Conference, Taipei, 2008

The commented manuscript was submitted for publication without informing at least four of the other authors, viz. N. Severijns, O. Zimmer, H.-F. Wirth and D. Rich. This violates our rights as collaborators. The analysis presented and the manuscript itself have not been discussed and have also not been approved by the entire collaboration prior to submission. Besides this formal incorrectness, we also criticise the content of the paper.

We review the current status of precision measurements in allowed nuclear beta decay, including neutron decay, with emphasis on their potential to look for new physics beyond the standard electroweak model. The experimental results are interpreted in the framework of phenomenological model-independent descriptions of nuclear beta decay as well as in some specific extensions of the standard model. The values of the standard couplings and the constraints on the exotic couplings of the general beta decay Hamiltonian are updated. For the ratio between the axial and the vector couplings we obtain C_A/C_V = -1.26992(69) under the standard model assumptions. Particular attention is devoted to the discussion of the sensitivity and complementarity of different precision experiments in direct beta decay. The prospects and the impact of recent developments of precision tools and of high intensity low energy beams are also addressed., 52 pages, 33 figures; submitted to Review of Modern Physics; uses aps.sty and rmp.sty

The aim of this work is experimental discovery and research of a rare neutron mode, the radiative beta-decay, where a new particle, the radiative gamma-quantum, is formed along with the expected decay products beta-electron, recoil proton and antineutrino. The discovery of this rare neutron decay mode was conducted through identification of triple coincidences events: simultaneous registration of beta electron, proton and radiative gamma-quantum. The ordinary neutron decay was registered by double coincidences of beta electron and recoil proton. The ratio of triple to double coincidences is connected with relative intensity of radiative neutron decay (branching ratio - B.R.) B.R. = (3.2+1.6)10-3 (with 90 % C.L. and in the gamma energy region greater than 35 keV), which we are the first in the world to measure during the second and third cycles on FRMII (TUM, Germany) in Summer 2005. This value of B.R. is consistent with standard electroweak theory., Corrected version of this paper was published in JETP Letters 83(1)

In the Standard Model of elementary particles, quark-mixing is expressed in terms of a 3 x 3 unitary matrix V, the so called Cabibbo-Kobayashi-Maskawa (CKM) matrix. Significant unitarity checks are so far possible for the first row of this matrix. This article reviews the experimental and theoretical information on these matrix elements. On the experimental side, we find a 2.2 sigma to 2.7 sigma deviation from unitarity, which conflicts with the Standard Model., accepted by EPJ C