Neutron Electric Dipole Moment Research Articles

Baryon asymmetric Universe from spontaneous CP violation

Spontaneous CP violation, such as the Nelson-Barr (NB) mechanism, is an attractive scenario for addressing the strong CP problem while realizing the observed phase of the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix. However, not only the CKM phase but also the baryon asymmetric Universe requires sources of CP violation. In this study, we show that a supersymmetric NB mechanism can naturally accommodate the Affleck-Dine (AD) baryogenesis within a CP-invariant Lagrangian. The model provides flat directions associated with new heavy quarks. Focusing on one of the directions, we find that the correct baryon asymmetry is obtained with a sufficiently low reheating temperature which does not cause the gravitino problem. Some parameter space is consistent with the gravitino dark matter. We assess radiative corrections to the strong CP phase induced by gauge-mediated supersymmetry breaking and CP-violating heavy fields and show that the strong CP problem is solved in a viable parameter space where the visible sector supersymmetric particles must be lighter than mathcal{O} (100) TeV. Even in the case that they are heavier than the TeV scale, our model predicts the neutron electric dipole moment within the reach of the near future experiments. Our model addresses the electroweak naturalness problem, strong CP problem, baryon asymmetric Universe, and dark matter. Then, the model may give a new compelling paradigm of physics beyond the Standard Model.

Simulations of the nEDM@SNS light collection system efficiency

A system for collecting the scintillation light produced by the capture of ultra-cold neutrons (UCN) on polarized 3He is discussed and results from simulations of its performance are presented. This system will be implemented in nEDM@SNS, the experiment searching for the neutron electric dipole moment (nEDM) at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. Simulation results show that the light collection system detects on average 17 photoelectrons per UCN-3He capture event (sufficient to generate a robust signal), reconstructs the event location in the beam direction to approximately 3 cm accuracy, detects capture events with a high and spatially uniform efficiency (0.95 with 1% variation), and rejects greater than 50% of beta decay background events.

A Search for Neutron to Mirror Neutron Oscillation Using Neutron Electric Dipole Moment Measurements

Baryon number violation is a key ingredient of baryogenesis. It has been hypothesized that there could also be a parity-conjugated copy of the standard model particles, called mirror particles. The existence of such a mirror universe has specific testable implications, especially in the domain of neutral particle oscillation, viz. the baryon number violating neutron to mirror-neutron (n−n′) oscillation. Consequently, there were many experiments that have searched for n−n′ oscillation, and imposed constraints upon the parameters that describe it. Recently, further analysis on some of these results have identified anomalies which could point to the detection of n−n′ oscillation. All the previous efforts searched for n−n′ oscillation by comparing the relative number of ultracold neutrons that survive after a period of storage for one or both of the two cases: (i) comparison of zero applied magnetic field to a non-zero applied magnetic field, and (ii) comparison where the orientation of the applied magnetic field was reversed. However, n−n′ oscillations also lead to variations in the precession frequency of polarized neutrons upon flipping the direction of the applied magnetic field. Precession frequencies are measured, very precisely, by experiments searching for the electric dipole moment. For the first time, we used the data from the latest search for the neutron electric dipole moment to constrain n−n′ oscillation. After compensating for the systematic effects that affect the ratio of precession frequencies of ultracold neutrons and cohabiting 199Hg-atoms, chief among which was due to their motion in non-uniform magnetic field, we constrained any further perturbations due to n−n′ oscillation. We thereby provide a lower limit on the n−n′ oscillation time constant of τnn′/|cos(β)|>5.7s,0.36T′<B′<1.01T′ (95% C.L.), where β is the angle between the applied magnetic field and the ambient mirror magnetic field. This constraint is the best available in the range of 0.36T′<B′<0.40T′.

Dark matter from axion strings with adaptive mesh refinement

Axions are hypothetical particles that may explain the observed dark matter density and the non-observation of a neutron electric dipole moment. An increasing number of axion laboratory searches are underway worldwide, but these efforts are made difficult by the fact that the axion mass is largely unconstrained. If the axion is generated after inflation there is a unique mass that gives rise to the observed dark matter abundance; due to nonlinearities and topological defects known as strings, computing this mass accurately has been a challenge for four decades. Recent works, making use of large static lattice simulations, have led to largely disparate predictions for the axion mass, spanning the range from 25 microelectronvolts to over 500 microelectronvolts. In this work we show that adaptive mesh refinement simulations are better suited for axion cosmology than the previously-used static lattice simulations because only the string cores require high spatial resolution. Using dedicated adaptive mesh refinement simulations we obtain an over three order of magnitude leap in dynamic range and provide evidence that axion strings radiate their energy with a scale-invariant spectrum, to within ~5% precision, leading to a mass prediction in the range (40,180) microelectronvolts.

Ultracold neutron storage simulation using the Kassiopeia software package

The Kassiopeia software package was originally developed to simulate electromagnetic fields and charged particle trajectories for neutrino mass measurement experiments. Recent additions to Kassiopeia also allow it to simulate neutral particle trajectories in magnetic fields based on their magnetic moments. Two different methods were implemented: an exact method that can work for arbitrary fields and an adiabatic method that is limited to slowly-varying fields but is much faster for large precession frequencies. Additional interactions to simulate reflection of ultracold neutrons (UCNs) from material walls and to allow spin–flip pulses were also added. These tools were used to simulate neutron precession in a room temperature neutron electric dipole moment experiment and predict the values of the longitudinal and transverse relaxation times as well as the trapping lifetime. All three parameters are found to closely match the experimentally determined values when simulated with both the exact and adiabatic methods, confirming that Kassiopeia is able to accurately simulate neutral particles. This opens the door for future uses of Kassiopeia to prototype the next generation of atomic traps and UCN experiments.

Composite neutrinos and the QCD axion: Baryogenesis, dark matter, small Dirac neutrino masses, and vanishing neutron electric dipole moment

We consider a chiral gauge theory from which light composite Dirac neutrinos dynamically emerge, augmented by a QCD axion to solve the strong CP problem. We show that an interplay between the composite sector and the axion can also simultaneously lead to successful baryogenesis and generate a correct dark matter abundance via "co-genesis" without contradicting present constraints. We predict $\Delta N_{\rm eff} \geq 0.14$ and $m_a$ in the range $O(1)\>{\rm \mu eV}$--$O(10)\>{\rm meV}$, which can be firmly tested by upcoming CMB experiments and axion searches.

Electric dipole moments of neutron and heavy quarks c, t in CP violating U(1)XSSM

We study and analyze the electric dipole moments (EDMs) of neutron and heavy quarks c, t in a CP-violating (CPV) supersymmetric U(1) extension of the standard model called U(1)XSSM, whose local gauge group is U(1)X×SU(3)C×SU(2)L×U(1)Y. The contributions from the one loop diagrams and the Weinberg operators are taken into account with the new introduced CPV phases beyond the minimum supersymmetric standard model (MSSM) being non-zero. Our numerical results show that in U(1)XSSM the neutron EDM can be smaller than its experimental upper bound 1.8×10−26 e⋅cm without a small CPV phase. The magnitudes of EDMs of heavy quarks c and t can reach 10−24 e⋅cm and 10−22 e⋅cm respectively. This is favorable to the study of CPV.

CP asymmetry in the angular distributions of \u03c4 \u2192 KS\u03c0\u03bd\u03c4 decays. Part II. General effective field theory analysis

In this work, we proceed to study the CP asymmetry in the angular distributions of τ → KSπντ decays within a general effective field theory framework including four-fermion operators up to dimension-six. It is found that, besides the commonly considered scalar-vector interference, the tensor-scalar interference can also produce a non-zero CP asymmetry in the angular distributions, in the presence of complex couplings. Using the dispersive representations of the Kπ form factors as inputs, and taking into account the detector efficiencies of the Belle measurement, we firstly update our previous SM predictions for the CP asymmetries in the same four Kπ invariant-mass bins as set by the Belle collaboration. Bounds on the effective couplings of the non-standard scalar and tensor interactions are then obtained under the combined constraints from the CP asymmetries measured in the four bins and the branching ratio of τ−→ KSπ−ντ decay, with the numerical results given respectively by operatorname{Im}left[{hat{upepsilon}}_Sright] = −0.008 ± 0.027 and operatorname{Im}left[{hat{upepsilon}}_Tright] = 0.03 ± 0.12, at the renormalization scale μτ = 2 GeV in the overline{mathrm{MS}} scheme. Using the best-fit values, we also find that the distributions of the CP asymmetries can deviate significantly from the SM expectation in almost the whole Kπ invariant-mass region. Nevertheless, the current bounds on operatorname{Im}left[{hat{upepsilon}}_Sright] and operatorname{Im}left[{hat{upepsilon}}_Tright] are still plagued by large experimental uncertainties, but will be improved with more precise measurements from the Belle II experiment as well as the proposed Tera-Z and STCF facilities. Assuming further that the non-standard scalar and tensor interactions originate from a weakly-coupled heavy new physics well above the electroweak scale, the SU(2)L invariance of the resulting SMEFT Lagrangian would indicate that very strong limits on operatorname{Im}left[{hat{upepsilon}}_Sright] and operatorname{Im}left[{hat{upepsilon}}_Tright] could also be obtained from the neutron electric dipole moment and the {D}^0-{overline{D}}^0 mixing. With the bounds from these processes taken into account, it is then found that, unless there exist extraordinary cancellations between the new physics contributions, neither the scalar nor the tensor interaction can produce any significant effects on the CP asymmetries (relative to the SM predictions) in the processes considered, especially under the “single coefficient dominance” assumption.

Superfluid helium based ultracold neutron source for the PIK reactor

The PIK reactor at NRC "Kurchatov Institute"-PNPI is going to be equipped with a high-flux Ultra Cold Neutron (UCN) source for fundamental physics researches. The UCN source will use superfluid helium, which will make possible to achieve the density of UCN 2.2·10^3 cm-3, that has not yet been achieved anywhere in the world. The UCN source will be installed on the GEK-4 channel, which will make possible to obtain a low value of heat influx to cryogenic vessels from reactor radiation. The heat removal from the UCN source vessel will be implemented by using a heat exchanger. The calculated UCN density in the EDM spectrometer chamber at the PIK is going to be 200 cm-3, which is 20 times higher than the existing UCN densities in the world. For a new UCN source based on superfluid helium, an extensive research program has been developed in field of the physics of fundamental interactions, including the search for a nonzero neutron EDM, precision measurement of the neutron lifetime, and search for mirror dark matter. Keywords: ultracold neutrons, neutron sources, superfluid helium, neutron EDM, neutron decay.

Prospects for a neutron EDM measurement with an advanced ultracold neutron source at TRIUMF

The TUCAN (TRIUMF Ultra-Cold Advanced Neutron) collaboration is building a new high-intensity ultracold neutron (UCN) source at TRIUMF with the aim of measuring the neutron electric dipole moment (nEDM) with unprecedented precision. TUCAN employs a spallation-driven superthermal UCN production scheme which has been demonstrated with a prototype UCN source. In this article, recent progress on the major components of a new upgraded UCN source and developments in thee context of installation of the nEDM spectrometer are reported.

Источник ультрахолодных нейтронов на основе сверхтекучего гелия для реактора ПИК

The PIK reactor at NRC "Kurchatov Institute" - PNPI is going to be equipped with a high-flux Ultra Cold Neutron (UCN) source for fundamental physics researches. The UCN source will use superfluid helium, which will make possible to achieve the density of UCN 2.2·10^3 cm-3, that has not yet been achieved anywhere in the world. The UCN source will be installed on the GEK-4 channel, which will make possible to obtain a low value of heat influx to cryogenic vessels from reactor radiation. The heat removal from the UCN source vessel will be implemented by using a heat exchanger. The calculated UCN density in the EDM spectrometer chamber at the PIK is going to be 200 cm-3, which is 20 times higher than the existing UCN densities in the world. For a new UCN source based on superfluid helium, an extensive research program has been developed in field of the physics of fundamental interactions, including the search for a nonzero neutron EDM, precision measurement of the neutron lifetime, and search for mirror dark matter.

Vanishing or non-vanishing rainbow? Reduction formulas of electric dipole moment

In this paper, we derive a simplified formula of electric dipole moments (EDMs) of a fermion. In the Standard Model, it is well-known that non-trivial cancellations between some rainbow-type diagrams induced by W boson exchanges occur in the calculation of the neutron EDM at the two-loop level due to the gauge symmetry. The fermion self-energy and the vertex correction are related through the Ward-Takahashi identity, and this relation causes the exact cancellation of the EDM. We derive EDM formulas for a more general setup by introducing the form factors for the fermion self-energy and the vertex correction so that the derived formulas can be applicable to a larger class of models. We conclude that the non-zero EDM contributions are induced from rainbow-type diagrams with the chirality flipping effects for internal fermions. We also discuss the other possible generalization of the EDM calculation which is applicable to the other classes of models.

The search for the neutron electric dipole moment at PSI

The existence of a nonzero permanent electric dipole moment (EDM) of the neutron would reveal a new source of CP violation and shed light on the origin of the matter–antimatter asymmetry of the Universe. The sensitivity of current experiments using stored ultracold neutrons (UCN) probe new physics beyond the TeV scale. Using the UCN source at the Paul Scherrer Institut, the nEDM collaboration has performed the most sensitive measurement of the neutron EDM to date, still compatible with zero (|d_n|<1.8\times 10^{-26} \, e {cm}|dn|<1.8×10−26ecm, C.L.,90%). A new experiment designed to improve the sensitivity by an order of magnitude, n2EDM, is currently under construction.

Beautiful and charming chromodipole moments

In the context of the Standard Model effective field theory we derive direct and indirect bounds on chromodipole operators involving the bottom and charm quark. We find that the experimental upper limit on the neutron electric dipole moment puts severe constraints on the imaginary parts of the Wilson coefficients of both chromodipole operators. The magnitudes of the Wilson coefficients are instead only weakly constrained by dijet searches and Z-boson production in association with bottom-quark jets. Flavour physics does not provide meaningful bounds.

Electric dipole moments of three-nucleon systems in the pionless effective field theory

We calculate the electric dipole moments (EDMs) of three-nucleon systems at leading order in pionless effective field theory. The one-body contributions that arise from permanent proton and neutron EDMs and the two-body contributions that arise from charge-parity-odd nucleon-nucleon interactions are taken into account. Neglecting the Coulomb interaction, we consider the triton and $^{3}\mathrm{He}$, and also investigate them in the Wigner-SU(4) symmetric limit. We also calculate the electric dipole form factor and find numerically that the momentum dependence of the electric dipole form factor in the Wigner limit is, up to an overall constant (and numerical accuracy), the same as the momentum dependence of the charge form factor. Finally, we study the cutoff dependence of these observables and find that they are properly renormalized.

Search for Low-Mass Axion Dark Matter with ABRACADABRA-10cm.

Two of the most pressing questions in physics are the microscopic nature of the dark matter that comprises 84% of the mass in the Universe and the absence of a neutron electric dipole moment. These questions would be resolved by the existence of a hypothetical particle known as the quantum chromodynamics (QCD) axion. In this work, we probe the hypothesis that axions constitute dark matter, using the ABRACADABRA-10cm experiment in a broadband configuration, with world-leading sensitivity. We find no significant evidence for axions, and we present 95% upper limits on the axion-photon coupling down to the world-leading level g_{aγγ}<3.2×10^{-11} GeV^{-1}, representing one of the most sensitive searches for axions in the 0.41-8.27neV mass range. Our work paves a direct path for future experiments capable of confirming or excluding the hypothesis that dark matter is a QCD axion in the mass range motivated by string theory and grand unified theories.

New-physics searches in CP -violating η muonic decays

In this work we investigate the possibility to observe CP-violation in η decays containing muons at the proposed REDTOP experiment. Employing the SMEFT to parametrize the new-physics CP-violating effects, we find that a single operator exists for which is possible to observe CP-violation at REDTOP in η → μ+μ− decays, while evading bounds from the neutron electric dipole moment and D−→μν‾ meson decays.

CP violation from charged Higgs bosons in the three Higgs doublet model

We demonstrate a new type of cancellation of contributions to the electron and neutron electric dipole moments (EDMs) that occurs in three Higgs doublet models (3HDMs) when CP violation appears in the charged Higgs sector. The cancellation becomes exact when the two physical charged Higgs bosons in the model are degenerate in mass. Depending on the model parameters, degeneracies at the 10% level are however sufficient to evade current bounds on the electron and neutron EDMs. We demonstrate that viable parameter space remains with both charged Higgs bosons lighter than 500 GeV and large CP-violating phases while also satisfying theoretical constraints from perturbativity and experimental ones from overline{B} → Xsγ and direct searches.

The CKM phase and overline{theta} in Nelson-Barr models

We analyze the Nelson-Barr approach to the Strong CP Problem. We derive the necessary conditions in order to simultaneously reproduce the CKM phase and the quark masses. Then we quantify the irreducible contributions to the QCD topological angle, namely the corrections arising from loops of the colored fermion mediators that characterize these models. Corrections analytic in the couplings first arise at 3-loop order and are safely below current bounds; non-analytic effects are 2-loop order and decouple as the mediators exceed a few TeV. We discuss collider, electroweak, and flavor bounds and argue that most of the parameter space above the TeV scale is still allowed in models with down-type mediators, whereas other scenarios are more severely constrained. With two or more families of mediators the dominant experimental bound is due to the neutron electric dipole moment.

Nucleon matrix element of Weinberg's CP-odd gluon operator from the instanton vacuum

We calculate the nucleon matrix element of Weinberg's dimension-6 CP-odd gluon operator fabc(F˜μν)a(Fμρ)b(Fρν)c in the instanton vacuum. In leading order of the instanton packing fraction, the dimension-6 operator is effectively proportional to the topological charge density (F˜μν)a(Fμν)a, whose nucleon matrix element is given by the flavor-singlet axial charge and constrained by the U(1)A anomaly. The nucleon matrix element of the dimension-6 operator is obtained substantially larger than in other estimates, because of the strong localization of the nonperturbative gluon fields in the instanton vacuum. We comment on the neutron electric dipole moment induced by CP violation proportional to the dimension-6 operator.