Constraints on short-range spin-dependent interactions from scalar spin-spin coupling in deuterated molecular hydrogen

Searches for exotic spin-dependent interactions with spin sensors

Numerous theories have postulated the existence of exotic spin-dependent interactions beyond the Standard Model of particle physics. Spin-based quantum sensors, which utilize the quantum properties of spins to enhance measurement precision, emerge as powerful tools for probing these exotic interactions. These sensors encompass a wide range of technologies, such as optically pumped magnetometers, atomic comagnetometers, spin masers, nuclear magnetic resonance, spin amplifiers, and nitrogen-vacancy centers. These technologies stand out for their ultrahigh sensitivity, compact tabletop design, and cost-effectiveness, offering complementary approaches to the large-scale particle colliders and astrophysical observations. This article reviews the underlying physical principles of various spin sensors and highlights the recent theoretical and experimental progress in the searches for exotic spin-dependent interactions with these quantum sensors. Investigations covered include the exotic interactions of spins with ultralight dark matter, exotic spin-dependent forces, electric dipole moment, spin-gravity interactions, and among others. Ongoing and forthcoming experiments using advanced spin-based sensors to investigate exotic spin-dependent interactions are discussed.

New Constraints on Axion-Mediated Spin Interactions Using Magnetic Amplification

Axions are highly motivated hypothetical particles beyond the standard model that can be dark matter candidates and address the strong CP problem. Here we search for axion-mediated interactions generated between two separated ^{129}Xe gas ensembles, monitoring and polarizing the ^{129}Xe nuclear spins through spin-exchange interactions with Rb vapor. Our method exploits the magnetic amplification through effective fields from Rb-Xe collisions, increasing the sensitivity for axion-mediated interactions by up to 145-fold relative to conventional methods. Moreover, we employ template filtering to extract exotic interactions with a maximum signal-to-noise ratio. By combining two techniques, axion-mediated interactions are constrained to be less than 10^{-5} of normal magnetic interactions on a length scale of 60 mm. We establish new constraints on the neutron-neutron pseudoscalar couplings for a mass range that expands into the well-motivated "axion window" (10  μeV-1  meV), improving previous constraints by up to 50-fold within it and 118-fold outside it. We further discuss promising applications in searches for other axion-nucleon interactions, including axion dark matter and black hole axion bursts with sensitivity well beyond astrophysical limits by several orders of magnitude.

Constraining CP Violating Nucleon-Nucleon Long-Range Interactions in Diatomic eEDM Searches

The searches for CP violating effects in diatomic molecules, such as HfF^{+} and ThO, are typically interpreted as a probe of the electron's electric dipole moment (eEDM), a new electron-nucleon interaction, and a new electron-electron interaction. However, in the case of a nonvanishing nuclear spin, a new CP violating nucleon-nucleon long-range force will also affect the measurement, providing a new interpretation of the eEDM experimental results. Here, we use the HfF^{+} eEDM search and derive a new bound on this hypothetical interaction, which is the most stringent from terrestrial experiments in the 1 eV-10 keV mass range. These multiple new physics sources motivate independent searches in different molecular species for CP violation at low energy that result in model independent bounds, which are insensitive to cancellation among them.

Constraints on exotic spin-velocity-dependent interactions

Experimental searches for exotic spin-dependent forces are attracting a lot of attention because they allow to test theoretical extensions to the standard model. Here, we report an experimental search for possible exotic spin-dependent force, specifically spin-and-velocity-dependent forces, by using a K-Rb-²¹Ne co-magnetometer and a tungsten ring featuring a high nucleon density. Taking advantage of the high sensitivity of the co-magnetometer, the pseudomagnetic field from this exotic force is measured to be ≤7 aT. This sets limits on coupling constants for the neutron-nucleon and proton-nucleon interactions in the range of ≥0.1 m (mediator boson mass ≤2 μeV). The coupling constant limits are established to be [Formula: see text] and [Formula: see text], which are more than one order of magnitude tighter than astronomical and cosmological limits on the coupling between the new gauge boson such as Z' and standard model particles.

Limits on Axions and Axionlike Particles within the Axion Window Using a Spin-Based Amplifier

Searches for the axion and axionlike particles may hold the key to unlocking some of the deepest puzzles about our Universe, such as dark matter and dark energy. Here, we use the recently demonstrated spin-based amplifier to constrain such hypothetical particles within the well-motivated "axion window" (10  μeV-1  meV) through searching for an exotic dipole-dipole interaction between polarized electron and neutron spins. The key ingredient is the use of hyperpolarized long-lived ^{129}Xe nuclear spins as an amplifier for the pseudomagnetic field generated by the exotic interaction. Using such a spin sensor, we obtain a direct upper bound on the product of coupling constants g_{p}^{e}g_{p}^{n}. The spin-based amplifier technique can be extended to searches for a wide variety of hypothetical particles beyond the standard model.

Dark Matter Axions, Non-Newtonian Gravity and Constraints on Them from Recent Measurements of the Casimir Force in the Micrometer Separation Range

We consider axionlike particles as the most probable constituents of dark matter, the Yukawa-type corrections to Newton’s gravitational law and constraints on their parameters following from astrophysics and different laboratory experiments. After a brief discussion of the results by Prof. Yu. N. Gnedin in this field, we turn our attention to the recent experiment on measuring the differential Casimir force between Au-coated surfaces of a sphere and the top and bottom of rectangular trenches. In this experiment, the Casimir force was measured over an unusually wide separation region from 0.2 to 8μm and compared with the exact theory based on first principles of quantum electrodynamics at nonzero temperature. We use the measure of agreement between experiment and theory to obtain the constraints on the coupling constant of axionlike particles to nucleons and on the interaction strength of a Yukawa-type interaction. The constraints obtained on the axion-to-nucleon coupling constant and on the strength of a Yukawa interaction are stronger by factors of 4 and 24, respectively, than those found previously from gravitational experiments and measurements of the Casimir force but weaker than the constraints following from a differential measurement where the Casimir force was nullified. Some other already performed and planned experiments aimed at searching for axions and non-Newtonian gravity are discussed, and their prospects are evaluated.

Constraints on Theoretical Predictions beyond the Standard Model from the Casimir Effect and Some Other Tabletop Physics

We review the hypothetical interactions predicted beyond the Standard Model which could be constrained by using the results of tabletop laboratory experiments. These interactions are described by the power-type potentials with different powers, Yukawa potential, other spin-independent potentials, and by the spin-dependent potentials of different kinds. In all these cases the current constraints on respective hypothetical interactions are considered which follow from the Casimir effect and some other tabletop physics. The exotic particles and constraints on them are discussed in the context of problems of the quantum vacuum, dark energy, and the cosmological constant.

New Limits on Anomalous Spin-Spin Interactions

We report the results of a new search for long-range spin-dependent interactions using a Rb-^{21}Ne atomic comagnetometer and a rotatable electron spin source based on a SmCo_{5} magnet with an iron flux return. By looking for signal correlations with the orientation of the spin source we set new constraints on the product of the pseudoscalar electron and neutron couplings g_{p}^{e}g_{p}^{n}/ℏc<1.7×10^{-14} and on the product of their axial couplings g_{A}^{e}g_{A}^{n}/ℏc<5×10^{-42} to a new particle with a mass of less than about 1  μeV. Our measurements improve by about 2 orders of magnitude previous constraints on such spin-dependent interactions.

The State of the Art in Constraining Axion-to-Nucleon Coupling and Non-Newtonian Gravity from Laboratory Experiments

Constraints on the Yukawa-type corrections to Newton’s gravitational law and on the coupling constant of axionlike particles to nucleons obtained from different laboratory experiments are reviewed and compared. The constraints on non-Newtonian gravity under discussion cover the wide interaction range from nanometers to millimeters and follow from the experiments on neutron scattering, measuring the Casimir force and Cavendish-type experiments. The constraints on the axion-to-nucleon coupling constant following from the magnetometer measurements, Cavendish-type experiments, Casimir physics, and experiments with beams of molecular hydrogen are considered, which refer to the region of axion masses from 10−10 to 200 eV. Particular attention is given to the recent constraints obtained from measuring the Casimir force at nanometer separation distance between the test bodies. Several proposed experiments focussed on constraining the non-Newtonian gravity, axionlike particles and other hypothetical weakly interacting particles, such as chameleons and symmetrons, are discussed.

Optomechanical probe of an axial-vector mediated interaction in the quantum regime

We present an atomic, molecular, and optical physics based method to search for axial-vector mediated dipole-dipole interaction between electrons. In our optomechanical scheme, applying a static magnetic field and a pump beam and a probe beam to a hybrid mechanical system composed of a nitrogen-vacancy center and a cantilever resonator, we could obtain a probe absorption spectrum. Based on the study of the relationship between this spectrum and the exotic dipole-dipole interaction, we put forward our detection principle and then provide a prospective constraint most stringent at a rough interaction range from 4 × 10^-8 to 2 × 10^-7m. Our results indicate that this scheme could be put into consideration in relevant experimental searches.

Determination of the Interval between the Ground States of Para- and Ortho-H_{2}

Nuclear-spin-symmetry conservation makes the observation of transitions between quantum states of ortho- and para-H_{2} extremely challenging. Consequently, the energy-level structure of H_{2} derived from experiment consists of two disjoint sets of level energies, one for para-H_{2} and the other for ortho-H_{2}. We use a new measurement of the ionization energy of para-H_{2} [E_{I}(H_{2})/(hc)=124 417.491 098(31)  cm^{-1}] to determine the energy separation [118.486 770(50)  cm^{-1}] between the ground states of para- and ortho-H_{2} and thus link the energy-level structure of the two nuclear-spin isomers of this fundamental molecule. Comparison with recent theoretical results [M. Puchalski et al., Phys. Rev. Lett. 122, 103003 (2019)PRLTAO0031-900710.1103/PhysRevLett.122.103003] enables the derivation of an upper bound of 1.5 MHz for a hypothetical global shift of the energy-level structure of ortho-H_{2} with respect to that of para-H_{2}.

Unusual Indirect Nuclear Spin-Spin Exchange Coupling through Solvated Electron

Solvated electrons have been found to exist in various media which also exhibit more intriguing properties such as superconductivity, nonlinear optical response, and so on. However, how they affect the nuclear spin properties has not been proven. In this work, we present the first detailed study on solvated-electron-triggered indirect nuclear spin-spin J-coupling using density functional theory calculations. Taking ¹⁹F as a probe, we verify the presence of unusual J couplings between two distant F atoms in HF-containing anionic clusters. These couplings occur "through solvated electron", rather than through conventional covalent bonds or space. Solvated electron can serve as an additional channel to efficiently realize long-range J-coupling between far separated nuclei because of its dispersivity and Rydberg character. The coupling magnitude strongly depends on the unique distribution of solvated electron and its second-order interaction with solvating HF units. This work provides novel insights into the mediating roles of electrons, possibly opening up potential applications based on weakly bound electrons.

A method for measurement of spin-spin couplings with sub-mHz precision using zero- to ultralow-field nuclear magnetic resonance

We present a method which allows for the extraction of physical quantities directly from zero- to ultralow-field nuclear magnetic resonance (ZULF NMR) data. A numerical density matrix evolution is used to simulate ZULF NMR spectra of several molecules in order to fit experimental data. The method is utilized to determine the indirect spin-spin couplings (J-couplings) in these systems, which is achieved with precision of 10^-2-10^-4Hz. The simulated and measured spectra are compared to earlier research. Agreement and improved precision are achieved for most of the J-coupling estimates. The availability of fast, flexible fitting method for ZULF NMR enables a new generation of precision-measurement experiments for spin-dependent interactions and physics beyond the Standard Model.

Indirect Spin-Spin Coupling Constants in the Hydrogen Isotopologues

The results of experimental and theoretical studies of indirect spin-spin coupling constants for hydrogen deuteride (HD), hydrogen tritide (HT), and deuterium tritide (DT) are described. The reduced coupling constants obtained from the gas-phase NMR (nuclear magnetic resonance) experiment conducted at 300 K are 2.338(1), 2.334(3), and 2.316(1) × 10(20) T(2) J(-1), while the ab initio values computed at the full configuration interaction level of theory equal 2.349(3), 2.343(3), and 2.322(3) × 10(20) T(2) J(-1) for HD, HT, and DT, respectively. The agreement of the experimental and theoretical results is improved when proper treatment of the influence of nuclear relaxation on the NMR spectrum is applied. However, there is a minor discrepancy between experiment and theory, exceeding the estimated error bars; potential sources of this discrepancy are discussed.

Constraints on Exotic Dipole-Dipole Couplings between Electrons at the Micrometer Scale

New constraints on exotic dipole-dipole interactions between electrons at the micrometer scale are established, based on a recent measurement of the magnetic interaction between two trapped 88Sr(+) ions. For light bosons (mass≤0.1  eV) we obtain a 90% confidence interval for an axial-vector-mediated interaction strength of |g(A)(e)g(A)(e)/4πℏc|≤1.2×10(-17). Assuming CPT invariance, this constraint is compared to that on anomalous electron-positron interactions, derived from positronium hyperfine spectroscopy. We find that the electron-electron constraint is 6 orders of magnitude more stringent than the electron-positron counterpart. Bounds on pseudoscalar-mediated interaction as well as on torsion gravity are also derived and compared with previous work performed at different length scales. Our constraints benefit from the high controllability of the experimental system which contained only two trapped particles. It therefore suggests a useful new platform for exotic particle searches, complementing other experimental efforts.

Using geoelectrons to search for velocity-dependent spin-spin interactions

We use the recently developed model of the electron spins within Earth to investigate all of the six possible long-range velocity-dependent spin-spin interactions associated with the exchange of an ultralight (mz'<10(-10) eV) or massless intermediate vector boson. Several laboratory experiments have established upper limits on the energy associated with various fermion-spin orientations relative to Earth. We combine the results from three of these experiments with the geoelectron-spin model to obtain bounds on the velocity-dependent interactions that couple electron spin to the spins of electrons, neutrons, and protons. Five of the six possible potentials investigated were previously unbounded. In the long-range limit we have improved the bound on the sixth potential by 30 orders of magnitude.

Using the Earth as a polarized electron source to search for long-range spin-spin interactions

Many particle-physics models that extend the standard model predict the existence of long-range spin-spin interactions. We propose an approach that uses the Earth as a polarized spin source to investigate these interactions. Using recent deep-Earth geophysics and geochemistry results, we create a comprehensive map of electron polarization within the Earth induced by the geomagnetic field. We examine possible long-range interactions between these spin-polarized geoelectrons and the spin-polarized electrons and nucleons in three laboratory experiments. By combining our model and the results from these experiments, we establish bounds on torsion gravity and possible long-range spin-spin forces associated with the virtual exchange of either spin-one axial bosons or unparticles.