Asymmetric dark matter is a well-motivated approach to explain the apparent coincidence between the relic densities of visible and dark matter, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="normal">Ω</mi><mi>D</mi></msub><mo>≃</mo><mn>5.4</mn><msub><mi mathvariant="normal">Ω</mi><mi>b</mi></msub></math>. A complete explanation requires two components, a relation between the particle masses of the dark and visible matter, and a second relation between the number densities in each sector. In this work, we propose a new mechanism to address the former. We consider an extended <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>S</mi><mi>U</mi><mo stretchy="false">(</mo><mn>3</mn><msub><mrow><mo stretchy="false">)</mo></mrow><mrow><mn>1</mn></mrow></msub><mo>×</mo><mi>S</mi><mi>U</mi><mo stretchy="false">(</mo><mn>3</mn><msub><mrow><mo stretchy="false">)</mo></mrow><mrow><mn>2</mn></mrow></msub></mrow></math> color group in the visible sector, with QCD embedded as the diagonal subgroup. A <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="double-struck">Z</mi><mn>2</mn></msub></math> exchange symmetry then relates <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>S</mi><mi>U</mi><mo stretchy="false">(</mo><mn>3</mn><msub><mo stretchy="false">)</mo><mn>2</mn></msub></math> to a dark, confining <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>S</mi><mi>U</mi><mo stretchy="false">(</mo><mn>3</mn><msub><mo stretchy="false">)</mo><mi>D</mi></msub></math> sector. The dark matter is a composite state of dark fermions transforming in the fundamental representation of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>S</mi><mi>U</mi><mo stretchy="false">(</mo><mn>3</mn><msub><mo stretchy="false">)</mo><mi>D</mi></msub></math>. The spontaneously broken <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="double-struck">Z</mi><mn>2</mn></msub></math> symmetry ultimately leads to a relation between the QCD and dark gauge couplings which, for suitable field content, gives rise to confinement scales of the same order of magnitude. The mechanism leads to a rich particle spectrum above the TeV scale which could be probed at future experiments. The model also naturally includes an axion solution to the strong <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>C</mi><mi>P</mi></math> problem.

Australia

Peter Cox

Rafael E. Pérez

Raymond R. Volkas

New idea for relating the asymmetric dark matter mass scale to the proton mass

We present a new scenario of freeze-in, where the dark matter is produced exclusively via the annihilation of photons. We study fermionic and scalar dark matter models with a focus on cosmological histories with low reheat temperatures. Photonic freeze-in can be probed via direct detection, dark matter production in SN1987A, and via the production of new electromagnetically charged states at the LHC. We briefly discuss UV-complete models that can realize this scenario.

Matthew J. Dolan

Frederick J. Hiskens

Photonic freeze-in

The neutron portal operator provides a theoretically motivated connection between the visible and dark sectors and features in several well-studied asymmetric dark matter models. This operator leads to dark matter induced nucleon decays that mimic the experimental signature of “ordinary” nucleon decays. In this work, we reinterpret Super-Kamiokande nucleon decay searches for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>n</mi><mo stretchy="false">→</mo><msup><mi>π</mi><mn>0</mn></msup><mi>ν</mi></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>p</mi><mo stretchy="false">→</mo><msup><mi>π</mi><mo>+</mo></msup><mi>ν</mi></math> to constrain dark matter induced nucleon decays. For GeV-scale dark matter, we obtain lower bounds of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="script">O</mi><mo stretchy="false">(</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi><mo stretchy="false">)</mo></math> on the scale of the effective neutron portal operator. We also discuss the prospects for future searches at Hyper-Kamiokande and highlight the importance of a dedicated experimental analysis with reduced systematic uncertainties.

Nicole F. Bell

Jayden L. Newstead

Michael B.G. Verde

Dark matter induced nucleon decay through the neutron portal

We derive new bounds on hadronically interacting, sub-GeV mass dark matter. First, we show that one-loop interactions with photons can be sufficient to maintain equilibrium between the dark matter and Standard Model sectors at MeV temperatures, resulting in constraints from big bang nucleosynthesis. Using chiral perturbation theory, we find that this leads to an upper bound on the dark-matter–nucleon scattering cross section that is orders of magnitude stronger than existing astrophysical constraints. Furthermore, we show that even if these interactions remain out of equilibrium, there is an irreducible freeze-in abundance of dark matter that can easily overclose the universe. We also compute new bounds from rare kaon decays that can provide even stronger constraints. Our results have significant implications for future direct detection experiments aiming to search for MeV-scale dark matter.

Joshua Wood

New limits on light dark matter-nucleon scattering

We propose the Optomechanical Dark-matter Instrument (ODIN), based on a new method for the direct detection of low-mass dark matter. We consider dark matter interacting with superfluid helium in an optomechanical cavity. Using an effective field theory, we calculate the rate at which dark matter scatters off phonons in a highly populated, driven acoustic mode of the cavity. This scattering process deposits a phonon into a second acoustic mode in its ground state. The deposited phonon (<math><mrow><mi>μ</mi><mi>eV</mi></mrow></math> range) is then converted to a photon (eV range) via an optomechanical interaction with a pump laser. This photon can be efficiently detected, providing a means to sensitively probe keV scale dark matter. We provide realistic estimates of the backgrounds and discuss the technical challenges associated with such an experiment. We calculate projected limits on dark matter–nucleon interactions for dark matter masses ranging from 0.5 to 300 keV and estimate that a future device could probe cross sections as low as <math><mrow><mi>O</mi><mrow><mo>(</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>32</mn></mrow></msup><mo>)</mo></mrow><mtext> </mtext><mtext> </mtext><msup><mrow><mi>cm</mi></mrow><mrow><mn>2</mn></mrow></msup></mrow></math>.

Christopher G. Baker

Warwick P. Bowen

Maxim Goryachev

Glen Harris

Optomechanical dark matter instrument for direct detection

The absorption of fermionic dark matter has recently been studied as a signature for the direct detection of dark matter. We construct the first UV completion of the scalar effective operator associated with this signature. We calculate the constraints on the model and demonstrate there is viable parameter space which can be probed by a next-generation experiment such as XLZD. We also consider the cosmological history of our model and show that the correct relic abundance can be obtained via freeze-out in the dark sector. However, within this minimal model, we find that the absorption signal is highly suppressed in the parameter space that yields the correct relic abundance.

Absorption of fermionic dark matter via the scalar portal

An ongoing challenge in dark matter direct detection is to improve the sensitivity to light dark matter in the MeV–GeV mass range. One proposal is to dope a liquid noble-element direct-detection experiment with a lighter element such as hydrogen. This has the advantage of enabling larger recoil energies compared to scattering on a heavy target, while leveraging existing detector technologies. Direct-detection experiments can also extend their reach to lower masses by exploiting the Migdal effect, where a nuclear recoil leads to electronic ionization or excitation. In this work, we combine these ideas to study the sensitivity of a hydrogen-doped LZ experiment (HydroX) and a future large-scale experiment such as XLZD. We find that HydroX could have sensitivity to dark matter masses below 10 MeV for both spin-independent and spin-dependent scattering, with XLZD extending that reach to lower cross sections. Notably, this technique substantially enhances the sensitivity of direct detection to spin-dependent proton scattering, well beyond the reach of any current experiments.

Alexander C. Ritter

Exploring light dark matter with the Migdal effect in hydrogen-doped liquid xenon

We provide an exhaustive classification of three-family DFSZ axion models that have no cosmological domain wall problem. This result is obtained by making the Peccei-Quinn symmetry flavour dependent in certain specific ways, thus reinforcing a possible connection between the strong CP problem and the flavour puzzle. Known DFSZ flavour variants such as the top-specific model emerge as special cases. Key features of the phenomenology of these models are briefly discussed.

Matthew Dolan

Maaz Hayat

United States

Andrea Thamm

Raymond Volkas

Classification of three-family flavoured DFSZ axion models that have no domain wall problem

We identify the QCD axion and right-handed (sterile) neutrinos as bound states of an SU(5) chiral gauge theory with Peccei-Quinn (PQ) symmetry arising as a global symmetry of the strong dynamics. The strong dynamics is assumed to spontaneously break the PQ symmetry, producing a high-quality axion and naturally generating Majorana masses for the right-handed neutrinos at the PQ scale. The composite sterile neutrinos can directly couple to the left-handed (active) neutrinos, realizing a standard see-saw mechanism. Alternatively, the sterile neutrinos can couple to the active neutrinos via a naturally small mass mixing with additional elementary states, leading to light sterile neutrino eigenstates. The SU(5) strong dynamics therefore provides a common origin for a high-quality QCD axion and sterile neutrinos.

Tony Gherghetta

Arpon Paul

A common origin for the QCD axion and sterile neutrinos from SU(5) strong dynamics

The scattering of neutral particles by an atomic nucleus can lead to electronic ionization and excitation through a process known as the Migdal effect. We revisit and improve upon previous calculations of the Migdal effect, using the Dirac-Hartree-Fock method to calculate the atomic wave functions. Our methods do not rely on the use of the dipole approximation, allowing us to present robust results for higher nuclear recoil velocities than was previously possible. Our calculations provide the theoretical foundations for future measurements of the Migdal effect using neutron sources, and searches for dark matter in direct detection experiments. We show that multiple ionization must be taken into account in experiments with fast neutrons, and derive the semi-inclusive probability for processes that yield a hard electron above a defined energy threshold. We present results for the noble elements up to and including xenon, as well as carbon, fluorine, silicon and germanium. The transition probabilities from our calculations are publicly available.