<p>Ultralight dark matter may couple quadratically to Standard Model particles. Such quadratic interactions give rise to both coherent and stochastic signals in pulsar timing array (PTA) observations. In this work, we characterize these signals, including the effects of dark matter propagation in a finite-density medium, and assess the sensitivity of current and upcoming PTA observations to their detection. For coherent signals, we find that the sensitivity of current PTA observations competes with and sometimes exceeds that of other probes, such as equivalence principle tests and atomic clocks. For stochastic signals, we find that PTA sensitivities underperform equivalence principle constraints for both existing and upcoming PTA datasets.</p>

Germany

Xucheng Gan

Hyungjin Kim

Andrea Mitridate

Probing quadratically coupled ultralight dark matter with pulsar timing arrays

Metric perturbations induced by ultralight dark matter (ULDM) fields have long been identified as a potential target for pulsar timing array (PTA) observations. Previous works have focused on the coherent oscillation of metric perturbations at the characteristic frequency set by the ULDM mass. In this work, we show that ULDM fields source low-frequency stochastic metric fluctuations and that these low-frequency fluctuations can produce distinctive detectable signals in PTA data. Using the NANOGrav 12.5-yr dataset and synthetic datasets mimicking present and future PTA capabilities, we show that the current and future PTA observations provide the strongest probe of ULDM density within the Solar System for masses in the range of <math><mrow><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>18</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi><mo>−</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>16</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></mrow></math>.

Stochastic ultralight dark matter fluctuations in pulsar timing arrays

Single phonon excitations, with energies in the 1–100 meV range, are a powerful probe of light dark matter (DM). Utilizing effective field theory, we derive a framework to compute DM absorption rates into single phonons starting from general DM-electron, proton, and neutron interactions. We apply the framework to a variety of DM models: Yukawa coupled scalars, axionlike particles with derivative interactions, and vector DM coupling via gauge interactions or Standard Model electric and magnetic dipole moments. We find that GaAs or <math><msub><mi>Al</mi><mn>2</mn></msub><msub><mi>O</mi><mn>3</mn></msub></math> targets can set powerful constraints on a <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mrow><mi>B</mi><mo>−</mo><mi>L</mi></mrow></msub></math> model, and targets with electronic spin ordering are similarly sensitive to DM coupling to the electron magnetic dipole moment. Lastly, we make the code, phonodark-abs (an extension of the existing phonodark code which computes general DM–single phonon scattering rates), publicly available.

United States

Kris Pardo

Tanner Trickle

Kathryn M. Zurek

Effective field theory for dark matter absorption on single phonons

We present a recast in different benchmark models of the recent CMS search that uses the end cap muon detector system to identify displaced showers produced by decays of long-lived particles (LLPs). The exceptional shielding provided by the steel between the stations of the muon system drastically reduces the Standard Model background that limits other existing ATLAS and CMS searches. At the same time, by using the muon system as a sampling calorimeter, the search is sensitive to LLPs energies rather than masses. We show that, thanks to these characteristics, this new search approach is sensitive to LLPs masses even lighter than a GeV, and can be complementary to proposed and existing dedicated LLP experiments.

Michele Papucci

Christina W. Wang

Cristián Peña

Si Xie

Energetic long-lived particles in the CMS muon chambers

Pulsar timing arrays (PTAs) are exceptionally sensitive detectors in the frequency band <math><mrow><mi>nHz</mi><mo>≲</mo><mi>f</mi><mo>≲</mo><mi>μ</mi><mi>Hz</mi></mrow></math>. Ultralight dark matter (ULDM), with mass in the range <math><msup><mn>10</mn><mrow><mo>−</mo><mn>23</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi><mo>≲</mo><msub><mi>m</mi><mi>ϕ</mi></msub><mo>≲</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>20</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></math>, is one class of DM models known to generate signals in this frequency window. While purely gravitational signatures of ULDM have been studied previously, in this work we consider two signals in PTAs which arise in the presence of direct couplings between ULDM and ordinary matter. These couplings induce variations in fundamental constants, i.e., particle masses and couplings. These variations can alter the moment of inertia of pulsars, inducing pulsar spin fluctuations via conservation of angular momentum, or induce apparent timing residuals due to reference clock shifts. By using mock data mimicking current PTA datasets, we show that PTA experiments outperform torsion balance and atomic clock constraints for ULDM coupled to electrons, muons, or gluons. In the case of coupling to quarks or photons, we find that PTAs and atomic clocks set similar constraints. Additionally, we discuss how future PTAs can further improve these constraints, and detail the unique properties of these signals relative to the previously studied effects of ULDM on PTAs.

David E. Kaplan

Constraining fundamental constant variations from ultralight dark matter with pulsar timing arrays

Semiconductors with <math><mi>O</mi><mo>(</mo><mi>meV</mi><mo>)</mo></math> band gaps have been shown to be promising targets to search for sub-MeV mass dark matter (DM). In this paper we focus on a class of materials where such narrow band gaps arise naturally as a consequence of spin-orbit coupling (SOC). Specifically, we are interested in computing DM-electron scattering and absorption rates in these materials using state-of-the-art density functional theory techniques. To do this, we extend the DM interaction rate calculation to include SOC effects which necessitates a generalization to spin-dependent wave functions. We apply our new formalism to calculate limits for several DM benchmark models using an example <math><msub><mtext>ZrTe</mtext><mn>5</mn></msub></math> target and show that the inclusion of SOC can substantially alter projected constraints.

Japan

Hsiao-Yi Chen

Zhengkang Zhang

Marco Bernardi

Dark matter direct detection in materials with spin-orbit coupling

We revisit the calculation of bosonic dark matter absorption via electronic excitations. Working in an effective field theory framework and consistently taking into account in-medium effects, we clarify the relation between dark matter and photon absorption. As is well-known, for vector (dark photon) and pseudoscalar (axion-like particle) dark matter, the absorption rates can be simply related to the target material’s optical properties. However, this is not the case for scalar dark matter, where the dominant contribution comes from a different operator than the one contributing to photon absorption, which is formally next-to-leading-order and does not suffer from in-medium screening. It is therefore imperative to have reliable first-principles numerical calculations and/or semi-analytic modeling in order to predict the detection rate. We present updated sensitivity projections for semiconductor crystal and superconductor targets for ongoing and proposed direct detection experiments.

Kathryn Zurek

Dark matter absorption via electronic excitations

Models of Dark Matter (DM) can leave unique imprints on the Universe’s small scale structure by boosting density perturbations on small scales. We study the capability of Pulsar Timing Arrays to search for, and constrain, subhalos from such models. The models of DM we consider are ordinary adiabatic perturbations in ΛCDM, QCD axion miniclusters, models with early matter domination, and vector DM produced during inflation. We show that ΛCDM, largely due to tidal stripping effects in the Milky Way, is out of reach for PTAs. Axion miniclusters may be within reach, although this depends crucially on whether the axion relic density is dominated by the misalignment or string contribution. Models where there is matter domination with a reheat temperature below 1 GeV may be observed with future PTAs. Lastly, vector DM produced during inflation can be detected if it is lighter than 10 −16 GeV. We also make publicly available a Python Monte Carlo tool for generating the PTA time delay signal from any model of DM substructure.

Vincent Lee

Probing small-scale power spectra with pulsar timing arrays

Dirac materials, because of their small <math><mi>O</mi><mo>(</mo><mi>meV</mi><mo>)</mo></math> band gap, are a promising target for dark photon-mediated scattering and absorption of light dark matter. In this paper, we characterize the daily modulation rate of dark matter interacting with a Dirac material due to anisotropies in their crystal structure. We show that daily modulation is an <math><mi>O</mi><mo>(</mo><mn>1</mn><mo>)</mo></math> fraction of the total rate for dark matter scattering in the Dirac material <math><mrow><msub><mrow><mi>ZrTe</mi></mrow><mrow><mn>5</mn></mrow></msub></mrow></math>. When present, the modulation is dominated by the orientation of the material’s dielectric tensor with respect to the dark matter wind and is maximized when the crystal is oriented such that the dark matter wind is completely aligned with the largest and smallest components of the dielectric tensor at two different times of the day. Because of the large modulation, any putative dark matter scattering signal could be rapidly verified or ruled out by changing the orientation of the crystal with respect to the wind and observing how the daily modulation pattern changes.

Ahmet Coskuner

Italy

United Kingdom

Andres Olivares

Directional dark matter detection in anisotropic Dirac materials

Collective excitations in condensed matter systems, such as phonons and magnons, have recently been proposed as novel detection channels for light dark matter. We show that excitation of (i) optical phonon polaritons in polar materials in an <math><mrow><mi>O</mi><mo>(</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><mi>T</mi><mo>)</mo></mrow></math> magnetic field (via the axion-photon coupling), and (ii) gapped magnons in magnetically ordered materials (via the axion wind coupling to the electron spin), can cover the difficult-to-reach <math><mrow><mi>O</mi><mo>(</mo><mn>1</mn><mi>–</mi><mn>100</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>meV</mi></mrow></math> mass window of QCD axion dark matter with less than a kilogram-year exposure. Finding materials with a large number of optical phonon or magnon modes that can couple to the axion field is crucial, suggesting a program to search for a range of materials with different resonant energies and excitation selection rules; we outline the rules and discuss a few candidate targets, leaving a more exhaustive search for future work. Ongoing development of single photon, phonon, and magnon detectors will provide the key for experimentally realizing the ideas presented here.