The dilaton, a pseudo-Nambu-Goldstone boson (pNGB) of broken scale invariance, is an appealing ultralight dark matter (DM) candidate. Its mass is protected by conformal invariance and it can be searched for in tabletop experiments. However, contrary to standard pNGBs of internal symmetries, the dilaton generically has a large non-derivative self-coupling, leading to radiative contributions to its mass of the order of its decay constant. Hence typical ultralight dilatons should also have sub-eV decay constants, which would incur significant deviations from standard DM behavior at structure formation times, in severe tension with observations. Therefore, a fine-tuning is required to generate a hierarchy between the mass and the decay constant. In this work, we consider whether supersymmetry (SUSY) can be used to protect this hierarchy from quantum corrections. To ensure an ultralight dilaton mass robust against realistic SUSY-breaking contributions, we must consider a novel dilaton stabilization mechanism. The observed DM abundance can be produced by the misalignment mechanism for dilaton masses ranging from 10 to 1 eV. Unfortunately, irreducible SUSY-breaking corrections due to gravity restrict the couplings between the dilaton and the Standard Model to be extremely small, beyond the reach of any current or proposed experiments. Our work demonstrates that constructing a consistent model of ultralight dilaton DM is quite involved.

United States

Abhishek Banerjee

Csaba Csáki

Israel

Michael Geller

Zamir Heller-Algazi

Ameen Ismail

Ultralight Dilatonic Dark Matter

<p>We show that coupling of ultralight dark matter (UDM) to quarks and gluons would lead to an oscillation of the nuclear charge radius for both the quantum chromodynamic (QCD) axion and scalar dark matter, an effect which is of particular importance for heavy elements. Consequently, the resulting oscillation of electronic energy levels could be resolved with optical atomic clocks, and their comparisons can be used to investigate UDM nuclear couplings, which were previously only accessible with other platforms. We demonstrate this idea using the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mmultiscripts><mrow><mi>S</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>2</mn></mrow></mmultiscripts></mrow><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msub><mo stretchy="false">(</mo><mi>F</mi><mo>=</mo><mn>0</mn><mo stretchy="false">)</mo><mo stretchy="false">↔</mo><msub><mrow><mmultiscripts><mrow><mi>F</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>2</mn></mrow></mmultiscripts></mrow><mrow><mn>7</mn><mo>/</mo><mn>2</mn></mrow></msub><mo stretchy="false">(</mo><mi>F</mi><mo>=</mo><mn>3</mn><mo stretchy="false">)</mo></mrow></math> electric octupole and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mmultiscripts><mrow><mi>S</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>2</mn></mrow></mmultiscripts></mrow><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msub><mo stretchy="false">(</mo><mi>F</mi><mo>=</mo><mn>0</mn><mo stretchy="false">)</mo><mo stretchy="false">↔</mo><msub><mrow><mmultiscripts><mrow><mi>D</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>2</mn></mrow></mmultiscripts></mrow><mrow><mn>3</mn><mo>/</mo><mn>2</mn></mrow></msub><mo stretchy="false">(</mo><mi>F</mi><mo>=</mo><mn>2</mn><mo stretchy="false">)</mo></mrow></math> electric quadrupole transitions in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mmultiscripts><mrow><msup><mrow><mi>Yb</mi></mrow><mrow><mo>+</mo></mrow></msup></mrow><mprescripts></mprescripts><none></none><mrow><mn>171</mn></mrow></mmultiscripts></mrow></math>. Based on the derived sensitivity coefficients for these two transitions and a long-term comparison of their frequencies using a single trapped <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mmultiscripts><mrow><msup><mrow><mi>Yb</mi></mrow><mrow><mo>+</mo></mrow></msup></mrow><mprescripts></mprescripts><none></none><mrow><mn>171</mn></mrow></mmultiscripts></mrow></math> ion, we find bounds on the scalar UDM-nuclear couplings and the QCD axion decay constant. These results are at a similar level compared to the tightest spectroscopic limits, and future investigations, also with other optical clocks, promise significant improvements.</p>

Germany

Dmitry Budker

Melina Filzinger

Nils Huntemann

Gil Paz

Gilad Perez

Sergey Porsev

Marianna Safronova

Oscillating Nuclear Charge Radii as Sensors for Ultralight Dark Matter

<p>We consider axions lighter than what their QCD couplings might otherwise suggest. Starting with a <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="double-struck">Z</mi><mi>N</mi></msub></math> axion, we introduce a small explicit <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="double-struck">Z</mi><mi>N</mi></msub></math> symmetry-breaking coupling between the Standard Model Higgs boson and a reheaton. This small explicit breaking allows us to populate a large portion of the light axion <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><mi>a</mi></msub><mo>−</mo><msub><mi>f</mi><mi>a</mi></msub></math> plane, removes the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>1</mn><mo>/</mo><mi>N</mi></math> tuning in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="double-struck">Z</mi><mi>N</mi></msub></math>-axion, and explains why only our sector was reheated. Because of finite temperature effects, axions of this sort undergo either “rigged” misalignment, where the axion misalignment angle is effectively <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>π</mi></math> regardless of its initial value, or “shuffled” misalignment, where the initial angle is effectively randomized.</p>

Manuel A. Buen-Abad

Anson Hook

Constructing a light QCD axion without <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>1</mn><mo>/</mo><mi>N</mi></math> tuning

The anomalous magnetic moment of the muon ($\textit{a}$ ) provides a stringent test of the quantum nature of the Standard Model (SM) and its extensions. To probe beyond the SM physics, one needs to be able to subtract the SM contributions, which consists of a non-perturbative part, namely, the hadronic vacuum polarization (HVP) of the photon. The state of the art is to predominantly use two different methods to extract this HVP: lattice computation, and dispersion relation-based, data-driven method. Thus one can construct different forms of the “$\textit{a}$ test” which compares the precise measurement of $\textit{a}$ to its theory prediction. Additionally, this opens the possibility for another subtle test, where these two “theory” predictions themselves are compared against each other, which is denoted as the “HVP-test”. This test is particularly sensitive to hadronic scale new physics. Therefore, in this work, we consider an SM extension consisting of a generic, light ~ (100 MeV – 1 GeV) vector boson and study its impact on both tests. We develop a comprehensive formalism for this purpose. We find that in the case of data-driven HVP being used in the $\textit{a}$ test, the new physics contributions effectively cancel for a flavor-universal vector boson. As an illustration of these general results, we consider two benchmark models: i) the dark photon ($\textit{A}$ ) and ii) a gauge boson coupled to baryon-number ($\textit{B}$). Using a combination of these tests, we are able to constrain the parameter space of $\textit{B}$ and $\textit{A}$ , complementarily to the existing limits. As a spin-off, our preliminary analysis of the spectrum of the invariant mass of 3$\textit{π}$ in events with ISR at the $\textit{B}$− factories (BaBar, Belle) manifests the value of such a study in searching for $\textit{B}$ → 3$\textit{π}$ decay, thus motivating a dedicated search by experimental collaborations.

Kaustubh Agashe

Minyuan Jiang

Shmuel Nussinov

Kushan Panchal

Srijit Paul

Yotam Soreq

Searching for hadronic scale baryonic and dark forces at ($\textit{g}$ − 2)

We propose a relaxation mechanism for the initial misalignment angle of the pre-inflationary QCD axion with a large decay constant. The proposal addresses the challenges posed to the axion dark matter scenario by an overabundance of axions overclosing the Universe, as well as by isocurvature constraints. Many state-of-the-art experiments are searching for QCD axion dark matter with a decay constant as large as 10 GeV, motivating the need for a theoretical framework such as ours. In our model, hidden sector magnetic monopoles generated in the early Universe give the axion a large mass via the Witten effect, causing early oscillations that reduce the misalignment angle and axion abundance. As the hidden gauge symmetry breaks, its monopoles confine via cosmic strings, dissipating energy into the Standard Model and leading to monopole-antimonopole annihilation. This removes the monopole-induced mass, leaving only the standard QCD term. We consider the symmetry breaking pattern of SU(2) → U(1) → 1, leading to monopole and string formation respectively. We calculate the monopole abundance, their interactions with the axion field, and the necessary conditions for monopole-induced axion oscillations, while accounting for UV instanton effects. We present three model variations based on different symmetry breaking scales and show that they can accommodate an axion decay constant of up to 10 GeV with an inflationary scale of 10 GeV. The required alignment between monopole-induced and QCD axion potentials is achieved through a modest Nelson-Barr mechanism, avoiding overclosure without anthropic reasoning.

Mexico

Manuel Buen-Abad

Dynamical axion misalignment from the Witten effect

We discuss models of ultralight scalar Dark Matter (DM) with linear and quadratic couplings to the Standard Model (SM). In addition to studying the phenomenology of linear and quadratic interactions separately, we examine their interplay. We review the different experiments that can probe such interactions and present the current and expected future bounds on the parameter space. In particular, we discuss the scalar field solution presented in [A. Hees, O. Minazzoli, E. Savalle, Y. V. Stadnik and P. Wolf, Phys.Rev.D 98 (2018) 6, 064051], and extend it to theories that capture both the linear and the quadratic couplings of the Dark Matter (DM) field to the Standard Model (SM). Furthermore, we discuss the theoretical aspects and the corresponding challenges for natural models in which the quadratic interactions are of phenomenological importance.

Inbar Savoray

Aviv Shalit

The phenomenology of quadratically coupled ultra light dark matter

We highlight general issues associated with quality and naturalness problems in theories of light QCD axions, axionlike particles, and relaxions. We show that the presence of Planck-suppressed operators generically lead to scalar coupling of axions with the Standard Model. We present a new class of <math><msub><mi>Z</mi><mi>N</mi></msub></math> QCD relaxion models that can address both the QCD relaxion <math><mi>C</mi><mi>P</mi></math> problem as well as its quality problem. This new class of models also leads to interesting experimental signatures, which can be searched for at the precision frontier.

Japan

Joshua Eby

From axion quality and naturalness problems to a high-quality <math><msub><mi>Z</mi><mi>N</mi></msub></math> QCD relaxion

We present a search for fundamental constant oscillations in the range 20 kHz–100 MHz that may arise within models for ultralight dark matter (UDM). Using two independent optical-spectroscopy apparatuses, we achieve up to <math><mrow><mo>×</mo><mn>1000</mn></mrow></math> greater sensitivity in the search relative to previous work [D. Antypas et al., Phys. Rev. Lett. 123, 141102 (2019).]. We report no observation of UDM and thus constrain respective couplings to electrons and photons within the investigated UDM particle mass range <math><mrow><mn>8</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>11</mn></mrow></msup><mi>–</mi><mn>4</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>7</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></mrow></math>. The constraints significantly exceed previously set bounds from atomic spectroscopy and, as we show, may surpass in future experiments those provided by equivalence-principle (EP) experiments in a specific case regarding the combination of UDM couplings probed by the EP experiments.

Oleg Tretiak

Xue Zhang

Nataniel L. Figueroa

Dionysios Antypas

Andrea Brogna

Improved Bounds on Ultralight Scalar Dark Matter in the Radio-Frequency Range

To solve the hierarchy problem, the relaxion must remain trapped in the correct minimum, even if the electroweak symmetry is restored after reheating. In this scenario, the relaxion starts rolling again until the backreaction potential, with its set of local minima, reappears. Depending on the time of barrier reappearance, Hubble friction alone may be insufficient to retrap the relaxion in a large portion of the parameter space. Thus, an additional source of friction is required, which might be provided by coupling to a dark photon. The dark photon experiences a tachyonic instability as the relaxion rolls, which slows down the relaxion by backreacting to its motion, and efficiently creates anisotropies in the dark photon energy-momentum tensor, sourcing gravitational waves. We calculate the spectrum of the resulting gravitational wave background from this new mechanism and evaluate its observability by current and future experiments. We further investigate the possibility that the coherently oscillating relaxion constitutes dark matter and present the corresponding constraints from gravitational waves.

Eric Madge

Wolfram Ratzinger

Pedro Schwaller

Gravitational wave echo of relaxion trapping

We discuss the sensitivity of the present and near-future axion dark matter experiments to a halo of axions or axion-like particles gravitationally bound to the Earth or the Sun. Such halos, assuming they are formed, can be searched for in a wide variety of experiments even when the axion couplings to matter are small, while satisfying all the present experimental bounds on the local properties of dark matter. The structure and coherence properties of these halos also imply novel signals, which can depend on the latitude or orientation of the detector. We demonstrate this by analyzing the sensitivity of several distinct types of axion dark matter experiments.