Light dark matter with sub-eV masses has a high number density in our Galaxy, and its scattering cross section with macroscopic objects can be significantly enhanced by coherence effects. Repeated scattering with a target object can induce a measurable acceleration. Torsion balance experiments with geometric asymmetry are, in principle, capable of detecting such signals. Our analysis shows that existing torsion balances designed to test the equivalence principle already place the most stringent constraints on DM-nucleon scattering in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo stretchy="false">(</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>2</mn></mrow></msup><mo>,</mo><mn>1</mn><mo stretchy="false">)</mo><mtext> </mtext><mtext> </mtext><mi>eV</mi></math> mass range.

Japan

Shigeki Matsumoto

China

Jie Sheng

Chuan-Yang Xing

Lin Zhu

Torsion Balance Experiments Enable Direct Detection of Sub-eV Dark Matter

In the search for beyond the Standard Model (SM) physics, long-lived particles (LLPs) have emerged as potential candidates and are being explored in various ongoing experiments. Future lepton colliders, such as the FCC-ee, shall provide an excellent opportunity to probe LLPs, owing to their clean environment and improved particle identification. This study investigates the potential of the proposed Innovative Detector for an Electron-Positron Accelerator (IDEA) detector at FCC-ee in the detection of LLPs produced from <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>B</mi></math>-meson and Higgs boson decays. We explore benchmark scenarios for different final states resulting from LLP decays, including a detailed analysis of the SM long-lived hadronic background. Additionally, we propose dedicated LLP detectors with different configurations, dimensions, and locations with respect to the IDEA detector. DELIGHT B, originally proposed as a dedicated LLP detector for the FCC-hh, stands out as the detector with the maximum efficiency for detecting LLPs produced at FCC-ee. We find that cylindrical detector configurations, if feasible to construct around the IDEA detector, would also enhance sensitivity for LLPs mostly decaying outside it.

India

Biplob Bhattacherjee

Camellia Bose

Germany

Herbi K. Dreiner

Nivedita Ghosh

Rhitaja Sengupta

Long-lived light mediators in a Higgs portal model at the FCC-ee

We explore whether the unprecedented energy resolution of upcoming gamma-ray telescopes can uncover relativistic effects in photon spectra resulting from dark matter (DM) annihilation or decay near the supermassive black hole (SMBH) at the Galactic Center (GC), specifically, gravitational redshift, Doppler broadening due to Lorentz boosts, and kinetic energy enhancements arising from high DM velocities. By modeling DM density and velocity profiles under various SMBH formation scenarios and DM properties, we calculate the corresponding gamma-ray spectra and identify the conditions under which SMBH-induced spectral distortions become observable. We find that, in favorable cases, the observed spectra encode the DM velocity distribution near the SMBH, enabling potential discrimination among annihilation mechanisms with different velocity dependencies. Even when SMBH-induced effects are modest, the upcoming COSI mission, with sub-percent energy resolution surpassing the typical DM velocity dispersion at the GC, $$ \mathcal{O}\left({10}^{-3}\right) $$ , may still be able to detect subtle Doppler broadening. These results highlight a promising pathway for determining the origin of gamma-ray signals and probing DM properties through high-resolution spectral measurements.

Yu Watanabe

United States

Alexander Kusenko

Indirect detection of dark matter around a supermassive black hole with high energy-resolution gamma-ray telescopes

We explore the indirect detection of sterile neutrino dark matter within the gauged U(1) extension of the Standard Model, in which three right-handed neutrinos account for neutrino masses, the baryon asymmetry, and dark matter. Focusing on the MeV mass range, we investigate two decay channels: the radiative decay $\textit{N}$ → $\textit{νγ}$, which produces a monochromatic photon, and the three-body decay $\textit{N}$ → $\textit{e}$ $\textit{e}$ $\textit{ν}$, which leads to a 511 keV photon signal from positronium decay. Taking the upcoming COSI mission as a case study, we show that both signals are experimentally accessible and complementary, with the 511 keV channel extending the sensitivity reach up to $$\mathcal{O}(100)$$ MeV. We propose a novel analysis strategy in Compton data space to isolate the diffuse 511 keV emission. Furthermore, we incorporate, for the first time, the Sommerfeld enhancement in the decay width of $\textit{N}$ → $\textit{e}$ $\textit{e}$ $\textit{ν}$, enabling more accurate predictions of the signal near the kinematic threshold. The combined observation of both channels would provide a distinctive and testable signature of the sterile neutrino dark matter hypothesis.

Subaru Fujisawa

Tatsuya Hayashi

Yuki Watanabe

Detecting sterile neutrino dark matter at MeV gamma-ray observatories

Dark matter with mass in the crossover range between wave dark matter and particle dark matter, around <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo stretchy="false">(</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>3</mn></mrow></msup><mo>,</mo><msup><mn>10</mn><mn>3</mn></msup><mo stretchy="false">)</mo><mtext> </mtext><mtext> </mtext><mi>eV</mi></math>, remains relatively unexplored by terrestrial experiments. In this mass regime, dark matter scatters coherently with macroscopic objects. The effect of the coherent scattering greatly enhances the accelerations of the targets caused from dark matter collisions by a factor of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo>∼</mo><msup><mn>10</mn><mn>23</mn></msup></math>. We propose a novel torsion balance experiment with test bodies of different geometric sizes to detect such dark-matter-induced acceleration. This method could provide the strongest sensitivity on the scattering cross section between the dark matter and a nucleon in the mass range of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo stretchy="false">(</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>3</mn></mrow></msup><mo>,</mo><mn>1</mn><mo stretchy="false">)</mo><mtext> </mtext><mtext> </mtext><mi>eV</mi></math>.

Pengshun Luo

Zhi-Jie Zhuge

Detecting meV-scale dark matter via coherent scattering with an asymmetric torsion balance

Sub-GeV dark matter (DM) has been attracting more attention due to the negative results searching for traditional weakly interacting massive particles (WIMPs). Such particles are expected to produce gamma rays from annihilation in the MeV energy region. Advancements in technology have opened up possibilities to precisely detect MeV gamma rays, leading to the upcoming space-based mission of the Compton Spectrometer and Imager (COSI). We comprehensively and quantitatively study the phenomenology of sub-GeV DM to determine if the COSI observations will probe their viable model parameter regions. We first construct models to describe sub-GeV DM based on the minimality and renormalizability of quantum field theory. Next, we impose various constraints on the models obtained from cosmological observations (CMB, BBN) and DM searches (accelerator, underground, astrophysical experiments, etc.). Finally, we identify viable parameter regions in each model and discuss whether or not COSI will be sensitive to the parameter regions. We find that a velocity-dependent annihilation cross-section is predicted in some regions, enabling COSI to detect the DM signal while avoiding severe constraints from cosmological observations.

Christopher Karwin

Tom Melia

Michela Negro

Thomas Siegert

Hiroki Yoneda

Tadayuki Takahashi

Sub-GeV dark matter and MeV gamma-ray detection with COSI

The new gauge boson introduced in the minimal extension of the standard model (SM) by gauging the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi mathvariant="normal">U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></mrow><mrow><mi mathvariant="normal">B</mi><mo>−</mo><mi mathvariant="normal">L</mi></mrow></msub></mrow></math> symmetry plays the role of dark matter when the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi mathvariant="normal">U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></mrow><mrow><mi mathvariant="normal">B</mi><mo>−</mo><mi mathvariant="normal">L</mi></mrow></msub></mrow></math> gauge coupling is highly suppressed. This dark matter, named the Féeton dark matter is known to be efficiently created in the early Universe by inflationary fluctuations with minimal gravity coupling, hence the framework, the gauged <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi mathvariant="normal">U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></mrow><mrow><mi mathvariant="normal">B</mi><mo>−</mo><mi mathvariant="normal">L</mi></mrow></msub></mrow></math> extended <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>SM</mi><mo>+</mo><mtext>inflation</mtext></mrow></math>, solves the four major problems of the SM; neutrino masses/mixings, dark matter, baryon asymmetry of the Universe, and the initial condition of the Universe (inflation). We comprehensively study the phenomenology of the dark matter when it is heavier than the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>−</mo></msup><msup><mi>e</mi><mo>+</mo></msup></math> threshold, namely twice the electron mass, considering the threshold effect on the dark matter decay into <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>−</mo></msup><msup><mi>e</mi><mo>+</mo></msup></math>. The viable parameter region is found only in the threshold region, while the branching fraction of the decay into <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>−</mo></msup><msup><mi>e</mi><mo>+</mo></msup></math> (i.e., the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>−</mo></msup><msup><mi>e</mi><mo>+</mo></msup></math> signal) never vanishes even at the threshold due to the effect. As a result, the pure <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi mathvariant="normal">U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></mrow><mrow><mi mathvariant="normal">B</mi><mo>−</mo><mi mathvariant="normal">L</mi></mrow></msub></mrow></math> extension without the kinetic mixing between the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi mathvariant="normal">U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></mrow><mrow><mi mathvariant="normal">B</mi><mo>−</mo><mi mathvariant="normal">L</mi></mrow></msub></mrow></math> and hypercharge gauge bosons have already been excluded by the present observation of the 511 keV photon from the galactic center. So, the Féeton dark matter requires a nonzero kinetic mixing to be a viable dark matter candidate and will be efficiently explored by future MeV-gamma ray telescopes thanks to the nonvanishing decay process into <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>−</mo></msup><msup><mi>e</mi><mo>+</mo></msup></math>.

Tsutomu T. Yanagida

Gauge <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi mathvariant="normal">U</mi><mo stretchy="false">(</mo><mn>1</mn><msub><mrow><mo stretchy="false">)</mo></mrow><mrow><mi mathvariant="normal">B</mi><mo>−</mo><mi mathvariant="normal">L</mi></mrow></msub></mrow></math> dark matter above the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>−</mo></msup><msup><mi>e</mi><mo>+</mo></msup></math> threshold

The decay of the mediator particle into standard model (SM) particles plays a significant role in exploring the dark sector scenario. We consider such a decay, taking the dark photon mediator as an example that mixes with the SM photon. We find that it requires a careful analysis of the decay rate in the presence of an SM vector boson (e.g., $\textit{Z}$ boson, $\textit{ρ}$ meson, and true muonium, etc.) nearly degenerate with the mediator particle in mass. The decay rate of the mediator particle calculated in the mass eigenstate basis agree with the correct result, given by the imaginary parts of the poles for the vector boson propagators, when the mixing parameter is smaller than a specific value. In such a case, the decay rate calculated by treating the mixing as a perturbative parameter is in agreement with the correct result. We clarify specific values for the mixing parameter quantitatively using several concrete examples of the SM vector bosons degenerate with the dark photon. When the mass mixing between the vector boson and dark photon is smaller (larger) than the decay width of the vector boson, the latter (former) method to calculate the decay rate of the mediator particle gives the correct result.

Poland

Ayuki Kamada

Takumi Kuwahara

Mediator decay through mixing with degenerate spectrum

In this paper, we propose a dedicated forward detector, FOREHUNT (FORward Experiment for HUNdred TeV), for 100 TeV FCC-hh for the detection of light long-lived particles (LLP) coming from <math><mi>B</mi></math>-meson decay. We calculate the signal acceptance as a function of mass and proper decay length of the LLP for 100 TeV and interpret our result in terms of model parameters for models of dark Higgs scalar and heavy neutral leptons. We also compare the sensitivity with proposed transverse detectors like MATHUSLA, CODEX-b for HL-LHC, and DELIGHT (Detector for long-lived particles at high energy of 100 TeV) for FCC-hh. Our analysis reveals that if the LLP is light (<math><mo>≲</mo><mn>4.4</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>) and has a low proper decay length (<math><mo>&lt;</mo><mn>10</mn><mtext> </mtext><mtext> </mtext><mi>m</mi></math>), a forward detector like FOREHUNT is the best option to look for the decaying LLP, while DELIGHT is preferable for higher proper decay lengths.

Prabhat Solanki

Light long-lived particles at the FCC-hh with the proposal for a dedicated forward detector FOREHUNT and a transverse detector DELIGHT

This study explores a minimal renormalizable dark matter (DM) model, incorporating a sub-GeV Majorana DM and a singlet scalar particle ϕ. Using scalar and pseudo-scalar interactions (couplings c  s  and c  p ), we investigate implications for DM detection, considering s-wave, p-wave, and combined (s+p wave) contributions in DM annihilation cross-section, as well as loop-correction contributions to DM-nucleon elastic scattering. Identifying a broad parameter space (10 MeV &lt; m  χ  ≲ m  ϕ ) within the 2σ allowed region, we explore scenarios (|c  s | ≫ |c  p |, |c  s | ≪ |c  p |, and |c  s | ≈ |c  p |). We find that (i) a non-zero pseudo-scalar coupling alleviates direct detection constraints as a comparison with the previous pure scalar coupling case; (ii) CMB observations set stringent limits on pseudo-scalar interaction dominant cases, making s-wave annihilation viable only for m  χ  &gt; 1 GeV; (iii) the preferred ϕ-resonance region can be tested in the future indirect detection experiments, such as e-ASTROGAM.

Yu-Tong Chen

Tian-Peng Tang

Yue-Lin Tsai

Lei Wu

Light thermal dark matter beyond p-wave annihilation in minimal Higgs portal model