⚛️ phenomenology

Sub-TeV Singlino Dark Matter in light from Sagittarius A $^\ast$ and LUX-ZEPLIN Nuclear-Recoil Event

This paper investigates the potential for detecting sub-TeV Singlino-dominated dark matter, motivated by a recent LUX-ZEPLIN nuclear-recoil event, by analyzing gamma-ray signals from dark matter density spikes surrounding the supermassive black hole Sgr A* and the stellar-mass black hole in XTE J1118+480.

Original authors: Utpal Chattopadhyay, Debottam Das, Rahul Puri, Joydeep Roy

Published 2026-09-04

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CC BY 4.0

Original authors: Utpal Chattopadhyay, Debottam Das, Rahul Puri, Joydeep Roy

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The universe is filled with invisible matter that we cannot see, touch, or smell, yet it holds galaxies together with its gravity. Scientists call this dark matter, and while they have strong evidence for its existence, they have never directly caught a single particle of it. One leading theory suggests that dark matter is made of a specific type of heavy, slow-moving particle that rarely interacts with normal matter. In the most popular versions of this theory, these particles are predicted to be very light or very heavy, but a middle ground has remained elusive. Recently, a sensitive detector deep underground in the United States, designed to catch these elusive particles, recorded a single, strange event that might be a sign of a dark matter particle with a mass between two hundred and one thousand times that of a proton. This observation has sparked a new search for a specific kind of dark matter candidate known as a "Singlino," a particle that is notoriously difficult to find with current methods.

A team of researchers has now turned their attention to the most extreme environments in our galaxy to see if they can find evidence of this elusive particle. They focused their search on two very different cosmic locations: the supermassive black hole at the very center of our Milky Way galaxy, known as Sagittarius A*, and a smaller, stellar-mass black hole located in a binary star system called XTE J1118+480. The logic behind this search is that black holes act like cosmic vacuum cleaners for dark matter. As a black hole forms and grows within a cloud of dark matter, its immense gravity pulls the surrounding particles in, compressing them into a tiny, incredibly dense region right around the black hole. This compressed region is called a "spike." In these spikes, the dark matter particles are packed so tightly that they are much more likely to collide with each other and annihilate, releasing high-energy gamma rays that telescopes on Earth can detect.

The researchers investigated whether a Singlino-dominated dark matter particle, with a mass between two hundred and one thousand GeV, could produce enough gamma rays in these spikes to be seen by our telescopes. They used a set of five specific theoretical models, known as benchmarks, which were carefully chosen to match all existing experimental data, including the recent strange event from the underground detector and the known mass of the Higgs boson. These models describe a universe where the lightest dark matter particle is almost entirely made of the Singlino type, which usually makes it very hard to detect because it interacts so weakly with normal matter. However, the team calculated that if these particles exist, the extreme density of the spikes around the black holes could boost the signal enough to be visible.

When they simulated the gamma-ray signals expected from the supermassive black hole at the center of our galaxy, the results were promising. The researchers found that for several of their models, the density of the dark matter spike could be high enough to produce a gamma-ray signal that matches what we actually observe from the galactic center. The strength of this signal depends heavily on how steep the density spike is; a steeper spike means more particles packed into a smaller space, leading to more collisions and a brighter signal. The study showed that if the dark matter density rises sharply enough near the black hole, the predicted gamma-ray flux from the Singlino particles would align with the data collected by the Fermi-LAT and HESS telescopes. This suggests that the mysterious event seen by the underground detector could indeed be caused by this type of dark matter, and that the gamma rays from the galactic center could be the smoking gun that confirms it.

In contrast, the search around the smaller black hole in the XTE J1118+480 system yielded a different result. While the physics of the dark matter spike is the same, the smaller black hole has a much weaker gravitational pull, meaning it cannot gather as much dark matter into a dense spike. The researchers calculated that even under the most optimistic assumptions about how dense the spike could become, the resulting gamma-ray signal would be far too faint to be seen against the background noise of the galaxy. Their simulations showed that the predicted light from this smaller black hole falls well below the levels actually observed by telescopes. This effectively rules out the possibility of detecting this specific type of dark matter around this particular stellar-mass black hole, regardless of how the dark matter is distributed.

The study concludes that the supermassive black hole at the center of our galaxy offers a unique and powerful laboratory for testing the existence of this specific dark matter candidate. If the recent underground detection is indeed a sign of a Singlino particle, then the gamma-ray observations of the galactic center should be able to confirm it, provided the dark matter spike is steep enough. The research highlights that while some cosmic environments are too quiet to reveal these hidden particles, others are so extreme that they might finally allow us to see the invisible. The findings do not prove the existence of the Singlino, but they provide a clear path forward: if future observations of the galactic center match the predicted signals, it would strongly support the idea that the dark matter making up our universe is composed of these elusive particles.

Technical Summary: Sub-TeV Singlino Dark Matter in light from Sagittarius A and LUX-ZEPLIN Nuclear-Recoil Event*

Problem Statement
The paper addresses the challenge of detecting Singlino-dominated neutralino dark matter (DM) within the Next-to-Minimal Supersymmetric Standard Model (NMSSM). Such DM candidates typically exhibit very small annihilation cross-sections and suppressed spin-independent direct detection (SIDD) cross-sections, rendering them difficult to probe via conventional indirect detection (ID) methods (e.g., dwarf spheroidal galaxies) or at the Large Hadron Collider (LHC). Furthermore, while recent observations by the LUX-ZEPLIN (LZ) collaboration identified a 248 keV nuclear-recoil event potentially consistent with a WIMP, a Singlino-like DM with mass $>200$ GeV is a viable candidate for this signal. The authors investigate whether the enhanced DM density in "spikes" surrounding supermassive black holes (SMBH) and stellar-mass black holes (sBH) can boost the annihilation signal of these elusive Singlino candidates to observable levels.

Methodology
The authors employ a multi-faceted approach combining particle physics modeling with astrophysical density profiling:

Particle Physics Framework:
- The study utilizes the $Z_3$ -symmetric NMSSM, where the lightest supersymmetric particle (LSP) is predominantly Singlino-like ( $\tilde{S}$ ).
- Five Benchmark Points (BMPs) are constructed using NMSSMTools-v6.1.2 and micrOMEGAs-7.1.4. These points satisfy constraints from the LHC, flavor physics, the observed Higgs mass ($122-128$ GeV), and the Planck relic density ( $\Omega_{DM}h^2 \approx 0.12$ ).
- The BMPs cover a mass range of $\sim 115$ GeV to $\sim 990$ GeV. They are tested against LUX-ZEPLIN (LZ) 2024 direct detection limits and Fermi-LAT/HESS indirect detection limits from dwarf spheroidal galaxies.
Astrophysical Modeling (DM Spikes):
- The authors model DM density profiles around the SMBH Sagittarius A (Sgr A)** and the stellar-mass black hole in the low-mass X-ray binary XTE J1118+480.
- The baseline halo profile is a generalized Navarro-Frenk-White (gNFW) profile with a variable inner cusp slope $\gamma_c$ .
- Adiabatic Growth & Spikes: The formation of a black hole within a pre-existing halo is assumed to create a DM spike. The spike slope $\gamma_{sp}$ is treated as a free parameter, bounded by the adiabatic limit $\gamma_{sp}^{ad} = (9-2\gamma_c)/(4-\gamma_c)$ .
- Saturation Effects: The model accounts for DM self-annihilation, which creates a saturation core ( $r < r_{in}$ ) where the density flattens to $\rho_{ann} = m_{\tilde{\chi}^0_1} / (\langle \sigma v \rangle \tau)$ .
- J-Factor Calculation: The authors calculate the astrophysical J-factor ( $J_{sp}$ ) for the spike region, integrating the squared density profile. They consider two definitions for the gravitational influence radius ( $r_h$ ): a fixed value ($2$ pc) and a dynamic value derived from the condition that the enclosed DM mass equals twice the black hole mass.
Flux Calculation:
- The differential photon flux is computed using Eq. (8), combining the thermally averaged annihilation cross-section $\langle \sigma v \rangle$ , the particle spectrum $dN/dE_\gamma$ (derived from micrOMEGAs including BSM cascade decays), and the calculated J-factors.
- The analysis includes contributions from the surrounding halo, convolved with the energy-dependent Point Spread Function (PSF) of the Fermi-LAT instrument.

Key Contributions and Results

Benchmark Viability: The selected BMPs successfully evade current LHC bounds and LZ direct detection limits. Notably, BMPs 1, 2, and 3 sit near "blind spots" where the DM-nucleon scattering cross-section vanishes, while BMPs 4 and 5 have suppressed cross-sections due to their high Singlino purity.
Sgr A Observations:*
- The study finds that the J-factor for Sgr A* can be enhanced by several orders of magnitude compared to the standard halo, depending on the spike slope $\gamma_{sp}$ , cusp slope $\gamma_c$ , and the influence radius $r_h$ .
- BMP Sensitivity: For steeper spikes (larger $\gamma_{sp}$ ), the J-factor becomes sensitive to particle physics inputs. BMPs 3 and 4, which have large annihilation cross-sections, show suppressed J-factors due to lower saturation densities ( $\rho_{ann}$ ), whereas BMPs with smaller $\langle \sigma v \rangle$ maintain higher densities closer to the black hole.
- Spectral Features: The predicted photon fluxes for BMPs 2, 3, and 4 are compared against the observed spectral energy distribution (SED) from Sgr A* (Fermi-LAT 4FGL J1745.6-2859 and HESS J1745-290). For BMP1, the flux is subdominant. For BMP2, a distinctive "box-like" spectral feature appears around 200 GeV due to $h_2 \to \gamma\gamma$ decays; however, the paper notes that for other features (e.g., $h_1$ and $A_1$ overlap), the distortion is subdominant to continuum spectra. While larger $\gamma_{sp}$ and dynamic $r_h$ increase the flux, the paper does not claim these scenarios definitively exceed the observed data, but rather explores the parameter space where they might become relevant.
- Conclusion for Sgr A:* The authors suggest that while the spike mechanism provides a significant boost, the detectability remains highly sensitive to astrophysical uncertainties. The results indicate that while the signal is enhanced, it does not necessarily surpass current observational limits in all scenarios, requiring specific combinations of $\gamma_{sp}$ , $\gamma_c$ , and $r_h$ to approach observability.
XTE J1118+480 Observations:
- For the stellar-mass black hole in XTE J1118+480, the gravitational influence radius is much smaller, resulting in a significantly lower J-factor.
- Even under the most optimistic assumptions (adiabatic spike formation with $\gamma_{sp} = 2.25$ ), the predicted photon flux remains well below the observed flux from 17 years of Fermi-LAT data.
- Conclusion for sBH: The authors explicitly conclude that observations of this sBH cannot probe any of the considered BMPs, even if a DM spike exists.

Significance and Claims
The paper claims that while Singlino-dominated DM is notoriously difficult to detect via conventional direct or indirect methods, the presence of DM density spikes around the Galactic Center's SMBH offers a potential pathway for discovery, though not a guaranteed one. The authors posit that if the recent LUX-ZEPLIN 248 keV event is indeed a DM signal, the specific Singlino scenarios explored in this work (masses $>200$ GeV) are compatible with that observation. Consequently, gamma-ray observations of Sgr A* serve as a crucial complementary test for this hypothesis. The study highlights that the detectability is highly sensitive to astrophysical uncertainties (spike slope, cusp slope, and influence radius), suggesting that refining these astrophysical parameters is essential for future indirect detection efforts. The authors conclude that while Sgr A* offers a promising laboratory for enhancing signals, the fluxes generally remain below or subdominant to observed data under standard assumptions, and the sBH in XTE J1118+480 is unlikely to yield a detectable signal for these specific models.

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