Hunting Dark Matter with Gamma Rays

Author: Denis Avetisyan

A new study explores how the Cherenkov Telescope Array could reveal the presence of dark photon dark matter by observing subtle distortions in high-energy light from distant sources.

A simulated flux realization of Mrk 501, modeled across 25 logarithmically-spaced energy bins, demonstrates how the best-fit ECPL function <span class="katex-eq" data-katex-display="false">H_0</span> is altered by the attenuation effect of dark photon scattering-specifically for a dark photon with a coupling of <span class="katex-eq" data-katex-display="false">A^{\prime} = 1~\mathrm{eV}</span> and mixing parameter of <span class="katex-eq" data-katex-display="false">\varepsilon = 5 \times 10^{-8}</span>-as detailed in Table 3. — A simulated flux realization of Mrk 501, modeled across 25 logarithmically-spaced energy bins, demonstrates how the best-fit ECPL function $H_0$ is altered by the attenuation effect of dark photon scattering-specifically for a dark photon with a coupling of $A^{\prime} = 1~\mathrm{eV}$ and mixing parameter of $\varepsilon = 5 \times 10^{-8}$ -as detailed in Table 3.

The Cherenkov Telescope Array may constrain dark photon dark matter models through the detection of spectral attenuation in very-high-energy gamma rays, particularly from blazars with central dark matter spikes.

Despite compelling evidence for dark matter, its fundamental nature remains elusive, motivating searches for diverse candidates beyond the Standard Model. This paper, ‘Probing Dark Photon Dark Matter with CTAO’, investigates the potential to detect dark photon dark matter through the observation of very-high-energy (VHE) gamma rays, leveraging the unprecedented sensitivity of the Cherenkov Telescope Array Observatory (CTAO). By analyzing spectral attenuation resulting from $\gamma\gamma' \to e^+e^-$ scattering, we demonstrate that CTAO can probe kinetic mixing parameters as low as $\varepsilon \sim 10^{-8}$ for dark photon masses around $10^{-1}~\textrm{eV}$ . Will forthcoming observations of sources like the Crab Nebula and blazars with potential dark matter spikes finally reveal the nature of this mysterious component of the universe?

The Invisible Universe: A Matter of Perspective

The cosmos, as currently understood, is dominated by a mysterious substance known as dark matter, comprising roughly 85% of all matter. This isn’t matter as commonly conceived – it doesn’t interact with light, rendering it invisible to telescopes. Its presence is inferred solely through its gravitational influence on visible matter, such as stars and galaxies, and on the large-scale structure of the universe. Observations reveal galaxies rotate faster than predicted based on their visible mass, suggesting an unseen gravitational component. Similarly, the way light bends around massive objects, a phenomenon called gravitational lensing, indicates a greater mass concentration than what is directly observable. This disconnect between observed gravity and visible matter presents a profound challenge to cosmological models and necessitates a reevaluation of fundamental physics, pushing scientists to explore exotic particle candidates and alternative theories of gravity.

The persistent mystery of dark matter has driven physicists to consider physics beyond the Standard Model, the remarkably successful but incomplete framework describing fundamental particles and forces. The Standard Model, despite its predictive power, provides no viable candidate to explain the observed gravitational effects attributed to dark matter. This shortfall has spurred investigation into “hidden sector” extensions – theoretical frameworks proposing the existence of particles that interact very weakly with the Standard Model, and primarily with each other. These new particles, potentially ranging from weakly interacting massive particles (WIMPs) to axions or sterile neutrinos, could constitute the missing mass and explain the universe’s structure. The search for these hidden particles involves increasingly sophisticated direct and indirect detection experiments, as well as collider searches aiming to create and observe these elusive components of the cosmos.

Line-of-sight dark matter mass integrals for the host galaxies of Mrk 421 and Mrk 501 reveal a dependence on the size of the emitting region <span class="katex-eq" data-katex-display="false">R_{em}</span> relative to the Schwarzschild radius <span class="katex-eq" data-katex-display="false">R_S</span> of the central supermassive black hole. — Line-of-sight dark matter mass integrals for the host galaxies of Mrk 421 and Mrk 501 reveal a dependence on the size of the emitting region $R_{em}$ relative to the Schwarzschild radius $R_S$ of the central supermassive black hole.

A Shadow Force: The Allure of the Dark Photon

The dark photon is a hypothetical particle proposed as a force carrier, specifically a gauge boson, that would mediate interactions within the dark sector while also exhibiting a weak coupling to Standard Model particles. This coupling occurs not through direct interaction, but via a process known as kinetic mixing, where the dark photon “mixes” with the ordinary photon. The strength of this mixing determines the degree to which dark photons can be detected through interactions with Standard Model particles. This mechanism provides a potential explanation for dark matter self-interaction, as dark photons could facilitate collisions between dark matter particles, and offers avenues for direct and indirect detection experiments, including searches for resonant production of dark photons in accelerators or observation of their decay products.

Photon-dark photon scattering, a predicted consequence of kinetic mixing between photons and dark photons, results in a measurable attenuation of high-energy gamma-ray spectra. This process occurs when a gamma-ray interacts with a dark photon, effectively removing it from the observed spectrum. The resulting attenuation manifests as a dip or broadening in the gamma-ray energy distribution, with the specific shape and depth of this feature dependent on the dark photon’s mass and coupling strength to Standard Model particles. Detection of this induced gamma-ray attenuation would provide strong evidence for the existence of dark photons and a potential pathway to characterizing their properties; current and future gamma-ray telescopes are designed to search for these spectral distortions.

The photon-dark photon scattering cross section <span class="katex-eq" data-katex-display="false">\sigma_{A^{\prime}}</span> varies with photon energy and is significantly impacted by the coupling parameter <span class="katex-eq" data-katex-display="false">\varepsilon</span>, as demonstrated by the contrasting solid and dashed lines representing <span class="katex-eq" data-katex-display="false">\varepsilon = 10^{-3}</span> and <span class="katex-eq" data-katex-display="false">10^{-6}</span>, respectively. — The photon-dark photon scattering cross section $\sigma_{A^{\prime}}$ varies with photon energy and is significantly impacted by the coupling parameter $\varepsilon$ , as demonstrated by the contrasting solid and dashed lines representing $\varepsilon = 10^{-3}$ and $10^{-6}$ , respectively.

Simulating the Cosmos: A Test of Observation

Simulated observations are performed using the planned capabilities of the Cherenkov Telescope Array Observatory (CTAO) to predict the expected gamma-ray spectra from astrophysical sources. These simulations focus on sources including the active galactic nuclei Mrk 421 and Mrk 501, and the supernova remnant, the Crab Nebula. By modeling the anticipated signals, we can assess the CTAO’s potential for detecting subtle features indicative of new physics, and optimize observation strategies for maximizing sensitivity to specific spectral characteristics of these sources.

Simulations of gamma-ray signals rely on accurately characterizing how the Cherenkov Telescope Array Observatory (CTAO) detects incoming photons, a process defined by the CTA Instrument Response Functions (IRFs). These IRFs account for factors such as telescope effective area, angular resolution, and energy reconstruction accuracy. To model the intrinsic emission from astrophysical sources, spectral models are employed; common choices include the Exponentially Cutoff Power Law (ECPL) and the Log-Parabola. The ECPL model $\propto E^{- \alpha} e^{-E/E_{cut}}$ describes a power law with an exponential cutoff at energy $E_{cut}$ , while the Log-Parabola model $\propto E^{-\alpha - \beta \log(E)}$ offers a flexible alternative. Utilizing these IRFs and spectral models allows for the creation of simulated observations that closely match expected data, enabling realistic assessments of detection capabilities and parameter estimation.

Simulations using the Cherenkov Telescope Array Observatory (CTAO) indicate the potential to constrain the kinetic mixing parameter ε of dark photon dark matter. Specifically, limits as low as $ε \sim 10^{-8}$ are achievable for blazars exhibiting central dark matter spikes, while the Crab Nebula yields a constraint of $ε \sim 2 \times 10^{-4}$ . These projected sensitivity levels represent an improvement comparable to the CAST experiment within the 0.1-1 eV mass range for dark photons, offering a complementary approach to existing dark matter searches.

A simulated CTAO flux realization of the Crab Nebula, represented by black points, is well-fitted by a log-parabola (blue line), and exhibits attenuation consistent with dark photon scattering at <span class="katex-eq" data-katex-display="false">m_{A^{\prime}}=0.1~\mathrm{eV}</span> and <span class="katex-eq" data-katex-display="false">\varepsilon=5\times 10^{-4}</span> (red line), as detailed in Table 3. — A simulated CTAO flux realization of the Crab Nebula, represented by black points, is well-fitted by a log-parabola (blue line), and exhibits attenuation consistent with dark photon scattering at $m_{A^{\prime}}=0.1~\mathrm{eV}$ and $\varepsilon=5\times 10^{-4}$ (red line), as detailed in Table 3.

The Veil of the Cosmos: Accounting for the Unknown

The universe isn’t entirely transparent to high-energy gamma rays; a pervasive, though faint, glow known as the Extragalactic Background Light (EBL) absorbs some of this radiation as it travels across vast cosmic distances. This attenuation, stemming from the accumulated light of all galaxies throughout the universe’s history, presents a significant challenge when searching for subtle signals of new physics, such as interactions with potential dark photons. Researchers must meticulously model the EBL’s intensity and spectral characteristics to differentiate its natural dimming effect from any additional attenuation that might indicate the presence of these elusive particles. Failing to accurately account for the EBL could lead to false positives – mistaking the dimming of gamma rays by existing light for the signature of a new, undiscovered force, or conversely, obscuring a genuine signal altogether. Therefore, precise EBL modeling is crucial for reliable interpretation of gamma-ray observations in the quest to unveil the universe’s hidden components.

The expected distribution of dark matter significantly influences the detectability of subtle interactions, with current models providing crucial frameworks for prediction. The ‘Navarro-Frenk-White’ (NFW) profile, a commonly used description, posits a density that decreases with distance from a galaxy’s center, shaping the baseline expectation for dark matter distribution. However, supermassive black holes are theorized to further concentrate dark matter, creating localized ‘spikes’ of significantly enhanced density. These spikes, if they exist, would dramatically amplify the potential signal from dark matter interactions within their vicinity, offering a promising avenue for detection. Accurately characterizing these dark matter density profiles, including the possibility of such spikes, is therefore essential for interpreting observational data and refining the search for these elusive particles.

The pursuit of dark photons, hypothetical particles mediating a ‘dark force’, relies heavily on identifying minute alterations in high-energy gamma-ray spectra. However, accurately interpreting these spectra demands a sophisticated understanding of the universe’s confounding factors. Researchers are increasingly focused on precisely modeling the Extragalactic Background Light (EBL), the accumulated glow from all galaxies, as it attenuates gamma-ray signals. Simultaneously, the distribution of dark matter – often described by models like the Navarro-Frenk-White (NFW) profile – significantly influences signal strength, with localized ‘dark matter spikes’ near supermassive black holes potentially amplifying detectable features. By meticulously accounting for both EBL and the nuanced distribution of dark matter, scientists aim to diminish systematic uncertainties and sharpen the search for the subtle spectral signatures that would reveal the existence of these elusive dark photons and illuminate the nature of dark matter itself.

Dark matter density profiles for the host galaxies of Mrk 421 (blue) and Mrk 501 (red) exhibit central spikes, contrasting with the normalized NFW profile of the Milky Way (black dotted line).

The search for dark matter, as detailed in this study of dark photon detection with the CTAO, reveals the inherent fragility of theoretical constructs. Any prediction, even one built on elegant mathematics and extensive simulations, is ultimately a probability subject to the ‘gravity’ of observational constraints. As Igor Tamm once observed, “Any theory is only a stepping stone; the universe is always one step ahead.” The paper’s focus on spectral attenuation – the dimming of gamma rays as they traverse regions of concentrated dark matter – highlights how seemingly subtle effects can dismantle even the most robust models. The potential for dark matter spikes to amplify these signals, while promising for detection, also serves as a stark reminder that nature doesn’t negotiate; it simply is.

The Horizon Beckons

The search for dark photons, predicated on the assumption that a slight extension to the Standard Model will illuminate the darkness, feels… familiar. Each calculation of spectral attenuation, each attempt to resolve a gamma-ray excess, is an attempt to hold light in one’s hands, and it slips away. The promise of the Cherenkov Telescope Array is not necessarily in finding a signal, but in refining the boundaries of ignorance. Blazars, with their potentially concentrated dark matter halos, offer a marginally brighter hope, yet even a positive detection merely shifts the question-not to what the dark matter is, but to why this particular manifestation.

The assertion that spikes in dark matter density will predictably alter observed spectra rests on models, of course. And models, however elegant, are always approximations, destined to fail at some, as yet unknown, energy scale or spatial resolution. To speak of ‘constraints’ on dark photon models feels a touch optimistic. It’s more accurate to say that the forthcoming data will define a smaller region of permissible fantasy.

Perhaps the true value of this line of inquiry lies not in solving the dark matter puzzle, but in acknowledging its fundamental intractability. Each negative result, each refinement of the search parameters, brings one closer to accepting that the universe may not be entirely knowable-that the horizon of understanding is not a boundary to be breached, but a condition to be embraced.

Original article: https://arxiv.org/pdf/2602.21920.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

The Invisible Universe: A Matter of Perspective

A Shadow Force: The Allure of the Dark Photon

Simulating the Cosmos: A Test of Observation

The Veil of the Cosmos: Accounting for the Unknown

The Horizon Beckons

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