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Improved Heavy Dark Matter Annihilation Search from Dwarf Galaxies with HAWC

Using improved event reconstruction and an expanded dataset from the High Altitude Water Cherenkov (HAWC) Observatory, this study analyzed gamma-ray emissions from dwarf spheroidal galaxies to search for heavy dark matter annihilation, finding no evidence of a signal and consequently setting new upper limits on the annihilation cross-section for dark matter masses between 1 and 10,000 TeV.

Original authors: A. Albert, R. Alfaro, C. Alvarez, A. Andrés, E. Anita-Rangel, M. Araya, J. C. Arteaga-Velázquez, D. Avila Rojas, H. A. Ayala Solares, R. Babu, P. Bangale, E. Belmont-Moreno, A. Bernal, K. S. Caballero

Published 2026-05-14

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: A. Albert, R. Alfaro, C. Alvarez, A. Andrés, E. Anita-Rangel, M. Araya, J. C. Arteaga-Velázquez, D. Avila Rojas, H. A. Ayala Solares, R. Babu, P. Bangale, E. Belmont-Moreno, A. Bernal, K. S. Caballero-Mora, T. Capistrán, A. Carramiñana, F. Carreón, S. Casanova, A. L. Colmenero-Cesar, U. Cotti, J. Cotzomi, S. Coutiño de León, E. De la Fuente, C. de León, P. Desiati, N. Di Lalla, R. Diaz Hernandez, M. A. DuVernois, J. C. Díaz-Vélez, K. Engel, T. Ergin, C. Espinoza, N. Fraija, S. Fraija, A. Galván-Gámez, J. A. García-González, F. Garfias, N. Ghosh, A. Gonzalez Muñoz, M. M. González, J. A. González, J. A. Goodman, J. Gyeong, J. P. Harding, S. Hernández-Cadena, I. Herzog, D. Huang, F. Hueyotl-Zahuantitla, P. Hüntemeyer, A. Iriarte, S. Kaufmann, D. Kieda, K. Leavitt, W. H. Lee, J. Lee, H. León Vargas, J. T. Linnemann, A. L. Longinotti, G. Luis-Raya, C. Lundy, K. Malone, O. Martinez, J. Martínez-Castro, H. Martínez-Huerta, J. A. Matthews, P. Miranda-Romagnoli, P. E. Mirón-Enriquez, J. A. Morales-Soto, E. Moreno, M. Mostafá, M. Najafi, A. Nayerhoda, L. Nellen, M. U. Nisa, R. Noriega-Papaqui, N. Omodei, E. Ponce, Y. Pérez Araujo, E. G. Pérez-Pérez, C. D. Rho, A. Rodriguez Parra, D. Rosa-González, M. Roth, H. Salazar, D. Salazar-Gallegos, A. Sandoval, M. Schneider, J. Serna-Franco, M. Shin, A. J. Smith, Y. Son, R. W. Springer, O. Tibolla, K. Tollefson, I. Torres, R. Torres-Escobedo, F. Ureña-Mena, E. Varela, L. Villaseñor, X. Wang, Z. Wang, I. J. Watson, H. Wu, S. Yu, X. Zhang, H. Zhou

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

Imagine the universe is filled with a mysterious, invisible substance called Dark Matter. We know it's there because it acts like a giant, invisible glue holding galaxies together, but we've never seen it, touched it, or caught it. It's the ultimate ghost in the machine.

This paper is a report from a team of scientists using a giant "net" in the sky to try and catch a glimpse of these ghosts. Here is the story of their hunt, explained simply.

The Detective: HAWC

The main character in this story is HAWC (High Altitude Water Cherenkov Observatory). Think of HAWC as a massive, high-tech swimming pool located on top of a volcano in Mexico. It's filled with 300 giant water tanks.

When a high-energy particle from space (like a cosmic ray) hits the atmosphere, it creates a shower of smaller particles that rain down on Earth. If a gamma ray (a specific type of light particle) hits the water in HAWC's tanks, it creates a flash of blue light (like a sonic boom for light). HAWC's job is to watch these flashes to figure out where the particles came from and what they were.

The Hunting Ground: Dwarf Galaxies

The scientists decided to look for Dark Matter in specific places called Dwarf Spheroidal Galaxies.

The Analogy: Imagine you are looking for a needle in a haystack. Most places in the universe are like a haystack full of hay (regular stars and gas) and a few needles (Dark Matter).
The Strategy: Dwarf galaxies are like a pile of only needles. They are tiny, dim galaxies that are mostly made of Dark Matter and very few regular stars. Because they are so "clean" of regular starlight and gas, if you see a flash of light there, it's much more likely to be from Dark Matter than from a regular star.

The team looked at 17 of these dwarf galaxies that happen to be in the view of their Mexican telescope.

The Theory: The Ghost Collision

The scientists have a theory: Dark Matter particles are like invisible billiard balls. If two of them crash into each other, they might annihilate (destroy each other) and turn into a burst of energy, specifically gamma rays.

HAWC is listening for that specific "crash" sound (the gamma rays) coming from those 17 dwarf galaxies.

The Upgrade: A Sharper Net

In previous years, HAWC tried this same hunt, but their "net" had some holes.

Old Net: They used older software that wasn't great at telling the difference between a gamma ray (the signal they wanted) and a cosmic ray (the background noise). It was like trying to hear a whisper in a crowded room with bad headphones.
New Net: For this study, they upgraded their software. They used Artificial Intelligence (Machine Learning) to filter out the noise. They also looked at a much larger amount of data (3,070 days of watching the sky, compared to just a few hundred days before). This is like upgrading from a fishing net with big holes to a fine-mesh net that catches even the smallest fish.

They also expanded their search. Before, they only looked for Dark Matter up to a certain weight. Now, they looked for "Heavy" Dark Matter, up to 10,000 times heavier than a proton.

The Results: The Silence

After analyzing the data with their new, super-sharp net, the scientists found nothing.

The Verdict: They did not detect any gamma rays coming from the dwarf galaxies that could be explained by Dark Matter crashing into itself.
The Good News: Even though they didn't find the ghost, they learned something important. By not finding it, they can now say with high confidence: "If Dark Matter exists and has this specific weight, it cannot be crashing into itself this often."

They set strict upper limits. Think of it like a speed trap. They didn't catch a speeder, but they can now say, "No car in this area is going faster than 50 mph." If Dark Matter is a "speeder," they've proven it's not speeding as fast as some theories predicted.

The Takeaway

This paper is a report of a very careful, very sensitive search that came up empty-handed. However, in science, a "null result" is still a victory. It tells us that the universe is not as noisy as some theories suggested.

What they did: Used a giant water telescope in Mexico to watch 17 tiny, dark galaxies for 8+ years.
What they improved: Used AI to filter out noise and looked for much heavier particles than before.
What they found: No evidence of Dark Matter annihilation.
What they concluded: They have set the tightest rules yet on how heavy Dark Matter can be and how often it might be crashing into itself, effectively narrowing the search for the universe's biggest mystery.

They didn't find the needle, but they proved that if the needle is there, it's hiding much better than anyone thought.

Technical Summary: Improved Heavy Dark Matter Annihilation Search from Dwarf Galaxies with HAWC

Problem Statement
Despite robust astrophysical evidence for the existence of dark matter (DM), its fundamental particle nature remains unknown. A leading theoretical candidate is the Weakly Interacting Massive Particle (WIMP), which may annihilate into Standard Model (SM) particles, producing secondary gamma rays. Dwarf spheroidal galaxies (dSphs) are ideal targets for indirect detection due to their high dark matter-to-baryonic matter ratios, low astrophysical background, and status as Milky Way satellites. However, previous searches by the High Altitude Water Cherenkov (HAWC) Observatory were limited by older reconstruction algorithms, smaller datasets, and a restricted mass range (up to 1 PeV). This study addresses the need for improved sensitivity to heavy WIMP DM (up to 10 PeV) by leveraging advancements in HAWC's data reconstruction and expanding the sample of analyzed dSphs.

Methodology
The analysis utilizes 3,070 days of HAWC operational data, a significant increase over previous studies (507 days in [5] and 1,038 days in [9, 10]). The study focuses on 17 dSphs located within HAWC's field of view (-26° to +64°), including two previously excluded sources (Willman 1 and Draco II) now constrained by recent kinematic observations.

Key methodological improvements include:

Reconstruction and Event Selection: The analysis employs the newer "Pass 5" reconstruction algorithm, which offers superior angular resolution and sensitivity compared to the "Pass 4" used in prior work. A Neural Network (NN) based energy estimator replaces the previous fraction-of-PMT-hits ( $f_{hit}$ ) method, improving energy resolution and maximum energy sensitivity. Furthermore, Machine Learning (ML) optimization suites are used to select event cuts that better discriminate gamma rays from hadronic cosmic ray backgrounds.
Dark Matter Density Modeling: Two distinct catalogs of DM density parameters are utilized to assess model dependence: the Pace-Strigari (PS) catalog [7], which uses log-uniform priors, and the Ando-Geringer-Sameth (AGS) catalog [8], which incorporates kinematic priors. Both assume a Navarro-Frenk-White (NFW) spatial distribution.
Spectral Modeling: Gamma-ray spectra for 11 SM annihilation channels ( $\chi\chi \to e^+e^-, \mu^+\mu^-, \tau^+\tau^-, b\bar{b}, t\bar{t}, W^+W^-, Z^0Z^0, u\bar{u}, d\bar{d}, \nu_e\bar{\nu}_e, \gamma\gamma$ ) are generated using HDMSpectra (HDMS) [18]. This model includes electroweak corrections and higher-order loop corrections, extending validity to the Planck scale, which is critical for the heavy DM mass range (1 TeV to 10 PeV). Inverse Compton Scattering (ICS) effects were evaluated but found to be negligible for the limits derived.
Statistical Analysis: A likelihood-based approach (using HAL and threeML) is applied to each dSph. The results are stacked to produce combined upper limits on the velocity-weighted annihilation cross-section ( $\langle \sigma v \rangle$ ). Systematic uncertainties from detector response (PMT efficiencies, calibration epochs) and J-factor model choices are quantified.

Key Contributions

Extended Mass Range: The study extends the search for heavy WIMP DM from a previous limit of 1 PeV up to 10 PeV.
Expanded Source Sample: The inclusion of 17 dSphs (up from 15) and the integration of Willman 1 and Draco II, which were previously excluded due to large uncertainties.
Enhanced Sensitivity: The implementation of Pass 5 reconstruction, NN energy estimation, and ML-based background rejection significantly reduces hadronic contamination and improves energy estimation, leading to tighter constraints.
Dual-Catalog Analysis: By presenting results for both PS and AGS catalogs, the paper provides limits based on both general log-uniform priors and specific kinematic priors, offering a more robust assessment of systematic uncertainties related to J-factors.

Results

No Detection: The analysis found no evidence of a gamma-ray excess attributable to dark matter annihilation in any of the 17 dSphs or in the combined dataset.
Upper Limits: The study sets 95% confidence level upper limits on $\langle \sigma v \rangle$ $⟨ σ v ⟩$ for the 11 annihilation channels across the mass range of 1 TeV to 10 PeV.
- For the PS catalog, the most constraining limits are achieved for the $\chi\chi \to b\bar{b}$ channel in the 30 TeV to 10 PeV range and for $\chi\chi \to \tau^+\tau^-$ between 4 TeV and 1 PeV.
- The AGS catalog yields slightly weaker combined limits due to generally lower J-factors, but remains competitive, particularly near 1 PeV.
Statistical Consistency: The observed limits generally align with the expected background fluctuations (Brazil bands). The largest observed excesses were found for $\chi\chi \to b\bar{b}$ at $m_\chi = 1$ TeV, with pre-trial significances of 2.17 $\sigma$ (PS) and 2.53 $\sigma$ (AGS), which are well below the threshold for discovery.
Source Contributions: Segue 1 remains a dominant contributor to the combined limits in both catalogs due to its high J-factor and position overhead in HAWC's field of view. Willman 1 plays a significant role in defining limits at high masses in the PS catalog.

Significance
The paper claims that these results represent the most constraining limits to date for heavy WIMP DM annihilation from dSphs in the PeV mass range. By utilizing the full capabilities of the HAWC detector with advanced reconstruction and ML techniques, the study demonstrates that HAWC can effectively probe DM masses up to 10 PeV. The work establishes competitive constraints that complement and extend limits from other experiments (such as Fermi-LAT, H.E.S.S., MAGIC, VERITAS, and LHAASO), particularly in the high-mass regime where other instruments have limited sensitivity. The authors conclude that while no WIMP signal was detected, the improved methodology and expanded dataset provide a rigorous test of the heavy WIMP hypothesis.