[Paper Review] First constraints on axion-like particles from Galactic sub-PeV gamma rays

Christopher Eckner, Francesca Calore|arXiv (Cornell University)|Apr 26, 2022

Dark Matter and Cosmic PhenomenaPhysics and Astronomy125 references18 citations

TL;DR

This study presents the first direct constraints on axion-like particles (ALPs) using sub-PeV diffuse Galactic gamma-ray data from HAWC and Tibet ASµ, leveraging the connection between IceCube's extragalactic high-energy neutrino flux and ALP-induced gamma-ray signals. By modeling ALP conversion in extragalactic and Galactic magnetic fields, the authors set a 95% confidence level upper limit of gaγγ < 2.1 × 10−11 GeV−1 for ALP masses below 2 × 10−7 eV, closing a gap in the ALP parameter space and complementing existing limits from TeV gamma-ray and ADMX experiments.

ABSTRACT

Experimental refinements and technical innovations in the field of extensive air shower telescopes have enabled measurements of Galactic cosmic-ray interactions in the sub-PeV range, providing new avenues for the search for new physics and dark matter. For the first time, we exploit sub-PeV (1 TeV -- 1 PeV) observations of Galactic diffuse gamma rays by HAWC and Tibet AS$\gamma$ to search for an axion-like-particle (ALP) induced gamma-ray signal directly linked to the origin of the IceCube extragalactic high-energy neutrino flux. Indeed, the production of high-energy neutrinos in extragalactic sources implies the concomitant production of gamma rays at comparable energies. Within the magnetic field of the neutrino emitting sources, gamma rays may efficiently convert into ALPs, escape their host galaxy un-attenuated, propagate through intergalactic space, and reconvert into gamma rays in the magnetic field of the Milky Way. Such a scenario creates an all-sky diffuse high-energy gamma-ray signal in the sub-PeV range. Accounting for the guaranteed Galactic astrophysical gamma-ray contributions from cosmic-ray interactions with gas and radiation and from sub-threshold sources, we set competitive upper limits on the photon-ALP coupling constant $g_{a\gamma\gamma}$. We find $g_{a\gamma\gamma} < 2.1 imes10^{-11}$ GeV$^{-1}$ for ALP masses $m_a \leq 2 imes10^{-7}$ eV at a 95\% confidence level. Our results are comparable to previous limits on ALPs derived from the TeV gamma-ray domain and progressively close the mass gap towards ADMX limits. The code and data to reproduce the results of this study are available on GitHub \url{https://github.com/ceckner/subPeVALPs}.

Motivation & Objective

• To constrain the photon-ALP coupling constant gaγγ using sub-PeV diffuse Galactic gamma-ray observations.
• To exploit the synergy between the IceCube extragalactic high-energy neutrino flux and ALP-induced gamma-ray signals in the sub-PeV range.
• To close the gap in ALP parameter space between TeV gamma-ray and ADMX limits by probing a previously under-constrained mass range.
• To model and subtract guaranteed Galactic astrophysical gamma-ray contributions from cosmic-ray interactions and unresolved sources to isolate potential ALP-induced signals.

Proposed method

• Utilized diffuse gamma-ray flux measurements from HAWC (43° < l < 73°, |b| < 4°) and Tibet ASµ (25° < l < 100°, |b| < 5°) in the 1 TeV to 1 PeV energy range.
• Constructed a model for ALP production in extragalactic neutrino-emitting sources via p-p or p-γ interactions, followed by conversion into photons in the Milky Way's magnetic field.
• Applied the Primakoff effect formalism to describe photon-ALP oscillations in external magnetic fields, using the Hamiltonian evolution equation for the density matrix: idρ/dz = [Hdis, ρ] - i/2{Habs, ρ}.
• Incorporated dispersive and absorptive effects via matrix elements Δ⊥, Δ∥, Δa, and Δaγ, accounting for plasma, magnetic field, and radiation field contributions.
• Performed a statistical analysis comparing the total predicted gamma-ray flux (astrophysical + ALP-induced) to observed data, varying gaγγ and ma to derive exclusion limits.
• Used fiducial prescriptions for star-formation rate density evolution and magnetic field strength evolution in extragalactic sources to model the ALP flux.

Experimental results

Research questions

• RQ1 Can sub-PeV diffuse Galactic gamma-ray observations from HAWC and Tibet ASµ be used to constrain the photon-ALP coupling constant gaγγ?
• RQ2 What is the contribution of ALP-induced gamma rays to the diffuse Galactic emission in the 1 TeV to 1 PeV range, assuming ALPs originate from extragalactic neutrino sources?
• RQ3 How do the observed sub-PeV gamma-ray fluxes compare to the expected astrophysical background from cosmic-ray interactions and unresolved sources?
• RQ4 What is the sensitivity of this method to ALP masses below 2 × 10−7 eV, and how does it compare to existing limits from other experiments?
• RQ5 To what extent can the IceCube neutrino flux be used as a proxy for ALP production in extragalactic sources?

Key findings

• The study sets a 95% confidence level upper limit of gaγγ < 2.1 × 10−11 GeV−1 for ALP masses ma ≤ 2 × 10−7 eV.
• The derived constraint is competitive with previous limits from the TeV gamma-ray domain and progressively closes the gap toward ADMX limits in the low-mass ALP parameter space.
• The analysis demonstrates that sub-PeV gamma-ray observations can probe a previously under-constrained region of ALP parameter space, particularly for ma < 2 × 10−7 eV.
• The model accounts for guaranteed Galactic astrophysical contributions, including cosmic-ray interactions with gas and radiation fields, and unresolved TeV sources, ensuring robustness against background contamination.
• The results are reproducible using publicly available code and data from the GitHub repository: https://github.com/ceckner/subPeVALPs.
• The study provides the first direct constraint on ALPs using real sub-PeV diffuse gamma-ray data, validating the theoretical framework of ALP-induced gamma-ray signals from extragalactic neutrino sources.