ALPs searches with LST-1

¹ Universita degli Studi di Padova Dipartimento di Fisica e Astronomia Galileo Galilei, Via Francesco Marzolo 8, 35131, Padova, Italy
² Istituto Nazionale di Fisica Nucleare Sezione di Padova INFN, Via Francesco Marzolo 8, 35131, Padova, Italy
³ University of Bergen Department of Physics and Technology, Allégaten 55, 5007, Bergen, Norway

^* e-mail: ivana.batkovic@unipd.it

Published online: 6 March 2025

Abstract

Axions and axion-like particles (ALPs) are hypothetical particles predicted by several extensions of the Standard Model and viable candidates for solving one of the big mysteries of the Universe: Dark Matter. By exploring the spectra of astrophysical objects obtained from observations with the Large Size Telescope (LST-1), we can search for signatures that such particles may leave. In particular, we look for oscillations in the spectra due to conversions between ALPs and very-high-energy (VHE) gamma rays. Our targets of interest include the blazars Mrk 421, Mrk 501, BL Lac, and 1ES1959+650, extensively observed with LST-1. The photon-ALP conversion probability is impacted by external magnetic fields, and for that reason, all magnetic fields in the propagation path need to be evaluated. In addition to the magnetic field in the relativistic jet of the blazar, we consider the impact of the Extragalactic Background Light in the intergalactic magnetic field and the magnetic field of the Milky Way for each source separately. For ALP masses in the neV range and magnetic field strengths of O(μG), these oscillations are more likely with photons in the GeV range, making LST-1 an optimal instrument for testing the ALP hypothesis in the VHE gamma-ray energy range. By exploring the LST-1 data of several blazars, our aim is to combine constraints from each source at the likelihood level, ultimately creating unique constraints in the ALP parameter space. These would be the first combined constraints obtained with a dataset from a group of blazar sources.