Master thesis defense by Rikke Stougaard Klausen

Speaker: Rikke Stougaard Klausen

Time and space: Tue @ 11:00, Margrethe Bohr salen

Title: Searching for Axions in the Gamma-ray Sky

Abstract: Axion-like particles (ALPs), hypothetical pseudo-scalar bosons, arise naturally in many extensions of the Standard Model, including string theory and theories with spontaneously broken global symmetries. Though originally motivated by the strong CP problem in quantum chromodynamics, ALPs are now considered broadly as potential mediators of new physics, and in some cases, as viable dark matter candidates. Their coupling to photons allows for oscillations between photons and ALPs in external magnetic fields, offering an indirect observational handle on their properties through astrophysical measurements.

This thesis investigates possible ALP–photon mixing effects in gamma-ray spectra of active galactic nuclei (AGNs) that lie within or behind galaxy clusters. These clusters act as large-scale magnetic environments where such oscillations are expected to be most pronounced. 32 AGNs has been selected based on spatial coincidence (and redshift) with clusters, using data from the Fermi Large Area Telescope (LAT). After quality filtering, 30 sources are retained for a stacked spectral analysis aimed at identifying energy-dependent modulations in the photon flux consistent with ALP-induced dimming.

To model the expected signal, a phenomenological expression for the photon survival probability is constructed, guided by simulations from the \texttt{ALPro} framework. The model incorporates the effects of turbulent magnetic fields and plasma densities, capturing the incoherent mixing expected over many domains. The predicted spectral feature, a step-like dimming pattern, is used to define exclusion contours in the ALP parameter space of mass and coupling.

The resulting constraints exclude a previously unexplored region in the parameter space, extending sensitivity to ALP-photon couplings around g_aγ ∼ 10^{−12} GeV^{−1} for masses m_a ∼ neV, which is an improvement of an order of magnitude. The exclusion regions are two disjoint regions, revealing a cliff of a possible detection region.

Compared to individual-source studies, the stacked approach offers increased statistical power and robustness to spectral noise. The analysis also explores the role of AGN flaring activity, testing several flare-filtering schemes to determine their impact on the spectral shape. It is found that flaring modifies the normalization of the spectrum but does not introduce significant energy-dependent features that could mimic or erase ALP signals.