Audible axions

Abstract

Conventional approaches to probing axions and axion-like particles (ALPs) typically rely on a coupling to photons. However, if this coupling is extremely weak, ALPs become invisible and are effectively decoupled from the Standard Model. Here we show that such invisible axions, which are viable candidates for dark matter, can produce a stochastic gravitational wave background in the early universe. This signal is generated in models where the invisible axion couples to a dark gauge boson that experiences a tachyonic instability when the axion begins to oscillate. Incidentally, the same mechanism also widens the viable parameter space for axion dark matter. Quantum fluctuations amplified by the exponentially growing gauge boson modes source chiral gravitational waves. For axion decay constants f 10 17 GeV, this signal is detectable by either pulsar timing arrays or space/ground-based gravitational wave detectors for a broad range of axion masses, thus providing a new window to probe invisible axion models.

Gravitational wave probes of axionlike particles

“…Here, we give a brief overview of the model presented in Ref. [8], which consisted of an axion field ϕ and a massless dark photon X μ of an unbroken Uð1Þ X Abelian gauge group,…”

“…In Ref. [8], we showed that axions or axionlike particles coupled to a light dark photon can produce a stochastic gravitational wave background (SGWB) when the axion field begins to oscillate in the early Universe, allowing exploration of parameter space inaccessible to experiments that rely on the axion-photon coupling. The rolling axion induces a tachyonic instability that amplifies vacuum fluctuations of a single gauge boson helicity, sourcing chiral GWs.…”

Super-Planckian axions from near-conformality

“…Moreover, axionlike particles are abundant in string compactifications [14][15][16]. This motivates searching for axions on different fronts, including using a variety of techniques to directly detect axions [17], as well as collider probes [18][19][20][21][22], gravitational wave searches [23][24][25][26], and astrophysical searches [27].…”

Axion misalignment driven to the bottom

“…(1.2), for f a O(10 12 ) GeV, the universe is overclosed unless θ mis is sufficiently small (applicable only to the case of pre-inflationary PQ breaking). There exist late-time mechanisms such as dilution from entropy production by late decaying moduli fields [31,32] or the axion superpartner [33][34][35][36][37], and depletion to other species [38][39][40][41]. Meanwhile, it is crucial to explore natural explanations to a small θ mis .…”