INTRODUCTION
The standard model of particle physics has long been thought to be incomplete as it is, for example, unable to explain dark matter, the observed matter-antimatter asymmetry of the universe, or the hierarchy problem. Another major weakness is the lack of a natural mech- anism to explain the absence of charge-parity (CP) violation in strong interactions. A solution, intro- duced by Peccei and Quinn [1], postulates an additional global symmetry U (1)P Q that is spontaneously broken at some large energy scale, fa. This generates a Nambu- Goldstone boson, the Weinberg-Wilczek axion [2, 3], with a field that transforms as a(x) → a(x) + αfa, where α is the phase of the introduced scalar field. If there is more than one global symmetry and, therefore, more than one Nambu-Goldstone boson, the particle corresponding to the excitation of the field combination is then the axion. Axions arising from symmetry breaking at electroweak scales have been discounted, having been ruled out by experimental searches [4], but axions that result from a much higher energy scale, so-called “invisible” axions [5–7], remain viable. In addition to QCD axions, particle excitations of the fields orthogonal to this field combina- tion are called Axion-Like-Particles (ALPs), and indeed, numerous string-theory driven models predict ALP can- didates [8–11].
Both axions and ALPs make interesting dark matter candidates [12]: they are nearly collisionless, neutral, nonbaryonic, and may be present in sufficient quantities to provide the expected dark matter density. Axions may have been produced as a nonthermal relic by the mis- alignment mechanism [13, 14] and while very light, are predicted to be produced essentially at rest, thus satis- fying the criteria for cold dark matter. There are also possible thermal production mechanisms [15], although these are unlikely to result in significant contributions to the dark matter. ALPs may have been present during the early phases of the Universe, produced as stable or long-lived particles that are now slowly moving within our Galaxy [16].
Production of axions may arise in stellar environments leading to a constant rate of emission from stars. From the Sun, this provides a second possible source of axion signal, but the consistency of stellar behaviour with mod- els that exclude axion emission also leads to tight con- straints on their existence [17–19]. Additional constraints arise from searches for axion couplings to photons via the Primakoff effect [20, 21]. Axions and ALPs are also ex- pected to couple with electrons, so can be probed with a wider range of experimental techniques, such as instru- ments with germanium and xenon active targets [22, 23]. Here we present searches for axio-electric coupling with the LUX experiment for two specific scenarios: i) QCD
∗Corresponding author: m.marzioni@ed.ac.uk
axions emitted from the Sun, and ii) keV-scale galac- tic ALPs that could constitute the gravitationally bound dark matter.