Is the relaxion an axion?

Abstract

We consider the recently proposed cosmological relaxation mechanism where the hierarchy problem is ameliorated, and the electroweak (EW) scale is dynamically selected by a slowly rolling axion field. We argue that, in its simplest form, the construction breaks a gauge symmetry that always exists for pseudo-Nambu-Goldstone bosons (in particular the axion). The small parameter in the relaxion model is therefore not technically natural as it breaks a gauge symmetry rather than global symmetries only. The consistency of the theory generically implies that the cutoff must lie around the electroweak scale, but not qualitatively higher. We discuss several ways to evade the above conclusion. Some of them may be sufficient to increase the cutoff to the few-TeV range (and therefore may be relevant for the little-hierarchy problem). To demonstrate the ideas in a concrete setting we consider a model with a familon, the Nambu-Goldstone boson of a spontaneously broken chiral flavor symmetry. The model has some interesting collider-physics aspects and contains a viable weakly interacting dark matter candidate.

Realizing the relaxion from multiple axions and its UV completion with high scale supersymmetry

“…To incorporate a huge relaxion excursion, one may simply assume that the relaxion is a non-compact field variable. See [3][4][5][6][7][8][9][10] for recent discussions of the related issues. In this paper, we discuss an alternative scenario in which the relaxion corresponds to an exponentially long multi-helical flat direction in the compact field space spanned by N sub-Planckian periodic axions:…”

Shift-symmetries at higher order

“…One could hope to develop heuristic arguments along the lines of those in the introduction, and indeed there may be interesting overlaps with shift-symmetry restoration in certain limits of the type II systems in [30]. We should remark that shift-symmetries have also come to the fore because of their central role in schemes that try to explain the weak-Planck hierarchy by means of cosmological relaxation [31,32], a subject which has recently received much attention [33][34][35][36][37][38][39][40][41][42][43][44][45][46]. Although these often feature axionic (i.e.…”

Relaxion cosmology and the price of fine-tuning

“…Since its proposal, the relaxion mechanism has been thoroughly scrutinised and several analyses have improved on the original model [5][6][7][8][9][10][11][12][13][14][15][16]. In this work we extend the relaxion framework with a consistent scenario of inflationary cosmology that naturally provides 1 The relaxion mechanism can also be implemented with a generic axion-like particle.…”