Phenomenology of relaxion-Higgs mixing

Abstract

We show that the relaxion generically stops its rolling at a point that breaks CP leading to relaxion-Higgs mixing. This opens the door to a variety of observational probes since the possible relaxion mass spans a broad range from sub-eV to the GeV scale. We derive constraints from current experiments (fifth force, astrophysical and cosmological probes, beam dump, flavour, LEP and LHC) and present projections from future experiments such as NA62, SHiP and PIXIE. We find that a large region of the parameter space is already under the experimental scrutiny. All the experimental constraints we derive are equally applicable for general Higgs portal models. In addition, we show that simple multiaxion (clockwork) UV completions suffer from a mild fine tuning problem, which increases with the number of sites. These results favour a cut-off scale lower than the existing theoretical bounds.

Peccei-Quinn relaxion

“…In the present universe, the QCD axion obtains a mass dominantly from the QCD anomaly and is fixed at a CP preserving minimum. On the other hand, the relaxion becomes massive mainly due to the potential induced by the hidden gauge anomaly, and thus it has different properties from other models [11][12][13]. First, the relaxion can have a large mass…”

Detecting hidden particles with MATHUSLA

“…The light particles of these hidden sectors can potentially be produced in a variety of ways, including rare meson decays and exotic Higgs decays [18]. One of the most minimal hidden sectors contains a new scalar, S, coupled to the standard model via ϵjSj 2 H † H. This scalar mixes slightly with the standard model-like Higgs, and provides a simple target for new physics searches [10,11,19,20].…”

A tale of two portals: testing light, hidden new physics at future e + e − colliders

“…Both of these marginal operators have been studied autonomously at electron colliders in the hidden photon context [5][6][7][8][9][10][11][12][13][14][15][16][17] and the hidden scalar context [18][19][20][21][22][23]. Some works study both operators in tandem [24][25][26] or adopt an effective operator approach [27].…”