QCD axion on hilltop by a phase shift of π.

Abstract

We show that the initial misalignment angle of the QCD axion (or axion-like particles) can be set very close to π, if the QCD axion has a mixing with another heavy axion which induces the phase shift ≈ π after inflation. In the simplest case, the heavy axion plays the role of the inflaton, and we call such inflation as "πnflation." The basic idea was first proposed by Daido and the present authors in Ref.[1] in 2017 and more recently discussed in Ref. [2]. We show that the QCD axion with a decay constant f a 3 × 10 9 GeV can explain dark matter by the πnflation mechanism. A large fraction of the parameter region has an overlap with the projected sensitivity of ORGAN, MADMAX, TOORAD and IAXO. We also study implications for the effective neutrino species and isocurvature perturbations. The πnflation can provide an initial condition for the hilltop inflation in the axion landscape, and in a certain set-up, a chain of the hilltop inflation may take place. 1 arXiv:1908.06071v2 [hep-ph] 3 Oct 2019 5 The mass and kinetic mixings between the QCD axion and another axion were discussed in e.g. Refs. [41][42][43][44][45], where the level crossing between the two axions can reduce the axion abundance. The mixing can also induce the DM decay [1,46,47]. Also the mixing between axions plays an important role in inflation model building [48][49][50][51][52][53] 6 The axion isocurvature perturbations can be suppressed in this case [58]. See e.g. Refs. [14,26,[59][60][61][62][63][64][65][66][67][68][69][70][71][72][73] for other scenarios to suppress the axion isocurvature.

Axion Kinetic Misalignment Mechanism

“…This mass scale m a ¼ Oð0.1 − 100Þ meV is under extensive experimental investigation [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. Other known production mechanisms in this mass range are (1) parametric resonance from a PQ symmetry breaking field [39,40], (2) anharmonicity effects [41][42][43] when θ i approaches π due to fine-tuning or inflationary dynamics [44,45], (3) decays of unstable domain walls [46][47][48][49][50][51][52][53], and (4) production during a kination era [54]. Contrary to these mechanisms, kinetic misalignment offers an exciting theoretical connection with the baryon asymmetry of the Universe through so-called axiogenesis [55].…”

Stochastic inflation with an extremely large number of e-folds

“…From the observational point of view, the total number of e-folds, N , must be larger than ∼ 50-60, which weakly depends on the inflation scale and thermal history after inflation. On the other hand, an extremely long duration of inflation is often required in certain scenarios, e.g., the relaxion model [1], the stochastic axion scenario [2][3][4][5], etc. The purpose of this Letter is to provide a simple single-field, slow-roll inflation model whose typical number of e-folds is extremely large.…”

“…The stochasticity parameter is related to η cl by η cl = 1 2 |η V |ξ sto . Therefore, the conditions (4) can be satisfied in a highly stochastic region ξ sto 1 if η V 1, in which case the expected e-folds can be well approximated by the classical formula (5).…”

“…Thus, in this simple example, the averaged e-folding number is entirely determined by the classical motion as in Eq. (5).…”

What if ALP dark matter for the XENON1T excess is the inflaton

“…If there is a mixing between the ALP and other axions, on the other hand, this is not necessarily the case because the minimum could be shifted after inflation[86]. See also refs [41,[87][88][89][90]…”