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Cosmological abundance of the QCD axion coupled to hidden photons
Kitajima, Naoya,Sekiguchi, Toyokazu,Takahashi, Fuminobu North-Holland Pub. Co 2018 Physics letters. Section B Vol.781 No.-
<P><B>Abstract</B></P> <P>We study the cosmological evolution of the QCD axion coupled to hidden photons. For a moderately strong coupling, the motion of the axion field leads to an explosive production of hidden photons by tachyonic instability. We use lattice simulations to evaluate the cosmological abundance of the QCD axion. In doing so, we incorporate the backreaction of the produced hidden photons on the axion dynamics, which becomes significant in the non-linear regime. We find that the axion abundance is suppressed by at most O ( <SUP> 10 2 </SUP> ) for the decay constant <SUB> f a </SUB> = <SUP> 10 16 </SUP> GeV , compared to the case without the coupling. For a sufficiently large coupling, the motion of the QCD axion becomes strongly damped, and as a result, the axion abundance is enhanced. Our results show that the cosmological upper bound on the axion decay constant can be relaxed by a few hundred for a certain range of the coupling to hidden photons.</P>
Level crossing between the QCD axion and an axionlike particle
Daido, Ryuji,Kitajima, Naoya,Takahashi, Fuminobu American Physical Society 2016 Physical Review D Vol.93 No.7
<P>We study a level crossing between the QCD axion and an axionlike particle, focusing on the recently found phenomenon, the axion roulette, where the axionlike particle runs along the potential, passing through many crests and troughs, until it gets trapped in one of the potential minima. We perform detailed numerical calculations to determine the parameter space where the axion roulette takes place, and as a result domain walls are likely formed. The domain wall network without cosmic strings is practically stable, and it is nothing but a cosmological disaster. In a certain case, one can make domain walls unstable and decay quickly by introducing an energy bias without spoiling the Peccei-Quinn solution to the strong charge parity problem.</P>
Cosmological stochastic Higgs field stabilization
Gong, Jinn-Ouk,Kitajima, Naoya American Physical Society 2017 Physical Review D Vol.96 No.6
<P>We show that the stochastic evolution of an interacting system of the Higgs field and a spectator scalar field naturally gives rise to an enhanced probability of settling down at the electroweak vacuum at the end of inflation. Subsequent destabilization due to parametric resonance between the Higgs field and the spectator field can be avoided in a wide parameter range. We further argue that the spectator field can play the role of dark matter.</P>
The QCD axion from aligned axions and diphoton excess
Higaki, Tetsutaro,Jeong, Kwang Sik,Kitajima, Naoya,Takahashi, Fuminobu Elsevier 2016 Physics letters: B Vol.755 No.-
<P><B>Abstract</B></P> <P>We argue that the QCD axion can arise from many aligned axions with decay constants much smaller than the conventional axion window. If the typical decay constant is of O ( 100 ) GeV to 1 TeV , one or more of the axions or saxions may account for the recently found diphoton excess at ∼ 750 GeV . Our scenario predicts many axions and saxions coupled to gluons with decay constants of order the weak scale, and therefore many collider signatures by heavy axions and saxions will show up at different energy scales. In particular, if the inferred broad decay width is due to multiple axions or saxions, a non-trivial peak structure may become evident when more data is collected. We also discuss cosmological implications of the aligned QCD axion scenario. In the Appendix we give a possible UV completion and argue that the high quality of the Peccei–Quinn symmetry is naturally explained in our scenario.</P>