QCD Axion and Quintessential Axion

The axion solution of the strong CP problem is reviewed together with the other strong CP solutions. We also point out the quintessential axion(quintaxion) whose potential can be extremely flat due to the tiny ratio of the hidden sector quark mass and the intermediate hidden sector scale. The quintaxion candidates are supposed to be the string theory axions, the model independent or the model dependent axions. 1 The Strong CP Problem At this conference, the organizer asked to review on the axion [1] as the CDM candidate [2]. Therefore, I will review the QCD axion and the related problem, the strong CP problem. With supersymmetrization, an O(GeV) axino can be a CDM candidate also [3] in addition to the axion CDM, but we will not discuss this interesting class of CDM candidate here. It was reviewed in another DM conference [4]. Out of several CDM candidates, the axion is most attractive since it arises from the need to solve the strong CP problem. One crucial difference of the axion CDM from most other CDM candidates is that the axion is a classical field oscillation [2] while many other CDM candidates rely on their heavy particle nature. The classical axion potential is extremely flat as shown in Fig. 1. The axion value 〈a〉 stays at some point(the bullet) for a long time before it starts to oscillates, which occurs at T ≃ 1 GeV when the Hubble time 1/H becomes larger than the oscillation period ma.