Constraints on Very Light Axions from Cavity Experiments

In view of the ongoing galactic (or cosmic) axion detection experiments, we compare the axion-photon-photon coupling caγγ ’s for various invisible (or very light) axion models. 11.40-q, 95.35+d, 12.60 Fr Typeset using REVTEX ∗Permanent address 1 The θ̄ parameter of the standard model is naturally understood in axion models [1]. This axion interpretation has led to the very light (or invisible) axion’s role in the galaxy formation [2]. If the seed of our galaxy is indeed the density perturbations due to invisible axions, the cold dark matter might be these cold axions with ∼ 0.3 GeV/cm energy density in our galaxy, which for fμeV axion mass corresponds to ∼ 3×1014/f axions per cm around us. These ubiquitous axions can be detected using a cavity immersed in strong magnetic field [3]. Two groups have already reported on this type of experiments [4,5]. In addition, La M1 transition has been studied to get a clue on these galactic axions [6]. These previous experiments have given the upper bound on the detection rate, but have not reached to the level of detecting the galactic axions (or cosmic axions). Currently, there are two ongoing experiments, the Kyoto University experiment on Rydberg atoms [7] and the Lawrence Livermore National Laboratory (LLNL) experiment [8]. In particular, the sensitivity of LLNL experiment is at the level of distinguishing several invisible axion models. So far, the theoretical invisible axion models compared with data are not given with proper distinction. Thus it is very important at this stage to clarify the prediction of the axion-photon-photon coupling constant caγγ in various invisible (very light) axion models. It is known that the free energy V is minimum at θ̄ = 0 in a world without weak CP violation [9]. If θ̄ 6= 0, the QCD term θ̄ 32π F a μνF̃ aμν (1) violates P and CP symmetry, implying |θ̄| < 10 from the neutron electric dipole moment bound. This can be understood if we let θ̄ be a dynamical variable, i.e.