Electromagnetic Catalysis of the Radiative Decay of the Axion

The radiative decay of the axion a → γγ is investigated in an external electromagnetic field in DFSZ model in which axion couples to both quarks and leptons at tree level. The decay probability is strongly catalyzed by the external field, namely, the field removes the main suppression caused by the smallness of the axion mass. will be published in Phys. Lett. B Various extensions of the Standard Model (SM) predict the existence of light neutral pseudoscalars which are the subject of constant theoretical and experimental studies. The axion [1, 2] is the most widely discussed pseudoscalar particle proposed by R.Peccei and H.Quinn to solve a major theoretical problem of CP-conservation in strong interactions. Although the original axion, associated with Peccei-Quinn (PQ) symmetry breaking at the weak scale (fw), is excluded experimentally, many variant PQ models and their associated axions are of great interest (see, for example [3, 4]). If the breaking scale of PQ symmetry fa ≫ fw (the latest astrophysical data yield fa ≥ 10GeV ), the resulting, so called, invisible axion is very light (ma ∼ f a ) and very weakly coupled (coupling ∼ f a ). At present astrophysical and cosmological considerations leave a rather narrow window for the axion mass [3, 5, 6]: 10 eV ≤ ma ≤ 10 eV . (1) A survey of various processes involving axion production and astrophysical methods for obtaining constraints on the parameters of axion models is given in [3]. Unfortunately a problem of its possible existence which is very important for the theoretical description of elementary particles remains unresolved. It can be naturally explained by the fact that the axion lifetime in vacuum is very large [3]: τ(a → 2γ) ∼ 6, 3 · 10 s ( 10eV ma )6 ( Ea 1MeV ) . (2) On the other hand, one can get more precise constraints on weakly coupled particles properties with an influence of external electromagnetic fields taken into account. It is also important in some astrophysical and cosmological considerations to take strong external fields (of order the critical, so called, Schwinger value Fs = m 2 e/e ≃ 0.44 · 10 G) into account. Notice that the decay probability in an external field depends on not only typical kinematical invariants of type m, p, but field invariants |e2(FF )|1/2, |e2(F̃F )|1/2, |e2(pFFp)|1/3 as well (here m and pμ are the mass and the 4-momentum of the particles, Fμν is the external field tensor, pFFp = pFμνF pρ). In ultrarelativistic limit the field invariant |e2(pFFp)|1/3 can occur the largest one. This is due to the fact that in the relativistic particle rest frame the field may turn out to be of order of the critical one or even higher, appearing very close to the constant crossed field ( ~ E ⊥ ~ H,E = H ), where (FF ) = (FF̃ ) = 0. Thus, the calculation in this field is the relativistic limit of the calculation in an arbitrary relatively smooth field, possesses a great extent of generality and acquires interest by itself. In this work we study the double radiative axion decay a → γγ in the external crossed field. We consider Dine-Fischler-Srednicki-Zhitnitskii (DFSZ) axions [7] which couple to both quarks and leptons at tree level. The corresponding interaction lagrangian has the form: Laf = −igaf (f̄γ5f)a (3) where gaf = Cfmf/fa is a dimensionless Yukawa coupling constant, Cf is a model-dependent factor, mf is a fermion mass.