Redefining the Axion Window

A major goal of axion searches is to reach inside the parameter space region of realistic axion models. Currently, the boundaries of this region depend on somewhat arbitrary criteria, and it would be desirable to specify them in terms of precise phenomenological requirements. We consider hadronic axion models and classify the representations $R_Q$ of the new heavy quarks $Q$. By requiring that (i) the $Q$’s are sufficiently short lived to avoid issues with long-lived strongly interacting relics, (ii) no Landau poles are induced below the Planck scale; 15 cases are selected which define a phenomenologically preferred axion window bounded by a maximum (minimum) value of the axion-photon coupling about 2 times (4 times) larger than is commonly assumed. Allowing for more than one $R_Q$, larger couplings, as well as complete axion-photon decoupling, become possible.

Figure 1

Heavy $Q$ contribution to the cosmological energy density versus $m_Q$. The dotted (dashed) line corresponds to free annihilation for color triplets (octets). The solid line corresponds to annihilation via bound state formation.

Figure 2

The window for preferred axion models. The green band encompasses models with a single $R_Q$. With more $R_Q$’s, the region below the line $E/N=170/3$ becomes allowed. The two dashed lines enclose the usual window $|E/N-1.92|\in [0.07,7]$ [5]. Current (projected) exclusion limits are delimited by solid (dashed) lines.