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Observable windows for the QCD axion through $N_\text{eff}$
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abstract
We show that when the QCD axion is directly coupled to quarks with $c_q/f \, \partial_\mu a \, \bar{q} \gamma^\mu \gamma^5 q$, such as in DFSZ models, the dominant production mechanism in the early universe at temperatures $1 \, {\rm GeV}\lesssim T \lesssim 100 \,{\rm GeV}$ is obtained via $q \bar{q} \leftrightarrow g a$ and $q g \leftrightarrow q a$, where $g$ are gluons. Different heavy quarks $q_i$ can produce a thermal axion background that decouples at a temperature $T_i$: (1) top quark at $T_t \lesssim 100 \, {\rm GeV}$ for $f/c_t \lesssim 3\times 10^8 {\rm GeV}$; (2) bottom quark at $T_b \lesssim m_b$, for $f/c_b\lesssim 8\times 10^{7} {\rm GeV}$; (3) charm quark at $T_c \lesssim m_c $ for $f/c_c \lesssim 5\times 10^{7} {\rm GeV}$. Each of these cases corresponds to a contribution to the effective number of relativistic degrees of freedom, in the windows given by $0.027 \leq \Delta N_\text{eff}\leq 0.031$, $0.037 \leq \Delta N_\text{eff} \leq 0.039$ and $0.039 \leq \Delta N_\text{eff}$, respectively. These contributions are larger than the one obtained when thermalization happens only above the electroweak phase transition, $\Delta N_{\rm eff}\lesssim 0.027$, and are within reach of future CMB S4 experiments, thus opening an alternative window to detect the axion and to test the early universe at such temperatures.
Forward citations
Cited by 5 Pith papers
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