<p> The axion, being one of the most well motivated solutions to the strong CP problem (Peccei and Quinn, 1977), has been <span style="font-size: 13px;">phenomenologically tested in several different contexts. Despite its extremely feeble interactions with the Standard </span><span style="font-size: 13px;">Model (SM) particles, it could leave sizeable imprints on cosmological and astrophysical observables, providing a precious </span><span style="font-size: 13px;">insight on its properties. Depending on the axion mass, a relativistic axion population can be thermally produced </span><span style="font-size: 13px;">in the early universe. Such a population would act as warm dark matter, leaving imprints in the Cosmic Microwave </span><span style="font-size: 13px;">Background (CMB). Observations and data of the CBM provide therefore constrains on the axion mass and couplings. </span></p>
<p> Unlike the KSVZ model, the axion couples at tree level to SM fermions in the DFSZ model. For this reason the DFSZ <span style="font-size: 13px;">axion has been considered to explain the Xenon-1T excess (Aprile et al., 2020) and the Star cooling anomalies (Giannotti </span><span style="font-size: 13px;">et al., 2017). In the recent paper by Ricardo Ferreira, Alessio Notari and Fabrizio Rompineve (Ferreira et al., 2020), the </span><span style="font-size: 13px;">DFSZ axion has been analysed in light of the cosmological constraints coming from the CMB Planck 2018 data, BAO </span><span style="font-size: 13px;">measurements, the SH0ES 2019 measurement and the Pantheon supernovae dataset. More particularly, they determine </span><span style="font-size: 13px;">the range of DFSZ axion masses and couplings compatible with the current observations.</span></p>
<p> For masses <img alt="" src="/sites/default/files/images/Captura%20de%20pantalla%20(25).png" style="width: 75px; height: 16px;" />, the axion thermal population is produced through the axion-pion interactions at temperatures <span style="font-size: 13px;">below the QCD phase transition (T <img alt="" src="/sites/default/files/images/Captura%20de%20pantalla%20(27).png" style="width: 15px; height: 15px;" /> 200 MeV). Remarkably, considering the model dependent couplings of </span><span style="font-size: 13px;">the DFSZ axion, it is found that axion production from leptons, while negligible for DFSZ-I model, it can be the dominant </span><span style="font-size: 13px;">production channel in the DFSZ-II model assuming that the axion-pion coupling is suppressed. According to that, </span><span style="font-size: 13px;">for a maximal axion-pion coupling <img alt="" src="/sites/default/files/images/Captura%20de%20pantalla%20(29).png" style="width: 70px; height: 14px;" />, they derive from the latest data sets a common bound to the type I </span><span style="font-size: 13px;">and II models of <img alt="" src="/sites/default/files/images/Captura%20de%20pantalla%20(32).png" style="width: 70px; height: 15px;" />(95%CL), while in the pionphobic case <img alt="" src="/sites/default/files/images/Captura%20de%20pantalla%20(34).png" style="width: 70px; height: 15px;" /> the bound on the axion mass is <img alt="" src="/sites/default/files/images/Captura%20de%20pantalla%20(36).png" style="width: 120px; height: 15px;" /></span><span style="font-size: 13px;"> (95%CL) in the DFSZ-I model (pion production) and <img alt="" src="/sites/default/files/images/Captura%20de%20pantalla%20(37).png" style="width: 70px; height: 15px;" /> (95%CL) in the DFSZ-II (lepton </span><span style="font-size: 13px;">production). Moreover, combining these cosmological data to the hints from astrophysics (Isern et al., 2018; Viaux et al., </span><span style="font-size: 13px;">2013) and the Xenon-1T anomaly through a Gaussian likelihood on the axion-electron coupling, they find that most </span><span style="font-size: 13px;">of the parameter space suitable to explain (separately) the latter hints is in fact ruled out by cosmology, Fig.(1) (for a </span><span style="font-size: 13px;">combined analysis ruling out the axion solution to both the stellar and Xenon observations see (Di Luzio et al., 2020)). In </span><span style="font-size: 13px;">particular, for the Xenon-1T case, they restrict the axion mass to the range <img alt="" src="/sites/default/files/images/Captura%20de%20pantalla%20(39).png" style="width: 100px; height: 14px;" /> for the DFSZ-II, while </span><span style="font-size: 13px;">the bound in the DFSZ-I is <img alt="" src="/sites/default/files/images/Captura%20de%20pantalla%20(41).png" style="width: 70px; height: 14px;" /> For the stellar hint case, only the DFSZ-II model can be constrained since in </span><span style="font-size: 13px;">the DFSZ-I scenario the thermal axion production is small in the stellar hint band. The bound is <img alt="" src="/sites/default/files/images/Captura%20de%20pantalla%20(43).png" style="width: 100px; height: 16px;" /></span></p>
<p>It is finally interesting to note that the Xenon-1T band can be probed by next generation CMB surveys in the DFSZ-I <span style="font-size: 13px;">scenario (Fig.(1)).</span></p>
<p> </p>
<p><a href="/sites/default/files/images/BOUNDS.png"><img alt="" src="/sites/default/files/images/BOUNDS.png" style="width: 580px; height: 312px;" /></a></p>
<div>
<p><strong>Figure 1:</strong> Parameter region compatible with Xenon-1T excess (curvy gray) and stellar hints (hatched blue) for the <span style="font-size: 13px;">DFSZ-I (left) and DFSZ-II (right) axion. Regions where <img alt="" src="/sites/default/files/images/Captura%20de%20pantalla%20(46).png" style="width: 13px; height: 17px;" /> Neff is large enough to be probed at 2o by Planck18 and </span><span style="font-size: 13px;">CMB-S4 are shaded in dark and light gray, respectively (Ferreira et al., 2020).</span></p>
</div>
<p> </p>
<p> </p>
<p> </p>
<p><strong style="color: rgb(67, 67, 67); font-family: Arial, Helvetica, sans-serif; font-size: 12px; background-color: rgb(250, 250, 250);">REFERENCES</strong></p>
<p><span style="font-size: 13px;">E. Aprile et al. Excess electronic recoil events in XENON1T. Phys. Rev. D, 102(7):072004, 2020. doi: 10.1103/PhysRevD. </span><span style="font-size: 13px;">102.072004.</span></p>
<p> </p>
<p>L. Di Luzio, M. Fedele, M. Giannotti, F. Mescia, and E. Nardi. Solar axions cannot explain the XENON1T excess. Phys. <span style="font-size: 13px;">Rev. Lett., 125(13):131804, 2020. doi: 10.1103/PhysRevLett.125.131804.</span></p>
<p> </p>
<p>R. Z. Ferreira, A. Notari, and F. Rompineve. The DFSZ axion in the CMB. 12 2020.</p>
<p> </p>
<p>M. Giannotti, I. G. Irastorza, J. Redondo, A. Ringwald, and K. Saikawa. Stellar Recipes for Axion Hunters. JCAP, 10:010, <span style="font-size: 13px;">2017. doi: 10.1088/1475-7516/2017/10/010.</span></p>
<p> </p>
<p>J. Isern, E. Garcia-Berro, S. Torres, R. Cojocaru, and S. Catalan. Axions and the luminosity function of white dwarfs: the <span style="font-size: 13px;">thin and thick discs, and the halo. Mon. Not. Roy. Astron. Soc., 478(2):2569–2575, 2018. doi: 10.1093/mnras/sty1162.</span></p>
<p> </p>
<p>R. D. Peccei and H. R. Quinn. CP Conservation in the Presence of Instantons. Phys. Rev. Lett., 38:1440–1443, 1977. doi: <span style="font-size: 13px;">10.1103/PhysRevLett.38.1440.</span></p>
<p> </p>
<p>N. Viaux, M. Catelan, P. B. Stetson, G. Raelt, J. Redondo, A. A. R. Valcarce, and A. Weiss. Neutrino and axion bounds <span style="font-size: 13px;">from the globular cluster M5 (NGC 5904). Phys. Rev. Lett., 111:231301, 2013. doi: 10.1103/PhysRevLett.111.231301.</span></p>