Improved Hot Dark Matter Bound on the QCD Axion.

We obtain a reliable cosmological bound on the axion mass m_{a} by (1) deriving the production rate directly from pion-pion scattering data, which overcomes the breakdown of chiral perturbation theory and results in ∼30% differences from previous estimates; (2) including momentum dependence in the Boltzmann equations for axion-pion scatterings, which enhances the relic abundance by ∼40%. Using present cosmological datasets we obtain m_{a}≤0.24  eV, at 95% C.

High Quality QCD Axion at Gravitational Wave Observatories.

The axion solution to the strong CP problem is delicately sensitive to Peccei-Quinn breaking contributions that are misaligned with respect to QCD instantons. Heavy QCD axion models are appealing because they avoid this so-called quality problem. We show that generic realizations of this framework can be probed by the LIGO-Virgo-KAGRA interferometers, through the stochastic gravitational wave (GW) signal sourced by the long-lived axionic string-domain wall network and by upcoming measurements of the neutron and proton electric dipole moments.

COVID-19 transmission risk factors.

We analyze risk factors correlated with the initial transmission growth rate of the recent COVID-19 pandemic in different countries. The number of cases follows in its early stages an almost exponential expansion; we chose as a starting point in each country the first day with 30 cases and we fitted for 12 days, capturing thus the early exponential growth. We looked then for linear correlations of the exponents with other variables, for a sample of 126 countries.

Temperature dependence of COVID-19 transmission.

The recent COVID-19 pandemic follows in its early stages an almost exponential expansion, with the number of cases as a function of time reasonably well fit by N(t) ∝ e, in many countries. We analyze the rate α in different countries, starting in each country from a threshold of 30 total cases and fitting for the following 12 days, capturing thus the early exponential growth in a rather homogeneous way. We look for a link between the rate α and the average temperature T of each country, in the month of the initial epidemic growth.

Observable Windows for the QCD Axion Through the Number of Relativistic Species.

We show that when the QCD axion is directly coupled to quarks with c_{i}/f∂_{μ}aq[over ¯]_{i}γ^{μ}γ^{5}q_{i}, such as in Dine-Fischler-Srednicki-Zhitnitsky models, the dominant production mechanism in the early Universe at temperatures 1  GeV≲T≲100  GeV is obtained via q_{i}q[over ¯]_{i}↔ga and q_{i}g↔q_{i}a, where g are gluons. The production of axions through such processes is maximal around T≈m_{i}, where m_{i} are the different heavy quark masses. This leads to a relic axion background that decouples at such temperatures, leaving a contribution to the effective number of relativistic degrees of freedom, which can be larger than the case of decoupling happens the electroweak phase transition, ΔN_{eff}≲0.

Standard model false vacuum inflation: correlating the tensor-to-scalar ratio to the top quark and Higgs boson masses.

For a narrow band of values of the top quark and Higgs boson masses, the standard model Higgs potential develops a false minimum at energies of about 10(16)  GeV, where primordial inflation could have started in a cold metastable state. A graceful exit to a radiation-dominated era is provided, e.g.