Search

If the Dirac neutrino masses are generated by a new scalar doublet with a vacuum at keV order, there would be rare hope to probe the framework in low-energy flavor physics, such as the lepton-flavor violating processes. However, the predicted neutrino Yukawa couplings being around the order of electronic Yukawa can realize a purely thermal Dirac leptogenesis, and thus render a more direct link between the high- and the low-energy CP violation. Despite its inert property in the low-energy flavor processes, the keV-vacuum induced Dirac neutrino model can generate a significant shift of the effective neutrino number, which can in turn be probed by the big-bang nucleosynthesis and cosmic microwave background epochs. We show that such a keV-vacuum induced Dirac neutrino model is already constrained by the current observations and will be probed with forecast sensitivity, serving therefore as a complementary avenue to test the Dirac leptogenesis., Comment: 8 pages, and 3 figures

We update the Standard Model (SM) predictions for the branching ratios of the rare top-quark decays $t \to cg(g)$, and evaluate the maximum rates that can be reached in the aligned as well as in the four conventional two-Higgs-doublet models (2HDMs) with $\mathcal{Z}_2$ symmetries. Taking into account the relevant constraints on the model parameters resulting from a global fit obtained at the $95.5\%$ confidence level, we find that the branching ratios of $t \to cg$ and $t \to cgg$ decays can reach up to $3.36\times 10^{-9}$ and $2.95\times 10^{-9}$ respectively, being therefore of the same order, in the aligned 2HDM (A2HDM). This is obviously different from the SM case, where the predicted branching ratio of the three-body decay $t \to cgg$ is about two orders of magnitude larger than that of the two-body decay $t \to cg$. On the other hand, compared with the SM predictions, no significant enhancements are observed in the four conventional 2HDMs with $\mathcal{Z}_2$ symmetries for the branching ratios of these two decays. Nevertheless, the predicted branching ratios of $t \to cg$ and $t \to cgg$ decays in the A2HDM are still out of the expected sensitivities of the future high-luminosity Large Hadron Collider and the Future Circular Collider in hadron-hadron mode., Comment: 48 pages, 6 figures, and 4 tables; more references and discussions added, final version published in the journal

Motivated by the recent anomalies in $b \to s \ell^+ \ell^-$ transitions, we explore a minimal $Z^\prime$ scenario, in which the $Z^\prime$ boson has a flavour-changing coupling to charm and top quarks and a flavour-conserving coupling to muons. It is found that such a $Z^\prime$ boson can explain the current $b \to s \ell^+ \ell^-$ anomalies, while satisfying other flavour and collider constraints simultaneously. The $Z^\prime$ boson can be as light as few hundreds GeV. In this case, the $t \to c \mu^+ \mu^-$ decay and the $tZ^\prime$ associated production at the LHC could provide sensitive probes of such a $Z^\prime$ boson. As a special feature, the $Z^\prime$ contributions to all rare $B$- and $K$-meson processes are controlled by one parameter. This results in interesting correlations among these processes, which could provide further insights into this scenario. In addition, an extended scenario, in which the $Z^\prime$ boson interacts with the $SU(2)_L$ fermion doublets with analogous couplings as in the minimal scenario, is also investigated., Comment: 40 pages, 13 figures, typos fixed, comments and references added

The DArk Matter Particle Explorer (DAMPE) is well suitable for searching for monochromatic and sharp $\gamma$-ray structures in the GeV$-$TeV range thanks to its unprecedented high energy resolution. In this work, we search for $\gamma$-ray line structures using five years of DAMPE data. To improve the sensitivity, we develop two types of dedicated data sets (including the BgoOnly data which is the first time to be used in the data analysis for the calorimeter-based gamma-ray observatories) and adopt the signal-to-noise ratio optimized regions of interest (ROIs) for different DM density profiles. No line signals or candidates are found between 10 and 300 GeV in the Galaxy. The constraints on the velocity-averaged cross section for $\chi\chi \to \gamma\gamma$ and the decay lifetime for $\chi \to \gamma\nu$, both at 95% confidence level, have been calculated and the systematic uncertainties have been taken into account. Comparing to the previous Fermi-LAT results, though DAMPE has an acceptance smaller by a factor of $\sim 10$, similar constraints on the DM parameters are achieved and below 100 GeV the lower limits on the decay lifetime are even stronger by a factor of a few. Our results demonstrate the potential of high-energy-resolution observations on dark matter detection., Comment: 14 pages, 8 figures. Update the content to keep up with the published version