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Register a new userImproved Constraints on Sterile Neutrinos in the MeV to GeV Mass Range
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Improved upper bounds are presented on the coupling $|U_{e4}|^2$ of an electron to a sterile neutrino $ u_4$ from analyses of data on nuclear and particle decays, including superallowed nuclear beta decays, the ratios $R^{(pi)}_{e/mu}=BR(pi^+ to e^+ u_e)/BR(pi^+ to mu^+ u_mu)$, $R^{(K)}_{e/mu}$, $R^{(D_s)}_{e/tau}$, and $B^+_{e 2}$ decay, covering the mass range from MeV to GeV.
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A detailed discussion is given of the analysis of recent data to obtain improved upper bounds on the couplings $|U_{e4}|^2$ and $|U_{mu 4}|^2$ for a mainly sterile neutrino mass eigenstate $ u_4$. Using the excellent agreement among ${cal F}t$ values for superallowed nuclear beta decay, an improved upper limit is derived for emission of a $ u_4$. The agreement of the ratios of branching ratios $R^{(pi)}_{e/mu}=BR(pi^+ to e^+ u_e)/BR(pi^+ to mu^+ u_mu)$, $R^{(K)}_{e/mu}$, $R^{(D_s)}_{e/tau}$, $R^{(D_s)}_{mu/tau}$, and $R^{(D)}_{e/tau}$, and the branching ratios $BR(B^+rightarrow e^+ u_e)$ and $BR(B^+rightarrow mu^+ u_mu)$ decays with predictions of the Standard Model, is utilized to derive new constraints on $ u_4$ emission covering the $ u_4$ mass range from MeV to GeV. We also discuss constraints from peak search experiments probing for emission of a $ u_4$ via lepton mixing, as well as constraints from pion beta decay, CKM unitarity, $mu$ decay, leptonic $tau$ decay, and other experimental inputs.
The transition magnetic moment of a sterile-to-active neutrino conversion gives rise to not only radiative decay of a sterile neutrino, but also its non-standard interaction (NSI) with matter. For sterile neutrinos of keV-mass as dark matter candidates, their decay signals are actively searched for in cosmic X-ray spectra. In this work, we consider the NSI that leads to atomic ionization, which can be detected by direct dark matter experiments. It is found that this inelastic scattering process for a nonrelativistic sterile neutrino has a pronounced enhancement in the differential cross section at energy transfer about half of its mass, manifesting experimentally as peaks in the measurable energy spectra. The enhancement effects gradually smear out as the sterile neutrino becomes relativistic. Using data taken with germanium detectors that have fine energy resolution in keV and sub-keV regimes, constraints on sterile neutrino mass and its transition magnetic moment are derived and compared with those from astrophysical observations.
The existence of light sterile neutrinos, as predicted in several models, can help to explain a number of observations starting from dark mater to recent anomalies in short baseline experiments. In this paper we consider two models - Left-Right Symmetric Zee model and Extended Seesaw model, that can naturally accommodate the presence of light sterile neutrinos in the eV to MeV mass scale. We perform a detailed study on the neutrinoless double beta decay process which receives major contributions from diagrams involving these light sterile neutrinos. Considering a number of theoretical and experimental constraints, including light neutrino masses and mixings, unitarity of the mixing matrix etc., we compare our predicted values of the half-life of neutrinoless double beta decay with the experimental limits. This can put significant constraints on the neutrino mass, active-sterile neutrino mixing and several other important parameters in these models.
The QCD axion is one of the most compelling solutions of the strong CP problem. There are major current efforts into searching for an ultralight, invisible axion, which is believed to be the only phenomenologically viable realization of the QCD axion. Visible axions with decay constants at or below the electroweak scale are believed to have been long excluded by laboratory searches. Considering the significance of the axion solution of the strong CP problem, we revisit experimental constraints on QCD axions in the O(10 MeV) mass window. In particular, we find a variant axion model that remains compatible with existing constraints. This model predicts new states at the GeV scale coupled hadronically, and a variety of low-energy axion signatures, such as rare meson decays, nuclear de-excitations via axion emission, production in $e^+e^-$ annihilation and fixed target experiments. This reopens the possibility of solving the strong CP problem at the GeV scale.
The vector $U$-bosons, or so called dark photons, are one of the possible candidates for the dark matter mediators. They are supposed to interact with the standard matter via a vector portal due to the $U(1)-U(1)^prime$ symmetry group mixing which might make them visible in particle and heavy-ion experiments. While there is no confirmed observation of dark photons, the detailed analysis of different experimental data allows to estimate the upper limit for the kinetic mixing parameter $epsilon^2$ depending on the mass $M_U$ of $U$-bosons which is also unknown. In this study we present theoretical constraints on the upper limit of $epsilon^2(M_U)$ in the mass range $M_U le 0.6$ GeV from the comparison of the calculated dilepton spectra with the experimental data from the HADES Collaboration at SIS18 energies where the dark photons are not observed. Our analysis is based on the microscopic Parton-Hadron-String Dynamics (PHSD) transport approach which reproduces well the measured dilepton spectra in $p+p$, $p+A$ and $A+A$ collisions. Additionally to the different dilepton channels originating from interactions and decays of ordinary matter particles (mesons and baryons), we incorporate the decay of hypothetical $U$-bosons to dileptons, $Uto e^+e^-$, where the $U$-bosons themselves are produced by the Dalitz decay of pions $pi^0to gamma U$, $eta$-mesons $eta to gamma U$ and Delta resonances $Delta to N U$. Our analysis can help to estimate the requested accuracy for future experimental searches of light dark photons by dilepton experiments.
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