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Register a new userSterile neutrinos in the Milky Way: Observational constraints
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We consider the possibility of constraining decaying dark matter by looking out through the Milky Way halo. Specifically we use Chandra blank sky observations to constrain the parameter space of sterile neutrinos. We find that a broad band in parameter space is still open, leaving the sterile neutrino as an excellent dark matter candidate.
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The standard model of cosmology predicts the existence of cosmic neutrino background in the present Universe. To detect cosmic relic neutrinos in the vicinity of the Earth, it is necessary to evaluate the gravitational clustering effects on relic neutrinos in the Milky Way. Here we introduce a reweighting technique in the N-one-body simulation method, so that a single simulation can yield neutrino density profiles for different neutrino masses and phase space distributions. In light of current experimental results that favor small neutrino masses, the neutrino number density contrast around the Earth is found to be almost proportional to the square of neutrino mass. The density contrast-mass relation and the reweighting technique are useful for studying the phenomenology associated with the future detection of the cosmic neutrino background.
Secret contact interactions among eV sterile neutrinos, mediated by a massive gauge boson $X$ (with $M_X ll M_W$), and characterized by a gauge coupling $g_X$, have been proposed as a mean to reconcile cosmological observations and short-baseline laboratory anomalies. We constrain this scenario using the latest Planck data on Cosmic Microwave Background anisotropies, and measurements of baryon acoustic oscillations (BAO). We consistently include the effect of secret interactions on cosmological perturbations, namely the increased density and pressure fluctuations in the neutrino fluid, and still find a severe tension between the secret interaction framework and cosmology. In fact, taking into account neutrino scattering via secret interactions, we derive our own mass bound on sterile neutrinos and find (at 95% CL) $m_s < 0.82$ eV or $m_s < 0.29$ eV from Planck alone or in combination with BAO, respectively. These limits confirm the discrepancy with the laboratory anomalies. Moreover, we constrain, in the limit of contact interaction, the effective strength $G_X$ to be $ < 2.8 (2.0) times 10^{10},G_F$ from Planck (Planck+BAO). This result, together with the mass bound, strongly disfavours the region with $M_X sim 0.1$ MeV and relatively large coupling $g_Xsim 10^{-1}$, previously indicated as a possible solution to the small scale dark matter problem.
Neutrino physics is nowadays receiving more and more attention as a possible source of information for the long-standing problem of new physics beyond the Standard Model. The recent measurement of the mixing angle $theta_{13}$ in the standard mixing oscillation scenario encourages us to pursue the still missing results on leptonic CP violation and absolute neutrino masses. However, puzzling measurements exist that deserve an exhaustive evaluation. The NESSiE Collaboration has been setup to undertake conclusive experiments to clarify the muon-neutrino disappearance measurements at small $L/E$, which will be able to put severe constraints to models with more than the three-standard neutrinos, or even to robustly measure the presence of a new kind of neutrino oscillation for the first time. To this aim the use of the current FNAL-Booster neutrino beam for a Short-Baseline experiment has been carefully evaluated. Its recent proposal refers to the use of magnetic spectrometers at two different sites, Near and Far ones. Their positions have been extensively studied, together with the possible performances of two OPERA-like spectrometers. The proposal is constrained by availability of existing hardware and a time-schedule compatible with the undergoing project of a multi-site Liquid-Argon detectors at FNAL. The experiment to be possibly setup at Booster will allow to definitively clarify the current $ u_{mu}$ disappearance tension with $ u_{e}$ appearance and disappearance at the eV mass scale.
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.
Nuclear reactors are strong, pure and well localized sources of electron antineutrinos with energies in the few MeV range. Therefore they provide a suitable environment to study neutrino properties, in particular neutrino oscillation parameters. Recent predictions of the expected antineutrino flux at nuclear reactors are about 6% higher than the average rate measured in different experiments. This discrepancy, known as the reactor antineutrino anomaly, is significant at the 2.5{sigma} level. Several new experiments are searching for the origin of this observed neutrino deficit. One hypothesis to be tested is an oscillation to another neutrino state. In a three flavor model reactor neutrinos do not oscillate at baselines below 100 m. Hence, if such an oscillation is observed, it would imply the existence of at least one light sterile neutrino state not participating in weak interactions. Such a discovery would open the gate for new physics beyond the Standard Model.
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