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Register a new userDetecting a Secondary Cosmic Neutrino Background from Majoron Decays in Neutrino Capture Experiments
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We consider theories in which the generation of neutrino masses is associated with the breaking of an approximate global lepton number symmetry. In such a scenario the spectrum of light states includes the Majoron, the pseudo-Nambu Goldstone boson associated with the breaking of the global symmetry. For a broad range of parameters, the Majoron decays to neutrinos at late times, after the cosmic neutrinos have decoupled from the thermal bath, resulting in a secondary contribution to the cosmic neutrino background. We determine the current bounds on this scenario, and explore the possibility of directly detecting this secondary cosmic neutrino background in experiments based on neutrino capture on nuclei. For Majoron masses in the eV range or below, the neutrino flux from these decays can be comparable to that from the primary cosmic neutrino background, making it a promising target for direct detection experiments. The neutrinos from Majoron decay are redshifted by the cosmic expansion, and exhibit a characteristic energy spectrum that depends on both the Majoron mass and its lifetime. For Majoron lifetimes of order the age of the universe or larger, there is also a monochromatic contribution to the neutrino flux from Majoron decays in the Milky Way that can be comparable to the diffuse extragalactic flux. We find that for Majoron masses in the eV range, direct detection experiments based on neutrino capture on tritium, such as PTOLEMY, will be sensitive to this scenario with 100 gram-years of data. In the event of a signal, the galactic and extragalactic components can be distinguished on the basis of their distinct energy distributions, and also by using directional information obtained by polarizing the target nuclei.
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We forecast constraints on neutrino decay via capture of the Cosmic Neutrino Background on tritium, with emphasis on the PTOLEMY-type experiment. In particular, in the case of invisible neutrino decay into lighter neutrinos in the Standard Model and invisible particles, we can constrain not only the neutrino lifetime but also the masses of the invisible particles. For this purpose, we also formulate the energy spectra of the lighter neutrinos produced by 2-body and 3-body decays, and those of the electrons emitted in the process of the detection of the lighter neutrinos.
We propose to search for light $U(1)$ dark photons, $A$, produced via kinetically mixing with ordinary photons via the Compton-like process, $gamma e^- rightarrow A e^-$, in a nuclear reactor and detected by their interactions with the material in the active volumes of reactor neutrino experiments. We derive 95% confidence-level upper limits on $epsilon$, the $A$-$gamma$ mixing parameter, $epsilon$, for dark-photon masses below 1$sim$MeV of $epsilon~< ~1.3times 10^{-5}$ and $epsilon~<~2.1times 10^{-5}$, from NEOS and TEXONO experimental data, respectively. This study demonstrates the applicability of nuclear reactors as potential sources of intense fluxes of low-mass dark photons.
Oscillations in the electron-capture (EC) decay rate observed in storage-ring experiments are reconsidered in connection with the neutrino mass difference. Taking into account that - according to Relativity Theory - time is slowed down in the reference frame of the orbiting charged particles as compared to the neutral particles (neutrinos) moving on a rectilinear path after the EC decay, we derive a value of $Delta m^{2}_{21}=(0.768pm0.012)cdot10^{-4} eV^{2}$ for the neutrino mass-squared difference which fully agrees with that observed in other neutrino-oscillation experiments. To further check the connection between EC-decay oscillations and $Delta m^{2}_{21}$ we suggest experiments with different orbital speeds, i.e., different values of the Lorentz factor.
We study the possibility to use line-intensity mapping (LIM) to seek photons from the radiative decay of neutrinos in the cosmic neutrino background. The Standard Model prediction for the rate for these decays is extremely small, but it can be enhanced if new physics increases the neutrino electromagnetic moments. The decay photons will appear as an interloper of astrophysical spectral lines. We propose that the neutrino-decay line can be identified with anisotropies in LIM clustering and also with the voxel intensity distribution. Ongoing and future LIM experiments will have -- depending on the neutrino hierarchy, transition and experiment considered -- a sensitivity to an effective electromagnetic transition moment $sim 10^{-12}, -,10^{-8}, (m_ic^2/{0.1 rm eV})^{3/2}mu_{rm B}$, where $m_i$ is the mass of the decaying neutrino and $mu_{rm B}$ is the Bohr magneton. This will be significantly more sensitive than cosmic microwave background spectral distortions, and it will be competitive with stellar cooling studies. As a byproduct, we also report an analytic form of the one-point probability distribution function for neutrino-density fluctuations, obtained from the Quijote simulations using symbolic regression.
The recent tension between local and early measurements of the Hubble constant can be explained in a particle physics context. A mechanism is presented where this tension is alleviated due to the presence of a Majoron, arising from the spontaneous breaking of Lepton Number. The lightness of the active neutrinos is consistently explained. Moreover, this mechanism is shown to be embeddable in the Minimal (Lepton) Flavour Violating context, providing a correct description of fermion masses and mixings, and protecting the flavour sector from large deviations from the Standard Model predictions. A QCD axion is also present to solve the Strong CP problem. The Lepton Number and the Peccei-Quinn symmetries naturally arise in the Minimal (Lepton) Flavour Violating setup and their spontaneous breaking is due to the presence of two extra scalar singlets. The Majoron phenomenology is also studied in detail. Decays of the heavy neutrinos and the invisible Higgs decay provide the strongest constraints in the model parameter space.
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