Abstract
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.
Highlights
While oscillation experiments have conclusively established that neutrinos have small but nonvanishing masses, the dynamics that generates these masses remains a mystery
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 gramyears of data
We find that for Majoron masses in the eV range, direct detection experiments based on neutrino capture on tritium (e.g., PTOLEMY) will be sensitive to this scenario with 100 gramyears of data even if the standard model (SM) neutrinos are lighter than 0.1 eV
Summary
While oscillation experiments have conclusively established that neutrinos have small but nonvanishing masses, the dynamics that generates these masses remains a mystery. Mν represents the neutrino mass, while f denotes the Majoron decay constant, which corresponds to the scale at which the global lepton number symmetry is broken. The global symmetry need not be exact but only approximate, in which case the Majoron will acquire a mass. It follows from the couplings of the Majoron to neutrinos, Eqs. Precision measurements of the cosmic microwave background (CMB) place severe limits on any such contribution, with the result that the number density of neutrinos from Majoron decays is always expected to be smaller than the CνB. Current cosmological observations allow roughly 5% of the total energy density in dark matter to have decayed to dark radiation before today [21,22,23].1 if neutrinos are Majorana, the number density of neutrinos (nJν) and antineutrinos (nJν ) from Majoron decay is given by
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