Primordial high energy neutrinos: theoretical/observational constraints and sharp spectral features

TL;DR

PHENU investigates whether primordial relics decaying or annihilating in the early Universe could produce high-energy neutrinos observable today as sharp spectral features. It classifies five spectral topologies (neutrino lines, 3-body decays, box-shaped spectra, out-of-equilibrium annihilation) and models their propagation including redshift and radiative corrections, then derives regions in $(m_P,\tau_P,f_P)$ constrained by $\Delta N_{\text{eff}}$, BBN, and CMB alongside current isotropic neutrino flux limits to identify observable, undistorted regions. The work provides a reconstruction framework to infer source parameters from an observed flux and discusses democratic production scenarios, revealing that PHENU could probe physics up to $m_P \sim 10^{6}$–$10^{11}$ GeV with lifetimes $\sim 10^{9}$–$10^{13}$ s, though in-flight scatterings can erase sharp features in parts of parameter space. Overall, the study highlights a novel channel to test early-Universe physics and high-scale particle interactions with existing and upcoming neutrino detectors, while outlining avenues for more precise scattering analyses and dedicated experimental searches.

Abstract

Among the few ways that allow or could allow us to probe the early Universe from the observation of a flux of primordial particles, there is one possibility which has been little studied: the observation today of high energy neutrinos which could have been emitted shortly after the Big Bang, from the decay or annihilation of early universe relics. We perform a general study of such a possibility. To this end, we first emphasise that these neutrinos could display various kinds of sharp spectral features, resulting from the primary energy spectrum at emission, and from how this spectrum is smoothed by redshift and radiative correction effects. Next, we determine the ranges of mass (from a fraction of eV all the way to the Planck scale) and lifetime of the source particles along which we do not/we do expect that the sharp spectral feature will be altered by interactions of the neutrinos on their way to the detector, mainly with the cosmic neutrino background or between themselves. We also study the theoretical (i.e. mainly BBN and CMB) and observational constraints which hold on such a possibility. This allows us to delineate the regions of parameter space (mass, lifetime and abundance) that are already excluded, hopeless for future observation or, instead, which could lead to the observation of such neutrinos in the near future.

Figures (16)

• Figure 1: Neutrino flux as a function of the observed energy today for the 4 types of sharp spectral features considered. The upper panels give it for a two-body decay and a 3-body decay of a particle of mass $m_P = 10^{14}$ MeV and lifetime $\tau_P = 1$ s. The lower panels give it for an example of box-shaped spectrum and for an out-of-equilibrium annihilation. Also given (in dashed lines) are the original spectra at source, rescaled in energy by an arbitrary factor (which we take so that the energy spectrum maxima coincide, explicitly showing the effect of redshift on the spectrum).
• Figure 2: Left: The energy of today's neutrino at the maximum of the flux in the $m_P$-$\tau_P$ plane. Right: Mean energy (solid) and energy at the maximum (dashed) of today's neutrino flux as a function of the source particle mass $m_P$ for various values of the lifetime $\tau_P$ for a 2-body decay.
• Figure 3: Same as figure \ref{['fig:meanmax_energy']}, for a 3-body decay.
• Figure 4: Energy spectrum with (solid) and without (dashed) final state radiation for a two-body decay of source particles with masses $m_P=1, 10, 100, 1000$ TeV.
• Figure 5: Minimum neutrino energy at injection $E_{\text{inj}}$ needed to create the specified SM particle pair at a given injection time $t_{\text{inj}}$ through scattering over a thermal neutrino (considering that the thermal neutrino has an energy equal to its average energy).