Flavors of Astrophysical Neutrinos with Active-Sterile Mixing

Abstract

We revisit the flavor composition of high-energy astrophysical neutrinos observed at neutrino telescopes. Assuming unitary time evolution of the neutrino flavor states, the flavor composition observable at Earth is related to the initial composition at their sources via oscillation-averaged flavor transitions. In a previous study we derived general bounds on the flavor composition of TeV-PeV astrophysical neutrinos assuming three-flavor unitary mixing. We extend these bounds to the case of active-sterile neutrino mixing. Our bounds are analytical, derived based only on the unitarity of the mixing, and do not require sampling over the values of the unknown active-sterile mixing parameters. These bounds apply to any extended active-sterile neutrino mixing scenario where energy-dependent nonstandard flavor mixing dominates over the standard mixing observed in accelerator, reactor, and atmospheric neutrino oscillations.

The future of high-energy astrophysical neutrino flavor measurements

“…To show this, we make detailed, realistic projections of how the uncertainty in the predicted flavor composition at Earth of the isotropic flux of high-energy neutrinos and its measurement will evolve over the next two decades. Our main finding is that, by 2040, we will be able to precisely infer the flavor composition at the sources, including possibly identifying the contribution of multiple neutrino-production mechanisms, even if oscillations are non-unitary [49,60,63,[102][103][104][105][106]. Further, we illustrate the upcoming power of flavor measurements to probe BSM neutrino physics using neutrino decay [35,45,47,52,55,[107][108][109][110][111][112].…”

“…In this case, U is a 3 × 3 submatrix of the larger, true mixing matrix. Relaxing the assumption of the unitarity of U leads to a broader range of allowed flavor composition at Earth due to the active neutrinos mixing with the new states [49,63,[102][103][104][105][106][166][167][168]. This is true even if the new states are too massive to be kinematically accessible.…”

The Future of High-Energy Astrophysical Neutrino Flavor Measurements

“…In this case, U is a 3 × 3 submatrix of the larger, true mixing matrix. Relaxing the assumption of the unitarity of U leads to a broader range of allowed flavor composition at Earth due to the active neutrinos mixing with the new states [49,63,[102][103][104][105][106][165][166][167]. This is true even if the new states are too massive to be kinematically accessible.…”

Oscillations of sterile neutrinos from dark matter decay eliminates the IceCube-Fermi tension

“…where U αa is an element of the mixing matrix U that connects the flavor and propagation states [38]. For the 3+1 scenario under consideration U is a 4 × 4 unitary mixing with 16 degrees of freedom: 6 mixing angles and 10 phases [39][40][41][42]. Unitarity ensures conservation of the total number of neutrinos of all flavors.…”

“…after all terms depending on mass squared differences are averaged out over cosmological distances. Assuming a general unitary mixing in the 3+1 flavor scenario such that the mixing parameters relevant for high-energy neutrinos are unbounded [42], Monte Carlo samples show that for a source of 100% sterile neutrinos there is a maximum of 75% transformed into active flavors after oscillations, i.e., one is left with a minimum of 25% sterile neutrinos at Earth [44]. The effective fraction of active neutrinos on Earth resulting from oscillations of sterile neutrinos will be taken as a free parameter of the model, but constrained to be less than 75%.…”