decay

The search for Lorentz-invariance violation (LIV) is one of our best paths toward uncovering the quantum nature of spacetime. One of its most famous potential manifestations is the modification of neutrino propagation speeds.

For years, the community has known that superluminal (faster-than-light) neutrinos are heavily constrained. LIV would cause them to rapidly lose energy by radiating electron-positron pairs in a vacuum. Because of this, when anomalous time delays between cosmic neutrinos and gamma rays are observed, phenomenologists have naturally favored subluminal (slower-than-light) LIV propagation as the most viable explanation.

In a new paper co-authored with José Manuel Carmona, José Luis Cortés, Ardit Gkioni, and Maykoll A. Reyes, we demonstrate that this apparent subluminal loophole is actually an illusion.

We show that the same LIV modifications that slow down neutrinos inevitably render high-energy electrons unstable. Under subluminal LIV, electrons undergo a catastrophic decay process: e⁻ → e⁻ + ν + anti-ν. This triggers rapid and severe energy degradation.

By demanding that electrons survive up to the extreme energies we observe in astrophysics—specifically, the 15.5-TeV candidates from H.E.S.S. and the 2.34-PeV electrons inferred by LHAASO in the Crab Nebula—we placed stringent new limits on subluminal LIV.

Our results firmly invalidate the subluminal parameter space previously invoked to explain years-long cosmic neutrino time delays. Consequently, any observable delays must either have purely astrophysical origins, rely on a universal LIV deformation across all particle species, or require physics well beyond the standard effective-field-theory framework.

Read more at:

Electron stability constrains neutrino time delays
Mauricio Bustamante, José Manuel Carmona, José Luis Cortés, Ardit Gkioni, Maykoll A. Reyes
2607.01339 hep-ph

In the Standard Model, neutrinos are effectively stable, their lifetimes orders of magnitude longer than the age of the Universe. In proposed extensions of the Standard Model, however, neutrinos might decay faster, and so observing them decay would constitute evidence of new neutrino physics.

Regardless, neutrino lifetimes, even augmented by new physics, are likely very long, and the effects of decay are likely manifest only in neutrinos that travel a long distance, during which the chances of them decaying becomes appreciable even if they are long-lived.

In a new paper, we have searched for signs of neutrino decay using the neutrinos from farthest away: the high-energy astrophysical neutrinos detected by IceCube, which travel cosmological-scale distances of Mpc-Gpc from their sources to Earth. Neutrino decay, in principle, alters the shape of the neutrino energy spectrum—introducing a step-like jump—and the flavor composition of the neutrinos upon reaching Earth—taking it outside the region expected from standard oscillations alone.

We find no signs of decay in present-day IceCube data, but place new, competitive lower limits on the lifetimes of the nu_2 and nu_3 neutrino mass eigenstates. We report, for the first time, limits inferred using the neutrinos from the first candidate steady-state astrophysical source of high-energy neutrinos, the active galaxy NGC 1068, and limits inferred from the diffuse flux of high-energy neutrinos.

These are arguably the most robust neutrino lifetime bounds garnered from high-energy astrophysical neutrinos so far! In addition, we make forecasts for the year 2035, combining multiple upcoming neutrino telescopes.

While similar studies have been performed before, ours brings two new perspectives, often overlooked or understudied, that make our results robust.

First, we consider broadly the large astrophysical uncertainties that plague the prediction of the flux of high-energy astrophysical neutrinos. This includes the size and shape of the neutrino energy spectrum, the flavor composition of the neutrino flux, the number or source populations and their distribution in redshift, and whether we have prior constraints on the size of the neutrino flux normalization. The impact of considering these uncertainties ranges from appreciable to critical. In some cases, they nearly make the sensitivity to neutrino decay vanish! Surprisingly, with present data, it is not possible to constrain neutrino decay using neutrinos from NGC 1068 due to the astrophysical unknowns!

Second, we model in detail the detection of neutrinos in IceCube and other neutrino telescopes. Their limited resolution to measure the energy, direction, and flavor of detected neutrinos blurs potential signs of neutrino decay in the flux of high-energy astrophysical neutrinos. We model experimental nuance using either tools provided publicly by the IceCube Collaboration (for the diffuse flux, using High Energy Starting Events), or using the PLEnuM (for the flux from NGC 1068, using tracks).

Mauricio Bustamante

Associate Professor – Niels Bohr Institute, University of Copenhagen

Electron stability constrains neutrino time delays

New limits on neutrino decay from high-energy astrophysical neutrinos