A few years ago, Sanjib Agarwalla and I published a paper on looking for new, flavor dependent, long-range neutrino-electron interactions that could affect neutrino oscillations. The new interactions are introduced by gauging lepton-number symmetries Le-Lmu and Le-Ltau. They are mediated by new neutral vector bosons, Z’. Because we considered them to be ultra-light, with masses below 10^{-10} eV, the interaction range is ultra-long. This allowed us to use the large collections of electrons in the local and distant Universe — the Earth, Moon, Sun, Milky Way, and cosmological electrons — as sources of the new matter potential, and the measurement of the flavor composition of high-energy astrophysical neutrinos as IceCube as the observable:

Universe’s Worth of Electrons to Probe Long-Range Interactions of High-Energy Astrophysical Neutrinos
Mauricio Bustamante, Sanjib Kumar Agarwalla
Phys. Rev. Lett. 6, 061103 (2019) [1808.02042]

This week we uploaded to the arXiv two follow-up works, in collaboration with Sanjib’s students Masoom and Sudipta and postdoc Ashish:

Present and future constraints on flavor-dependent long-range interactions of high-energy astrophysical neutrinos
Sanjib Kumar Agarwalla, Mauricio Bustamante, Sudipta Das, Ashish Narang
arXiv:2305.03675

Flavor-dependent long-range neutrino interactions in DUNE & T2HK: alone they constrain, together they discover
Masoom Singh, Mauricio Bustamante, Sanjib Kumar Agarwalla
arXiv:2305.05184

In the first paper, we revisit, revamp, and extend the analysis that we did in 2018, using the flavor composition of high-energy astrophysical neutrinos. There two main improvements. First, we now use a Bayesian statistical analysis, which allows us to claim constraints at higher significance than in the original paper. It also allows us to consistently treat the uncertainties in neutrino mixing parameters and in the measurement of the flavor composition. Second, we now consider also new neutrino-neutron interactions, introduced by gauging Lmu-Ltau. The constraints that we get on the coupling strength are these:

From the abstract: We find that, already today, the IceCube neutrino telescope demonstrates potential to constrain flavor-dependent long-range interactions significantly better than existing constraints, motivating further analysis. 

In the second paper, we study the same problem, but now using next-generation long-baseline neutrino experiments DUNE and T2HK. The neutrino energies are much lower, MeV-GeV compared to the TeV-PeV of astrophysical neutrinos. However, the event rates are enormous, and that makes DUNE and T2HK sensitive to even subtle modifications of the oscillation probabilities. In this case, the constraints are these:

From the abstract: Alone, DUNE and T2HK may strongly constrain long-range interactions, setting new limits on their coupling strength for mediators lighter than 10^{−18} eV. However, if the new interactions are subdominant, then both DUNE and T2HK, together, will be needed to discover them, since their combination lifts parameter degeneracies that weaken their individual sensitivity.