INT Program INT-20-1a

Neutrinos from the Lab to the Cosmos

January 13 - February 7, 2020

Overview

We are entering an era where we are expanding our knowledge of the properties of neutrinos. Laboratory oscillation experiments are solidifying the basic properties of neutrinos. Astrophysical observations of neutrinos-whether of ultra high energy cosmic neutrinos or of supernovae-provide a new window to their properties and their origin. And cosmological observations-in conjunction with laboratory constraints-provide a more complete understanding on the total mass scale of the neutrinos. All of these efforts likely shall cement an emerging "standard" picture of neutrino physics. This program will take timely advantage of this culmination of knowledge, harnessing new results so as to provide a more complete theoretical understanding of the neutrino framework.

Such consolidation is coming from assembly of information from a number of sectors. Atmospheric oscillation observations as well as long baseline are narrowing the maximal-or near maximal-mixing θ₂₃ prevalent between the μ and t flavor neutrinos, while reactor and solar neutrino observations are precisely determining the solar mixing angle, θ₁₂, with reactors now well-determining θ₁₃ and have some indications of the value of δCP. On the other hand, the persistence of short baseline and reactor anomalies may lead to the emergence of a new paradigm with more than the standard three neutrino picture. In addition, the tritium endpoint experiment KATRIN is now taking data, with future kinematic experiments like Project-8 and ECHO are maturing to a competitive level. As oscillation experiments potentially will determine the ordering of the neutrino mass states, the implications for the reach of neutrinoless double beta decay experiments are significant.

Astrophysical high energy neutrino observations of extragalactic origin are entering the high-flux regime, with spatial and flavor structure of the observations. The IceCube experiment is determining the astrophysical flux of neutrinos at the 10 TeV to greater than 1000 TeV level. At the same time, new large water experiments such as ANTARES experiment are beginning to come online, providing a wider lens for astrophysical neutrino sources.

At lower energy, searches for supernova neutrinos continue, and will soon enter a new phase with the coming online of the largest liquid scintillator detector so far, JUNO, and of the upgraded Super-Kamiokande detector enhanced with Gadolinium. In parallel with observational searches, the role of neutrinos in core collapse supernovae is being clarified, with the advent of robust multi-dimensional numerical simulations of core collapse and subsequent protoneutron star cooling. Specifically, new numerical results will advance our understanding of the contribution of neutrinos to the explosion energetics and to nuclear processes in the interior of the collapsed star.

Cosmological observations have entered the high-precision regime and may simultaneously inform us of the neutrino ordering hierarchy. However, anomalies remain. The amplitude of fluctuations on small scales are lower than expected among several observations-the so-called σ8 problem. This may indicate massive neutrinos at the scale of a sum of masses ∑m ~ 0.3 eV. In addition, there is a persistent discrepancy between local and large scale measures of the Hubble expansion rate-the so-called H0 problem. This could indicate more effective relativistic degrees of freedom, or effective numbers of neutrinos.

The goals of the program is to advance the intellectual debate on a number of fundamental questions in neutrino physics: