Particle and nuclear physicists face a real dilema. Our "Standard Model" explains most of what we observe at accelerator and non-accelerator experiments, IceCube included. The Standard Model has been around for about 40 years. It's three generations of quarks and leptons, four forces, and the Higgs boson come together to provide a good description of the processes we observe at the Large Hadron Collider (LHC) and other accelerators, like Brookhaven's Relativistic Heavy Ion Collider, not to mention underground neutrino detectors. The only clear crack in the standard model is the fact that neutrinos oscillate between the different flavors, and therefore should have mass. But, most of us don't feel like this is a huge crack.
So, we have been looking for holes in the Standard Model for the past 40 years. With the discovery of the Higgs boson in the bag, this is now the main rationale for the LHC. Each year the four LHC experiments put out hundreds of new results; the search for "New" (beyond the standard model) physics is a major focus. Unfortunately, they have not found any clear evidence for any new physics.
There are some good reasons we know that there must be physics beyond the Standard Model. The evidence for both dark matter and dark energy is clear and convincing. Many theories of dark matter model it as a new particle that could very well be discovered at the LHC. Dark energy is even more mysterious. It is beyond the reach of any as-yet proposed laboratory scale experiments, but it is a cler reminder that the universe still has some deep secrets.
Although it is not our primary focus, IceCube is also searching for new physics, mostly that involving neutrinos. As part of this search, we continue to study neutrino oscillations (see my previous post here) in more detail. One of the things that we are looking for is a new type of neutrinos, which do not interact; these are called 'sterile neutrinos.' If regular neutrinos oscillated into sterile neutrinos, it would look just like these neutrinos disappeared. We search for sterile neutrinos by looking at how likely neutrinos produced in cosmic-ray air showers are to appear in IceCube. If sterile neutrinos exist, neutrinos traveling long distances through the Earth might disappear. In a recent study, published in Physical Review Letters (here, and also available here through the Cornell arXiv), we set strict limits on sterile neutrinos. We pub limits on the possible existence of sterile neutrinos with certain characteristics; the main characteristics are the mass difference between sterile neutrinos and regular neutrinos, and the mixing angle (strength of coupling) between sterile and regular neutrinos.
The IceCube limits are of particular interest because they rule out a region of parameter space (mass difference and mixing angle) that had been suggested by a couple of earlier experiments. These earlier results had attracted great attention, but we now know that they are unlikely to be correct.
So, we need to keep looking to find a different crack in the Standard Model, possibly including sterile neutrinos with different masses and couplings.