There is a limit to how big we can build particle colliders on Earth, whether that is because of limited space or limited economics. Since size is equivalent to energy output for particle colliders, that also means there’s a limit to how energetic we can make them. And again, since high energies are required to test theories that go Beyond the Standard Model (BSM) of particle physics, that means we will be limited in our ability to validate those theories until we build a collider big enough. But a team of scientists led by Yang Bai at the University of Wisconsin thinks they might have a better idea – use already existing neutrino detectors as a large scale particle collider that can reach energies way beyond what the LHC is capable of.
Neutrinos are notorious for very weakly interacting with things – there are trillions of them passing through you as you read this sentence. However, put enough matter in their way and eventually a special few will run directly into a proton or electron. The resulting particle spray, which is typically going faster than light in whatever medium the neutrino hit, creates a light known as Cherenkov radiation. But really what causes the Cherenkov radiation are the particles created by what is essentially a giant particle collider.
We already intentionally build neutrino detectors out of giant blocks of ice or vats of water. In traditional detectors, these massive areas of clear material are surrounded by photodetectors, which pick up any stray Cherenkov radiation simply as a source of photons. Projects IceCube in Antarctica, KM3NeT in the Mediterranean, and Baikal-GV in Lake Baikal, not to mention one of the most powerful one yet, JUNO, in Jiangmen China, that is just now coming online, were designed with that light detection in mind. But Dr. Bai and his colleagues think they can do more.
Fraser discusses the IceCube Experiment
They suggest using these massive laboratories as a “Large Neutrino Collider” (LvC – v is the symbol in particle physics for a neutrino). In these detectors, there are two types of neutrino interaction events – “tracks” and “showers”. The paper focuses on “track” events, which happen when a neutrino interacts with a muon, and create clear “tracks” of light that can easily be analyzed. “Showers” on the other hand, are caused by other types of reactions and show up as spherical bursts of light that are much harder to analyze. Importantly, many of the track events are caused by Ultra-High Energy neutrinos, which can release energies up to 220 peta-electron volts, like one that was recently detected at KM3NeT. That is almost 16,000 times more energy that the LHC can currently make with its collisions.
Operating in that high energy field would allow physicists to glimpse new particles that go beyond the standard model. In particular, there is a type of particle called a Leptogluon, which are both “colored” like gluons, but also interact leptons, and are part of “composite” models that theorize that leptons and gluons are made of the same material. These are an ideal target, as they are theoretically very heavy, but could be detected very effectively using the LvC, especially bigger versions that might come online soon.
Unfortunately, the authors also calculated that, for many other types of interactions, the LvC is either on par with or even lags behind what the LHC is capable of. Searching for leptoquarks, one of the particles suggested by some Grand Unified Theories, the LvC would be “comparable” to the LHC, but for a search for new heavy vector bosons it wouldn’t even be able to compete with the LHC as is.
Fraser discusses some of the issues with the standard model with UT contributor Dr. Paul Sutter
With all that being said, repurposing an already existing piece of physics infrastructure to do new and interesting detection work sounds promising. There are several new generations of neutrino detectors that scale up the detection area, and thereby would enhance the LvC’s capabilities. But they are still on the drawing board for now. Maybe this paper will inspire their designers to consider including some particle detection equipment alongside the photodetectors to really get the best use case out of these fascinating experiments.
Learn More:
Y Bai, K Xie, & B Zhou – Large Neutrino “Collider”
UT – Catching Ghost Particles in Real Time
UT – IceCube-Gen2: 8 Cubic Kilometers of Ice, 5 Times the Sensitivity