A few days ago I wrote a brief review of the Seesaw Model of particle physics. Being a theorist, I forgot to mention that the motivation for that review was a new set of results from the experimental community that constrains such models. And so I thought that today I would give a few details on these new results.

In the model I reviewed, known as a Type-I model, each of the species of neutrino that are part of the Standard Model are partnered with a second, very heavy neutrino that provides a small mass to its lighter partner. This mechanism allows the Standard Model neutrinos to have a very small mass without actually being massless, which is very difficult to achieve otherwise. 

However this model is still only a theoretical proposal, with no experimental evidence to support it or to disprove it. Experimental particle physicists have spent the last three decades searching through trillions of high energy reactions searching for a sign of the heavier neutrinos, but as yet nothing has been found. 

One possible reason is that the heavier neutrino is by its very nature difficult or impossible to detect. Neutrinos do not interact through electromagnetic or strong nuclear forces, and even a very heavy neutrino would be far too light to detect gravitationally. The Standard Model neutrinos only interact through weak nuclear forces, and the heavier neutrino may not even do that. It then becomes very difficult to detect a heavy particle that doesn't interact with anything around it.

However there is another type of seesaw model, known as the Type-III Seesaw, in which the heavy, sterile neutrino is replaced with a pair of heavy, charged particles as well as a slightly different neutral particle. These are even more exotic than the Type-I Seesaw models, and the mechanism by which they generate light neutrino masses is similar but more complicated. One advantage of the Type-III model is that it is potentially easier to detect.

The Type-III Seesaw model can be studied in particle accelerators through the production of these heavier, charged particles. If you collide protons at the LHC at high enough energies, you will produce high energy photons that will then produce these new particles. When these particle enter a detector, they will again interact and decay through electromagnetic forces and can be observed and studied in that way. (There are other production and detection channels, but the electromagnetic channel is a simple one for understanding the model.)

That is exactly what the team at the CMS detector at the LHC have just completed. By reviewing billions of particle reactions, and searching for signs of a very heavy particle that cannot be explained within the Standard Model, they were able to test the Type-III Seesaw model and look for signals of new physics.

Unfortunately they did not find anything. They have now placed limits on the mass of the exotic particles of 840GeV, which would make them at least five times heavier than the heaviest particle in the Standard Model. This certainly does not mean that the seesaw model is disproven, and in some ways a heavier particle is even better for the seesaw model, but it does improve the limits on the model.

It is certainly an interesting result. Even though they have not detected any new particles or physics beyond the Standard Model, it has still improved our understanding of high energy subatomic physics, and that is always a good thing!