Question: How does measuring the neutrino beam help answer why we're here?

Matthew Malek answered on 11 Mar 2014:

This is a fantastic question!

It starts with matter and anti-matter. For every matter particle, there is an anti-matter anti-particle with an opposite electric charge. So the negatively charged electron has a positively charged “positron”. And the positive proton has a negative “anti-proton”. Even the neutral neutron has an anti-matter counterpart, as the neutron is made up of three charged quarks; the anti-neutron has three quarks of opposite charge.

At first, it would be natural to think that the laws of physics are the same for matter and anti-matter. This is a symmetry called CP (for “charge-parity”, but don’t worry about the name). If CP were a true symmetry of nature, then equal amounts of matter and anti-matter would have been produced in the Big Bang. When matter and anti-matter collide, they annihiliate (via the electromagnetic force) and leave behind only energy. So if CP were a true symmetry, all that matter and anti-matter from the Big Bang would be gone, leaving only a universe full of energy!

That’s clearly not the case, or else we wouldn’t be here. So why didn’t it happen that way?

Well, the matter/anti-matter asymmetry in the universe (i.e., the fact that we live in a universe made out of matter with nearly no anti-matter) must be caused by some difference between matter and anti-matter, an effect called “CP violation”. Although it was once believed that the CP symmetry was a law of nature, we have known for a long time that CP violation exists. It was discovered in the 1960s by a former collaborator of mine, Jim Cronin, and he won the Nobel Prize for this. (I wasn’t involved in the research. Heck, I wasn’t even born yet!)

Well, if we know that CP violation exists, then what’s the problem? The problem is this: We have seen CP violation in particles called “mesons” that are made up of a quark and an anti-quark. However, this CP violation can be measured… and it is waaaaaaaaaaaay too small to account for the presence of matter. So CP violation in what we call the “quark sector” exists, but is too small to be the answer.

Where else could it be? If not in the quark sector, the other logical place is the neutrino sector! We know that neutrinos can change forms (“oscillate”) and it may be possible that neutrinos and anti-neutrinos do this differently — another source of CP violation. This source of CP violation has not yet been definitively been measured, but early indications (such as from my experiment) show that it may be very large. Possibly large enough to explain why you and I exist, rather than having a universe simply filled with nothing but radiation!