Particle Physics Developments

This article is adopted from an e-mail by mentor Steve Schnetzer to teachers and students in the Rutgers QuarkNet summer program.

There have been some interesting physics developments over the summer. They are mostly negative developments (things not found), but that doesn't mean they are bad. What's important is that we learn how the universe works. If the universe doesn't give the positive result that we expected or hoped for, we will still have learned something about the universe and that is what's important.

1) In early August the ATLAS and CMS experiments reported their results at a major conference in Chicago, the International Conference on High Energy Physics (ICHEP). Both experiments reported no evidence so far for supersymmetric particles. Supersymmetry (SUSY) is a very attractive theory to naturally explain why the Higgs mass and subsequently all of the other known particle masses are so light (≲100 GeV/c2) when vacuum fluctuations of quantum fields should drive the mass to be many orders of magnitude larger. 

2) A week or so after the ICHEP conference, there was a small meeting of mostly theoretical physicists at the Niels Bohr Academy in Copenhagen. At that conference, the SUSY Bet was resolved. In the video, you can watch several of the most prominent physicists in particle theory discuss in non-mathematical terms one of the outstanding issues in physics. 

3) On a related topic, LUX, a very large underground detector in a mine in South Dakota, announced in July that they saw no evidence of dark matter. Here is an article from Scientific American about it. LUX is a giant tank of liquid Xenon designed to be able to detect dark matter when it (very rarely) interacts with a nucleus of the detector. The lightest supersymmetric particle (LSP) is a very good candidate for dark matter but was not seen in LUX. Dark matter is definitely real; we know that from the rotational velocities of stars in galaxies and from gravitational lensing, but we still have no idea what it is. It seems likely now that it is not the LSP. 

4) Now for some news of a positive result. There was an announcement in early July at the 27th International Conference on Neutrino Physics and Astrophysics at Imperial College in London that the T2K experiment has found evidence for CP violation in neutrinos. This may be a really big deal. CP violation means that there is a difference in the laws of physics between matter and antimatter. If the laws of physics were symmetric between matter and antimatter, there would have been equal amounts and it would have all annihilated leaving the universe filled only with photons, neutrinos and maybe dark matter. That would have been very bad for us. The T2K experiment sends a beam of muon neutrinos produced by a proton accelerator on the Pacific Coast of Japan to detector inside a mountain several hundred kilometers away. On the way, some of the muon neutrinos oscillate into electron neutrinos. You can read about the results of the experiment here:

The key is Figure 1. Compare the upper and lower plots on the right. There are roughly three to four times fewer electron anti-neutrinos than electron neutrinos observed. This is a very exciting result. It is at about a 2 sigma confidence level, which means that statistically there is about a 95% change that the result is real. Over the next five years more data will be collected by T2K that should turn this, if it is real, into a statistically 99.9% certain result. Meanwhile, in the U.S., there is a plan to send a beam of neutrinos from Fermilab to a detector in the Homestake Mine at Sanford Lab in South Dakota (the same mine where the LUX dark matter detector mentioned above is currently taking data). Stay tuned. If you'd like to read more on this, you might start here: