Symmetry has a fairly new article on sterile neutrinos, explaining some of the basic ideas about what they are and why we’re interested in them. Measurements of the Z peak in e+/e- collisions at LEP showed that there are only 3 neutrinos, but there are some caveats to that. The measurements showing that there are 3 neutrinos really mean that there are 3 neutrinos that (1) have less than half the Z mass (91.2 GeV) and (2) interact with Standard Model particles via the weak force.
If we can instead add some “neutrinos” that don’t interact through the weak force, then there’s room for more neutrinos. One of the main ways people search for them is to find problems with the standard picture of 3-neutrino oscillations. If there are sterile neutrinos that mix with the three usual flavors (electron, muon, and tau), then maybe we can find evidence of neutrinos oscillating into sterile neutrinos. That is, regular neutrinos seemingly disappearing altogether rather than changing from one type to another.
SuperKEKB, an upgraded version of the KEK B electron/positron collider at the KEK lab in Tsukuba has started up. There are no collisions yet, but they are starting to run beams around the main ring. The Belle II experiment will use he SuperKEKB facility to study B physics – that is, the physics of B mesons (and other particles involving b quarks). These are particularly interesting because the lifetimes of many of these particles decay weakly with long enough lifetimes to actually measure how far they travelled. B physics allows for high precision tests of various aspects of the Standard Model, including things like CP violation in the quark sector (i.e. matter/antimatter differences), particle spectroscopy (measuring the properties of the various kinds of composite particles), and searches for various kinds of new physics.
Symmetry magazine has an ABCs of Particle Physics feature right now, which provides fun explanations of various topics in physics (with a focus on high energy physics). Everything is meant to be accessible to basically anyone. You can check it out here.
I finally found some time to read the LIGO paper. A couple things that I thought were interesting:
- The peak power from gravitational waves was 200 solar masses per second. The power didn’t stay there for very long since a total of 3 solar masses was radiated away.
- The rate of false positives the size of the signal seen is one in tens of thousands of years, so this is a signal that is enormously above any known backgrounds.
- LIGO also uses some complicated template fitting routine where they compare the measured signal to a library of pre-calculated theoretical curves. This only gives approximate results for physics parameters, so they then have to supplement this with an actual fit.
- The next biggest event had a false positive rate of only one every few years
Yesterday, Columbia professor Brian Greene was on Colbert’s show to talk about gravitational waves. Greene gives some nice explanations for laymen (with graphics!) about gravitational waves in general and about LIGO. He even brings out a Michelson interferometer to demonstrate how the LIGO setup works (though not with gravitational waves). Since Greene is a theorist, I would assume that someone else had to set up the interferometer for it to actually show some sensible results. You can find the video on Youtube here.
The ANTARES neutrino telescope has a new result looking for “secluded” dark matter, where dark matter annihilation is mediated through some new mediator that then decays into Standard Model particles. They claim that this can explain the high energy bump in the positron/electron ratio and can also still be a thermal relic from the Big Bang.
They look at several different channels, including one where the mediator actually lives long enough to reach Earth and decay in the atmosphere, and others where neutrinos in the final state are measured.
For this model, the result is actually stronger than direct detection experiments for spin-dependent interactions and is stronger at very high masses in the spin-independent channel. While this result isn’t particularly groundbreaking, the paper mentions that it is the first search of this kind for this type of dark matter, and I think the model, which I hadn’t heard much of previously, sounds quite interesting.
The DM-Ice experiment has released their first results for a search for an annually modulating signal of dark matter. They’re currently about an order of magnitude off from the purported DAMA/LIBRA signal but hopefully will improve significantly in the future.
DM-Ice is a NaI(Tl)-based experiment looking for signals of dark matter in their scintillating crystals. The big purpose of DM-Ice is to test various theories for why the DAMA/LIBRA experiment, a NaI(Tl) experiment at Gran Sasso, has seen an annual modulation in its event rate for many years. Various people have proposed that maybe DAMA/LIBRA is just seeing a seasonal effect. DM-Ice is in Antarctica, so any seasonal effects will be very different. If DM-Ice sees the same modulation as DAMA/LIBRA, then that would rule out many of the proposed explanation, since dark matter will modulate in the same way no matter where the detector is but most other things will be location dependent.