I can be terse

We now have yet another indication that neutrinos cannot travel faster than the speed of light after all, provided by a neighbor of the OPERA detector that set off the fuss in the first place. OPERA's detector sits deep underground at Gran Sasso in Italy, where it receives neutrinos from a beam generated at CERN, 730km away on the French-Swiss border. Because the neutrino beam spreads out over the intervening distance, it's possible to run multiple detectors at the same site, all listening in on the same beam. The team running one of Gran Sasso's other detectors (called ICARUS) has now performed time-of-flight measurements on neutrinos and determined that they don't seem to be moving faster than light.

Both ICARUS and OPERA use the same neutrino source, and so have the same clocking capabilities. ICARUS uses a slightly different technique to detect neutrinos, but without some compelling explanation for the new data, it is safe to put the OPERA results down as "likely experimental error." Relativity wins the day, again.

But until we get the results of OPERA’s proposed studies we can’t say for sure that their measurement is right or wrong. Suppose that they reduce the lead time of the neutrinos from 60ns to 40ns. That would still be a problem for special relativity! So let’s investigate how we can get faster than light neutrinos in special relativity, before we no longer have the luxury of an exciting result to play with.

AFAICT, no one knows if the OPERA corrections for the cable problem will drop the CERN neutrinos below the speed of light. This post takes a fun look at how it could be that photons might not travel as fast as is physically possible, and why neutrinos might.

What Cohen and Glashow did last week was to generalize this idea to point out a new physical phenomenon (new at least to me) and use it to argue that OPERA’s result is self-inconsistent. They argue that the very effect of faster-than-light travel that OPERA claims to observe would have caused distortions in its neutrino beam that clearly were not observed. Moreover, Cohen and Glashow also pointed out that at least two other experiments studying higher energy neutrinos put even stronger constraints on the possibility of anything similar to what OPERA observed.

The article is fascinating, so read the whole thing. Dr. Strassler describes an elegant approach to constraining the types of modifications to Relativity that are consistent with the OPERA data. This approach was developed by Andrew Cohen and Sheldon Glashow.

Cerenkov radiation is emitted by electrically charged particles moving faster than light in a medium. Relativity says we shouldn't see Cerenkov radiation in a vacuum, but it is an important effect in materials where light slows down, such as water, and particles can exceed the local speed of light. You may have seen that blue glow around a submerged reactor; that's Cerenkov radiation, and the effect takes energy from the emitting particle.

Neutrinos have no charge, so they would not emit Cerenkov radiation (well, they have a very, very tiny type of charge so they can emit a very, very dim form of Cerenkov radiation). But neutrinos interact via the weak force, and what Cohen and Glashow did was show that such particles can emit an analogous type of radiation if they exceed the speed at which electrons can travel in a medium. This radiation would remove energy from the neutrino beam in a way that would be very easy for the OPERA experiment to see. But OPERA's results do not show the energy removal signature of Cohen-Glashow radiation.

Observations of neutrinos from a distant supernova have put strong constraints on neutrino speed for lower energies than OPERA. Two other experiments have observed neutrinos 100 to 1000 times more energetic than OPERA's neutrinos, and they do not see the Cohen-Glashow radiation energy loss.

So, we must choose between OPERA's FTL neutrinos or Cohen and Glashow's weak force radiation effect. It is not impossible that both could be true, but if so, it will place strong constraints on the kind of modifications that can be made to Special Relativity.

In short, OPERA's FTL results became more unlikely, but have not yet been ruled out. I was struck by the elegance of the Cerenkov radiation analogy involving the weak force to put tighter constraints on the physics of FTL neutrinos, if they exist.

"It's a beautiful idea. It explains dark matter, it explains the Higgs boson, it explains some aspects of cosmology; but that doesn't mean it's right.

"It could be that this whole framework has some fundamental flaws and we have to start over again and figure out a new direction,"

This is how Science is supposed to work. Supersymmetry is a beautiful theory with lots of explanatory power. But if the predictions don't match with observational evidence, well, it is just pretty math.

Supersymmetry hasn't been completely ruled out, yet, but the versions that remain viable are more theoretically complex, and that is never good news. Nature usually sides with elegant ideas. Sadly, one of the elegant ideas to lose out will be String Theory, which requires Supersymmetry.

Back in July, I had the great opportunity to attend the 2011 CTEQ Summer School in Madison, Wi., where for 10 days we talked about this equation:

Now, this is not just any ordinary equation, it is arguably the most important equation for any physicist working at the Large Hadron Collider, the Tevatron, or any of the other half-dozen atom smashers on this planet. In fact, this equation is precisely what inspired the name Paper vs. Protons.

Since quantum physics is inherently statistical most calculations result in computing probabilities of things happening. The formula above allows you to compute the probability of what happens when you collide protons, something experimentalists can measure, by simply calculating the probability of something happening when you collide quarks, something undergraduates can do!

Richard Ruiz carefully walks us through the equation so we can understand what is happening at the conceptual level. The equation tells us the probability of producing an electron (e^-) and a positron (e⁺) if you smash two protons (p) into each other. And the results from this equation will tell us if the predictions for the Higgs particle match what Nature tells us.

Inside atomic nuclei, protons and neutrons fill space with a packing density of 0.74, meaning that only 26 percent of the volume of the nucleus in is empty.

That's pretty efficient packing. Neutrons achieve a similar density inside neutron stars, where the force holding neutrons together is the only thing that prevents gravity from crushing the star into a black hole.

Today, Felipe Llanes-Estrada at the Technical University of Munich in Germany and Gaspar Moreno Navarro at Complutense University in Madrid, Spain, say neutrons can do even better.

A new paper is out that claims under exceptionally high pressure, neutrons can switch form spherical symmetry to cubic symmetry. If true, this will have a major effect on our understanding of neutron stars massive enough to reach the necessary internal pressure.

My goal is to explain the sense in which the Standard Model is “chiral” and what that means. In order to do this, we’ll first learn about a related idea, helicity, which is related to a particle’s spin. We’ll then use this as an intuitive step to understanding the more abstract notion of chirality, and then see how masses affect chiral theories and what this all has to do with the Higgs.

I'm enjoying Dr. Tanedo's series on the Standard Model. Simple, informative, accessible, and even though physics-by-analogy is fraught, very accurate. I especially like the little spin diagram above.

If you want to start at the beginning, and it is a very good place to start, go to http://www.quantumdiaries.org/2010/02/14/lets-draw-feynman-diagams/