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Saturday, June 8, 2013
Science ...

Last year, the world of particle physics received some well-deserved public attention after several teams of physicists working at the Large Hadron Collider in Switzerland confirmed the existence of the Higgs boson. If you recall, the Higgs particle confirms that there is an energy field responsible for giving mass to certain types of sub-atomic particles. Higgs is a bit misunderstood in public circles; under the popular interpretation, you are tempted to curse the damned Higgs field as you step off the scale whenever trying to lose a few pounds. Mass (like food calories) is a very dense form of energy, and gravity gives its attention to all forms of energy; given the energy density of mass, gravity gives a LOT more attention to massive things. More attention than some of us might like, especially after a few big meals and some sinful deserts . . .

But actually, you shouldn’t really blame the Higgs field if you’re not ready yet to go on The Biggest Loser. Most of the mass energy in the atoms and molecules that make up the world that we live in comes not from the Higgs field, but from the attractive “strong” forces between the quarks within the protons and neutrons of the atomic nucleus. The Higgs-derived mass is just a small fraction of the mass of most atoms.

Nonetheless, the discovery of the Higgs field and particle was very important to physicists, as it solved a variety of problems and contradictions within the Standard Particle Model (e.g., the fact that the force-carrier particles for the weak nuclear force have mass, while other similar force particles like photons don’t), and at the same time opened up some new problems (e.g., the energy of each Higgs particle is tiny compared with the large energy of a black hole of the same size; on the face of it, both represent pure mass energy, so why do our measurements show one to be so big and the other to be so small? Scientists suspect some sort of offsetting mechanism, but don’t yet know what that mechanism is). In sum, the Higgs was the star of the show for science last year, something that grabbed the attention of the public; mostly for the wrong reason, but any positive attention that science gets is good.

Some of the more inquiring minds out there might wonder, what will be the next big thing from the world of the tiny? If I had to bet, I’d say it was the axion (not to be confused with the former detergent of same name). If you haven’t heard of the axion, remember that you heard it hear first (even though scientists have been talking about axions since 1977; recall that the Higgs particle was first proposed in 1964). Like the Higgs, the axion particle and field were proposed to fill a gap in the Standard Particle Model. The Higgs was meant to explain a broken symmetry (a pattern that stopped working at some point) involving the weak nuclear force (the force that mediates radioactivity and fusion reactions within the sun), whereas the axion is meant to deal with a broken symmetry that was expected but actually did NOT appear regarding the strong nuclear force, i.e. the “glue” that binds quarks into protons and neutrons within the atomic nucleus.

If this was all they amounted to, axions would be a real yawner to most people. But after a while, the scientists started to see that like the Higgs, the axion could do a lot more than originally imagined. If you’ve been following the related world of cosmology physics, i.e. the study of the universe as a whole (a rather radical inverse of particle physics, with its emphasis on the tiniest things possible), you might know that there is a “dark matter” problem. The stars and galaxies act as though there is a whole lot more mass out there in the heavens than our telescopes (and other invisible radiation probes) can account for. E.g., most galaxies seem to be spinning too fast. Also, the Cosmic Microwave Background from the early universe shows that there had to be a lot more mass involved in the formation of universe than we can presently detect. So, there appears to be matter out there that responds to gravity, but does not otherwise interact with the atoms and molecules and particles that we know so well. In fact, cosmologists are quite convinced that there is a lot more of this “cold dark matter” than the regular “hot” matter that we are familiar with — about 5 times as much of it!

But what the heck is it? It ultimately has to be composed of tiny particles, like everything else. But what are these particles? You’d think that the particle physicists would have some suspects. And they do — but the problem is, since these particles don’t reflect light or radiation, there aren’t many good options for detecting them. We can see their effects through gravity when viewing a galaxy of millions of stars; at such a scale, there are tremendous numbers of dark matter particles swirling around. But try to detect just one, and you’ve really got a needle in a haystack to look for.

At present, physics doesn’t know what particles make up dark matter. It has been an on-going mystery for over 30 years. But they have two good candidates: supersymmetrical particles, and axions. Super-symmetrical particles (“SUSY”, as they are sometimes called) would be a new, presently un-detected set of “cousins” to each of the known particles in the Standard Model (e.g. quarks, electrons, photons, neutrinos, gluons, and yes, the Higgs boson too). Most of them wouldn’t be stable and aren’t candidates for dark matter; but one or two of them theoretically would be light enough to have very long lives, and at the same time would be very shy of the usual electromagnetic forces that we mostly use to search the heavens with. SUSY is strongly (but not necessarily) related to superstring theory, the currently favored (but still not proven) theoretical method of uniting gravity under a common formulation that also contains electromagnetism and the weak and strong nuclear forces.

Axions, by contrast, are a bit more limited in their intent. They COULD fit in with supersymmetry and superstring theory, but don’t necessarily need to. Like the Higgs, they are a bit less “grandiose” than the SUSY particles. But there are many interesting differences. Given that axions address a specific problem observed in the Standard Model, they have certain (theoretical) characteristics that make them very small and very “cold” (don’t move much), as compared with SUSY particles (and also the Higgs). As such, they cannot be detected using the Large Hadron Collider, as with the Higgs and with SUSY. The LHC is a “brute force” device that racks up billions of high-energy particle collisions looking for a handful of special fragment patterns. Axions, by contrast, are more “contemplative”; because of their condensed stillness, they require a special type of detector that is extremely sensitive to a very weak and ghostly type of microwave signal; that detector is called the ADMX experiment, located at the University of Washington.

The ADMX has just gone through some upgrades and is slowly searching across the range of energy levels that axions might exist at. Unfortunately, the key word here is SLOW. Given how ghostly the axion would be and the limited resources available for this type of experiment, it could take up to 10 years to definitely search the entire possible range of axion energies. This is definitely a monastic approach to particle physics, compared to the “crank up the heat until she jumps out of the cauldron” approach of the LHC.

Given all the accelerator power and computer capacity available at the LHC in Geneva, you’d think that the SUSY approach has the advantage. But guess what . . . the LHC was looking for SUSY while it was looking for the Higgs, and it got lucky with the former but struck out on the latter. This is not so say that SUSY has been ruled out. The LHC is down now but will be back within 2 years with double the power. Then we will really see if SUSY can be forced out of hiding. But some physicists are a bit concerned, because the lowest-energy (and thus most stable) SUSY candidates should have been dropping some hints by now. Recall that there were hints of the Higgs for many years from both the LHC and the Tevatron in the US, but it took the power of the LHC to amass enough readings to be sure that what looked like the Higgs wasn’t something else. Supposedly, SUSY is being a bit more shy thus far.

There have been a few axion hints, but not very significant yet. So the race is on for the dark matter particle, which may turn out to be more like a tortoise race. One physicist calls the axion the “thinking man’s dark energy particle”, and I like that. As with the Higgs, they were thought up because of a very specific and limited disagreement between particle experiment results and the Standard Model theory. They don’t try to “start a revolution” within the Standard Model, like the SUSY theories do. They would solve a lot of problems without re-writing the books very much. And, they would (probably) be an American discovery! Perhaps a last taste of scientific glory for the good old USA.

So, I’m pulling for the humble axion as the thing that finally explains more than 3/4 of everything in the universe with mass. The discovery of axions wouldn’t change human life very much, as there won’t be any immediate applications for axions or axionic energy (thank goodness there won’t be an “axion bomb” anytime soon . . . I don’t think . . . ). Maybe someday in the far distant future, space probes or even space “arks” full of humans could use axion clouds to navigate the pathways between the stars or even galaxies, by utilizing their gravitational force (just as today, space probes to Mars, Jupiter or the other planets gather speed by using “slingshot” effects around the moon, the sun, or other planets along the way). At present, as we get better and better with manipulating the ghostly but somewhat more accessible neutrino, we don’t even have any practical applications for them in sight (no neutrino phones that could work even 20 miles below the surface of the earth — not yet, anyway!). Nonetheless, axions would represent a big step forward in the development of the Standard Model, and a giant leap in our understanding of the cosmos as a whole.

◊   posted by Jim G @ 3:28 pm      

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