Tuesday, May 18, 2010

CP violation observed in B-meson systems by the D0 detector at Fermilab's Tevatron!

While the eyes of the physics world have been glued on the Large Hadron Collider at CERN, the physicists toiling away at the LHC's predecesor in Batavia, Illinois have been hard at work making new discoveries. Team members of the D0 Collaboration at Fermilab's Tevatron have announced the observation of what appears to be a CP violation in the oscillation of neutral B-mesons into their own antiparticles.

Okay, now that I've made the eyes of any non-physicist reading this glaze over, perhaps a plain English explanation of this outcome and its significance is in order. It is perhaps easier if I start with the latter in order to provide historical context. To do that, we need to start at the beginning. The very beginning. The Big Bang.

Okay, perhaps we don't have to back up quite that far, but we should at least back up to the discovery of the Big Bang.  Back in 1929, astronomer Edwin Hubble was studying the electromagnetic spectra of distant astronomical objects, specifically galaxies, and noted that they were moving away from us. (Well, from the standpoint of General Relativity, it isn't so much that they are moving as it is that the intervening space is expanding.) In fact, the further away they were, the faster they were moving away from us, an observation which confirmed a theoretical prediction by the Belgian Roman Catholic priest Monsignor Georges Lemaître in 1927. By 1931, Lemaître had carried his theories regarding the expansion of the universe to their logical conclusion, that at some point in the past, the entirety of the universe was concentrated into a single point, a "Primeval Atom," which then exploded and formed the universe as we know it. The concept was referred to derisively by Fred Hoyle as the "Big Bang theory," and the name stuck. (One cannot help but be struck by the irony of the fact that the concept of the Big Bang, the established reality of which is frequently denied by Biblical literalists, was first posited by a priest.)

Now, no theory can properly be called a theory (as opposed to a hypothesis or conjecture) without meeting certain criteria, one of the most important of which is usefulness in making testable predictions.  The Big Bang theory was no exception, and one of the predictions of the theory was the existence of a sort of "after glow" of the Big Bang in the form of cosmic microwave background radiation, a prediction which was confirmed in 1964 by Arno Penzias and Robert Wilson of Bell Labs. (I had the pleasure of speaking with Robert Wilson on a few occasions while I was an undergrad back in the late 80's. Very nice guy. But I digress.)

All very well and good, but what does all of this have to do with the new discovery at Fermilab?  I'm getting to that....

As the Big Bang theory gained broader acceptance among cosmologists, particle physicists started piping up with an objection. They pointed out that matter and anti-matter particles are always produced in pairs, and that the Big Bang should have spewed out equal quantities of matter and anti-matter particles, which would have promptly annihilated each other, leaving a universe filled with nothing but a sea of gamma radiation. No matter. This conundrum is referred to the baryon asymmetry problem.

But we are here. And the Big Bang happened. Whodathunk?

A possible solution to this quandary was proposed in the mid-60's by Andrei Sakharov, a Soviet physicist as noted for his political activism as for his scientific accomplishments. (In 1980, he was arrested for protesting the Soviet invasion of Afghanistan.) Sakharov proposed a theoretical model in which there might be an asymmetry in the behavior of matter and anti-matter in  certain types of interactions, and that such an asymmetry could lead to a net quantity of matter remaining after the bulk of matter and anti-matter mutually annihilated itself in the wake of the Big Bang. Furthermore, Sakharov argued, such an asymmetry would manifest itself as a violation of CP conservation.

"And what in blue blazes," I hear you cry, "is that?"

Before I define "CP," I should digress just a bit and comment on the concept of symmetry and its relationship to conservation laws. The concept of symmetry is fundamental to theoretical physics. Monumentally fundamental. All conservation laws are rooted in symmetry. Conservation of momentum is the result of translation symmetry. Conservation of energy is rooted in time symmetry. Conservation of angular momentum is a consequence of directional symmetry. And the importance of symmetry goes even deeper than that. In quantum field theory, a special type of symmetry called "gauge symmetry" is imposed on the fields that the theory describes. Requiring gauge symmetry causes terms describing force interactions to pop right out of the field equations automatically, whether it is the electromagnetic force, the weak nuclear force governing beta decay, or the strong nuclear force which bind quarks to form protons and neutrons. It is a beautiful thing to see.

But just as important as symmetry is in theoretical physics, there is a related concept called "spontaneous symmetry breaking." It is this concept which allows the Higgs field to give fermions their mass, and which caused the electro-weak force to split off into the electromagnetic and weak nuclear forces as the universe cooled after the Big Bang. In theoretical physics, when a symmetry is broken, something interesting is happening.

Which leads us back to CP violation. The concept behind CP (charge-parity) conservation is as follows: perform an experiment with a collection of particles. Then swap all of the particles with equivalent particles of opposite charge (in other words, their anti-particles), then perform the experiment again, but watching it in a mirror. The outcome of the experiment should be identical.

But, as it turns out, CP symmetry is not always obeyed. Such an asymmetry as described by Sakharov was experimentally detected in 1964. Neutral K-mesons (also known as kaons) were observed to turn into their own anti-particles and back again repeatedly. This in of itself was not a surprise, but the rate at which each part of the oscillation took place was not the same, and this is where the "CP violation" was happening. There was subtle difference in the way matter and anti-matter were behaving, which was in line with Sakharov's solution to the issue of matter's dominance in our universe. But the asymmetry observed in kaon oscillations was not enough. Not by a factor of 10,000,000,000.

Which brings us back full-circle to the findings of the D0 team. The CP violation which they have observed in B-meson oscillations in the Tevatron is significantly larger than that observed in 1964 with kaon oscillations. Possibly large enough to explain the baryon asymmetry problem. Which would explain why we have a universe of stars, galaxies, planets, trees, puppies, Legos, and Bugatti Veyrons, and not a universe of pure energy.

For more info, take a look at the RESONAANCES blog and the Quantum Diaries Survivor blog, both of which have interesting discussions about this discovery. The New York Times also has an article about this discovery.

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