Violating Parity with Quarks and Gluons

Cosmic Variance
By Sean Carroll
Feb 16, 2010 7:30 PMNov 20, 2019 3:48 AM


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Hey, nobody told me that having a blog would involve homework. But here's Jerry Coyne, nudging me into talking about a story in this morning's New York Times. Fortunately it's interesting enough to be worth taking a swipe at. The news is an interesting result from RHIC, the Relativistic Heavy Ion Collider at Brookhaven Lab on Long Island. RHIC has been quite the source of surprising new results since it turned on in 2000. It's not the highest-energy collider in the world, nor did it ever aim to be; instead, it creates novel conditions by smashing together the nuclei of gold atoms. Gold nuclei have lots of particles -- 79 protons and 118 neutrons -- so the collisions make a soup known as the quark-gluon plasma. (We ordinarily think of a proton or neutron as consisting of three quarks, but those are just the "valence" quarks that are always there. There are also large numbers of quark-antiquark pairs popping in and out of existence, not to mention scads of force-carrying gluons that hold the quarks together. So you are actually create a huge number of quarks and gluons in each collision.)

We think we understand the basic rules of quarks and gluons very well -- they're described by the theory of quantum chromodynamics (QCD), and Nobel prizes have already been handed out. But knowing the basic rules is one thing, and knowing how they play out in reality is something very different. We understand the basic rules of electrons and electromagnetism very well, but chemistry and biology (not to mention atomic physics) are still surprising us. Likewise with quarks and gluons: the results at RHIC have yielded quite a few surprises. Most interestingly, in the aftermath of the collisions the hot plasma of quarks and gluons seems to behave more like a dense fluid than a bunch of freely-moving individual particles. Still much to be learned. This latest result has to do with a violation of parity -- the symmetry you get by reflecting around one axis, like when you view something in a mirror. (Unfortunately there is a completely different transformation known as mirror symmetry, which this new result has nothing to do with, despite potentially confusing titles.) Quarks and gluons interact in interesting ways, and in the many fluctuations that happen in these high-temperature collisions we can get "bubbles" that pick out a direction in space. In the presence of these bubbles, quarks treat left and right differently, even though they treat both directions exactly the same when they're in empty space. The phenomenon is known as the chiral magnetic effect -- "chiral" means "distinguishing left from right," and it happens when you put the quark-gluon plasma in a magnetic field. It's worth mentioning that, while this result is interesting and very helpful to our quest to better understand the strong interactions, it does not represent the overthrow of any cherished laws of physics. On the contrary, it was predicted by the laws of physics as we currently understand them -- and by human beings such as Dimitri Kharzeev and others. Parity is an important idea in physics, but it's broken all the time -- very famously by the weak interactions. Heck, even biologists know how to break parity -- most naturally occurring amino acids are left-handed, not right-handed. (I think the reasons why are still mysterious, but can be traced to accidents of history -- hopefully someone will correct me if that's off base.) The interesting thing is that the strong interactions don't seem to violate parity under ordinary circumstances; it would be very easy for them to do so, but they seem not to in Nature. When things could happen but don't, physicists are puzzled; this particular puzzle is known as the Strong CP Problem. ("CP" because the strong interactions could easily violate not only parity, but the combined operation of parity and charge conjugation, which switches particles with antiparticles.) This new result from RHIC doesn't change that state of affairs, but shows how quarks and gluons can violate parity spontaneously if they are in the right environment -- namely, a hot plasma with a magnetic field. So, okay, no new laws of physics. Just a much better understanding of how the existing ones work! Which is most of what science does, after all.

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