Violating Parity with Quarks and Gluons

By Sean Carroll | February 16, 2010 8:30 am

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.

  • Low Math, Meekly Interacting

    Technically, virtually all biogenic amino acids are L and not D. In a hypothetical “primordial soup” however, it’s not at all straightforward to come up with naturally-occurring conditions that yield anything but a racemic mixture of amino acid enantiomers. There may be clues from space; apparently some amino acids found in carbon-rich chondrites show an excess of L forms, but usually on the order of 10 or 20%, nowhere near the almost 100% seen in cells. However, it’s thought that this small difference in abundance, delivered from space, may have provided the selective pressure to yield the dominance of the L forms. Once cells got better at synthesizing their own amino acids, the legacy of L dominance (perhaps has old as the first ribosomes) became fixed. What the cause of the disparity in space might be, I don’t know for sure, but I recall it may have to do with polarized radiation from some stellar source. Maybe the molecular cloud that yielded our solar system was heavily irradiated by a rotating neutron star. At any rate, the disparity still appears to be completely accidental.

  • Joseph Smidt

    I’m not sure if it is the same experiment you mention, but also in the RHIC news was they brought a substance to a record 4 trillion degrees:

    That’s really hot! (Given the sun’s core is ~ 15 million.)

  • Baal

    Parity is not “the symmetry you get by reflecting around one axis”. It is the operation of flipping the signs of all three axes (x,y,z) simultaneously.

    Avoiding confusion about parity is indeed important.

  • Sean

    Avoiding confusion is indeed important. And defining parity by reflecting around one axis is a better definition, in my opinion, because it works in any number of dimensions, while reflecting around all axes only works in odd numbers of spatial dimensions. In three dimensions they are equivalent up to a rotation, but a quick blog post isn’t the place to get into that.

  • Sili

    I recall some people trying to link parity violation to biological handedness, but the difference in energy scales is utterly ridiculous. The meteoroid/-ite results sound interesting, but until I see more conclusive evidence, I favour accident of history as well. The first replicator just happened to made up of Ls – possibly catalysed by quarts or a similar optically active inorganic, but again on a random slab of the stuff.

    Incidentally D and L are only very very tenuously connected to handedness in the sense of how polarised light is rotated. (+)-glyceraldehyde (that is dextrorotary) was by definition assigned the absolute structure D (the hydroxygroup pointing right in a standard Fisher projection with the aldehyde on top). The rest is history.


    Why Gold and not Lead, Bismuth or Uranium? The more the merrier, I’d’ve thought.

    How ionised are the atoms? Is it Au79+ or just Au+?

  • marc

    Sili—yes, the difference in energy scales is huge, but a recent paper arxiv:1001.3849 suggests that neutrinos from a supernova will selectively destroy one orientation of N-14, thus causing a sufficient asymmetry. Not sure how this can be tested, though…

  • steeleweed

    Recall as a teen reading Isaac Asimov’s take on ” …most naturally occurring amino acids are left-handed, not right-handed.” Believe he attributed it to the spin of the electron and found that was also responsible for some of the unique properties of H2O.
    His info may now be outdated, proven wrong or expanded – anyone refresh my memory with the latest theories on this?

  • Peter H

    Does this mean we can finally stop looking for the anti-matter universe?

  • Ellipsis

    Gravity is also a self-coupling field (I believe this is ultimately tied to the fact that QCD is self-coupling) — would analogous spontaneous parity violations be a part of any conceivable quantum theory of gravity as well?

  • SLC

    Re Sean @ #4

    Coordinate reflection in three dimensions is in no way, shape, form, or regard equivalent to a rotation. If that were the case, then rotation invariance would be violated in weak interactions and hence angular momentum would also not be conserved in weak interactions. It is only equivalent to a rotation in Cartesian spaces with an even number of coordinates.

  • Pingback: 17 February 2010 « blueollie()

  • James

    Re SLC @ 10;
    I think you misunderstand Sean – in 3 dimensions, a reflection in all axes is equivalent to a reflection in one axis plus a rotation.

  • SLC

    Re James @ #12

    I agree. Point taken.

  • Nonnormalizable

    Sili: It looks like the LHC will collide fully stripped lead ions: I assume RHIC does the analogous thing; I’m thinking the difficulty in taking off all the electrons is probably trivial compared to the multiplicative increase in electromagnetic acceleration you get in the accelerator for doing so.

  • Jacques Distler

    Sean, could you maybe explain what the result is?

    I have been unable to make heads or tails of the Press reports.

    In e+e- experiments, there are lots of parity-violating effects that manifest themselves as a forward/backward asymmetry. What picks out a preferred direction when you’re colliding gold-on-gold?

  • Sean

    I haven’t read any actual experimental papers, so I’m probably not the best person to ask. I just glanced over this theory paper. The point seems to be that you can define a “reaction plane” using the beam momentum and the impact parameter. The collisions create magnetic fields perpendicular to the reaction plane (along the angular momentum). The parity violation is manifested by an electric current (vector) parallel to the magnetic field (axial vector), which shows up as a charge asymmetry. So it’s not a forward-backward asymmetry at all.

    Standard caveats that this ain’t my area of expertise.

  • Jacques Distler

    So it’s not a forward/backward asymmetry. It’s a parallel/anti-parallel to the angular momentum asymmetry. That’s fine, except I don’t understand how, experimentally, you arrange a preferred direction for the angular momentum. (The elliptic flow picks out a preferred axis for the angular momentum, but not the direction.)

  • Sean

    My impression (caveats still in force) is that the preferred axis is the point. With parity violation, you expect the electric dipole (charge asymmetry) to be oriented preferentially along the magnetic field, while otherwise you’d expect it to be perpendicular (if there were any at all). In other words,
    $latex vec{J} = alpha vec{B}$
    is a parity-violating condition, for any sign of α.

  • Jeff Harvey

    Sean, the analysis in the paper you cited seems to be based on the idea that in heavy ion collisions you can locally generate a non-zero value of the CP violating theta parameter. As far as I can tell the mechanism by which this is supposed to happen is not really explained, but if you assume it does, then the rest of the analysis is fairly standard axion electrodynamics except that the “axion” is not a true dynamical field, but rather associated with some fluctuation of the QCD vacuum.

  • Sili

    Thanks, Nonnormalizable , I suspected as much – but as a chemist I’m a bit unfamiliar with what is standard in high energy physics.

    I think I knew that LHC was gonna use lead (moon lighting as the LPC?). Interesting to see how modest we Europeans are, compared to the American tacky bling-bling.

    Though, I hope there is some real reason for using Au instead of Pb. But now I really really wanna see someone start smashing U-238 together.

  • Sean

    Jeff, I think that’s basically right. I didn’t quite follow where the effective θ(x) came from, but given that I think it all follows.

  • Dmitri Kharzeev

    The source of the local effective theta (at weak coupling) is the “sphaleron” solution of Quantum Chromo-Dynamics.
    The sphaleron rate (per unit volume, unit time) is proportional to the fourth power of the temperature, so the high
    temperature created at RHIC helps.

  • Jeff Harvey

    Thanks Dimitri, but can this sphaleron solution actually be used to do any kind of reliable calculation? In electroweak theory there is symmetry breaking and weak coupling and one can make sense of semiclassical solutions like sphalerons. In QCD there is no gauge symmetry breaking and one is at strong coupling. You can introduce a scale through temperature, but one is still at strong coupling so I don’t understand why one should take a particular classical configuration out of infinitely many and think it has any particular significance.

  • Dmitri Kharzeev

    Jeff, thanks for your comments – indeed the QCD sphalerons (unlike the electroweak ones) are under a quantitative control only at sufficiently high temperature when the coupling is weak due to the asymptotic freedom. Close to the QCD phase transition temperature the coupling is not weak enough to claim the dominance of classical solutions. Nevertheless, the rate of topological transitions is large even at moderate temperatures as seen on the lattice in QCD and computed analytically at strong coupling e.g. in N=4 SUSY theory. What field configurations are responsible for these transitions is not yet clear (at least, to me), and this is why at the moment we prefer to characterize their effect by an effective local theta.
    I do hope that the experimental result from RHIC will help to understand better what happens here.

  • Charles Daney

    Here’s a summary of thinking on amino acid chirality as of last April:

    There are plausible mechanisms, but no smoking gun to seal the verdict.

  • http://franklinhu.comtheory.html Franklin Hu

    I was just reading about parity violation in Leon Lederman’s book “The God particle”. What I wonder about is the effect that the magnetic fields have in this experiment. We know that magnetic fields can easily split apart groups of electrons in the classic Stern-Gerlach experiment. If we did such an experiment and only examined one of the spots and ignored the other, then we might also declare that parity is violated because we only see one type of electron emerge from our experiment when we know there are supposed to be 2. How do we know that something like this didn’t happen with this parity violation or the new one that you mention? I would imagine one easy test would be to flip all the magnetic fields around. If this parity violation was due to a magnetic steering effect, then you might see the particles start with left handedness, but when you switch around the poles, you might see all right handedness. On the otherhand if it isn’t a magnetic effect, then they should stay left handed regardless. I did not see any such discussions in the book, so I was wondering how they could be so sure that parity was really violated in this case or in the newer case you cited.


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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