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Rene Bellwied | USLHC | USA

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When did fundamental QCD go out of fashion ?

There is a lot of buzz about the world’s largest ‘atom smasher’, the LHC, and rightfully so. And if you peel away the glamour of the machine itself, there is some of the most exciting science still to discover. We have predictions paving the way to new frontiers, such as the Higgs, supersymmetry, Dark Energy, extra dimensions, and the list goes on and on.

But after some weeks of reading article after article I can’t stop wondering: what ever happened to good old fashioned Quantum Chromo Dynamics ? Have we unraveled the mysteries of asymptotic freedom, of deconfinement and chiral symmetry restoration, and I just missed it ? Have we experimentally verified a nobel prize winning theory that still left much to be understood ? Have we indeed cracked the fundamental question of matter formation in the universe ? I would venture to say ‘no’ to most of these questions.

And you might say, wait a minute, doesn’t electro-weak symmetry breaking and the potential discovery of the Higgs solve all our remaining questions about matter and mass. Well, does it ? I am talking about the good old proton or neutron here. The building blocks of nature. How do these hadrons come about, how does hadronization occur, this extreme jump in mass, this unique principle of confinement ? Somehow high energy physics has largely turned away from this fundamental process in QCD. Too complicated, not calculable, not first principle, not fundamental (?) etc. etc. And there are at least some factorizable theories, such as fragmentation, or some neat lattice QCD calculations, to answer most of it. Yup, tell that to your first year graduate student. Well, there once was a quark, very energetic and then suddenly it decided to break up into many hadrons just like that. Pleeease. This is where the phrase ‘…and then a miracle occurred..’ enters into our fundamental understanding of nature.

And people will argue with me that much more is understood and on good footing, and they are probably right, but a.) we have stopped investing a lot into the experimental verification of the phenomenon of hadronization and b.) as long as constituent quarks, instantons, sphalerons and even more exotic states are allowed as explanations for the most basic process in the evolution in our universe I don’t feel that the book is closed and that we have achieved a really deep understanding.

Admittedly there are QCD physics groups in all the three major experiments at the LHC, but the buzz seems to focus more on exotica and smartly labeled ‘new physics’ rather than on the questions that should interest all of us the most. 

And let’s not forget that there were some recent experimental breakthroughs on QCD physics. Mostly from RHIC, which now has unambiguously proven that a deconfined state of matter exists at a sufficiently high temperature. It behaves weird though, more like a perfect liquid with strongly coupled degrees of freedom, rather than a weakly coupled plasma.  But it is deconfined, so it unravels one of the QCD pillars experimentally. Its preferred re-confinement method seems to be recombination of quarks, though, rather than fragmentation, which is in blatant disagreement with one of the oldest accepted models in elementary particle physics. So maybe the hadronization in medium is different than the hadronization in vacuum ? And if so, which one is relevant for the generation of matter in the evolution of the universe ? Was the universe a vacuum or a deconfined medium at the time of hadronization ?

And in addition there is still very little evidence for the other pillar of QCD, chiral symmetry restoration, which tries to convince us that a massive universe will turn into a state than can be described by a massless symmetry theory (QCD) above a certain temperature. Pretty dramatic, hmmm ? And still not experimentally verified.

So let’s turn part of our attention back to the basics. It’s true that verification of the chiral transition will likely require heavy ions and is thus the strong suit of ALICE, but also the more limited heavy ion programs in CMS and ATLAS as well as the proton-proton programs in all three experiments can make fundamental measurements to the question of generation of QCD mass, not Higgs mass, in nature. And to me that is more interesting than all the exotica that might or might not prove Gene Rodenberry and Isaac Asimov right. Just call me old fashioned.

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5 Responses to “When did fundamental QCD go out of fashion ?”

  1. Joe Tuggle says:

    You know, I was just thinking about this a few days ago. Most of the mass we’re familiar with comes from QCD dynamics. Too bad we don’t know how to calculate with it. Perturbative expansions might be kinda cool when you learn how to do them, but I do wish there were another way to do calculations.

  2. MP says:

    I think that the reason QCD mysteries are not written about in LHC articles is because they have not been explained to the public in books and articles for the layman. The public probably knows about String Theory and Dark Energy, and the Higgs is easy enough to summarize in a sentence (it gives particles mass!) – not to mention that these all have short and catchy names.

    As a layman myself (advanced enough to know roughly what you’re talking about here) I would really appreciate some Scientific-American-level explanations of the questions you mention. There are many people who have a great interest in physics but have chosen not to dedicate our lives to its study. We count on “science popularizers” to give us a glimpse of what’s going on.

    Explanations for the layman not only generate interest, but as a consequence also help to support funding. (hint, hint)

  3. Seth Zenz says:

    Most particle physicists seem to be motivated by an intense drive toward the fundamental, a drive which has left many things half-understood along the way. I agree that somebody ought to go back and fill in the details, but that wouldn’t get me up in the morning, personally.

  4. So how far have lattice QCD efforts got towards explicitly finding the ground state wavefunction (complicated beast that it is) of the proton?

    It would put a lot of confidence back if your 938MeV could be *calculated*!

  5. peter cameron says:

    some possibility you might find a clue or two here
    http://redshift.vif.com/JournalFiles/V18NO2PDF/V18N2CAM.pdf

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