Earlier this week, the LHCb experiment at the Large Hadron Collider announced that, after painstakingly analyzing the data from earlier runs, they have decisive evidence of a previously unobserved particle: the pentaquark.
What’s a pentaquark? In simple terms, it’s five quarks stuck together. Stick two up quarks and a down quark together, and you get a proton. Stick two quarks together, you get a meson of some sort. Five, you get a pentaquark.
(In this case, if you’re curious: two up quarks, one down quark, one charm quark and one anti-charm quark.)
Artist’s Conception
Crucially, this means pentaquarks are not fundamental particles. Fundamental particles aren’t like species, but composite particles like pentaquarks are: they’re examples of a dizzying variety of combinations of an already-known set of basic building blocks.
So why is this discovery exciting? If we already knew that quarks existed, and we already knew the forces between them, shouldn’t we already know all about pentaquarks?
Well, not really. People definitely expected pentaquarks to exist, they were predicted fifty years ago. But their exact properties, or how likely they were to show up? Largely unknown.
Quantum field theory is hard, and this is especially true of QCD, the theory of quarks and gluons. We know the basic rules, but calculating their large-scale consequences, which composite particles we’re going to detect and which we won’t, is still largely out of our reach. We have to supplement first-principles calculations with experimental data, to take bits and pieces and approximations until we get something reasonably sensible.
This is an important point in general, not just for pentaquarks. Often, people get very excited about the idea of a “theory of everything”. At best, such a theory would tell us the fundamental rules that govern the universe. The thing is, we already know many of these rules, even if we don’t yet know all of them. What we can’t do, in general, is predict their full consequences. Most of physics, most of science in general, is about investigating these consequences, coming up with models for things we can’t dream of calculating from first principles, and it really does start as early as “what composite particles can you make out of quarks?”
Pentaquarks have been a long time coming, long enough that someone occasionally proposed a model that explained that they didn’t exist. There are still other exotic states of quarks and gluons out there, like glueballs, that have been predicted but not yet observed. It’s going to take time, effort, and data before we fully understand composite particles, even though we know the rules of QCD.
4 gravitons 
I recall reading once that a five-quark hadron was possible, but no one had ever seen one. Are there any implications for its sighting, or is this just more confirmation of the SM?
(Off topic: A question I’m asking scientists this week: Do you consider Einstein showed Newton wrong or that Einstein extended Newton to other domains?)
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Seeing the pentaquark rules out various pentaquark-less proposals, but otherwise it’s essentially just confirming the SM. It will likely provide very useful data for those who study hadron masses/structure, but it’s not evidence for any beyond-the-standard-model physics.
“Einstein showed Newton wrong” is generally considered bad pedagogy, in my experience. Newtonian mechanics still works fine in a wide variety of applications, most mechanical engineering is after all resolutely Newtonian. “Einstein extended Newton to other domains” is a little better, but still seems a bit off to me. I’d probably go for something like “Einstein showed that Newtonian physics was a limiting case of deeper rules.”
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I like your version! Thanks!
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I would note, just to be clear, that the “true pentaquark” interpretation of the results is not a consensus interpretation of the data. The alternative view would be the much less exciting possibility that there was a baryon and a meson which were linked to each other in a manner analogous to the linkage of atoms into a molecule (or perhaps of baryons into an unstable atomic nucleus), per Karliner and Rosner http://www.science20.com/a_quantum_diaries_survivor/marek_karliner_not_a_pentaquark_but_a_molecule_as_he_and_rosner_predicted-156535
FWIW, I’m incline to think that their criticism is well founded and probably correct.
Similarly, many proposed tetraquark candidates have been described more fruitfully instead as “meson molecules”.
What makes a “true tetraquark” or “true pentaquark” exceptional is the absence of this kind of substructure that breaks the composite object into distinct and familiar mesons and baryon subparts. In the “molecule” analysis a pentaquark is not a pentagram and is instead a lopsided bar bell with a baryon on one side and a meson on the other, and a tetraquark is a bar bell with a meson on each side of the connection between the two hadrons.
Similarly, a definitive glueball sighting remains elusive.
The failure of physicists after all of these years to make definitive sightings of glueballs, tetraquarks or pentaquarks, all of which are not obviously forbidden by QCD is quite notable (since in QM, everything that is not forbidden is mandatory), and could even point to some minor axiom of QCD that has been omitted from an otherwise extremely successful theory.
Yes, it could be that these phenomena are simply hard to see and nearly impossible to see except at very high energies such as those found at the LHC. But, it could be that something we haven’t included in the QCD model actively suppresses their appearance and actually prevents true tetraquarks, true pentaquarks or pure glueballs from coming into being. (And, while using QCD to calculate the properties of true tetraquarks or true pentaquarks, this isn’t the case for glueballs for which detailed calculations of their properties were calculated long before those of many classes of mesons and baryons were calculated from first principles).
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