Tuesday, July 14, 2015

Pentaquark discovery claimed by LHCb

Due to confinement, quarks are obsessive about the creation of bound states. Most typically, we have quark-antiquark pairs (mesons) and triplets of quarks (baryons). All of those states have lots of extra gluons and quark-antiquark pairs.

Quark bound states simpler than the pentaquark

The word "pentaquark" means "five quarks". They are hypothetical particles made out of five quarks-or-antiquarks. The Greek prefix is being used to remember the times when Greece was an advanced country, some 2,000 years ago. These bags of 5 particles have to contain 4 quarks and 1 antiquark, or vice versa, because \(4-1\) and \(1-4\) are the only multiples of 3 among the allowed numbers \(x-(5-x)\) and the divisibility by three is needed for the particle to be color-neutral.

This word has appeared once on this blog. About 9.7 years ago, I wrote about a seminar at which Peter Ouyang had claimed that pentaquarks didn't exist, for some subtle technical reasons. (Well, the plural has appeared thrice on TRF.)

Well, this Gentleman will surely find a today's paper by the LHCb collaboration controversial. The experimenters released this preprint
Observation of \(J/\psi p\) resonances consistent with pentaquark states in \(\Lambda^0_b\to J/\psi K^- p\) decays
The particular pentaquark that is apparently being observed is a "pentaquark-charmonium" state.

They looked at lots and lots of decays of the \(\Lambda_b^0\) hyperon, a well-known cousin of the neutron (or the proton). The neutron has the quark content \(udd\). Replace one \(d\) by the bottom quark \(b\) and you get \(udb\) which is the content of the neutral bottom Lambda hyperon.

This beast is created many times in the LHC collisions. And it often decays to three particles: the \(J/\psi\) meson, also called the charmonium (the content is \(c\bar c\)), discovered by Richter's and Ting's teams in 1974; the negative kaon with the \(s\bar u\) content; and our beloved \(uud\) proton.

This is a three-body final state but one may calculate the invariant mass of two of the final particles, the charmonium and the proton, and they experimentally find two clear peaks. They correspond to resonances with
  1. mass \(4.380\pm 0.008\pm 0.029 \GeV\), width \(205\pm 18\pm 86\MeV\)
  2. mass \(4.450\pm 0.002\pm 0.003 \GeV\), width \(39\pm 5\pm 19\MeV\)
Each of these two resonances is seen at a significance level exceeding nine sigma so there's no possibility that it's just some "fluke". The interpretation could hypothetically be different from a "pentaquark" – whose quark content is \(uds c\bar c\) – but the observed widths are rather large, several megaelectronvolts (the decay is fast) so there is not enough time for changes of the quark content.

I would think that these new resonances are pentaquarks, indeed, when it comes to the number of dominant valence quarks. Another question is whether all claims that have been made about pentaquarks in the theoretical literature are correct. I would be much less certain about that... There is a school of thought that interprets these new states as hadronic molecules. Because the counterpart of the fine-structure constant for the strong force is so close to one, I have a problem with the very concept of separation of multi-quark bound states and "molecules". The difference between them can't be parameterically separated. But just to be sure, it is plausible that the description of the states as "molecules" will turn out to be useful.

Those impatient people who love to talk about "deadlines" for discovery may want to know that the pentaquarks were first hypothesized in the 1960s, half a century ago. Even though they don't really require any extraordinary energies of the experiments, it took quite some time to observe them.

See e.g. the BBC for a short story or a CERN press release.

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