Quarks never travel alone for long, so they’re usually produced as part of a larger particle, containing one or two additional quarks. The authors got a computer to model what would happen if the decay went through an intermediate particle they called Pc+. Assuming the results hold up, the particle raises the same question that came up in response to the tetraquark: how is this put together?
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But there was nothing in our theories that prevented larger assemblages having even more quarks. And now, an experiment at the Large Hadron Collider (LHC) has come up with evidence of a five-quark behemoth.
The physics of these particles can potentially tell us about the limitations of the Standard Model or the asymmetry between matter and antimatter in the Universe. In this particular experiment, the LHCb team was looking at ?0b baryons, which contain an up quark, a down quark, and a bottom quark.
This particle contains two up and one down quarks, along with a charm and anticharm—pentaquark-charmonium, which can exist in a number of possible energy states. The model used to identify the particles had to be simplified down to reduce the number of free parameters, or the math would have been outrageous. The zoo of particles, along with the familiar proton and neutron, were built from combinations of the six quarks or their antiparticles. This simplification involved ignoring even more exotic possibilities, so it is probably reasonable, but you can expect theoreticians to look it over carefully.
One is that it’s put together like known particles, with multiple quarks interacting through gluons.
The system neatly explained the spin and charge of these particles and helped make sense of particles discovered as accelerators reached even higher energies. A buzz of excitement about a five-quark particle came and subsided after other accelerator teams couldn’t reproduce the result.
The authors were measuring this decay in part because it can tell us something about the half-life of the parent ?0b particle. There was just one problem: no particles could explain why the peaks were at the locations they were seen. The significance of the match was over nine sigma; only five sigma are required for physicists to announce discovery. And it’s somewhat surprising that a single analysis would turn up two members of a class of particles that had eluded detection previously. The alternative is that it’s a hybrid composed of two particles we already know about.
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