Wednesday, October 12, 2011

Loose Ends In The Observed Hadron Set

Aidan Randle-Conde notes in an October 10, 2011 blog post, the case of the X(3872) hadron, first glimpsed in 2003.

This new particle was a narrow resonance that liked to decay to the J/ψ and two pions. At first it just looked a little odd, but analysis showed that it didn’t fit in with what anyone would expect. For example, if it was part of the normal charmonium spectrum it could also decay by emitting a photon into other states in a predictable way. It’s heavy enough to decay directly to charmed mesons, but for some reason it prefers to put an extra pion in there instead. After analyzing the spin of the particle via angular analysis the spin and parity of the particle didn’t seem to make sense. The particle has been seen in at least six independent different experiments, and it doesn’t seem to be a statistical fluctuation. . . . Since discovering the X(3872) we’ve seen a plethora of similar particles which all have similarly imaginative names such as Y(4360). These states drive the heavy flavor theorists crazy. Some of the proposed models include tetraquarks (two quark and two antiquarks in a bound state) a molecule of mesons and a glueball (a bundle of gluons).

This is surprising, because there are only so many ways that you can put six kinds of color charge balanced quarks together, even accounting for chirality and antiparticles. We've found all of the Standard Model predicted mesons and all but a handful of the Standard Model predicted baryons, just where they belong. Then, this one pops up and it doesn't fit the model. This is a round peg in a square hole, and while it may not be as sexy as the Higgs boson, still provides ripe territory for new physics. It is a popular subject for conference papers at events like Charm 2010.

A quick review of the conference papers suggests the prevailing view is that the strange particles that don't fit the meson (two quark) and baryon (three quark) model are "molecules" of two mesons each, some easier to decode than others, rather than true tetraquarks that bind together in an undifferentiated four quark composite particle or "excited states" of existing mesons and baryons.

The binding force that holds these molecules together is presumably a bit like the nuclear binding force between protons and neutrons in an atomic nucleus that "spills over" from the strong force interactions between baryons and is transmitted via meson exchange within nuclei.

More particularly, the meson molecules show similarity to two related effects that probably have a common cause seen in atomic nuclei:

Nucleons appear to differ when they are tightly bound in heavier nuclei versus when they are loosely bound in light nuclei. In the first effect, experiments have shown that nucleons tightly bound in a heavy nucleus pair up more often than those loosely bound in a light nucleus.

"The first thing was the probability of finding two nucleons close together in the nucleus, what we call a short-range correlation," says Larry Weinstein, a professor at Old Dominion University. "And the probability that the two nucleons are in a short-range correlation increases as the nucleus gets heavier." . . . other experiments have shown a clear difference in how the proton's building blocks, called quarks, are distributed in heavy nuclei versus light nuclei. This difference is called the EMC Effect.

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