The quark model of matter is one of the most successful models in modern physics. It explains the structure of the protons and neutrons in matter, it explains why we see quarks in mesons and baryons, and it predicts a “periodic table” of hadrons. It was all going so well until the X(3872) came along and messed everything up. When Rabi heard the news of the discovery of the muon he replied “Who ordered that?!” It’s happened again, this time with hadrons.

The quark model predicts that quarks cannot appear alone. They must always come in groups, and there are only two allowed ways to organize quarks. They can appear in groups of three quarks or antiquarks (known as baryons), or they can appear as a quark-antiquark pair (known as mesons). Formally, they can also appear in combinations of baryons or mesons, and in fact the nuclei of atoms are bound states of baryons. So far there have been no definite candidates for bound states of mesons. By now, nearly all of the mesons have been discovered and fit into this framework, and most of the baryons have also been identified. In order to show the mesons and baryons properly we need to go beyond the usual two dimensional table, so the multiplets get shown in multidimensional diagrams. If we limit ourselves to just the four lightest quarks we can show the mesons on a three dimensional hexadecuplet:

Each one of those particles has been discovered and its properties measured. Our state of the art models get the masses about right. Whenever we expect to see a missing particle, we look in the data and see it. We’re still missing a few baryons, but we’re getting there.

Then the X came along. It made its first appearance at the e705 experiment at Fermilab, but from what I remember hearing, it was largely ignored. Belle announced the discovery of a mysterious new particle 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. After years of searching, here is everything we know about the X(3872), taken from the Particle Data Group:

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). In my more obscure moments I sometimes think it would be funny if it was actually the Higgs. Of course it isn’t, its properties are all wrong, but it would be ironic if we were searching for it in all the wrong places.

The X(3872) was discussed several times at the Charm 2010 conference. One of the best talks was given by Marina Nielson, and she shows us what the Charmonium spectrum would look like if we included these new particles. It’s a bit messy:

Since Charm 2010 I’ve moved away from Charm physics in favor of new physics searches at the LHC, so I don’t have any new information about the status of the X(3872). I imagine ATLAS, CMS and LHCb could all reconstruct it if they wanted to, but so far all I’ve seen is results from CMS, presented at Blois. If anyone has any more up to date news I’d love to hear it!