LEGO® Exotics: Pentaquarks

Five Quarks

It is not against the rule of colour strong charge addition to create particles comprising of more than three quarks, and example of this are particles made from four quarks and one antiquark. These particles can be made in one of two ways; all quarks and antiquark could be contained within some single particle, which we call a pentaquark; or they could be a bound pair of one Baryon and one Meson, known as a Baryon-Meson molecule.  The discussion, when talking of Baryons and Mesons, of the strong force colour charge means there should be nothing stopping us from creating pentaquarks or Baryon-Meson molecules. A white strong charge Baryon plus a white strong charge Meson would simply result in a white strong charge bound molecule. Also if we have 4 quarks and 1 antiquark we can also create a white charge pentaquark in a number of different ways:
red + green + blue + red + anti-red = white
red + green + blue + green + anti-green = white
red + green + blue + blue + anti-blue = white
One such particle, the theta-plus, is shown below comprised as either a pentaquark (left) or a Beyond-Meson molecule.
Theta_plus
In 1997 Dmitri Diakonov, Victor Petrov, and Maxim Polyakov [1]employed similar methods to Gell-Mann in his Eightfold way, using the symmetries of the quarks to predict not only the existence but also the expected mass of pentaquark particles. Again like Gell-Mann they predicted a pattern in these symmetries called an Exotic Baryon anti-decuplet; exotic because these particles (or combinations thereof) are not constructed in the same way as other Baryons; baryon because they have some properties common with Baryons (there is at least one baryon’s worth of quarks making up these particles); anti-decuplet because there were 10 particles, as in Gell-Mann’s decuplet, but pointing in the opposite direction. I have drawn one representation of this anti-decuplet below using my LEGO analogy. This is just one of a number of patterns that can be, and have been, drawn from quark symmetries.
Baryon-Meson_Molecule_antidecuplet

Exotic Baryon Anti-decuplet represented as Baryon-Meson molecules.

Pentaquark_Baryon_antidecuplet

Exotic Baryon Anti-decuplet represented as pentaquarks.

With the prediction out there it is was now the job of the experimentalists to smash particles into one another and sift through the debris to see if any of these particles existed. They chose to focus their searches upon those particles at the extreme points of the anti-decuplet triangle. The lighter particles produced when these pentaquarks decay can only be explained by these exotic states. Let us take the Θ+ as an example.
img8The Θ+ can be identified experimentally by the fact it is uniquely strange. The Θ+ contains an anti-strange quark while all three quark baryons can only contain a strange quark, because no baryon contains an antiquark. We can say that the Θhas an opposite strangeness to all traditional Baryons; this is something that can be identified in particle detectors. The Θ+ is similar to Baryons as it has the same quality known as baryon number; related to the colour charge of the quarks and antiquarks. Both pentaquarks and three quark Baryons have a baryon number of 1; each quark has baryon number +1/3 and the antiquark has baryon number -1/3. Experiments have shown that the strangeness and baryon number must be conserved when a particle decays to other lighter particles. By tracking strangeness and baryon number, experiments are able to pick out groups of particles which could only have come from the decay of a pentaquark. As we will discuss in future posts, this shows up in experimental data as a large amount of extra data around a single particle mass which sits on top of a broad number of other possible background data.

In 2003 the LEPS experiment in Japan published a paper [2] which suggested evidence that a particle with a mass the same as the Θ(within errors) had been seen within its detectors. Over the next year this claim was followed by some nine other experiments all saying that they too had seen an excess in their data around the predicted Θmass. The evidence for this pentaquark seemed compelling, but there were some problems and questions surrounding the data. In some cases the number of background events were underestimated, which exaggerated and excesses there might have been. Some experiments chose specific techniques to enhance data around the predicted mass of the Θ+. When considering the results of all ten experiments the range of masses determined by each, although similar, varied far more than one would expect from the given theory. It was obvious that further experiments were needed, with much more data, if the existence of the Θwere to be confirmed or refuted.

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