What is a pentaquark

The Pentaquark (from Greek penta, dt. five) is a hadron with a baryon number of +1.

In order to be able to understand its structure within the framework of the standard model of elementary particle physics, it must be made up of at least five quarks, more precisely four quarks and one antiquark.

In parlance, pentaquark is often used to denote the lightest of these particles, which has a strangeness of +1 and the proper name Θ+ (pronounced theta +) carries. Since a strangeness of +1 can only be achieved with an antistrange quark, at least 4 additional quarks are required for a baryon number of +1. So there is das+ based on the quark model made up of two up- and two-down quarks and an antistrange quark. Such a configuration is permitted within the framework of the standard model of elementary particle physics, but has not yet been observed. That is why one speaks of one exotic baryon.

Existential controversy

The unequivocal experimental proof of its existence is the object of intense research and debate and would be a breakthrough in the field of the physics of subatomic, strongly interacting particles (the so-called hadrons), in which so far only baryons consisting of three quarks and mesons consisting of two quarks were known. Atomic nuclei also represent stable quark structures, but these always consist of a multiple of three quarks, since they can be described very well as binding states of protons and neutrons, which in turn are baryons consisting of three quarks.

The existence of pentaquarks was originally predicted in a 1997 publication by Dmitri Diakonov, V. Petrov and M. Polyakov[1], however, this prediction was viewed very skeptically by her colleagues. The prediction predicts a particle with an unusually long lifespan, which would lead to a very small and therefore clearly observable total decay width of only 30 MeV. The mass should be 1530 MeV.

The first experimental observation of the Θ+ was reported in July 2003 by Takashi Nakano at the University of Osaka, Japan and endorsed by Ken Hicks at Jefferson Laboratory, Virginia, USA. This surprising discovery led to a wave of investigations into pre-existing data for signals for the pentaquark. Within a few months, around a dozen different groups also reported evidence for the Θ+ to have discovered. Some groups even claimed to be able to detect more pentaquarks.

However, doubts also arose about the results, both theoretical and experimental. About a dozen other experimental groups have no evidence of the existence of the Existenz+ found. In addition, the experiments found different masses, some of which were incompatible with one another. Particularly surprising was the small decay width, which was still well below the value predicted by Diakonov, Petrov and Polyakov. The pentaquark would thus live over 100 times longer than other particles of comparable mass.

The CLAS collaboration at the Jefferson Laboratory in Newport News, Virginia, USA, under the direction of Raffaella de Vita, has finally started a dedicated experiment, the main aim of which is to investigate the Pentaquark hypothesis. In this most comprehensive investigation to date, there was no evidence of the existence of pentaquarks. As a result, these scientists assume that the previous evidence of pentaquarks is based on misinterpreted data. This work is in the April 2005 issue of the journal Nature to find[2].

Overall, it is still unclear why some groups observe the particle, while others find a negative result. Certainly not to be neglected here is the experimenter effect.

It is interesting to note that most of the experiments that failed to detect pentaquarks were high-energy experiments. In this situation it is known that certain mechanisms are suppressed which could be responsible for the production of pentaquarks at lower energies. Therefore, the previous experiments cannot conclusively rule out the existence of pentaquarks.

Additional theoretical predictions of properties of the pentaquarks would in principle be possible with the help of computer simulations of quantum chromodynamics, so-called lattice range theories. However, these theoretical approaches are not yet very advanced, which is why different groups have so far been able to show contradicting results. The ultimate solution to this problem is likely to require significantly more computing power. Nevertheless, this field of work is only being promoted very cautiously, although this method only requires a fraction of the cost of an experiment and could possibly exclude the existence or provide valuable information for the experimenters.


  1. http://xxx.lanl.gov/abs/hep-ph/9703373
  2. http://www.nature.com/news/2005/050418/full/050418-1.html

Category: elementary particles