Exotic Pentaquark Particle Explanation


Explanation for the Scientist


The pentaquark is a particle made from four quarks plus one antiquark. All other particles (that interact via the strong nuclear force) are made from either three quarks (called baryons) or from one quark plus one antiquark (called mesons). The proton is a well-known example of a baryon. Of the particles called mesons, none are well-known but the best example is the pi-meson. This particle lives for less than a microsecond, but still particle beams of pi-mesons can be made in the laboratory and used in applications such as cancer radiation treatments. Many dozens of baryons and mesons are known to exist, and these two categories were the only ones (for particles made from quarks) until the pentaquark was discovered. As such, the pentaquark is a new type of quark-matter particle.

Of course, there are other non-quark-matter particles, such as the electron or the neutrino, but only a handful are known. These particles do not interact with other particles via the strong nuclear force, and hence they do not bind together like quarks. These particles are not the topic in the discussion here.

The existence of pentaquarks is not firmly established, but there is good experimental evidence that suggests the existence of one candidate, called the Theta. The experimental evidence comes from over a dozen different measurements using beams of photons, neutrinos and protons. The first evidence came from an experiment in Japan, led by Takashi Nakano (Osaka), where Ken Hicks (Ohio) made key contributions. However, not all experiments have seen the Theta, particularly those done with high-energy protons, and this remains a mystery. More details are given in the review articles given in the links below.


Experimental Outlook for the Pentaquark.

Experimental Review of the Theta+ Pentaquark .

If the Theta particle exists, it is made from two up quarks, two down quarks and one anti-strange quark, with a mass of about 5/3 times the mass of a proton. Unlike the proton, the Theta lives for less than a tiny fraction of a second, yet its lifetime can still be measured. The initial experiments indicate that the Theta has a lifetime about ten times longer than expected (based on 3-quark baryons of similar mass). This poses a challenge to theoretical models of quark interactions. In fact, we hope to learn a great deal about the theory of the force that binds quarks together (called QCD) from the Theta (if it exists).

Experiments to gather more data that will either firmly establish or refute earlier claims of the Theta are currently being done. One such experiment was recently presented at the April 2005 meeting of the APS, by Raffaella DeVita (INFN). Along with co-spokespersons Marco Battaglieri (INFN), Valerie Kubarovsky (Jefferson Lab) and Dennis Weygand (Jefferson Lab), they searched for a mass peak in the particles produced when high-energy photons are directed onto protons. Even though a similar experiment done earlier had shown promising evidence at the mass predicted theoretically for the Theta+ pentaquark particle, their new results (with about 10 times the statistical power) give a null result. The details are given on the link at the bottom of the main page.

In another experiment from Jefferson Lab, Ken Hicks (Ohio) and co-spokesperson Stepan Stepanyan (Jefferson Lab) are using a deuteron target. Daniel Carman and Tsutomu Mibe (Ohio) are also a big part of this project, as are collaborators David Tedeschi and Nathan Baltzell (South Carolina) and European collaborators Patrizia Rossi (INFN), Marco Mirazita (INFN), Silvia Niccolai (Orsay) and Bryan McKinnon (Glasgow). The data-taking has been completed, and the analysis of these data are in progess. Results are expected before the end of 2005.

Last modified: April 15, 2005
Kenneth Hicks