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.
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 web page linked
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).
This work is supported in part by the US Department of Energy and
the National Science Foundation.
These results were just presented at the annual Users Group Meeting
at Jefferson Lab. The results do not show any evidence for the
pentaquark. The actual mass spectra are shown in the link at the
bottom of the web page here .