Subatomic particles made from quarks have been found to fit into
two categories: (1) particles made from 3 quarks, called baryons, and
(2) particles made from one quark and one anti-quark, called mesons.
For example, the proton and the neutron are members of the baryon group.
The meson group does not have any stable members, and hence is less
well known. An example is the "pi" meson, which exists only for
tens of nanoseconds, but long enough to study in the laboratory.
Some combinations, such as a particle made from 4 quarks, are not
allowed by the rules of the strong force, Quantum Chromodynamics
(or QCD for short).
The pentaquark particle, if it exists, would be a combination of
4 quarks and one anti-quark. This has the same quarks as the
combination of baryon plus a meson, and so it satisfies the rules
of QCD. However, there is no experimental evidence for a stable
pentaquark particle. Recently, several experiments reported
evidence for a short-lived particle, called the "Theta", which
has characteristics of a pentaquark. On the other hand, there
are roughly an equal number of experiments that did not see
any evidence (but were expected to see pentaquarks). Now the
question is: does the "Theta" particle exist or not?
Recently, two experiments have dealt a blow to the possible existence
of the pentaquark. Both are high-statistics experiments done at
the CLAS detector
of Jefferson Lab .
Another question is: why should you care? One answer is that
if the pentaquark exists, then it is a new catagory of subatomic
particle and hence it will help us learn about the strong force
(QCD) that binds quarks together into matter. While this does
not have direct implications in today's society, we hope that
this will lead to new technologies in the future. Just consider
that about 100 years ago, the electron was considered to be a
useless piece of knowledge! Today, electronics are everywhere.
Another example is radiation. Fifty years ago, who would have
thought that radiation therapy would be a treatment for cancer?
While we can't predict the future, we hope that learning more
about the forces that bind matter together will lead to new