The Z(4430), a new subatomic particle that provides smoking gun evidence for a four-quark mesonUniversity of Hawaiʻi at Mānoa
Posted: Apr 3, 2008Researchers at the University of Hawaii at Manoa
in the Physics and Astronomy Department working
on the Belle experiment in Tsukuba, Japan reported
the discovery of a new class of subatomic particle in the April 11th
issue of Physical Review Letters, the leading US physics journal.
In 1964, Murray Gell-Mann proposed that the subatomic
particles seen in nature are comprised of different
configurations of fundamental constituents that
he called quarks. In Gell-Mann's scheme, quarks form
particles in two ways: three quark combinations
produce "baryons," particles like the proton and neutron that
form the nuclei of atoms in most matter,
while quark-antiquark combinations form "mesons," such as
the pi-meson and K-meson.
For the four decades that have elapsed since the birth of the quark idea, Gell-Mann's three-quark baryon quark-antiquark meson description has sufficed to explain all observed particles. Recently, however, researchers in the Belle experiment in Japan have found a meson that that defies classification as a simple quark-antiquark state. This meson, called the Z(4430), is seen to decay
into an electrically neutral psi' meson --a well established
charmed-quark anticharmed-quark state-- plus a charged pi meson.
The psi' among the decay products indicates that the parent
Z(4430) must contain a charmed-anticharmed quark pair. However,
this quark combination is necessarily electrically neutral. Since the Z(4430)
is electrically charged it must contain an additional
quark-antiquark pair (up-quark antidown-quark for the Z+(4430) and
down-quark antiup-quark for the Z-(4430).
mesons have been proposed by theorists and a number
of candidate "tetraquarks" have been identified in
experiments, most notably the X(3872), A meson that
was found by UH researchers in 2003. However, prior to
the Z(4430), all such candidates have been
electrically neutral, which allowed the possibility for
a quark-antiquark classification. This "out" does not
exist for the charged Z(4430), which appears
to be smoking-gun evidence for a tetra-quark meson state.
The Physical Review Letter on the Z(4430) is the result of work by Professor
Stephen Olsen (University of Hawaii) and Sookyung Choi (Geyongsung
University) in Korea and the Belle experimental collaboration.
Other University of Hawaii participants in Belle include
faculty members Tom Browder, Michael Jones, Mike Peters, Gary Varner,
postdoctoral fellows Herbert Hoedlmoser and Li Jin, graduate
students Hulya Guler, Kurtis Nishimura, Jamal Rorie
and Himansu Sahoo. The work in the Belle experiment,
an international collaboration of physicists from countries
in Asia, America, Europe and Australia is supported
by the US Department of Energy. The University of Hawaii group also plans
to participate in the Super KEKB factory upgrade that will start
operation in 2012 in Tsukuba, Japan.
For more information, visit: http://belle.kek.jp/