(14 July 2015) Today, the LHCb experiment at CERN’s Large
Hadron Collider has reported the discovery of a class of particles known as
pentaquarks. The collaboration has submitted a paper reporting these findings
to the journal Physical Review Letters.
“The pentaquark is not just any new particle,” said LHCb
spokesperson Guy Wilkinson. “It represents a way to aggregate quarks, namely
the fundamental constituents of ordinary protons and neutrons, in a pattern
that has never been observed before in over fifty years of experimental
searches. Studying its properties may allow us to understand better how
ordinary matter, the protons and neutrons from which we’re all made, is
constituted.”
Our understanding of the structure of matter was
revolutionized in 1964 when American physicist, Murray Gell-Mann, proposed that
a category of particles known as baryons, which includes protons and neutrons,
are comprised of three fractionally charged objects called quarks, and that
another category, mesons, are formed of quark-antiquark pairs. Gell-Mann was
awarded the Nobel Prize in physics for this work in 1969. This quark model also
allows the existence of other quark composite states, such as pentaquarks
composed of four quarks and an antiquark. Until now, however, no conclusive
evidence for pentaquarks had been seen.
LHCb researchers looked for pentaquark states by examining
the decay of a baryon known as Λb (Lambda b) into three other particles, a J/ψ-
(J-psi), a proton and a charged kaon. Studying the spectrum of masses of the
J/ψ and the proton revealed that intermediate states were sometimes involved in
their production. These have been named Pc(4450)+ and Pc(4380)+, the former
being clearly visible as a peak in the data, with the latter being required to
describe the data fully.