New particle discovered at Hadron Collider
14 Jul 2015
Scientists at CERN’s Large Hadron Collider (LHC) have announced the discovery of a new class of particles known as pentaquarks.
Experiments to detect pentaquarks form part of LHC’s “beauty” project, which - unlike other collider projects - uses a series of subdetectors to detect mainly forward particles - those thrown forwards by the collision in one direction.
“The pentaquark is not just any new particle,” said LHC beauty (LHCb) spokesman Guy Wilkinson.
“The pentaquark is not just any new particle
LHCb spokesman 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,” Wilkinson said.
“Studying its properties may allow us to understand better how ordinary matter, the protons and neutrons from which we’re all made, is constituted.”
During the 1960s, US physicist Murray Gell-Mann suggested 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.
According to Gell-Mann’s theory, this “quark model” also allows for the existence of other quark composite states, such as pentaquarks composed of four quarks and an antiquark.
To discover pentaquark states, the LHCb team studied the decay of a baryon known as ?b (Lambda b) into three other particles, a J/?- (J-psi) meson, a proton and a charged kaon meson.
Studying the spectrum of masses of the J/? and the proton revealed that intermediate states were sometimes involved in their production, an LHC statement said.
“Benefitting from the large data set provided by the LHC, and the excellent precision of our detector, we have examined all possibilities for these signals, and conclude that they can only be explained by pentaquark states”, says LHCb physicist Tomasz Skwarnicki of Syracuse University.
“More precisely the states must be formed of two up quarks, one down quark, one charm quark and one anti-charm quark,” he said.
The next stages of analysis will involve the study of how quarks are bound together within the pentaquarks.
“The quarks could be tightly bound,” said LHCb physicist Liming Zhang of Tsinghua University.
“Or they could be loosely bound in a sort of meson-baryon molecule, in which the meson and baryon feel a residual strong force similar to the one binding protons and neutrons to form nuclei.”
The research team has submitted a paper reporting its findings to the journal Physical Review Letters.
