Physicists have confirmed the existence of two rare pentaquark states. The discovery, which took place at the CERN Large Hadron Collider (LHC) in Geneva, Switzerland, could have major implications for the study of the structure of matter.
The finding also puts to rest a 51-year-old mystery, in which American physicist Murray Gell-Mann famously posited the existence of fundamental subatomic constituents called quarks, which form particles such as protons. In 1964, he said that—in addition to a constituent with three quarks—there could be one with four quarks and an anti-quark, known as a “pentaquark.” Until now, the search for pentaquarks has been fruitless.
“The statistical evidence of these new pentatquark states is beyond question,” says Sheldon Stone, professor of physics at Syracuse University, who helped engineer the discovery. “Although some positive evidence was reported around 10 years ago, those results have been thoroughly debunked. Since then, the LHCb [Large Hadron Collider beauty] collaboration has been particularly deliberate in its study.”
Stone credits Gell-Mann, a Nobel Prize-winning scientist who spent much of his career at Caltech, for postulating the existence of quarks, which are fractionally charged objects that make up matter.
“He predicted that strongly interacting particles [hadrons] are formed from quark-antiquark pairs [mesons] or from three quarks [baryons],” Stone says. “This classification scheme, which has grown to encompass hadrons with four and five quarks, underscores the Standard Model, which explains the physical make-up of the universe.”
Stone says that while his team’s discovery is remarkable, it also prompts many questions. One of them is the issue of how quarks bind together. The traditional answer has been a residual nuclear force, approximately 10 million times stronger than the chemical binding in atoms.
But not all bindings are created equal, says collaborator and physics professor Tomasz Swarnicki. “Quarks may be tightly bound or loosely bound in a meson-baryon molecule. The color-neutral meson and baryon feel a residual strong force [that is] similar to the one binding nucleons to form nuclei.”
Adds Stone: “The theory of strong interactions is the only strongly coupled theory we have. It is particularly important for us to understand, as it not only describes normal matter, but also serves as a precursor for future theories.”
Last year, Skwarnicki also helped prove the existence of a meson named Z(4430), with two quarks and two antiquarks.
Liming Zhang, a former Syracuse University research associate and associate professor at Tsinghua University in Beijing China, will present the findings at an LHCb workshop on Wednesday, July 22, at CERN.
Source: Syracuse University