CERN’s LHC helps find Pentaquark

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The Large Hadron Collider (LHC) has put to rest 51-year-old mystery involving quarks by discovering a pattern that has never been observed before in over fifty years of experimental searches – pentaquark.

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’s 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 at CERN looked for pentaquark states by examining the decay of a baryon known as Lambda b into three other particles J-psi, a proton and a charged kaon. On studying the spectrum of masses of the J-psi and the protonm, they found 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.

“The statistical evidence of these new pentaquark states is beyond question,” says Sheldon Stone, Distinguished Professor of Physics, 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.”

Physicist Tomasz Skwarnicki of Syracuse University added that the large data set provided by the LHC, and the excellent precision of the detector have enabled them to examine all possibilities for these signals, and the conclusion is that they can only be explained by pentaquark states

“More precisely the states must be formed of two up quarks, one down quark, one charm quark and one anti-charm quark”, Skwarnicki added.

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 makeup of the Universe.”

Though scientists have tried to search for pentaquarks, they haven’t been able to search for them conclusively. This is where LHC’s data set and LHCb experiment differ as they have been able to look for pentaquarks from many perspectives, with all pointing to the same conclusion.

Illustration of the possible layout of the quarks in a pentaquark particle such as those discovered at LHCb. The five quarks might be tightly bonded. © CERN

Illustration of the possible layout of the quarks in a pentaquark particle such as those discovered at LHCb. The five quarks might be tightly bonded. © CERN

“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.”

Stone says that, while his team’s discovery is remarkable, it still raises 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, Skwarnicki says. “Quarks may be tightly bound or loosely bound in a meson-baryon molecule,” he explains. “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.”

The discovery is the latest in a string of successes for the Department of Physics, which made international headlines last year, when Skwarnicki helped prove the existence of a meson named Z(4430), with two quarks and two antiquarks.

More studies will be needed to distinguish between these possibilities, and to see what else pentaquarks can teach us. The new data that LHCb will collect in LHC run 2 will allow progress to be made on these questions.

The findings have been published on ArXiv.