LHCb’s Pentaquark Discovery Named Top 10 Breakthrough of 2015
The Top 10 is chosen by a panel of Physics World editors and reporters. Qualified research must be of fundamental importance, represent a significant advancement in knowledge, demonstrate a strong link between theory and practice and be of interest to all physicists.
Physics professors Tomasz Skwarnicki and Sheldon Stone, along with Ph.D. student Nathan Jurik G’16 and former research associate Liming Zhang (now a professor at Tsinghua University in Beijing), made international headlines in July when they discovered two rare pentaquark states.
The accolade is the latest in a series for the team, putting to rest a 51-year-old mystery, in which American physicists Murray Gell-Mann and George Zweig independently proposed that all baryonic matter is composed of three quarks, or four quarks and an antiquark known as a pentaquark. While many three-quark baryons have been found, the pentaquark sighting is a first.
“This discovery explains not only how protons and neutrons are bound together, but also how matter is constituted,” says Alan Middleton, professor and chair of physics. “Pentaquarks force us to rethink how atomic nuclei behave under the most extreme conditions, from the super-dense Early Universe to neutron stars. It’s a milestone for particle physics in general.”
The discovery occurred at the European Organization for Nuclear Research, also known as CERN. Based in Geneva, Switzerland, CERN is home to the Large Hadron Collider (LHC), the biggest, most powerful particle accelerator in the world. Each year, thousands of scientists and engineers flock to CERN, in hopes of better understanding the makeup of the universe.
Among them are some 800 scientists—including a team of 20 Syracuse researchers, headed up by Stone—who are involved with the LHCb experiment. (The “b” stands for “beauty,” which is a type of quark found in protons and neutrons. While absent in today’s universe, b quarks were prevalent after the Big Bang, nearly 14 billion years ago, and have been generated extensively by LHCb.) The LHCb team uses the accelerator to fling particles together to see what happens.
“Energy is sometimes converted into heavy particles that are not normally found in nature,” says Skwarnicki, a fellow of the American Physical Society (APS). “By examining the debris from these high-impact collisions, we’re able to learn more about the building blocks of matter and the forces controlling them.”
“All convincing tetraquark and pentaquark candidates have a heavily charmed quark-antiquark pair inside them,” Skwarnicki says, adding that charm quarks are among the most massive of all quarks.
“A lot of different quarks are created by the LHC, before they fall apart or decay into other forms,” says Stone, who spends an average of three months a year at CERN, working on LHCb. “Our goal is to catch these b quarks, which are usually part of some baryon [with three quarks] or meson [three quarks and an antiquark], and to analyze their decays.”