Cracking the quark code: Purdue physicists uncover the true makeup of a mysterious particle

2025-08-29

Nuclear physics group at Purdue helps solve a half-century-old puzzle that involves the fundamental building blocks of matter and antimatter.

Professor Fuqiang Wang (left) and graduate student An Gu (right) worked with the CMS detector apparatus in the background. Photos were added to the CMS detector image by Wang. (Photo provided by Fuqiang Wang)

A group of physicists at Purdue University has cracked a decades-old mystery surrounding a tiny particle called the f0(980). Discovered in the 1970s, the f0(980) particle puzzled scientists for decades because they couldn’t agree on how many smaller particles (quarks and antiquarks) made it up.

Quarks are the fundamental building blocks of matter, and antiquarks are their antimatter counterparts. These tiny particles usually combine in groups of three (as in protons and neutrons) or in pairs of a quark and an antiquark. However, physics doesn’t rule out the possibility of more complex combinations, such as four-particle (tetraquark) states. The f0(980) particle was suspected to be one of these unusual combinations, but no one could directly measure its composition.

Now, graduate student An Gu, Prof. Fuqiang Wang, and Prof. Wei Xie from the Department of Physics and Astronomy at Purdue University, along with other collaborators with the Compact Muon Solenoid (CMS) experiment, have been able to “directly” count the number of quarks and antiquarks inside of the f0(980) particle and found that it was two, not four as previously thought by some theoretical models. Their discovery has been recently published in Nature Communications.

“We conduct basic research using high-energy nucleus-nucleus (heavy-ion) collisions,” explains Fuqiang. “The primary goal is to study the state of matter at very high temperatures, those found in the early universe right after the Big Bang, and/or matter densities found in the cores of neutron stars. We use high energy heavy ion collider facilities.”

The team used the (CMS) detector at the European Organization for Nuclear Research (CERN)’s Large Hadron Collider in Geneva, Switzerland, to conduct high-speed collisions between protons and lead nuclei at speeds nearly 99.99999% of the speed of light. These collisions recreated extremely high temperatures like those in the early universe roughly 10 microseconds after the Big Bang.

“A unique feature of these collisions is that the produced quarks and antiquarks have a preferred direction of emission because of the fluctuations in the way the collision happened, the degree of which is encoded in their anisotropies,” explains Fuqiang. Anisotropy is the property of being directionally dependent, as opposed to isotropy, which means homogeneity in all directions. “If a quark-antiquark pair forms a particle, then the particle’s anisotropy will be twice that of the quarks and antiquarks. If two quark-antiquark pairs form a particle, then the particle’s anisotropy will be four times. The anisotropy of quarks and antiquarks can be calibrated by many types of particles whose quark contents are precisely known.”

This picture illustrates the formation of particles in heavy ion collisions. Particles tend to form when the constituent quarks have similar positions and momenta. (Figure provided by Fuqiang Wang)

That said, the team analyzed how the f0(980) particle behaved and discovered that its motion matched that of a simple quark-antiquark pair, not the more complex tetraquark structure some had theorized. “This is the most “direct” counting of quarks known to physicists. It solved a half-century-old puzzle,” says Fuqiang.

The CMS experiment performs the present work. The detector was built by thousands of engineers and scientists over a period of more than 15 years. It is a large international collaboration comprised of around 240 institutions from over 50 countries. Purdue University, a collaborating institution composed of high-energy particle and nuclear physicists, is one of the participating institutions. The research is supported by the U.S. Department of Energy.

 

About the Department of Physics and Astronomy at Purdue University   

Purdue’s Department of Physics and Astronomy has a rich and long history dating back to 1904. Our faculty and students are exploring nature at all length scales, from the subatomic to the macroscopic and everything in between. With an excellent and diverse community of faculty, postdocs and students who are pushing new scientific frontiers, we offer a dynamic learning environment, an inclusive research community and an engaging network of scholars.   

Physics and Astronomy is one of the seven departments within the Purdue University College of Science. World-class research is performed in astrophysics, atomic and molecular optics, accelerator mass spectrometry, biophysics, condensed matter physics, quantum information science, and particle and nuclear physics. Our state-of-the-art facilities are in the Physics Building, but our researchers also engage in interdisciplinary work at Discovery Park District at Purdue, particularly the Birck Nanotechnology Center and the Bindley Bioscience Center. We also participate in global research including at the Large Hadron Collider at CERN, many national laboratories (such as Argonne National Laboratory, Brookhaven National Laboratory, Fermilab, Oak Ridge National Laboratory, the Stanford Linear Accelerator, etc.), the James Webb Space Telescope, and several observatories around the world.   

 

Written by: David Siple, communications specialist, Purdue University Department of Physics and Astronomy

Contributors:

Fuqiang Wang, professor, Purdue University Department of Physics and Astronomy