The APS Topical Group on Hadronic Physics

Credit: Maximilien Brice/CERN

The Large Hadron Collider in Geneva, Switzerland, smashes subatomic particles called hadrons—mostly protons, a type of hadron—together at extraordinarily high speeds.

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It’s a physics 101 principle: The strong force binds quarks and gluons together into the protons, neutrons, and nuclei that form most visible matter in the universe. But the laws governing these interactions—known as quantum chromodynamics or “QCD”—are far from fully understood: How are protons and neutrons bound together? How do quarks and gluons give a proton its mass and spin? How densely can they be packed in atomic nuclei?

With more than 600 members, the Topical Group on Hadronic Physics (GHP) is a hub for physicists seeking answers to these questions.

To unravel the mysteries of hadrons, physicists use some of the world’s largest and most sophisticated particle accelerators to conduct two types of experiments: hot and cold. Key to the study of “hot” hadronic matter is Brookhaven National Lab’s Relativistic Heavy Ion Collider, which smashes together two beams of gold ions traveling at nearly the speed of light, melting the protons and neutrons and momentarily freeing quarks and gluons. To study “cold” hadronic matter, many physicists rely on the particle accelerator at Thomas Jefferson National Accelerator Facility, which fires a stream of electrons at proton and neutron targets, causing their particles to scatter into an array of detectors. Other projects are going on at Fermilab and CERN’s Large Hadron Collider, and physicists are excited about the Electron-Ion Collider, which will be sited at Brookhaven National Laboratory.

“At the most basic level, we’re trying to understand how quarks and gluons interact,” explained David Gaskell, chair of GHP and a staff scientist at the Thomas Jefferson National Accelerator Facility.

Despite the breadth and depth of the field, hadronic physicists lacked a home within APS until GHP’s establishment in 2001. “Hadronic physics used to fall into the realm of high energy particle physics, but that got left behind as the focus in particle physics shifted to searching for the Higgs Boson,” Gaskell explained. “Hadronic physics can also be considered a part of nuclear physics, but this is quite a broad field—lots of topics fall under that umbrella.” Accordingly, many hadronic physicists are dispersed between the Divisions of Particles and Fields or Nuclear Physics, huge fields in which hadronic physics can slip through the cracks. GHP provides a community where hadronic physics is front and center.

GHP’s activity is centered on APS’s April Meeting and GHP’s Biennial Meeting. At the 2022 April Meeting, GHP’s sessions—which included talks from invited experts and mini-symposia—covered topics ranging from heavy-ion collisions from a QCD perspective, the multidimensional structure of hadrons, and briefings on experimental programs at the Brookhaven and CERN colliders.

GHP’s Biennial Meeting is cozier, drawing between 100 and 130 participants. The meeting is held in odd-numbered years and at the same venue as the April Meeting, so attendees can attend both conferences in one trip.

At GHP Biennial Meetings, longer time slots enable more comprehensive discussions. “At the April Meeting, contributed talks are only 10 minutes long,” Gaskell explained. “It’s difficult to address a topic in any real depth in this length of time.” In contrast, talks at the Biennial Meeting clock in at 20-25 minutes. “It’s fun as a speaker to be able to go into more detail about a topic you’re interested in, and as an audience member, you can get a much deeper understanding of others’ work,” he said.

The 2023 GHP Biennial Meeting—which will take place in Minneapolis, Minnesota, in the days leading up to the April Meeting on April 15-18—marks the first time the hadronic physics community will gather in-person since the Covid-19 pandemic began.

Still, the pandemic did expand meeting accessibility: The fully remote 2021 Biennial Meeting drew nearly 300 participants. “It opened my eyes to the fact that there’s a huge community of people who want to participate in our meetings, give talks, and get information about the latest findings in hadronic physics, but for whatever reason can’t make the trip. We haven’t been serving them appropriately,” Gaskell explained.

In addition to expanding access to meetings with hybrid options, another goal of the GHP executive committee is to elevate students and early career scientists in hadronic physics, who comprise 36% of the group’s ranks. To this end, GHP offers young researchers travel grants to attend meetings, as well as prizes and awards, including for best dissertation, to recognize excellent work in the field.

Prospective members have plenty to gain from joining GHP. “The main reason to join is the physics, to join a community that is trying to understand QCD and the strong force,” Gaskell said.

Visit the GHP website to learn more.

Abigail Dove is a writer based in Stockholm, Sweden.