A rather unassuming particle is playing an important role in the hunt for subatomic oddities. Similar to protons and neutrons, mesons are composed of quarks bound together by the strong nuclear force. But these short-lived particles have different characteristics that can reveal new information about the atomic nucleus and how the universe works.
Advancing this understanding could one day enable new discoveries in many fields, ranging from nuclear power to medicine and materials engineering.
The so-called a2 meson is a relatively lightweight system of quarks. It is produced in experiments at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility.
Now, for the first time, scientists at Jefferson Lab have measured the probability of the a2 being produced by a polarized photon beam hitting a proton target. This measurement is called a cross section and was recently published in the journal Physical Review C. The result enables the search for the lightest spin-exotic meson, the pi1, with an ultimate goal of mapping the mass spectrum of hybrid systems.
“We took a well-understood particle and measured a quantity that is new, using a sophisticated technique that we will need to study exotics, such as hybrids,” said Malte Albrecht, a Jefferson Lab staff scientist who is part of the Gluonic Excitations Experiment (GlueX). “It’s a physics result on its own but also a step on our roadmap toward exotic results with GlueX.”
GlueX at Jefferson Lab
Gluons are fundamental particles that carry the strong nuclear force, or the “glue” that clumps quarks into composites such as protons, neutrons, mesons and other hadrons.
The theory of quantum chromodynamics (QCD) describes this strong force. It is the main research thrust of the GlueX Collaboration, an international team of scientists who conduct their research at Jefferson Lab.
“One thing GlueX is specifically interested in understanding is the quark-gluon degrees of freedom,” said Sean Dobbs, an associate professor at Florida State University and a member of the GlueX collaboration. “The excitation of the gluons that hold hybrid mesons together contributes directly to their properties.”
To explore QCD, GlueX takes advantage of photons produced from the electrons of the Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Office of Science user facility enabling the research of more than 1,650 physicists worldwide.
The GlueX apparatus uses a thin wafer of diamond to convert CEBAF’s electrons into a beam of photons. These light particles are linearly polarized, meaning their electric field oscillates in a particular direction as they hurtle toward their atomic target. A specialized detector system measures the spectral spray from the resulting collisions, allowing scientists to peer into the “black box” of particle production and decay.
“What goes on inside that box could be a lot of things, but the polarization gives us a hint at what it could be,” said Lawrence Ng, a postdoctoral researcher at Jefferson Lab who studied under Dobbs at Florida State. “It tells us a little bit about how these mesons are produced.”
Piecing the Pi
The pi1 meson is a close cousin of the a2, just a little weirder.
Both are QCD systems that exist in nearby energy regions. They have similar quark content – some combination of the “up” and “down” flavors of quark and antiquark. They have relatively light masses, with a2 checking in at 1,320 million electron volts (MeV) and pi1 at around 1,600 MeV. But they have different spin states, represented by the numerals after their names, and inside a pi1 meson, the gluons behave differently.
“Imagine quarks as billiard balls and gluons as rubber bands holding them together,” Dobbs said. “The difference between a ‘normal’ meson like the a2 and hybrids like the pi1 is that in the latter, the rubber band is excited. You pluck it and let it vibrate, giving you extra energy.”
The pi1(1600) has been difficult to pin down experimentally, but the a2(1320) is relatively well understood. That made a2 the ideal candidate.
In the search for the pi1 at GlueX, physicists refer to the a2 as their “standard candle.” The term is borrowed from astronomy and describes an interstellar object with a known brightness that provides a stable reference signal from which additional information, like cosmic distances, could be inferred.
To identify the a2(1320) among the many other particles produced in the photon-on-proton collisions, the GlueX team used an elaborate technique called a partial-wave analysis. This method filters out contributions from other reactions, allowing researchers to home in on the a2.
Now that the concept is proven using GlueX’s machinery, software and extraction technology, the group can use the measurement as a reference for exploring the pi1(1600) and reaching for an established spectrum of hybrid states.
According to Albrecht, the next step is to continue the hunt for these rare meson systems to not only confirm their existence but also find evidence for systems not previously observed.
“Proving our ability to perform a partial-wave analysis and get something new out, with a particle that is well known, is the first step toward understanding contributions that are possibly much smaller and more elusive, because they are exotic.”