Art Courtesy of Luna Aguilar.
Blink, and you’ll miss it—scientists at the STAR Collaboration, an international effort, are smashing metal ions together at nearly the speed of light, unleashing an unfathomable amount of energy with each impact. For a brief fraction of a second, the colliding atoms are broken down not merely into protons and neutrons, but into an even more primordial form: quarks and gluons, the basic building blocks of all matter. The resulting soup of particles is so hot and dense that it is thought to resemble conditions in the early universe, just a few microseconds after the Big Bang. As the ball of quark-gluon plasma expands, spreading out at a dizzying clip, it rapidly cools and settles into larger and more complex structures. By studying what emerges from their Big-Bang-in-a-jar, the researchers seek to probe not only the nature of matter and the laws of physics, but also the underlying logic of the universe itself.
The key to this exploration lies in one particular kind of object: antimatter. In August of 2024, the STAR team announced that they had detected the largest group, or nucleus, of antiparticles ever found. Their work, the culmination of decades of effort, pushes the boundaries of particle physics and sheds light on the fundamental axioms that govern our world. STAR is one of the largest and longest-running physics collaborations in the world: since its inception in 1991, thousands of scientists from across the globe have worked together on the project.
One of STAR’s most essential researchers is Helen Caines, the Horace D. Taft Professor of Physics at Yale. Caines chaired the STAR Collaboration as its spokeswoman from 2017 to 2023 and studies the quark-gluon plasma by examining the particles it creates. Working alongside Caines is fellow STAR researcher Fernando Flor, a postdoctoral fellow at Yale who develops computational models to help interpret the high-energy collision data generated by their particle accelerator.
One Particle, Two Charges
But hold on a second—what even is antimatter? Though the word is often bandied about by science fiction writers and futurists, few people understand what it really means.
The concept of antimatter was first proposed over a century ago by the mathematical physicist Paul Dirac. In 1929, Dirac was interested in combining the new theories of quantum mechanics and special relativity, which both offered insights into the nature of matter and energy. He began by working on a seemingly simple case study, the electron. But just as the square root of a number can yield both a positive and negative answer, Dirac’s equations predicted that the electron can possess either a negative or positive charge. Electrons were universally known to be negatively charged particles; the notion of a positively charged counterpart—later named the anti-electron, or “positron”—was nonsensical. The scientific community twisted itself into knots trying to disprove the finding. Yet, only three years later, Dirac was vindicated. The existence of the positron was confirmed, unlocking a whole new world of antiparticles: objects that share the same properties as their ordinary counterparts, but have opposite charges. For objects that lack charge, like neutrons, their antiparticle equivalents have other properties reversed, such as their quantum number, which describes the particle’s possible energy states.
Dirac’s work quickly revealed a deeper problem. The universe had started with a net charge of zero, so there should have been equal amounts of positively and negatively charged objects, elementary particles and antiparticles. “We should see an equal number of quarks and antiquarks come out of this process, because the quantum numbers and charge have to be conserved. For every charge you should make an anti-charge, and for every quark you should make an antiquark,” Caines said.
Yet, when we look out into the universe today, there is almost no antimatter. The proof? When matter and antimatter interact, they destroy one another, converting into pure energy in a process known as annihilation. This process is extremely violent: if just one gram of matter came in contact with one gram of antimatter, the resulting explosion would release enough energy to destroy a city. If there were any significant amounts of antimatter in our environment, we’d be in serious trouble. At some point between the beginning of the universe and now, something must have wiped out most of the antimatter—without touching regular matter in the process.
Why Antimatter Matters
This asymmetry between matter and antimatter has puzzled physicists for decades as they sought to understand how the properties of antimatter might differ from those of ordinary matter. One theory suggests that antiparticles might not bond in the same way as ordinary matter and are therefore left in smaller, higher-energy forms that are more vulnerable to decay.
“Is this the reason we’re in our matter universe—because, for some reason, antimatter simply cannot form?” Caines said. “Perhaps we can make antiprotons and positrons, but they just don’t combine into nuclei in the same way.”
But if this was true, it would challenge one of the most fundamental axioms of modern physics, an idea called charge, parity, and time reversal (CPT) symmetry. Much like how a sphere appears the same from every angle, CPT symmetry asserts that particles should look the same if you swap their charge, direction of motion, and orientation in time. This implies that particles and their corresponding antiparticles are fundamentally identical, just moving in opposite directions. Therefore, they should be governed by the same physical laws, and should bond in the same way. Thus, the study of antiparticle bonding has the potential to confirm or undermine CPT symmetry, one of our most essential notions about the nature of reality itself.
At first, antimatter researchers looked to space for their investigations. “Originally, we were limited to studying extraterrestrial sources of antiparticles, like cosmic rays or particles from the Sun,” Flor said. Since these particles spend their lives in a near-perfect vacuum, they never have a chance to be annihilated, and occasionally make it to Earth’s surface unscathed, where they can be detected. But such events are rare—in order to observe an antimatter bonding event, two cosmic rays would have to collide, a nearly impossible event. So, in order to investigate antimatter bonding, it’s become more feasible for us to first create the antiparticles ourselves.
The Antimatter Hypernucleus Is Born
This is where STAR comes in. STAR is based at the relativistic heavy ion collider (RHIC), a particle accelerator at Brookhaven National Laboratory. There, scientists accelerate gold, uranium, zirconium, and ruthenium to 99.996 percent the speed of light—fast enough to traverse the four-kilometer circumference of the device over seventy-five thousand times per second. When two of these massive atoms collide, they are heated to over seven trillion degrees Fahrenheit, scrambling their constituent particles into quark-gluon plasma. Out of this quark-gluon plasma emerges antiparticles, which can then bond to form nuclei.
However, some nuclei—including the newly discovered nucleus—decay very quickly, making them challenging to detect. These are called “hypernuclei” because they contain an extra particle: an unstable twin of the proton called a lambda hyperon. Hypernuclei provide another critical avenue to explore antimatter bonding, and ultimately probe CPT symmetry. “The question becomes: can you bind a lambda particle with an antiproton and an antineutron in the same way that you can bind their matter equivalents?” Caines said.
Because the lambda hyperon is unstable, it causes its parent nuclei to decay rapidly, splitting up into smaller particles after just a fraction of a second. Therefore, STAR looks not for the hypernuclei themselves but for their byproducts. The detector at STAR, which stands for the Solenoidal Tracker at RHIC, is effectively an enormous container filled with gas. As the byproduct particles zip through the gaseous medium, they ionize it, releasing electrons. The new electrons are exposed to electric and magnetic fields, causing them to move in different directions. “If we can measure [the electrons’] location and drift velocity at the detector, we can figure out what position they came from in the detector,” Caines said. By tracing the byproducts in this way, STAR can determine where and what they came from.
The new antimatter hypernucleus consists of a lambda hyperon, an antiproton, and two antineutrons: the most massive antimatter configuration ever discovered. However, its complexity means that it forms extremely rarely. In order to detect the new antimatter hypernucleus, the STAR team had to register over seven billion collisions. Out of this sample, the team is only confident that they saw the new nucleus fifteen or sixteen times. “For context, this would be like going around the planet, polling everyone that exists, and asking if a dozen or two of them have something in common,” Flor said.
The discovery of this new hypernuclei suggests that CPT symmetry works—for now. The STAR team combed through all seven billion collisions, searching for clues about the properties of their new hypernucleus. They found that it forms and decays at the same rate as its matter equivalent, confirming that they both obey the same laws of physics.
Conclusions and Future Possibilities
The STAR scientists are exhilarated. The creation of the new hypernucleus opens the door to new possibilities, like exploring the inner structure of antimatter atoms. “Going forward, we’re very interested to see if the proton, neutron and lambda in this nucleus are arranged in a different way from how we’d expect,” Caines said. As STAR approaches its technical limits, new experiments are on the horizon. Brookhaven is currently constructing a new electron-ion collider, which will help scientists probe the inner structure of atoms with unprecedented detail. “We’re moving into a new and exciting space, limited only by math and our own imagination,” Flor said.