STAR Highlights II: Study of Small Systems and the Search for New, Exotic Physics

For a quarter of a century, scientists have been smashing heavy atomic nuclei together at nearly the speed of light to recreate the conditions that existed just millionths of a second after the Big Bang. This extreme environment, created inside a massive particle accelerator known as the Relativistic Heavy Ion Collider, is hot and dense enough to melt the protons and neutrons that make up ordinary matter. When these particles melt, they form a new state of matter called a quark-gluon plasma, a soup of fundamental particles that flows with almost no friction. By studying how this plasma behaves, researchers hope to understand the fundamental forces that hold the universe together. A key question for physicists has been determining exactly how small a collision can be before this exotic, fluid-like state of matter stops forming. Does the plasma require a massive collision between two heavy gold atoms, or can it emerge from the much smaller collision of a single proton with a heavy nucleus?

A recent report from the STAR collaboration, a researcher operating a large detector at the collider, offers a comprehensive look at twenty-five years of data to answer these questions and explore other mysteries of the subatomic world. The team analyzed collisions involving dozens of different combinations of atomic species, ranging from tiny protons to heavy uranium atoms, and across a wide range of energy levels. Their work confirms that the signature of this hot, fluid plasma appears even in very small collision systems, provided the conditions are right. They also discovered new ways to measure the internal structure of atomic nuclei and found evidence of exotic particles that exist for only a fleeting moment before decaying.

One of the most significant findings concerns the size of the system needed to create this plasma. For years, scientists debated whether the fluid-like behavior seen in large collisions was a true phase of matter or just a collection of independent particle interactions. The STAR team investigated this by colliding oxygen nuclei with other oxygen nuclei, a system much smaller than the gold-on-gold collisions used in the past. They looked for several distinct signs that a plasma had formed. They observed that high-energy particles were being slowed down as they passed through the medium, a phenomenon known as jet quenching. They also saw that the production of certain heavy particles was suppressed, suggesting they were being broken apart by the intense heat. Furthermore, the radiation emitted by the collision indicated a temperature consistent with a hot plasma, and the production of strange particles increased in a way that matched predictions for a fluid medium. When they compared these oxygen collisions to collisions involving deuterium, a light isotope of hydrogen, they found that the fluid behavior depended on the shape and internal structure of the colliding nuclei, confirming that the signal comes from a collective response of the matter rather than random chance.

Beyond the plasma itself, the researchers used these collisions to study the fleeting existence of unstable atomic nuclei. In low-energy collisions, they successfully measured the production of lithium isotopes that are so unstable they decay almost instantly. By tracking how these short-lived particles interacted with the surrounding dense matter, the team was able to test theories about how atomic nuclei are built from smaller pieces. They found that the shorter-lived particles were produced in lower numbers than their more stable counterparts, a result that directly reflects the competition between the formation of these nuclei and their destruction by the surrounding environment. In a separate search for exotic matter, the team looked for muonic atoms, which are rare structures where a muon, a heavy cousin of the electron, orbits a hadron. They found a clear signal for these atoms in the collision debris, providing a new method to study the production of soft muons that are otherwise impossible to detect directly.

The study also delved into the spin, or intrinsic rotation, of particles, a property that reveals deep secrets about the quantum vacuum. The team measured the alignment of the spin of phi mesons, a type of particle made of a strange quark and its antiparticle. By analyzing the full two-dimensional distribution of the particles they decay into, they detected a non-zero signal in the off-diagonal elements of the spin matrix. This indicates that the spins of the quark and antiquark are correlated in a specific way. To understand the origin of this correlation, they compared it to measurements of lambda-antilambda pairs in proton collisions, where they found a strong correlation that suggests the quantum vacuum imprints its structure onto the particles as they are created. The team is now working to see if this correlation survives in the hot plasma of heavy-ion collisions, which would tell them whether the extreme heat melts the vacuum's structure or if it persists.

Finally, the researchers developed new ways to measure the flow of the matter created in these collisions. They examined how the average momentum of particles fluctuates from one collision to the next, which reveals how the size of the initial fireball changes. These measurements allowed them to estimate the speed of sound within the quark-gluon plasma, a value that describes how stiff or compressible the medium is. Their results suggest that the speed of sound changes with temperature in a way that matches theoretical calculations from lattice quantum chromodynamics. They also found that the way particles flow depends on their mass, with heavier particles gaining more momentum from the expanding fluid, a pattern that holds true across all the different collision systems they studied. This detailed mapping of the plasma's properties provides a clearer picture of how the universe behaved in its earliest moments and sets the stage for future discoveries in the decades to come.