ALP photo-production in the parton and hadron cascade model PACIAE

Deep within the fabric of the universe, physicists suspect there are hidden particles that could solve some of the most stubborn mysteries in nature. Among these potential candidates are axion-like particles, ghostly entities that might explain why the universe behaves the way it does and could even make up the invisible dark matter that holds galaxies together. While these particles have never been directly seen, scientists know that if they exist, they should be able to transform into pairs of light particles, known as photons, and vice versa. This theoretical possibility has driven researchers to look for them in high-energy collisions, hoping to catch a glimpse of their creation or decay. The challenge lies in the fact that these particles are elusive, and their signals are often buried under a chaotic storm of other subatomic debris produced in the same collisions.

To explore this possibility, a team of researchers turned to a powerful computer simulation to recreate the conditions of a massive particle crash. They focused on collisions between gold nuclei, smashing them together at an energy level of 200 GeV, a speed and force comparable to what happens in the most intense experiments conducted at major particle accelerators. The team used a sophisticated digital model called PACIAE to simulate five million of these collisions. This model acts like a virtual laboratory, tracking how the fundamental building blocks of matter, known as partons, scatter and interact before settling down into a final cloud of stable particles. In this simulated aftermath, the researchers found a vast number of photons, the particles of light, scattered throughout the debris.

The core of this study involved a new method to see if these scattered photons could be reassembled into the hidden axion-like particles. The researchers applied a statistical approach, looking for pairs of photons in their simulated data that were close enough in energy and direction to potentially merge back into a single axion-like particle. They set specific rules for this merging process, focusing on pairs where the combined energy was less than twice the mass of a specific type of particle called a kaon. By applying a probability factor based on how often such a transformation is expected to happen, they were able to count how many of these hidden particles might have been formed in their virtual collisions. This was the first time this specific technique of recombining photons from a hadronic state into axion-like particles had been attempted in this manner.

The results of these simulations provided a clear picture of what to expect if these particles are indeed being created. The researchers calculated how many axion-like particles would appear for different masses, ranging from very light to heavier values within the MeV and GeV ranges. They found that as the mass of the particle increased, the number of particles produced dropped significantly. For the lightest mass they tested, the simulation predicted a yield of nearly 28 particles across the full range of possible outcomes, while for the heaviest mass, this number fell to just over three. The team also mapped out how these particles would move and where they would appear in the collision zone. They discovered that the way the particles spread out sideways did not change much regardless of their mass, but the overall number of particles decreased as the mass got heavier. Furthermore, the spread of the particles along the direction of the collision beam narrowed as the mass increased, creating a tighter cluster for heavier particles.

These findings offer a concrete set of predictions for experimentalists to test. The researchers have provided specific numbers for how many of these particles should be seen, how fast they should be moving, and where they should be located within the collision debris. While the study itself is a simulation and not a direct observation, it establishes a baseline for what a real experiment should look for. The authors suggest that future investigations could use these predictions to search for the particles in actual data from heavy-ion collisions. They also plan to expand their work in the future by looking at how gluons, another type of fundamental particle, might contribute to the creation of these axion-like particles, and how this process might be linked to the phase transitions of matter that occurred in the early universe. For now, the work stands as a detailed theoretical map, guiding the search for a particle that remains one of the most intriguing possibilities in modern physics.