Probing axion-like particles through associated gauge-boson recoil and polarization at the LHC

The universe is filled with invisible actors. While the Standard Model of particle physics successfully describes the known particles and forces, it leaves out the nature of dark matter and the reason why the universe is made of matter rather than antimatter. To fill these gaps, physicists propose the existence of new, heavier particles that have not yet been seen. Among the most compelling candidates are axion-like particles. These are hypothetical, ghostly particles that could explain dark matter and other cosmic mysteries. They are expected to interact very weakly with ordinary matter, but they might occasionally reveal themselves by turning into pairs of light particles called photons, or by interacting with the heavy carriers of the weak nuclear force, known as W and Z bosons. If these particles exist, they would leave behind a unique fingerprint in the debris of high-energy collisions, distinct from the fingerprints left by known particles like the Higgs boson.

At the Large Hadron Collider, the world's most powerful particle accelerator, scientists smash protons together at nearly the speed of light to recreate the conditions of the early universe. A team of researchers has now proposed a new way to hunt for these elusive axion-like particles using the upcoming High-Luminosity LHC, an upgraded version of the machine that will operate at a center-of-mass energy of 14 tera-electronvolts with an integrated luminosity of 3 inverse attobarns. Their strategy focuses on a specific scenario: a proton collision that produces an axion-like particle alongside a W or Z boson. The axion-like particle would then decay into two photons, while the accompanying W or Z boson would decay into a charged lepton, such as an electron or muon, and possibly a neutrino. The researchers simulated billions of these events to see if the unique behavior of the axion-like particle could be distinguished from the overwhelming background of known Standard Model processes.

The key to this search lies in how the axion-like particle interacts with the W or Z boson. Unlike the Higgs boson, which interacts with these force carriers in a straightforward, non-derivative way, the axion-like particle interacts through a more complex, momentum-dependent mechanism. This difference in the underlying physics dictates how the particles fly apart after the collision. Specifically, the axion-like particle forces the accompanying W or Z boson to spin in a very specific direction, known as transverse polarization, and to recoil with significantly higher momentum than a Higgs boson would. The researchers realized that by measuring the angle at which the decay products of the W or Z boson emerge and by tracking how hard the boson is kicked back, they could separate the signal of a new particle from the noise of known physics.

To test this idea, the team performed detailed computer simulations of the collisions, modeling the behavior of the detector and the complex interactions of the particles. They focused on a mass range for the axion-like particle between 20 and 200 gigaelectronvolts. They found that by selecting events where the two photons and the charged lepton had specific energy and angle characteristics, they could suppress the background noise. The most powerful tool in their kit was the "recoil" of the W or Z boson. Because the axion-like particle interaction pushes the boson harder, events with a very energetic boson were much more likely to be a signal than background noise. Additionally, the angle of the charged lepton relative to the direction of the boson's flight revealed the spin state of the boson. In the case of an axion-like particle, the boson was almost exclusively transversely polarized, whereas a Higgs-like particle would produce a mix of spin states, including a significant longitudinal component.

The researchers combined these observations using a sophisticated machine-learning algorithm called XGBoost, which is designed to find complex patterns in large datasets. They trained the algorithm to distinguish between the simulated axion-like signal and the various background processes, such as the production of a W or Z boson with two photons from standard quantum processes. The results were promising. Even without relying on the mass of the diphoton pair to identify the particle, the algorithm could separate the signal from the background with high efficiency. When they included the mass information, the separation became even sharper. For a benchmark scenario where the axion-like particle has a mass of 100 gigaelectronvolts, the study suggests that the High-Luminosity LHC could detect these particles if their coupling to the W boson is stronger than approximately 5.6 times 10 to the power of minus 3 inverse tera-electronvolts. This represents a significant improvement over current limits, potentially opening a new window into the physics of the early universe.

Perhaps most importantly, the study demonstrated that these techniques could do more than just find a new particle; they could help identify what that particle is. If a peak in the data appeared, it could be caused by an axion-like particle or by a different, Higgs-like scalar particle of the same mass. The researchers showed that by analyzing the polarization and recoil of the accompanying boson, they could tell the difference between the two. The axion-like particle would leave a distinct signature of transverse polarization and high recoil, while the Higgs-like particle would show a different pattern. This means that if a new resonance is discovered, the same data used to find it could be used to understand its fundamental nature, distinguishing between different theories of new physics.

The work highlights that the accompanying gauge boson is not just a tag to mark an event, but a diagnostic tool that encodes the details of the interaction. While the researchers noted that experimental cuts and detector limitations can blur these subtle features, their simulations show that the underlying physics is robust enough to be detected. They concluded that by exploiting the unique kinematic and polarization properties of the associated production, the High-Luminosity LHC will be able to probe the existence of axion-like particles with unprecedented sensitivity, potentially revealing a new layer of reality hidden within the collisions of protons.