Axion-like particle at the 10 GeV scale: Higgs decays to wide jets and photons

In the vast landscape of modern physics, the Standard Model serves as our most reliable map, cataloging the fundamental particles that make up the universe and the forces that govern their interactions. For decades, this map has been remarkably accurate, yet physicists remain certain it is incomplete. They suspect there are hidden territories beyond the known borders, populated by particles that are too light or too elusive to have been caught by current experiments. One of the most compelling candidates for these missing pieces is the axion-like particle. Unlike the heavy, well-known particles such as the Higgs boson, these hypothetical entities are predicted to be very light and interact very weakly with ordinary matter. Their existence is tied to a deep symmetry in nature, a kind of hidden rule that, if broken, would give them a tiny mass. Because they are so light and so shy, finding them is like searching for a whisper in a hurricane; they are easily drowned out by the overwhelming noise of known particle collisions. This makes the search for them one of the most challenging frontiers in experimental physics, requiring scientists to look for subtle, unusual patterns in the debris of high-energy crashes rather than waiting for a loud, obvious signal.

A recent study by researchers at Fermilab and Argonne National Laboratory offers a fresh perspective on how to find these elusive particles, specifically focusing on a mass range around ten billion electron volts. The team proposed that if these axion-like particles exist, they might be hiding in plain sight within the decay patterns of the Higgs boson. The Higgs boson, discovered in 2012, is a heavy particle that can break apart into lighter pairs. The researchers theorized that if an axion-like particle is light enough, the Higgs could decay into two of them at once. Once created, these new particles would quickly transform into other things. If the axion-like particles interact with heavy, colored particles that carry electric charge, they would mostly turn into pairs of gluons, which are the particles that bind quarks together, or occasionally into pairs of photons, which are particles of light. The unique signature of this process would be a Higgs boson splitting into two axion-like particles, where one pair of axion-like particles turns into a spray of gluons and the other pair turns into a flash of light.

The researchers used computer simulations to trace the path of this specific decay chain through the detectors at the Large Hadron Collider. They found that the resulting pattern is distinct from the background noise of ordinary particle collisions. When the Higgs boson decays into two axion-like particles, the resulting particles are moving so fast that their decay products are squeezed together. The two gluons from one axion-like particle merge into a single, wide jet of energy that has a specific internal structure, looking like a single object with two distinct cores. Meanwhile, the two photons from the other axion-like particle fly out as a pair of light beams. This creates a rare event where a detector sees a pair of light beams and a wide, structured jet of energy all coming from the same point. The team calculated that if the axion-like particle has a mass of about 13.6 billion electron volts, this specific combination of a light pair and a wide jet would appear at a rate that matches a puzzling signal recently observed by the CMS experiment at the Large Hadron Collider. That experiment had reported an excess of events at this exact mass, a statistical fluctuation that was significant enough to catch the attention of physicists but not strong enough to be declared a discovery on its own.

The study suggests that this excess is not a random glitch but could be the first evidence of the Higgs boson decaying into these new particles. The researchers showed that their model fits the data from both the CMS and ATLAS experiments, two of the main detectors at the collider. While the ATLAS experiment saw a much smaller, less significant bump at the same mass, the researchers demonstrated that their proposed signal is consistent with both sets of observations. The key to this explanation is that the Higgs boson is not decaying into a single new particle, as many previous searches assumed, but into a pair. This changes the geometry of the event, boosting the light particles so they move fast enough to pass the strict filters used by the experiments, while the wide jet from the other side provides a hidden clue that was previously ignored. The team also noted that if the axion-like particles were slightly heavier or lighter, or if they decayed differently, the signal would look different, but the 13.6 billion electron volt mass fits the current data surprisingly well.

Beyond explaining a specific anomaly, the paper highlights a broader opportunity for discovery. The researchers pointed out that if the axion-like particles decay almost entirely into gluons, the Higgs boson would produce two wide jets, each with a two-core structure, creating a resonance at the mass of the Higgs itself. This "four-gluon" decay mode has never been systematically searched for, largely because the background noise from ordinary particle collisions is immense. However, the researchers argued that the unique shape of these wide jets and the fact that they come from a boosted Higgs boson could allow scientists to separate the signal from the noise. They urged the experimental teams to look for these specific patterns, suggesting that the current data might already contain the answer, waiting to be recognized by the right search strategy. The work does not claim to have proven the existence of these particles, but it provides a concrete, testable roadmap for how they could be found, turning a vague statistical bump into a potential window into new physics.

The implications of this work extend to the future of particle physics. If these axion-like particles exist, they could be the first sign of a deeper structure underlying the Higgs boson, potentially pointing toward theories where the Higgs is not a fundamental particle but a composite of even smaller constituents. The study emphasizes that the search for new physics does not always require building bigger machines or waiting for higher energies; sometimes, it requires looking at the data we already have with a different set of eyes. By focusing on the specific kinematic features of how particles move and merge, the researchers have identified a path that could lead to a breakthrough. The fact that their model aligns with a real, observed excess in the data makes the case compelling, suggesting that the next step is for the experimental collaborations to re-examine their data with these specific signatures in mind. If confirmed, this would be a monumental step, revealing a new family of particles that have been hiding in the shadow of the Higgs boson for years.