Search for resonant and nonresonant production of pairs of dijet resonances with b jets in the final state in proton-proton collisions at $\sqrt{s}$ = 13 TeV

The universe is built on a set of rules known as the Standard Model, a framework that successfully describes the fundamental particles and forces that make up everything we see. Yet, physicists know this picture is incomplete. It cannot explain gravity, the nature of dark matter, or why there is more matter than antimatter in the cosmos. To find the missing pieces, scientists at the Large Hadron Collider smash protons together at nearly the speed of light, creating a shower of new particles that might reveal a hidden layer of reality. Among the most promising theories are those predicting the existence of heavy, unstable particles that decay into jets of ordinary matter. While scientists have long searched for single heavy particles breaking apart, a more complex scenario involves the creation of pairs of these heavy particles, which then break apart into even more debris. This specific search looks for a rare signature: two pairs of particle jets, where each pair contains a heavy bottom quark, a type of matter that is heavier and rarer than the common protons and neutrons in our everyday world.

In a recent study, the CMS Collaboration at CERN examined a vast collection of collision data to hunt for these elusive pairs. They analyzed 138 units of data, known as femtobarns, gathered from proton-proton collisions at an energy of 13 tera-electronvolts. The team was looking for two distinct possibilities. The first was a nonresonant process, where two heavy particles are created independently and simultaneously, each decaying into a bottom quark and a lighter quark. The second possibility involved a resonant process, where a single, even heavier parent particle is created first, which then splits into the two intermediate particles before they decay further. In both scenarios, the final result would be four distinct jets of energy, with two of them originating from bottom quarks. The researchers used sophisticated algorithms to identify these bottom quarks and to reconstruct the mass of the original particles from the energy of the resulting jets.

The analysis focused on finding a "bump" or a localized excess in the data that would stand out against the smooth, predictable background of ordinary particle collisions. The background noise is immense, generated by the constant production of jets in standard quantum interactions, but the researchers developed a method to model this background directly from the data itself, rather than relying solely on theoretical predictions. They sorted the events based on the ratio of the mass of the two-jet pairs to the total mass of all four jets, a technique that helped separate potential signals from the overwhelming background. After sifting through the data, the team found no definitive evidence of new particles. The results did not show a clear, statistically significant peak that would confirm the existence of the predicted heavy resonances.

However, the absence of a discovery allowed the scientists to set strict boundaries on what could exist. They determined that if certain types of heavy particles, specifically those predicted by a model involving the violation of a symmetry called R-parity, do exist, they must be heavier than 0.8 tera-electronvolts. This finding effectively rules out the existence of these particles in the lower mass range that previous experiments had not yet fully explored. Similarly, for the resonant search involving heavy diquark particles, the data excluded masses between 2 and 7 tera-electronvolts. These limits are the first of their kind for this specific type of particle decay involving bottom quarks, pushing the frontier of what is known about the possible mass of new physics.

While the main search came up empty, the data did contain a few intriguing fluctuations. In the search for nonresonant production, the most noticeable deviation from the expected background appeared at a mass of 1.0 tera-electronvolt, with a statistical significance of 2.1 standard deviations. In the resonant search, a slightly larger fluctuation was observed at a four-jet mass of 3.1 tera-electronvolts, corresponding to a significance of 2.6 standard deviations. In the language of particle physics, these numbers indicate that while the data looked slightly different from the background prediction at these specific points, the effect is not strong enough to be considered a discovery. Such fluctuations are expected to occur occasionally due to random statistical variations, and the researchers emphasize that they do not constitute proof of new particles.

The study also highlighted a specific event with the highest energy recorded in the dataset, where the four jets combined to a mass of 5.5 tera-electronvolts. This single event, which occurred in 2018, contained two pairs of jets that were nearly identical in mass and aligned in a way that matched the theoretical expectations for the signal. Despite its striking appearance, this event was not enough to overturn the overall conclusion, as the rest of the data did not support a consistent signal at that energy level. The researchers noted that this event appeared in the high-mass tail of the distribution, a region where statistical fluctuations are more likely to occur.

Ultimately, this work represents a significant step forward in the search for physics beyond the Standard Model. By extending the search to include bottom quarks and by covering a wide range of masses, the collaboration has closed the door on several theoretical possibilities that were previously open. The results provide a clearer map of the energy landscape, telling scientists where new particles are not, which is just as valuable as finding where they might be. The search continues, with the Large Hadron Collider collecting more data, but for now, the universe remains silent on the existence of these specific heavy particle pairs.