Branching fraction measurements of and anomalous decays
Using 13 TeV $pp$ collision data from the LHCb experiment, this paper reports the first experimental search for decays and the most precise measurement to date of the branching fraction, while also providing new upper limits and measurements for several other anomalous decays of the and mesons.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Deep within the subatomic world, particles known as mesons act as fleeting messengers, carrying forces between the fundamental building blocks of matter. Among these, the eta and eta-prime mesons are particularly interesting because they are heavy, unstable cousins of the pions, and their behavior is governed by the strong nuclear force, the glue that holds atomic nuclei together. Physicists have long used these particles to test the limits of their best theories about how the universe works at its smallest scales. One specific puzzle involves rare decays, where these mesons transform into pairs of muons, which are heavier, unstable versions of electrons. According to the standard model of particle physics, these transformations should happen at a very predictable rate, driven by a subtle quantum effect involving the creation and destruction of virtual photons. However, if the actual rate of these decays differs from the prediction, it could signal the presence of new, invisible particles or forces that have not yet been discovered.
A team of researchers using the Large Hadron Collider beauty (LHCb) experiment has now taken a fresh look at these rare events. By analyzing a massive collection of data from high-speed collisions of protons, they have measured how often eta mesons decay into two muons with unprecedented precision. They also searched for the same decay in the heavier eta-prime meson, a process that had never been observed before. While they found a clear signal for the eta meson, they did not find enough evidence to confirm the eta-prime decay, instead setting a strict upper limit on how often it might occur. Furthermore, the team looked for a specific type of four-particle decay involving two pions and two muons, finding a clear signal for the eta-prime version but only an upper limit for the eta version. These results sharpen the picture of how these particles behave, confirming some theoretical expectations while leaving the door open for future discoveries.
The researchers conducted their study using data collected between 2016 and 2018 at the LHCb detector, which is designed to spot particles containing heavy quarks. They focused on collisions where the energy was equivalent to 13 trillion electron volts, a scale high enough to produce vast numbers of charm mesons. These charm mesons are crucial to the experiment because they frequently decay into the eta and eta-prime mesons the scientists wanted to study. By tracking the debris from these collisions, the team could reconstruct the paths of the resulting particles and identify when an eta or eta-prime meson had transformed into the specific combinations of muons and pions they were looking for. To ensure their measurements were accurate, they compared these rare events against a well-understood reference process involving a different particle called the phi meson, which also decays into muons. This comparison allowed them to cancel out many of the uncertainties related to how the detector works.
The analysis revealed a clear signal for the decay of the eta meson into two muons. The team calculated that this happens in approximately 5.3 out of every million eta mesons. This measurement is the most precise ever recorded for this specific process, improving the accuracy of previous results by more than half. The precision of this number is significant because it allows theorists to test their calculations of the strong force with greater confidence. If the theoretical predictions for the strength of the interaction between the meson and the photons do not match this new, sharper measurement, it could point to a flaw in the current understanding of particle physics. The result stands as a solid confirmation of the standard model for this particular decay channel, with the uncertainty reduced to just a few percent.
In contrast, the search for the eta-prime meson decaying into two muons yielded no definitive signal. The researchers examined the data and found that if this decay happens at all, it must be extremely rare, occurring less than 1.9 times in every ten million eta-prime mesons. This is the first time such a limit has been established for this particle. The lack of a clear signal does not mean the decay is impossible, but it does mean that if it exists, it is far less frequent than some theories might have suggested. The team also looked for a more complex decay where the eta-prime meson breaks apart into two pions and two muons. Here, they found a strong signal, measuring the rate at which this happens to be about 2.7 times in every hundred thousand eta-prime mesons. This is the first time this specific four-particle decay has been measured for the eta-prime, providing new data on how these particles interact with pions.
The study also turned its attention to the eta meson in the four-particle decay mode, searching for the same combination of two pions and two muons. In this case, no signal was found. The researchers determined that if this decay occurs, it must happen less than 5.4 times in every ten million eta mesons. This result is consistent with theoretical predictions that suggest the decay is heavily suppressed due to the lower mass of the eta meson compared to the eta-prime. The difference in mass limits the available space for the decay products to move, making the process much less likely for the lighter particle. By establishing these upper limits, the team has effectively ruled out the possibility that this decay is happening at a rate that would have been easily detectable with their current data.
Beyond testing the standard model, the researchers also used their data to search for signs of new physics, specifically looking for hypothetical particles like dark photons or axion-like particles that might mediate these decays. They scanned the data for any unexpected patterns that would suggest an invisible particle was being created and then decaying into the muons. After a thorough search across a range of possible masses for these hypothetical particles, they found no evidence of such new physics. They set new, stricter limits on how often these exotic particles could be involved in the decays, effectively narrowing the search space for future experiments. The absence of these signals reinforces the current view that the standard model holds up well in this regime, though the door remains open for more sensitive searches in the future.
The precision of these measurements is limited not by the amount of data collected, but by the uncertainty in the known properties of the reference particle used for comparison. The team relied on the known decay rate of the phi meson into muons, which carries a small margin of error. Until that reference value is measured with greater precision, the accuracy of these new results will remain capped at its current level. Despite this, the work represents a major step forward in understanding the behavior of eta and eta-prime mesons. By providing the most precise measurement of the eta-to-muon decay and the first measurements and limits for the eta-prime channels, the LHCb collaboration has provided a clearer map of the subatomic landscape. These results will serve as a benchmark for theorists refining their models and for experimentalists planning the next generation of searches for the unknown.
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