Search for meson decays to multimuon final states
Using LHCb data corresponding to an integrated luminosity of , this study searches for meson decays into four- and six-muon final states via various intermediate particles, finding no evidence for these processes and setting upper limits on their branching fractions ranging from to .
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
The universe is built from a small set of fundamental particles that interact through forces, a framework physicists call the Standard Model. This theory has been incredibly successful at predicting how matter behaves, yet it leaves several profound questions unanswered. It cannot explain why the universe contains dark matter, why particles have mass, or why there is more matter than antimatter. Because of these gaps, scientists suspect there are hidden particles or forces waiting to be discovered. One promising idea is that new, heavy particles might exist but are too heavy to be created directly in current experiments, or they might be so weakly connected to ordinary matter that they slip past our detectors. If such particles exist, they could be produced in rare decays of heavier particles, acting as intermediaries that briefly appear and then vanish into lighter, detectable debris.
In 2026, researchers at the Large Hadron Collider beauty experiment, known as LHCb, turned their attention to a specific search for these hidden intermediaries. They focused on the decay of B mesons, which are particles containing a heavy bottom quark. The team looked for a very specific and rare outcome: a B meson breaking apart to produce either four or six muons. Muons are heavy cousins of the electron, and finding them in such large numbers from a single decay is highly unusual in the known laws of physics. The researchers were hunting for a scenario where a B meson first decays into two new, invisible scalar particles, which then travel a short distance before decaying into pairs of muons. If these new particles exist, they could explain some of the universe's mysteries, but only if they leave a trace in the data.
To find these rare events, the team analyzed data collected from proton-proton collisions at an energy of 13 tera-electronvolts, corresponding to a total amount of data known as an integrated luminosity of 5.4 inverse femtobarns. This dataset represents billions of collisions recorded by the LHCb detector between 2016 and 2018. The challenge was immense because the signal they sought was expected to be buried under a mountain of ordinary background events. The detector is designed to track charged particles with extreme precision, measuring their momentum and how far they travel before decaying. The researchers had to distinguish between muons that appeared immediately at the collision point and those that traveled a measurable distance, indicating they came from a long-lived intermediate particle. They also had to filter out known processes that mimic the signal, such as decays involving well-known particles like the J/psi or phi mesons, by carefully checking the mass of the particle pairs.
After applying rigorous selection criteria and sophisticated statistical tools to separate potential signals from background noise, the team found no evidence of the new decays. They did not see a single instance where a B meson decayed into four or six muons in the way predicted by the new theories. Instead of finding a discovery, the researchers set strict upper limits on how often these decays could possibly occur. They determined that if these decays happen at all, they must be incredibly rare, occurring less than once in every few hundred million to a few billion B meson decays, depending on the mass and lifetime of the hypothetical intermediate particles. These limits are the most stringent constraints to date for many of the scenarios they tested, effectively ruling out a wide range of theoretical models that predicted these particles would be more common.
The results cover a broad spectrum of possibilities, testing intermediate particles with masses ranging from a few hundred million electronvolts to several billion electronvolts, and lifetimes spanning from a fraction of a picosecond to a full nanosecond. For some configurations, the researchers found that the decay could happen no more than about 0.6 times in every billion attempts, while for others, the limit was slightly higher, around 5.4 times in every billion. These numbers represent the boundary of what is currently possible to detect with the existing data. The absence of a signal does not mean the new particles do not exist, but it does mean that if they do, they are either much heavier, much shorter-lived, or much more weakly coupled to ordinary matter than the most optimistic theories suggested.
This work serves as a powerful guide for future research. By narrowing down the space where these new particles might hide, the LHCb collaboration has told theorists where not to look and where they might need to refine their models. The search for physics beyond the Standard Model is a process of elimination, and this study has successfully eliminated a large region of the theoretical landscape. The team's findings stand as a testament to the precision of modern particle physics, demonstrating that even when a discovery is not made, the act of measuring the limits of nature provides essential knowledge. The search continues, with the hope that future data or more sensitive experiments will eventually reveal the hidden sector that completes our understanding of the universe.
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