Evidence for the decay and improved measurements of the decay
Using 18 fb of Run 3 LHCb data, this paper reports the first single-experiment evidence for the rare decay and achieves the world's most precise measurements of the branching fraction and effective lifetime, all of which align with Standard Model predictions.
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 author. 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 we can measure and describe. For decades, physicists have used a framework called the Standard Model to explain how these particles behave, a theory that has passed every test thrown at it with remarkable precision. Yet, this model feels incomplete. It cannot explain why the universe is filled with more matter than antimatter, nor does it offer a candidate for dark matter, the invisible substance that holds galaxies together. To find the missing pieces, scientists do not always need to build bigger machines to smash particles harder; sometimes, they need to look closer at the rarest, most delicate events. By watching how certain heavy particles decay, or break apart, into lighter ones, researchers can spot tiny deviations that hint at invisible forces or new particles lurking just beyond the edge of current detection.
One such rare event involves a particle called the B meson, which contains a heavy beauty quark. In the Standard Model, a specific type of B meson can occasionally transform into a pair of muons, which are heavy cousins of the electron. This transformation is so difficult to achieve that it happens only a few times in every billion decays. Because the process is so suppressed, even a tiny contribution from a new, undiscovered particle could change the rate at which it happens or how long the B meson lives before it vanishes. Measuring these decays is like listening for a single whisper in a roaring stadium; if the whisper sounds different than expected, it suggests a new voice is speaking in the crowd.
A team of researchers working with the LHCb detector at CERN has now listened to this whisper with unprecedented clarity. Using data collected during the most recent run of the Large Hadron Collider, they have found the first strong evidence that a specific type of neutral B meson, known as B0, does indeed decay into two muons. While this decay was predicted by the Standard Model, it had never been seen clearly by a single experiment before. The team also refined their measurements of a similar, slightly more common decay involving a different B meson, called Bs, finding that it happens at a rate and with a lifetime that perfectly match the predictions of the established theory. These results, derived from analyzing billions of proton collisions, suggest that the current model of particle physics remains robust, even as scientists continue to hunt for the cracks that might reveal a deeper reality.
The work was carried out using the upgraded LHCb detector, a massive instrument designed to track particles moving at nearly the speed of light. The detector sits along the path of the collider's beams, capturing the debris of collisions that occur at energies of 13.6 trillion electron volts. Between 2024 and 2025, the machine delivered a vast amount of data, equivalent to 18 inverse femtobarns of integrated luminosity, a measure of the total number of collisions recorded. The upgraded detector is far more sensitive than its predecessors, capable of operating in a much busier environment where many collisions happen at once. This improvement allowed the team to sift through the noise of the collider to find the specific signatures of the decays they were seeking.
To find the rare decays, the researchers had to distinguish the signal from a sea of background events. Most collisions produce a chaotic spray of particles, and occasionally, random combinations of these particles can mimic the appearance of a B meson decaying into two muons. The team used a sophisticated computer algorithm, known as a boosted decision tree, to learn the subtle differences between a genuine signal and a random background. This tool analyzed the paths and properties of the particles, such as how far they traveled from the collision point and how they were angled, to assign a score to each candidate event. By selecting only the events with the highest scores, the team isolated a clean sample of potential decays.
Once the candidates were selected, the researchers measured the mass of the two muons in each event. If the two muons came from the decay of a B meson, their combined mass would cluster around a specific value, creating a peak in the data. The team found a clear peak at the expected mass for the B0 meson, indicating the presence of the decay. The statistical significance of this finding reached three standard deviations, a level that physicists describe as evidence. While this is not yet the gold standard of a discovery, which requires five standard deviations, it is a strong indication that the signal is real and not a fluke of the data. The team also measured the rate of the Bs meson decay, finding it to be consistent with previous observations but with much greater precision.
The researchers also measured how long the Bs meson lives before it decays. This effective lifetime provides a different kind of information than the decay rate alone. In the Standard Model, the decay is expected to come almost entirely from a specific version of the particle that lives longer. If new physics were involved, the mix of particle versions could change, altering the measured lifetime. The team found that the lifetime of the Bs meson is 1.60 picoseconds, a value that aligns perfectly with the predictions of the Standard Model. This agreement suggests that the decay is proceeding exactly as the current theory describes, with no obvious signs of new particles interfering with the process.
To ensure their results were reliable, the team combined the new data from the recent run with measurements from earlier runs of the collider. This combined analysis, which includes data from 2011 through 2025, provided the most precise determination of these decay rates to date. The final numbers show that the Bs meson decays into muons about 3.25 times in every billion attempts, while the B0 meson does so about 1.14 times in every ten billion attempts. These numbers are remarkably close to what the Standard Model predicts. The ratio between the two rates, which depends on the properties of the particles and the forces acting on them, also matches the theoretical expectation.
The findings have important implications for the search for new physics. Many theories that attempt to extend the Standard Model predict that these rare decays would happen more frequently or with different lifetimes than observed. The fact that the measurements align so closely with the Standard Model places tight constraints on these alternative theories. It suggests that if new particles exist, they are either too heavy to influence these decays or they interact in ways that do not disturb the delicate balance of the Standard Model. While the discovery of the B0 decay is a triumph of experimental skill, the lack of deviation from the theory means the hunt for new physics must continue in other directions.
The success of this measurement highlights the power of the upgraded LHCb detector. The ability to collect such a large amount of data and to distinguish rare signals from background noise with high precision demonstrates that the facility is ready for the next phase of exploration. The team's work provides a solid baseline for future studies, ensuring that any future deviations found in these decays will be genuine signs of new physics rather than statistical fluctuations. For now, the universe continues to behave exactly as the Standard Model says it should, even in its most elusive corners. The silence of the data, in this case, is as informative as a shout would be, telling physicists that the known laws of nature hold firm, even as they look for the cracks that might lead to a deeper understanding of the cosmos.
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