Observation of the and study of the decays
Using LHCb proton-proton collision data, this paper reports the first observation of the rare decay with a measured branching fraction of and provides a new measurement of the branching fraction ratio between and .
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
In the subatomic world, nature follows a strict set of rules known as the Standard Model. This framework explains how the smallest building blocks of the universe interact, but it also contains rare loopholes. One such loophole involves a specific type of transformation where a heavy particle called a bottom quark changes into a lighter strange quark. According to the Standard Model, this switch is incredibly difficult to pull off; it can only happen through complex, indirect pathways that suppress the event, making it a very rare occurrence. Because these events are so rare, they act as a sensitive test for the laws of physics. If the universe behaves slightly differently than the Standard Model predicts during these rare moments, it could be a sign of new, undiscovered forces or particles hiding in the shadows.
For decades, scientists have focused their search for these rare events on particles called mesons, which are made of two quarks. However, there is another family of particles called baryons, which are made of three quarks. These baryons offer a different perspective because they spin, adding a layer of complexity that mesons do not have. Until now, most research into these rare transformations in baryons had been limited to just one type, leaving a gap in our understanding of how these rules apply to heavier, more complex particles. The LHCb experiment at CERN, a massive detector designed to study particles containing heavy quarks, has now turned its attention to a heavier baryon known as the . This particle is much rarer to produce than the ones previously studied, making the search for its rare decays a significant challenge.
A team of researchers using the LHCb detector has successfully observed a rare decay of the baryon for the first time. In this process, the heavy baryon transforms into a lighter particle called a and a pair of muons, which are heavy cousins of the electron. This event is driven by the rare transition of a bottom quark into a strange quark. To find this signal, the team analyzed a vast amount of data collected from proton-proton collisions at the Large Hadron Collider over several years. They looked for a specific signature: a cascade of decays where the heavy baryon first splits into a and two muons, and then the itself decays into a lambda particle and a pion, with the lambda particle further decaying into a proton and another pion. This unique chain of events creates a distinct path through the detector that is very difficult for background noise to mimic.
The researchers found clear evidence of this decay, observing a signal that is statistically significant enough to be considered a discovery. They counted the number of times this event occurred and compared it to a more common, similar decay where the transforms into a and a J/psi particle, which is a well-known state made of a charm quark and its antiparticle. By using this common decay as a reference point, the team could cancel out many uncertainties related to how often these particles are produced and how the detector sees them. This method allowed them to measure the probability of the rare event with much greater precision. They determined that the branching fraction, which is the probability of this specific decay happening out of all possible ways the particle can decay, is approximately 118 times 10 to the power of negative 8. This means that for every ten million decays, this rare event happens roughly once, a rate that is surprisingly high compared to other similar rare processes involving baryons.
The study also looked at how this decay behaves at different energy levels, specifically by examining the mass of the two muons produced in the event. The researchers divided the data into three distinct ranges of this mass. In the highest energy range, they found a very strong signal, confirming the decay happens frequently there. In the middle range, they found evidence of the decay, though with less certainty. In the lowest energy range, the signal was too weak to confirm, so they set an upper limit on how often it could be happening. These measurements were compared to predictions made by theoretical models based on lattice quantum chromodynamics, a powerful method for calculating the behavior of quarks. The results showed good agreement with these predictions, particularly in the high-energy region, suggesting that the current understanding of the Standard Model holds up even for these heavy, complex particles.
In addition to the rare muon pair decay, the team also studied a related process where the decays into a and a heavier cousin of the J/psi particle called the . They measured the ratio of how often this happens compared to the more common J/psi decay. They found that the decay occurs about half as often as the J/psi decay. This result is consistent with measurements made on other types of baryons and helps scientists understand the role of complex interactions that occur when these particles decay. The fact that the rare muon pair decay happens at a rate comparable to, or even higher than, similar decays in lighter particles suggests that the heavy mass of the might partially compensate for its low production rate, making it possible to observe these elusive events.
This discovery opens a new chapter in the study of heavy baryons. By confirming that these rare decays occur and measuring their rates, the researchers have provided a new benchmark for testing the Standard Model. The ability to observe these events in a heavier baryon system, with its unique spin properties, offers a complementary way to check for deviations that might hint at new physics. As the LHCb experiment continues to collect more data in the coming years, scientists will be able to study the detailed angular patterns of these decays, potentially uncovering even deeper secrets about the fundamental forces that govern the universe. The observation of the decay stands as a testament to the power of precision measurement in the quest to understand the building blocks of reality.
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