Evidence for the semileptonic decay
Using 4.5 fb⁻¹ of collision data collected by the BESIII detector, researchers performed the first search for the Cabibbo-suppressed semileptonic decay , measuring its branching fraction to be with a statistical significance of .
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In the subatomic world, matter is built from a small family of particles called quarks. These tiny constituents bind together to form larger particles known as baryons. One such baryon, the Lambda-c, is a heavy, short-lived particle containing a charm quark. Because it is unstable, the Lambda-c does not last long; it quickly transforms into other, lighter particles. This transformation is governed by the weak force, one of the four fundamental forces of nature. When a Lambda-c decays, it can sometimes spit out an electron and a neutrino, a process known as a semileptonic decay. These specific decays are like clean windows for physicists. Unlike other messy decays where many particles fly off in chaotic directions, semileptonic decays leave a clear trail that allows scientists to peer inside the structure of the original particle and understand how its internal components rearrange themselves during the breakup.
For decades, physicists have studied the most common ways a Lambda-c decays, but they have been eager to find rarer, more difficult-to-spot variations. One such rare variation involves the Lambda-c turning into a proton, a pion, an electron, and a neutrino. This specific path is suppressed by the rules of particle physics, making it much harder to occur than the standard routes. Finding it is crucial because it acts as a test for our theories about how these particles behave. If the rate at which this rare decay happens matches our predictions, it confirms our current understanding of the subatomic world. If it differs, it could hint at new physics or require us to rewrite the rules of how these particles interact.
A team of researchers using the BESIII detector at the Beijing Electron Positron Collider has now taken a significant step toward answering this question. They analyzed a massive collection of data, amounting to 4.5 inverse femtobarns of collision records, gathered over a range of energy levels between 4.600 and 4.699 GeV. In these collisions, electron and positron beams smash together to create pairs of Lambda-c and anti-Lambda-c particles. The team used a clever technique to isolate the rare decay they were looking for. They first identified one particle in the pair, the anti-Lambda-c, by reconstructing its decay into known hadronic modes. By knowing exactly how this partner particle was moving, they could infer the properties of its twin, the Lambda-c, even though the Lambda-c itself decayed into a mix of particles that included an invisible neutrino.
To find the specific signal of the Lambda-c turning into a proton, a pion, and an electron, the researchers had to filter out a sea of background noise. Many other processes can produce similar-looking particles, and the invisible neutrino makes the reconstruction tricky. The team defined a specific kinematic variable, essentially a measure of missing energy and momentum, which should be zero for the true signal events. They applied a series of strict filters to their data, checking the paths of the charged particles, identifying their types with high precision, and rejecting events that looked like they came from more common decays. They also used a machine learning tool, a boosted decision tree, to distinguish the subtle differences between the signal they wanted and the remaining background clutter.
After sifting through the data, the team found a small but distinct excess of events that matched their signal expectations. They counted 15.4 events, with an uncertainty of 4.9, above the expected background. This result corresponds to a branching fraction, or the probability of this specific decay occurring, of roughly 2.96 times 10 to the power of negative 4. In statistical terms, the chance that this result is just a random fluctuation of the background is extremely low, corresponding to a significance of 4.2 standard deviations. In the language of particle physics, this is considered strong evidence, though not yet a definitive discovery, which typically requires a higher threshold.
The researchers also looked closely at the mass of the proton and pion system to see if the decay happened through an intermediate excited state, a temporary resonance that might have formed before breaking apart. With the current amount of data, they found no clear signs of such intermediate states. This lack of evidence for specific intermediate particles helps to constrain theoretical models. Some previous calculations predicted that this decay would happen much more frequently if it proceeded through a specific excited nucleon state. The new measurement suggests those higher predictions are likely incorrect, as the observed rate is too low to support them. While the study does not yet pinpoint the exact mechanism of the decay, it provides a vital new data point that narrows the field of possibilities for theorists.
This work represents a careful and rigorous effort to map the landscape of charmed baryon decays. By confirming the existence of this rare decay channel and measuring its rate, the BESIII collaboration has provided a new constraint for the scientific community. The result is a solid piece of the puzzle, helping to refine our understanding of the weak force and the structure of matter. As the detector continues to collect more data in the coming years, the sensitivity of these measurements will improve, potentially allowing scientists to isolate the contributions of intermediate states and deepen their insight into the complex dynamics of subatomic particles.
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