Measurement of the branching fractions of the decays , and
Using 4.5 fb of collision data collected with the BESIII detector, this study reports the first observation of the singly Cabibbo-suppressed decay , provides the most precise measurements of the branching fractions for both and , and finds evidence for a contribution in the latter decay.
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In the subatomic world, matter is built from a small set of fundamental particles called quarks. These tiny constituents bind together in groups of three to form particles known as baryons. One of the most common baryons is the proton, which sits at the heart of every atom. However, nature also produces heavier, unstable cousins of the proton, such as the charmed baryon, which contains a heavy charm quark. Because these heavy particles are unstable, they do not last long; they quickly break apart into lighter, more stable particles. This process of breaking apart is called decay, and it is governed by the weak force, one of the four fundamental forces of nature. By watching how these heavy particles decay and measuring how often specific decay paths occur, physicists can test their understanding of the rules that govern the universe. For decades, scientists have been very successful at mapping these rules for heavy particles made of two quarks, known as mesons. Yet, the behavior of three-quark baryons remains much harder to predict and understand, leaving a significant gap in our knowledge of how the strong and weak forces interact in these complex systems.
A team of researchers using the BESIII detector in Beijing has now taken a major step toward filling that gap. They analyzed a massive collection of collision data, corresponding to an integrated luminosity of 4.5 fb⁻¹, to study how the charmed baryon, specifically the , falls apart. The team focused on three specific ways this particle can decay, all of which involve the baryon breaking down into a lambda particle (a lighter baryon) and other particles containing strange quarks. Two of these decay paths had been seen before, but one had never been observed directly. The researchers successfully measured the frequency of all three decays, providing the most precise numbers ever recorded for these events. Most notably, they confirmed the existence of a decay mode that had only been predicted by theory, and they found evidence that a specific intermediate particle, the , plays a role in one of the processes, though the exact nature of its involvement depends on how the quantum waves of the particles interfere with one another.
The experiment took place at the Beijing Electron Positron Collider, where beams of electrons and their antimatter counterparts, positrons, were smashed together at carefully controlled energies. When these particles collide, they can momentarily create heavy charmed baryons and their antiparticles. The BESIII detector acts like a giant, high-speed camera that records the tracks and energies of every particle flying out from the collision. The researchers sifted through this data to find the rare instances where a decayed into a lambda particle, a neutral kaon (which itself quickly turns into two pions), and either a charged kaon or a charged pion. Because these decay chains involve several steps and many different particles, the team had to be extremely careful to distinguish the true signal from the background noise of other random particle collisions. They used sophisticated computer simulations to understand how their detector would respond to these specific events and to calculate how efficiently they could spot them.
The study yielded clear results for all three decay modes. The team measured the probability of the decaying into a lambda, a neutral kaon, and a charged kaon to be approximately 3.04 in every thousand decays. This result matches previous measurements but with much greater precision, narrowing the range of uncertainty significantly. More importantly, they observed the decay into a lambda, a neutral kaon, and a charged pion for the very first time. This is a "singly Cabibbo-suppressed" decay, a term describing a process that is theoretically allowed but happens less frequently than the most common decay paths. The researchers found this occurs about 1.73 times in every thousand decays. This number is notably lower than what some theoretical models based on symmetry principles had predicted, suggesting that our current understanding of these heavy baryon decays may need refinement.
In addition to measuring the overall rates, the team investigated the internal structure of the decay involving the charged pion. They found strong evidence, with a statistical significance of 4.7 standard deviations, that this decay often proceeds through an intermediate stage where a short-lived particle called a is formed before breaking apart. However, because this intermediate particle is broad and the data sample is limited, the researchers could not determine a single, definitive rate for this specific path. Instead, they calculated the probability under three different scenarios regarding how the quantum waves of the different decay paths might overlap and interfere. Depending on the specific phase of this interference, the probability of the contribution ranges from about 1.29 to 5.21 in every thousand decays. The fact that the measured rate for the direct decay into a lambda, neutral kaon, and pion is lower than theoretical expectations, combined with the complex behavior of the intermediate , indicates that the forces at play inside these heavy baryons are more intricate than previously thought. As the researchers note, future data sets with even larger numbers of collisions will be essential to pin down these mechanisms and fully understand the decay patterns of charmed baryons.
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