Measurements of the Absolute Branching Fraction of the Semileptonic Decay and the Axial Charge of the
Using a large sample of events collected by the BESIII detector, this study reports the first measurement of the absolute branching fraction and axial-vector to vector coupling ratio for the semileptonic decay , achieving a branching fraction significantly lower than the world average and an axial charge precision comparable to previous experiments despite using only 5% of their statistics.
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In the subatomic world, matter is built from a small family of particles called quarks, which bind together to form protons and neutrons. But there is a larger, more exotic family of particles known as hyperons. These are unstable cousins of the proton and neutron, carrying a property called "strangeness" that causes them to decay rapidly into lighter particles. For decades, physicists have studied how these hyperons transform, hoping to uncover the hidden rules that govern the weak nuclear force—the force responsible for radioactive decay. A central piece of this puzzle is a specific number, known as the axial charge, which acts like a fingerprint for how the internal spins of the quarks are arranged inside the particle. Understanding this number is crucial because it helps scientists test a fundamental symmetry in nature called SU(3) flavor symmetry. If this symmetry holds perfectly, the axial charges of different hyperons should follow a strict, predictable pattern. However, if the symmetry is broken, even slightly, it reveals deeper truths about how the universe is constructed and how the spin of the proton is distributed among its constituent quarks.
For over sixty years, the only way to measure these elusive particles was to fire beams of protons at fixed targets, creating a chaotic spray of debris where hyperons were born and died in fractions of a second. These experiments were difficult, often yielding small samples of data with significant uncertainty. Now, a team of researchers using the BESIII detector at the Institute of High Energy Physics in Beijing has taken a different approach. They utilized a massive collection of events generated by colliding electrons and positrons to create a pristine environment where hyperons are produced in pairs, entangled with their antiparticles. By sifting through more than ten billion such events, they focused on a specific, rare decay where a negatively charged hyperon, the , transforms into a lambda particle, an electron, and an invisible neutrino. This process had never been observed at an electron-positron collider before, and the team's goal was to measure exactly how often it happens and to extract the precise value of the axial charge.
The researchers employed a clever technique called "double-tagging" to isolate their signal from the overwhelming noise of other particle interactions. First, they identified one half of a hyperon pair by fully reconstructing its decay into a lambda particle and a pion. Once this "tag" was secured, they knew exactly where the partner hyperon was and how it was moving, allowing them to look for the specific decay of the partner into an electron and a neutrino. Because the neutrino cannot be detected directly, the team relied on the laws of conservation of energy and momentum to infer its presence. They calculated the missing energy and momentum in the event; if the numbers balanced perfectly with a single missing particle, it was a strong candidate for the decay they were seeking. To ensure they were not fooled by background particles that mimicked the signal, they used advanced statistical tools to filter out false alarms, such as pions that were mistakenly identified as electrons.
After applying these rigorous filters, the team found a clear signal. They determined that the probability of this specific decay occurring is approximately . This result is significantly lower than the previously accepted world average, differing by nearly four standard deviations, which suggests that the old measurements may have been inaccurate or that the theoretical models need adjustment. More importantly, by analyzing the angles at which the decay products emerged, the team was able to calculate the axial-vector to vector coupling, a key parameter that describes the strength of the weak force in this transition. They found this value to be $0.18$, with a small margin of error. When they translated this into the axial charge of the hyperon, they arrived at a value of $0.22$.
This measurement is a significant achievement because it was accomplished with a dataset that is only five percent the size of those used in previous major experiments, yet it achieved a comparable level of precision. This leap in efficiency comes from the unique advantages of the electron-positron collider environment, where the quantum entanglement of the produced particle pairs allows for a much more sensitive analysis of the decay angles than was possible with traditional fixed-target methods. The new value for the axial charge aligns well with predictions from chiral perturbation theory, a sophisticated mathematical framework used to describe how quarks interact at low energies. However, the fact that the measured branching fraction is lower than expected remains a point of interest. These results provide a fresh, high-precision benchmark for testing the limits of symmetry in the subatomic world, offering a clearer view of the fundamental forces that shape the structure of matter.
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