Production and Decay Dynamics of the Charmed Baryon in Annihilations near Threshold
Using 6.4 fb⁻¹ of collision data collected by the BESIII detector, this study presents the first observation of transverse polarization in baryons produced near threshold, enabling the simultaneous extraction of decay asymmetry parameters and the testing of -violating observables in the charmed baryon sector.
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
Matter as we see it is built from a small family of particles called baryons. The most familiar of these are protons and neutrons, which form the nuclei of every atom in the universe. While scientists have spent decades mapping the behavior of these common particles, a whole class of heavier cousins remains much more mysterious. These are baryons that contain at least one "charm" quark, a heavy building block that makes the particle unstable and short-lived. Because these particles vanish almost instantly after they are created, they cannot be studied in a beam or a target like their stable relatives. Instead, researchers must create them in high-energy collisions and watch how they fall apart, piecing together their secrets from the fragments they leave behind. Understanding these heavy particles is crucial because they act as a testing ground for the fundamental forces that hold matter together and the rules that govern how matter transforms.
A team of physicists using the BESIII detector in Beijing has now taken a major step forward in understanding one of these elusive particles, the baryon. By analyzing billions of collisions between electrons and positrons, they have observed a specific type of motion in the newly created baryon that had never been seen before. This motion reveals that the particle is not just spinning randomly; it is tilted in a specific direction relative to the plane where it was born. This discovery is significant because it confirms that the electromagnetic forces creating the particle are more complex than previously thought, involving a subtle phase shift between different types of forces. Furthermore, by studying how this tilted particle decays, the researchers were able to measure the internal angles of its breakup with high precision, providing new data to test the standard model of particle physics and search for hidden violations of symmetry.
The experiment took place at the Beijing Electron Positron Collider, where beams of electrons and their antimatter counterparts, positrons, are smashed together at energies ranging from 4600 to 4951 million electron volts. When these particles collide, they can annihilate each other, converting their energy into a single virtual photon that briefly exists before transforming into a pair of baryons: a and its antimatter twin, the . The researchers collected data corresponding to 6.4 inverse femtobarns of collisions, a massive dataset that allowed them to study 13 different energy points just above the threshold where these particles can be created. The key to their success was looking at the angles at which the particles flew apart and how they subsequently decayed into lighter particles like protons, pions, and kaons.
The central discovery is the observation of transverse polarization. In simple terms, when the baryon is created, it does not just move forward; it also spins. The researchers found that this spin is not random but is oriented perpendicular to the plane defined by the incoming electron beam and the outgoing baryon. This specific orientation is caused by a non-zero relative phase between two complex numbers that describe how the particle interacts with the electromagnetic force. Think of these two numbers as describing the particle's electric and magnetic character; the fact that they are out of step with each other forces the particle to spin in a specific direction. This effect had been predicted but never definitively measured for this type of particle. The team measured this polarization with a statistical significance exceeding ten standard deviations, meaning the chance of this result being a random fluke is virtually zero.
Once the researchers established that the particle was polarized, they used this information to measure how the particle decays. The is unstable and quickly breaks apart into other particles. The way it breaks apart depends on its spin and the internal dynamics of the decay. By analyzing the joint angles of the decay products in channels such as , , , and , the team extracted the decay asymmetry parameters. These parameters describe the preference of the decay products to fly in certain directions relative to the parent particle's spin. The researchers found that the decay of the channel has a negative asymmetry parameter, confirming a sign that had been debated in previous studies. They also measured the phase difference between the different ways the particle can decay, known as the S-wave and P-wave amplitudes.
The results challenge existing theoretical models. The team compared their measurements of the phase difference between the electric and magnetic form factors against predictions based on the vector meson dominance model, a framework that assumes the interaction is mediated by intermediate particles. The data showed a clear deviation from these predictions, suggesting that the internal structure of the and the dynamics of its production are more complicated than the simple models allow. This discrepancy indicates that our current understanding of how heavy quarks interact with light quarks inside the baryon is incomplete. Additionally, the researchers tested for violations of charge-parity (CP) symmetry, a fundamental symmetry in physics. While no significant CP violation was found in these specific decay channels, the precision of the measurement establishes a new method for searching for such effects in the future.
This work provides the most detailed map to date of how the phase of the form factors changes with energy for any baryon. The researchers measured the sine of the phase difference at 13 distinct energy points, revealing a structure that varies significantly across the energy range. The root-mean-square value of the transverse polarization reached a maximum of approximately 18 percent at an energy of 4661 MeV. These measurements of the decay asymmetry parameters and the phase differences between the decay amplitudes are now available to theorists to refine their models of the strong and weak interactions. By confirming the existence of transverse polarization and measuring its effects on decay, the study opens a new avenue for exploring the fundamental symmetries of nature in the realm of heavy baryons, proving that electron-positron colliders are powerful tools for uncovering the hidden dynamics of the subatomic world.
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