Improved search for radiative transitions
Using an improved dataset of from the BESIII detector, this study finds no significant signal for the radiative transitions and sets new, tighter 90% confidence level upper limits on their branching fractions and partial decay widths, reducing previous constraints by factors of approximately three and two, respectively.
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Deep within the subatomic world, particles known as charmonium exist as fleeting pairs of a heavy charm quark and its antimatter twin, bound together by the strong nuclear force. Among these, the is a particularly interesting state, sitting just above the energy threshold where these quark pairs can break apart to form open charm particles. Physicists have long expected this particle to decay almost exclusively into those open charm pairs, yet observations have hinted at a more complex reality where it occasionally transforms into other, lighter particles. One specific transformation that theorists have been eager to confirm involves the emitting a flash of light, or a photon, and turning into a different, tightly bound state called the . This process is a radiative transition, a way for the heavy particle to shed energy and change its internal structure. Understanding whether and how often this happens is crucial because the rate of this decay can reveal whether the particle behaves exactly as simple models predict or if more complex quantum effects, involving virtual particles popping in and out of existence, are playing a significant role.
A team of researchers using the BESIII detector, a massive instrument designed to capture the debris of high-energy collisions, set out to find evidence of this specific light-emitting transformation. They analyzed a vast collection of data containing billions of electron-positron collisions, a sample seven times larger than what had been used in previous attempts to find this signal. By carefully reconstructing the paths of charged particles and the energy of photons, they looked for the specific signature of the turning into a photon and an particle, which then immediately decayed into a specific combination of kaons and pions. Despite the immense size of their data set and the precision of their instruments, the researchers found no significant signal. The data did not show the clear peak that would indicate the presence of these new particles; instead, the observed events were consistent with known background processes that mimic the signal but do not involve the transformation in question.
Because no signal was found, the team could not measure a specific rate for this decay. Instead, they calculated the strictest possible upper limits on how often this event could possibly occur without contradicting their data. They determined that the probability of the decaying into a photon and the lighter state is less than 5.0 times 10 to the negative 6th power, and for the slightly heavier version of the , the limit is even lower at 3.7 times 10 to the negative 6th power. In terms of the energy released during the decay, they established that the rate for the lighter transition must be less than 5.5 keV, and for the heavier one, less than 29.4 keV. These new limits are significantly tighter than previous measurements, effectively ruling out a wide range of theoretical predictions that suggested the decay should happen much more frequently.
The results provide a sharper boundary for theoretical physicists to work within. While some models that include complex interactions with open charm channels predicted rates that are still compatible with these new, stricter limits, other models that rely on specific relativistic corrections predict rates that are far too small to be detected even with this improved sensitivity. The absence of a signal suggests that if this transformation happens at all, it is a very rare event, occurring far less often than some early calculations suggested. This finding helps to refine our understanding of the forces that hold quarks together, confirming that the behavior of the is more subtle than a simple decay into open charm, yet perhaps not as dramatic as some theories of exotic quantum interference had hoped. The study stands as a testament to the power of accumulating more data to push the boundaries of what we can see, even when the answer is a quiet "no" rather than a loud discovery.
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