Search for new physics with charm data at BESIII

The universe we see around us is made of matter, yet the laws of physics that govern the smallest particles suggest that the Big Bang should have created equal amounts of matter and antimatter, which would have annihilated each other instantly. This mismatch is one of the great unsolved puzzles of modern science, alongside the mystery of why neutrinos, ghostly particles that pass through everything, have mass at all. The Standard Model, our best current map of the subatomic world, describes how particles interact but cannot explain these gaps. To find the missing pieces, physicists look for rare events where the rules seem to break, such as processes that violate the conservation of lepton number or baryon number. Lepton number is a count of certain light particles like electrons, while baryon number counts heavier particles like protons and neutrons; in the Standard Model, these counts are strictly preserved. If scientists can catch a particle doing something that changes these counts, or if they see a particle transform into a different type of lepton without a neutrino to balance the books, it would be a clear sign of new physics beyond our current understanding.

At the Beijing Electron Positron Collider, a machine that smashes electrons and their antimatter counterparts together, the BESIII experiment has gathered a massive collection of data from collisions that create charmonium, a heavy particle made of a charm quark and its antiquark. This dataset, containing billions of events, acts as a high-precision microscope for hunting these forbidden transformations. The researchers focused on three main avenues: looking for signs that neutrinos are their own antiparticles, searching for particles that change their identity in ways the Standard Model forbids, and checking if heavy charmonium particles can decay through weak forces in ways that are usually impossible. By analyzing these billions of collisions, the team was able to set the most stringent limits to date on how often these strange events might occur, effectively narrowing the search space for new theories.

In the hunt for lepton number violation, the team looked for a process where a heavy particle called a cascade baryon decays into a lighter sigma baryon and two electrons, a reaction that would imply the neutrino is a Majorana particle, meaning it is its own antiparticle. Using a technique that reconstructs the event from both sides of the collision to ensure accuracy, they examined over ten billion collisions. They found no evidence of this decay happening. Instead of finding a signal, they calculated that if this decay does occur, it must happen less than two times in every one hundred thousand attempts. They extended this search to other particles, including mesons like the eta and omega, looking for similar decays where a neutral particle turns into two pions and two electrons. Again, no signals appeared, and the researchers established that these decays are even rarer, occurring less than a few times in a million attempts for the lighter mesons and less than two times in a billion for the heavier ones.

The search also turned to baryon number violation, specifically looking for a phenomenon where a lambda baryon spontaneously turns into its antimatter twin, an anti-lambda, and back again. This oscillation would break the rule that matter and antimatter are distinct and conserved separately. The team analyzed the same massive dataset of collisions to find events where the final state suggested such a flip had occurred. They found zero instances of this happening. Based on the absence of any signal, they determined that if this oscillation does take place, it must happen less than once in every million attempts. This result sets a new, stricter limit on how fast such a transformation could occur, pushing the minimum time required for the flip to be longer than three-tenths of a microsecond, a timescale that is incredibly fast but still far beyond what the Standard Model allows.

The experiment also investigated charged lepton flavor violation, a process where a heavy particle decays into two different types of charged leptons, such as an electron and a muon, without the usual neutrino partners. In the Standard Model, an electron and a muon are distinct families that do not mix in this way. The researchers looked for this mixing in the decay of the psi(3686) particle and in the decays of eta and eta-prime mesons. In the case of the psi(3686), they observed a handful of candidate events, but the number was consistent with what would be expected from random background noise rather than a genuine signal. For the eta and eta-prime mesons, the data showed no excess of events at all. Consequently, the team concluded that these flavor-changing decays are extremely rare, occurring less than once in every ten million to ten billion attempts, depending on the specific particle. These limits are significantly tighter than previous measurements, effectively ruling out many theoretical models that predicted these decays would be more common.

Finally, the team searched for weak decays of charmonium that involve a change in the flavor of the charm quark, a process that is heavily suppressed in the Standard Model but could be enhanced by new physics. They looked for decays where the charmonium particle transforms into a D meson and other particles, or where it emits a photon or a pair of muons. Across all the channels they examined, from those involving a D meson and a rho meson to those involving a D meson and an electron, no significant signals were found. The researchers set upper limits on how often these decays could happen, finding that they occur less than a few times in every ten million to ten billion attempts. These results provide a comprehensive map of where new physics is not hiding, forcing theorists to refine their models to fit these increasingly strict constraints.

The work done by the BESIII collaboration does not reveal a new particle or a broken law of nature, but it does something equally important for science: it draws a sharper boundary around what is possible. By using the world's largest samples of charm data, the team has shown that the universe is even more obedient to the Standard Model than previously thought, at least in these specific channels. The absence of signals is a powerful result, as it eliminates a wide range of theoretical possibilities and guides future research toward the few remaining corners where new physics might still be waiting to be discovered. As more data is collected, these limits will continue to tighten, either revealing a crack in the current model or pushing the search for new physics to even more extreme frontiers.