Search for charmonium(like) states in at BESIII
Using a data sample of 1667.4 pb collected at GeV with the BESIII detector, a search for charmonium-like states decaying into via the process yielded no significant signal, resulting in the establishment of 90% confidence level upper limits on the product of the production cross section and branching fraction for potential -even states including and various excited charmonia.
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
In the subatomic world, matter is built from a small family of fundamental particles called quarks. Most of the matter we see every day, like the protons and neutrons in our bodies, is made of quarks bound together in groups of three. However, the laws of physics also allow for more exotic combinations, such as four quarks held together in a single package. These rare formations are known as tetraquarks, and they behave differently than the ordinary matter we are familiar with. For decades, physicists have been hunting for these unusual states, particularly those involving a heavy type of quark called the "charm" quark. One famous example, discovered twenty years ago, is a particle named X(3872). It sits right at the edge of where two heavy particles can just barely stick together, leading scientists to believe it might be a loose molecule made of two smaller particles rather than a tight knot of four quarks. If this molecular idea is correct, then physics predicts there should be a heavier partner to this particle, sitting slightly higher in energy, waiting to be found.
A team of researchers using the BESIII detector at the Beijing Electron Positron Collider set out to find this predicted partner. They focused their search on a specific energy level where theory suggested a new particle, often called X(4013), might exist. This hypothetical particle would be a heavy cousin to the X(3872), potentially made of two neutral D-mesons bound together. To catch a glimpse of it, the scientists smashed electrons and positrons together at a precise energy of 4.682 GeV. When these particles collide, they can sometimes produce a flash of light—a photon—and leave behind a heavy, unstable particle that quickly falls apart. The researchers looked specifically for events where a photon was produced alongside a particle that immediately decayed into a pair of neutral D-mesons, which themselves broke down into other particles that the detector could track.
The team analyzed a massive collection of data, equivalent to 1,667.4 inverse picobarns of collisions, to see if any of these events clustered around the mass where the X(4013) was expected to appear. They also searched for several other known types of heavy particles that could exist in this same energy range, just to be thorough. After carefully sorting through the collisions and filtering out background noise from other common particle interactions, the researchers found no evidence of the X(4013) or the other predicted states. The data showed no significant spike or signal that would indicate the presence of these new particles. Instead of finding a discovery, the experiment established strict upper limits on how often these particles could possibly be produced. For the X(4013), the team determined that if it exists, it is produced so rarely that its production rate must be less than 5.6 events per unit of collision energy, with a high degree of statistical confidence.
This result is significant because it tells us what is not there, narrowing the path for future theories. While the X(3872) remains a mystery that could be a molecular state, its heavier partner, if it follows the same rules, should have been visible in this data. The fact that it was not seen suggests that either this particle does not exist, or it is much harder to create and detect than current models predicted. The researchers also set limits for other potential heavy particles, such as excited versions of the eta-c and chi-c families, finding no signs of them either. By ruling out these possibilities with high precision, the study provides a clearer map for other scientists, showing that the search for these specific heavy quark combinations must look elsewhere or consider different theoretical explanations. The work demonstrates that the current detectors are sensitive enough to find these particles if they were present in the predicted quantities, and their absence is a valuable piece of the puzzle in understanding how the strong force binds matter together.
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