A strange visitor came to light in a fleeting flash. A beam of photons struck a deuterium nucleus (heavy hydrogen), and in the cascade of produced particles, detectors noticed something unusual. At a certain combination of masses and momenta, the signal sharply intensified, as if a new particle had appeared for a brief moment.
This clump of matter is a candidate for a bound state of two nucleons (a proton and a neutron, or two protons) and a strange antikaon (a particle containing a strange quark). Such objects are called kaonic clusters. They live for infinitesimal fractions of a second: the measured decay width (0.055 GeV/c²) corresponds to a lifetime of about 10⁻²³ seconds. For comparison, in that time, light travels a distance three times smaller than the diameter of a proton.
The statistical significance of the signal reached 7.3 sigma — far beyond a random fluctuation. The particle's mass is determined to be 2.354 GeV/c², slightly below the threshold for producing a free proton, neutron, and kaon. This 'shortfall' is precisely what indicates their mutual attraction, gluing the three particles into a single entity.
The captured signal is not just a point on a graph. It is a window into the world of strong interactions, where quarks weave into intricate combinations. The next step is to verify the finding in other reactions and attempt to measure the cluster's shape directly.
A statistical significance of 7.3 sigma means that the probability of a random fluctuation mimicking the signal is less than one in ten billion.