Particle physicists have reported the likely first-ever sighting of a long-theorized subatomic process known as a triangle singularity. The process involves a pair of particles, called kaons, exchanging one of their two constituent particles before moving away from each other. When depicted in a Feynman diagram—a pictorial representation of particles and their interactions—the kaons and their paths form two corners and two legs of a triangle, respectively, while the two swapped particles comprise the third corner. Discovering this process could offer new insights into the strong force, one of the four fundamental forces of nature. The strong force governs interactions of the elementary particles, called quarks, that comprise kaons, as well as more familiar protons and neutron that constitute atoms. See also: Atom; Elementary particle; Feynman diagram; Fundamental interaction; Neutron; Physics; Proton; Quark; Strong nuclear interactions
Researchers working with the COMPASS (Common Muon and Proton Apparatus for Structure and Spectroscopy) experiment made the findings. COMPASS is built around a section of a particle accelerator at the European Organization for Nuclear Research, known as CERN, near Geneva, Switzerland. The experimental setup involves colliding a beam of particles called pions (similar to kaons) into hydrogen-atom targets. These high-energy collisions break the strong force bonds between quarks in the various particles. The energy released by this breaking of bonds then forms new particles, including kaons. Tracking the generated particles and recording their decays into other particles within COMPASS helps reveal new aspects of the behavior of the strong force. The new explanation of a triangle singularity better fits an anomalous signal seen in COMPASS data, versus an initial explanation provided by the generation of an exotic type of tetraquark, or four-quark particle. However, more data will need to be collected to firm up either the triangle singularity or tetraquark explanation. See also: Collision (physics); Energy; Meson; Particle accelerator
Although the strong force is reasonably well-characterized by the theory of quantum chromodynamics, much remains unexplained about its interactions and manifestations. For example, the quarks composing protons and neutrons provide only about 1% of each particle's mass. Instead, most of the particles' mass comes from poorly understood interactions among gluons, carrier particles of the strong force that—as their name implies—"glue" quarks together into larger particles. Theoretical physicists hope to better understand these interactions for immediate insights into the behavior and structure of matter. Continuing deep study of the nature of the strong force could perhaps one day lead to its manipulation, akin to the electromagnetism—another fundamental force—that humankind has succeeded in harnessing over the last two centuries. See also: Electromagnetism; Gluons; Mass; Quantum chromodynamics; Theoretical physics