Lee Billings in Scientific American:
Physicists using the Large Hadron Collider beauty (LHCb) experiment at CERN in Geneva, Switzerland, have discovered a new kind of heavy particle, they announced this week at a conference in Venice.
The particle, known as Xi-cc++ (pronounced “Ksī-CC plus-plus”), is composed of three smaller elementary particles called quarks—specifically, one lighter-weight “up” quark like those found in protons and neutrons as well as two “charm” quarks, which are a heavier and more exotic variety. (The designations “up” and “charm” are two of the six “flavors” physicists assigned to quarks based on the particles’ varying masses and charges.) The Standard Model of particle physics predicts Xi-cc++ and many other possible particles with various configurations of the six known flavors of quarks. But until now such “doubly charmed” particles had eluded conclusive detection. Further studies of the new particle—and other members of the doubly charmed particle family—could reinforce the Standard Model or lead to new vistas in particle physics. Either way, the new particle could be a tool to unlock a deeper understanding of the fundamental “strong” force that binds quarks together to form protons and neutrons, which in turn form atoms—as well as planets, stars, galaxies and people.
Any particle made of quarks is called a hadron. The world’s largest and most powerful particle accelerator, CERN’s Large Hadron Collider (LHC), slams these particles together in search of new particles and interactions. Hadrons fall into two broad families: mesons, exotic particles with one quark and one antiquark; and baryons, particles composed of three quarks. The new Xi-cc++ particle is a baryon. But due to its doubly charmed nature it is almost four times heavier than more familiar baryons such as protons and neutrons, which are made up entirely of light quarks rather than heavy ones. “Finding a doubly heavy quark baryon is of great interest, as it will provide a unique tool to further probe quantum chromodynamics [QCD]—the theory that describes the strong [force], one of the four fundamental forces,” LHCb spokesperson Giovanni Passaleva said in a statement. “Such particles will thus help us improve the predictive power of our theories.”