A new and particularly charming “exotic matter” particle has been discovered. Although new subatomic particles are now found so often it's hard for the interested amateur to keep up, this one distinguishes itself by having the longest decay time of any exotic matter found so far. As such, it takes physicists close to a long-held dream: the production of stable exotic particles (those that exist outside the original formulation of two and three quark particles).
Discovered by CERN physicists during the Large Hadron Collider beauty (LHCb) experiment, the new Tcc+ particle is a tetraquark, a particle that consists of two quarks and two antiquarks. The new discovery was presented yesterday at the European Physical Society Conference on High Energy Physics.
Quarks’ very name comes from the fact they are normally found in threes; a line in James Joyce's daunting Finnegans Wake reads: “Three quarks for Muster Mark!” In trios they make up the protons and neutrons that make up almost all of our mass. Later we learned mesons, the carriers of the strong nuclear force, consist only of a quark paired with an antiquark, a particle with the mass of its equivalent quark but opposite charge
More recently the “particle zoo” has grown to include tetra and pentaquarks, particles formed from four or five quarks/antiquarks respectively. The “flavors” of their constituent quarks/antiquarks determine their properties. Tcc+ consists of two charm quarks, an up and a down antiquark.
Charm quarks and antiquarks cancel out their charm component, like positive and negative electrical charges. The presence of two charm quarks with nothing to balance them makes Tcc+ twice as charming. Jokes aside, Tcc+’s significance comes from how closely it approaches stability, compared to the highly unstable tetraquarks created previously.
Tetraquarks split into two mesons. Like a heavy element fissioning into lighter ones, the mesons produced in this way have a combined mass less than the tetraquark they come from; the “lost” mass is released as energy. However, an article in the CERN courier reports the combined mass of Tcc+'s products is so close to the mass of a Tcc+ particle only 273 keV energy is left over, where other tetraquarks decay energy is measured in tens or hundreds of millions of electron volts. With less energy ready to be released, the impetus for a Tcc+ to decay is reduced, extending its life.
Although far less massive than top quarks, charm quarks have masses hundreds of times up and down quarks. Tcc+ is the first tetraquark discovered with such a mismatch between its component quarks and antiquarks. The measurement of its small decay energy increases confidence a tetraquark formed of two bottom quarks and an up and down antiquark would have negative decay nuclear force decay energy, making it highly stable.
A double bottom tetraquark “Is produced rarely and is out of reach of the current luminosity of the Large Hadron Collider,” the LHCb beauty experiment’s Dr Ivan Polyakov told the CERN Courier. However, forthcoming upgrades may change this.
Besides its status as a stepping stone to stable exotic particles, Tcc+ has charmed its discoverers for other reasons. The short lives of existing tetraquarks prevent us from studying them directly, leaving us to study the mesons they produce to infer their nature. Tcc+’s mesons are relatively easy to study, and the low energy release facilitates precise measurements. These make it easier to test the accuracy of theoretical models of physics at a subatomic level.