Three new particles were discovered by the international LHCb collaboration at Large Hadron Collider, (LHC). They include a new pentaquark type and a pair of tetraquarks that includes a new type. Today's findings were presented at a CERN seminar. They add three new members to the growing number of hadrons discovered at the LHC. These findings will allow physicists to better understand the mechanism by which quarks are bound together into these composite particles.
Quarks, elementary particles, come in six flavors: charm, strange and top. They are often combined in groups of twos or threes to create hadrons, which are the protons and neutrons found within atomic nuclei. They can combine to form four-quark and five quark particles, known as "tetraquarks", and "pentaquarks," but this is less common. Although these exotic hadrons were predicted about six decades ago by theorists, they have only been observed recently by LHCb and other experiments.
The majority of exotic hadrons found in the last two decades have been tetraquarks and pentaquarks that contain a charm quark as well as a charm antiquark. The remaining quarks are either an up, down, or strange quark or their respective antiquarks. LHCb discovered many different exotic hadrons over the last two years. The collaboration discovered a tetraquark that contained two charm quarks and one charm antiquark. It also had two "open-charm tetraquarks" which included a charm antiquark as well as an up quark, down quark, and strange antiquark. It also discovered the first ever "double-open-charm" tetraquark, which had two charm quarks as well as an up- and down-antiqueark. An open charm is a particle that contains a charm quark but no equivalent antiquark.
The second type is a doubly electricly charged tetraquark. It is an open-charm, electrically charged tetraquark that consists of a charm quark, an antiquark, an up quark, an antiquark, as well as an antiquark and an antiquark. The neutral counterpart was also spotted in the joint analysis of decays between positively charged and neutral mesons. These new tetraquarks have a statistical significance of 8 (neutral particle), and 6.5 (doubly-charged particle). This is the first observation of a pair.
Niels Tuning, LHCb's physics coordinator, says that the more we do analyses, the more exotic hadrons we discover. "We are witnessing a period similar to 1950s when a particle zoo of hadrons was discovered. This eventually led to the quark model for conventional hadrons in 1960s. We're creating 'particle zoo 2.0.'"
Chris Parkes, spokesperson for LHCb, stated that "Finding new types of tetraquarks or pentaquarks will help theorists create a unified model exotic hadrons. The exact nature of which is largely unknow" It will also allow us to understand more conventional hadrons.
Some theories describe exotic hadrons in single units of tightly bound nuarks. Others envision them as pairs of standard quarks loosely bound in a molecule like structure. It will take more research into exotic hadrons to determine if they are one or the other, or both.