THE international LHCb collaboration at the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research, has observed a new kind of “pentaquark” and the first-ever pair of “tetraquarks”, which includes a new type of tetraquark. These will be additions to the growing list of new particles found at the atom smashing machine. The findings were reported recently at a CERN seminar and at the International Conference in High Energy Physics in Italy.
Quarks, elementary particles, come in six types, or “flavours”: up (u), down (d), charm (c), strange (s), top (t), and bottom (b). They usually combine in groups of twos and threes to form “hadrons”, which include protons and neutrons that make up atomic nuclei. More rarely, they form composites of four- and five-quark particles, or tetraquarks and pentaquarks. These exotic hadrons were predicted in the 1960s when Murray Gell-Mann and George Zweig proposed the quark model for strongly interacting particles and it was experimentally verified. But only in the last 20 years have these been observed (“Exotic arrival”, Frontline, May 22, 2009).
Most of the exotic hadrons discovered in the past two decades are tetraquarks or pentaquarks containing a c-quark and a c-antiquark, with the remaining two or three quarks being u, d, or s-quarks and an s-antiquark. But the latest discoveries by LHCb include new kinds of exotic hadrons. The first kind, observed in the analysis of “decays” of negatively charged particles called B mesons, is a pentaquark made up of a c-quark and a c-antiquark and a u-, a d-, and an s-quark. It is the first pentaquark found to contain a strange quark.
The second kind is a doubly electrically charged tetraquark. It is an open-charm tetraquark composed of a c-quark, an s-antiquark, and a u-quark and a d-antiquark, and it was seen along with its neutral counterpart in a combined analysis of decays of positively charged and neutral B mesons. This is the first-ever observation of a pair of tetraquarks.
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“We’re witnessing a period of discovery similar to the 1950s, when a ‘particle zoo’ of hadrons started being discovered and ultimately led to the quark model of conventional hadrons in the 1960s,” said LHCb physics coordinator Niels Tuning. “We’re creating ‘particle zoo 2.0’.”
There is no unified description of such exotic hadrons. Some theoretical models describe them as single units of tightly bound quarks. Others picture them as pairs of conventional hadrons loosely bound in a molecule-like structure.