A new type of “pentaquark” particle and the first-ever pair of “tetraquarks” have been discovered by the international LHCb collaboration at the Large Hadron Collider (LHC), expanding the list of exotic hadrons found at CERN.
The vast majority of hadrons – particles made of quarks – are protons and neutrons, which have three quarks each. There are six types of quarks divided into three families: Up and down, charm and strange, and top and bottom. Respectively, each has a charge of two-thirds and minus one-third compared to the proton.
The proton is made of two up quarks and one down quark, making it positively charged. One up and two down make a neutron. Particles such as these, with an odd number of quarks, are known as baryons. If they have an even number, they are known as mesons, which commonly are made of one quark and its antiquark, which has the same mass but opposite charge.
Over the last 20 years, the Belle collaboration in Japan, Fermilab, and LHCb has shown that it is possible to create heavier baryons and mesons, something that had been predicted six decades ago. The pentaquark just discovered is made of a charm quark and a charm antiquark and an up, a down, and a strange quark. This is the first pentaquark that features a strange quark and it was determined with a statistical significance of 15 sigmas. The gold standard for discoveries is 5 sigmas.
The second discovery is of a tetraquark pair. The first one has twice the charge of the proton and is made of a charm quark, a strange antiquark, an up quark, and a down antiquark. It was discovered together with its neutral counterpart, which is made of a charm quark, a strange antiquark, an up antiquark, and a down quark.
“The more analyses we perform, the more kinds of exotic hadrons we find,” LHCb physics coordinator Niels Tuning said in a statement. “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. We’re creating ‘particle zoo 2.0’.”
The discovery of new and exotic hadrons helps us understand the strong nuclear force, which keeps the quarks within a particle together. It is unclear if in exotic hadrons, the quarks all stick tightly together, or they form triples and couples that are loosely bound like a molecule. More analysis is necessary.
“Finding new kinds of tetraquarks and pentaquarks and measuring their properties will help theorists develop a unified model of exotic hadrons, the exact nature of which is largely unknown,” says LHCb spokesperson Chris Parkes. “It will also help to better understand conventional hadrons.”
Just yesterday, the LHC began its third run which, will run for almost four years. The newly updated LHCb detector is expected to collect 10 times more data than in the previous run.