Physicists from the Large Hadron Collider beauty (LHCb) Collaboration at CERN’s Large Hadron Collider (LHC) announced today the observation of a strange pentaquark, a doubly charged tetraquark and its neutral partner.
Quarks are elementary particles and come in six flavors: up, down, charm, strange, top and bottom.
They usually combine together in groups of twos and threes to form hadrons such as the protons and neutrons that make up atomic nuclei.
More rarely, however, they can also combine into four-quark and five-quark particles, or tetraquarks and pentaquarks.
These exotic hadrons were predicted by theorists at the same time as conventional hadrons, about six decades ago, but only relatively recently, in the past 20 years, have they been observed by LHCb and other experiments.
Most of the exotic hadrons discovered in the past two decades are tetraquarks or pentaquarks containing a charm quark and a charm antiquark, with the remaining two or three quarks being an up, down or strange quark or their antiquarks.
In 2020, the LHCb physicists discovered a tetraquark made up of two charm quarks and two charm antiquarks, and two ‘open-charm’ tetraquarks consisting of a charm antiquark, an up quark, a down quark and a strange antiquark.
And in 2021, they found the first-ever instance of a ‘double open-charm’ tetraquark with two charm quarks and an up and a down antiquark.
The discoveries announced today by the LHCb Collaboration include new exotic hadrons: the strange pentaquark PΛψs(4338)0the doubly charged tetraquark Thascs0(2900)++ and its neutral partner Thascs0(2900)0.
Observed in an analysis of decays of negatively charged B mesons, PΛψs(4338)0 is a pentaquark made up of a charm quark and a charm antiquark and an up, a down and a strange quark. It is the first pentaquark found to contain a strange quark.
Thascs0(2900)++ is an open-charm tetraquark composed of a charm quark, a strange antiquark, and an up quark and a down antiquark.
It was spotted together with its neutral counterpart, Thascs0(2900)0in a joint analysis of decays of positively charged and neutral B mesons.
“The more analyzes we perform, the more kinds of exotic hadrons we find,” said LHCb physics coordinator Dr. Niels Tuning.
“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’,” he added.
“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,” said Dr. Chris Parkes, spokesperson of the LHCb Collaboration.
“It will also help to better understand conventional hadrons.”