CERN Physicists Discover Four-Charm-Quark Particle
Physicists from CERN’s LHCb Collaboration have discovered a new tetraquark particle, named X(6900), composed of two charm quarks and two charm antiquarks.
Quarks are point-like elementary particles that typically come in packages of two (mesons) or three (baryons), the most familiar of which are the proton and neutron — each is made of three quarks.
There are six types — or flavors — of quark to choose from: up, down, strange, charm, bottom and top. Each of these also has an antimatter counterpart.
In their fundamental 1964 papers, American physicists Murray Gell-Mann and George Zweig proposed the quark model and mentioned the possibility of adding a quark-antiquark pair to a minimal meson or baryon quark configuration to form tetra- and pentaquarks.
It took 50 years, however, for physicists to obtain unambiguous experimental evidence of the existence of these exotic particles.
In April 2014, the LHCb Collaboration published measurements that demonstrated that the Z(4430)+ particle is composed of four quarks.
A year later, the LHCb physicists reported the observation of two pentaquarks, Pc(4450)+ and Pc(4380)+.
“Particles made up of four quarks are already exotic, and the one we have just discovered is the first to be made up of four heavy quarks of the same type, specifically two charm quarks and two charm antiquarks,” said Dr. Giovanni Passaleva, the outgoing spokesperson of the LHCb Collaboration.
“Up until now, the LHCb and other experiments had only observed tetraquarks with two heavy quarks at most and none with more than two quarks of the same type.”
The LHCb team found the X(6900) tetraquark using the particle-hunting technique of looking for an excess of collision events, known as a ‘bump,’ over a smooth background of events.
Sifting through the full LHCb datasets from the first and second runs of the Large Hadron Collider, which took place from 2009 to 2013 and from 2015 to 2018 respectively, they detected a bump in the mass distribution of a pair of J/ψ particles, which consist of a charm quark and a charm antiquark.
The bump has a statistical significance of more than five standard deviations, the usual threshold for claiming the discovery of a new particle, and it corresponds to a mass at which particles composed of four charm quarks are predicted to exist.
“These exotic heavy particles provide extreme and yet theoretically fairly simple cases with which to test models that can then be used to explain the nature of ordinary matter particles, like protons or neutrons,” said Dr. Chris Parkes, the incoming spokesperson of the LHCb Collaboration.
“It is therefore very exciting to see them appear in collisions at the Large Hadron Collider for the first time.”
The discovery is described in a paper posted on the arXiv.org preprint server.
R. Aaij et al (LHCb Collaboration). 2020. Observation of structure in the J/ψ-pair mass spectrum. CERN-EP-2020-115, LHCb-PAPER-2020-011; arXiv: 2006.16957