In March 2019, physicists with the LHCb Collaboration at the Large Hadron Collider near Geneva, Switzerland, announced that they have discovered an ultra rare particle that was first predicted over 50 years ago.
That particle, called a pentaquark, is a hadron that is comprised of five quarks. Hadrons are particles made of two or more quarks and held together by the strong force. The strong nuclear force keeps protons within the nucleus of atoms together by the exchange of particles called mesons. This constant exchange is what holds the nucleus together.
Until the early 2000s, scientists had seen only two types of hadron:
Baryons - containing three quarks, for example, protons and neutrons
Mesons containing one quark and one antiquark, for example the pi-meson
"Three quarks for Muster Mark!"-James Joyce, Finnegans Wake
Quarks are the fundamental building blocks of matter, they cannot be further broken down, and they come in six flavors: Up, Down, Strange, Charm, Top, Bottom. Each of these flavors has an antimatter companion that is identical to it, but with the opposite charge.
In 2016, scientists discovered tetraquarks that contain two quarks and two antiquarks. The recent discovery of pentaquarks, which contain one down quark, two up quarks, a charm quark and a charm antiquark, was first proposed by physicist Murray Gell-Mann in 1964 when he realized that quark-antiquark pairs could be added to mesons and baryons to create heavier particles. In 1969, Gell-Mann received the Nobel Prize in physics for his work on the theory of elementary particles.
In 2015, evidence greater than 5 sigma was first seen of the first two pentaquarks, named Pc(4450)+ and Pc(4380)+. The four-digit number indicates the mass of the pentaquark in MeV/c2, making these pentaquarks more than four times heavier than a proton. Pc(4450)+ might actually be two separate pentaquarks, Pc(4440)+ and Pc(4457)+.
After a second run of the proton beam at CERN, scientists discovered a third pentaquark called Pc(4312)+, which had a statistical significance of 7 sigma. The pentaquarks could be made of five quarks, but they could also be a baryon and a meson that are stuck together.
The Ultimate Fate of Neutron Stars
LHCb spokesperson Guy Wilkinson was quoted in The Guardian newspaper as saying, "One place where pentaquarks may be relevant is when stars collapse and form neutron stars, the final stage of collapse before some go on to make black holes. In that environment, it’s quite possible that pentaquarks are formed, and if that’s so, it could have significant consequences for what happens to the stars, what they look like and what is their ultimate fate."