The Large Hadron Collider (LHC) of the European Organisation for Nuclear Research (much better known as Cern) has scored another major success with the discovery of a new class of exotic subatomic particles known as pentaquarks. The discovery was made by the LHCb experiment, one of the six experiments in the LHC. (LHCb is the abbreviation for Large Hadron Collider beauty, which was originally focused on a particle known as a b quark or beauty quark.) The LHCb is a major and very sophisticated machine in its own right, being 21 m long, 13 m wide and 10 m high and weighing 5 600 t.

Back in 1964, US physicist Murray Gell-Mann theorised that baryons (a category of particle which includes protons and neutrons) and mesons were made up of even smaller particles, called quarks or antiquarks. There are six different types of quarks (designated up, down, top, bottom, strange and charmed), each with its equivalent antiquark. Each baryon is composed of three quarks while each meson contains one quark and one antiquark. Gell-Mann’s theory allowed for the existence of pentaquarks composed of four quarks and an antiquark, but no such “composite state” (as it is called) had ever been observed – until now. (Gell-Mann won the Nobel Physics Prize in 1969 for his work.)

“The pentaquark is not just any new particle,” explained LHCb spokesperson Guy Wilkinson. “It represents a way to aggregate quarks, namely the fundamental constituents of ordinary protons and neutrons, in a pattern that has never before been observed in over 50 years of experimental searches. Studying its properties may allow us to understand better how ordinary matter, the protons and neutrons from which we’re all made, is constituted.”

The LHCb experiment succeeded because it was able to search for the pentaquark from multiple perspectives. “It’s as if the previous searches were looking for silhouettes in the dark, whereas the LHCb conducted the search with the lights on, and from all angles,” stated Cern in its press release about the discovery. All previous searches for the particle had proved inconclusive.

“Benefiting from the large data set provided by the LHC and the excellent precision of our detector, we have examined all possibilities for these signals [detected by the LHCb] and conclude that they can only be explained by pentaquark states,” stated LHCb team member and Syracuse University physicist Tomasz Skwarnicki. More precisely, the states must be formed of two up quarks, one down quark, one charm quark and one anticharm quark.”

The next step in the research will be to ascertain how the quarks are organised within the pentaquark.“The quarks could be tightly bound or they could be loosely bound in a sort of meson-baryon molecule, in which the meson and baryon feel a residual strong force similar to the one binding protons and neutrons to form [atomic] nuclei,” suggested LHCb and Tsinghua University physicist Liming Zhang.

The LHCb experiment involves 704 scientists from 69 universities and laboratories in 17 countries, supported by some 400 engineers and technicians. It is run by a Collaboration Board (CB) which is composed of one representative from each of the participating institutions as well as the LHCb management team. The CB elects a chairperson, who has a two-year term.

The LHC itself is located in an underground tunnel on the Franco-Swiss frontier, near Geneva, which is some 27.36 km in circumference. It originally cost about £2.6-billion to build. The world’s most powerful particle accelerator, it restarted operations in April after a two-year period of maintenance and upgrading, making it even more powerful.