CERN’s Large Hadron Collider confirms newest particle: the pentaquark

If you want to see some funny nerd-runs over the next couple of days, go hang out at your local university’s physics department. The physical scientists there will be running up and down the halls, excited as they tend to get, because the latest fundamental constituent of the universe has now been confirmed: with a new paper in Physics Review Letters, CERN scientists working at the Large Hadron Collider claim to have found the long sought-after pentaquark.

The pentaquark was actually predicted way back in 1964, by the very same math that predicted the rest of the “hadrons” — particles made of quarks and held together by the strong nuclear force. Two-quark hadrons are called mesons. Three-quark hardons are called baryons, and include familiar high-school particles like protons and neutrons. Yet there’s no physical law restraining hadrons from getting larger than three quarks; hypothetical four-quark hadrons are called tetraquarks, and five-quark hadrons are called (wait for it) pentaquarks.


The six types of quark, and six the corresponding anti-quarks.

Note that by this definition, “pentaquark” is not a particle, but a type of particle. There could potentially be as many different pentaquark particles as there are five-part combinations of the six types of quark — though theory restricts this a bit by requiring that one of those five quarks be an anti-quark. Looking for the most easily detectable types of hypothetical pentaquarks led to repeated observations in the mid-2000’s, but later work called the reliability of those findings into question.

That’s why these findings from CERN are likely to stand up to peer review and scientific skepticism: the researchers are well aware of the pentaquark’s history of eluding capture, and of embarrassing reputable scientific teams. CERN’s Patrick Koppenburg told the BBC that the pentaquark “seems to be cursed somehow, because there have been many discoveries that were then superseded by new results that showed that previous ones were actually fluctuations and not real signals.”

They are confident that won’t happen to their results, which come with high statistical significance. In parlance, scientists working at the Large Hadron Collider’s LHCb experiment confirmed the existence of pentaquarks made of two up quarks, a down quark, a charm quark, and an anti-charm quark.


The LCHb (LHC beauty) experiment, diagrammed.

So what does this discovery accomplish, beyond validating some scientists’ career paths? Well, there’s no way of knowing just yet, but one long-theorized source of pentaquarks in the real world is the collapse of a large regular star into a super-dense neutron star. Neutron stars are of great interest to astronomers for all sorts of reasons, and understanding their life cycles could be crucial to continuing to advance our understanding of how the universe developed.

By confirming that quarks can indeed come together into an all-new conformation, CERN has provided a platform for physicists to improve the basic models of how matter forms and behaves. Quarks underlie all “normal” matter as we tend to think about it, so understanding how they can and cannot form aggregate particles could be of unparalleled importance. As mentioned, there’s no way to predict just how much importance until the new and updated theories start pouring in.

CERN characterized these experiments rather poetically, saying that prior searches for the pentaquark were like “searching for silhouettes in the dark,” while the LCHb experiment could look at the particle with the lights on, from all directions.​