Particle discovered at CERN solves a 20-year-old mystery

A new particle has popped into existence at CERN’s Large Hadron Collider, a heavier proton-like particle that contains two charm quarks.

Protons and neutrons are examples of a class of particles called baryons, which each contain three fundamental subatomic particles called quarks that come in a variety of so-called flavours. In the case of a proton, there are two “up” quarks and one “down” quark that make up the particle.

But heavier quarks, like those known as charm quarks, can also combine to make baryons. However, because these unusual quark combinations are heavier and so more unstable, they often have fleetingly short lifetimes and quickly decay into other particles.

In 2017, physicists working at CERN’s LHCb experiment glimpsed one of these exotic baryons, memorably named Xi_cc⁺⁺, that was made up of two charm quarks and an up quark. This particle lived for only a trillionth of a second. Now, physicists working on the LHCb experiment have spotted the charm-filled sister particle to Xi_cc⁺⁺, called the Xi_cc⁺particle, which contains a down quark instead of an up, making it a heavier analogue of the proton.

This particle had a predicted lifetime of six times shorter than that of the Xi_cc⁺⁺, making it much harder to detect. It was found only after the LHCb experiment was upgraded to carry out more sensitive particle searches. The finding has a statistical significance of over 7 sigma, a measure that physicists use to state how confident they are that the result isn’t a random fluke, which easily clears the 5-sigma bar required to claim a discovery.

“Not only is it interesting discovering the particle in its own right – the Xi_cc⁺ has been searched for for a long time – but it also really shows the power that these upgrades to the LHC are having,” says Chris Parkes at the University of Manchester in the UK. “In one year’s data sample, we were able to see something that we couldn’t see with 10 years of data from the previous generation.”

Spotting this particle could teach us about how the strong nuclear force, which describes how quarks bind together, glues together heavier quarks than those we see in protons and neutrons, says Parkes. But it also resolves a 20-year-old mystery.

In 2002, physicists working on the SELEX experiment at the Fermi National Accelerator Laboratory in Illinois thought that they had spotted a particle that looked very much like Xi_cc⁺, but with a much lower mass than predicted at only a 4.7 sigma level of confidence. “Now we’ve found it, but it’s at a mass which is similar to its partner [Xi_cc⁺⁺] that we found a few years ago, and not at the mass that was predicted by SELEX,” says Parkes. The strength of the new discovery closes the door on the question of this particle’s mass.

“It’s a very interesting measurement, but it’s unclear what we learn from it,” says Juan Rojo at Vrije University Amsterdam in the Netherlands. “There is no rule in quantum chromodynamics which prevents this hadron from existing, but now we’ve measured it exists, we are left not particularly illuminated.”

Part of this, says Rojo, is because our current theories don’t predict well how heavier quarks inside baryons should interact or what their masses should be. “The data is now ahead of the theory for these kinds of particles, but it could be that in five years from now, this measurement is able to answer some very important theory questions,” says Rojo, such as what different combinations of quarks mean for particle masses.