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

A new particle has appeared at CERN’s Large Hadron Collider, a heavier proton-like particle containing two charm quarks.

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

But heavier quarks, such as those called charm quarks, can also combine to make baryons. But because these unusual quark combinations are heavier and thus 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⁺⁺which 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 discovered the charming sister particle of Xi_cc⁺⁺called Xi_cc⁺particle, which contains a down quark instead of an up, making it a heavier analogue of the proton.

This particle had an expected lifetime six times shorter than Xi_cc⁺⁺which makes it much more difficult to detect. It was found only after the LHCb experiment was upgraded to perform more sensitive particle searches. The finding has a statistical significance of over 7 sigma, a measure physicists use to say how confident they are that the result is not a fluke, easily clearing the 5-sigma line required to claim a discovery.

“Not only is it interesting to discover the particle itself – Xi_cc⁺ has been sought for a long time – but it also really shows the power that these upgrades to the LHC have,” says Chris Parkes at the University of Manchester in the U.K. “In one year’s sample of data, we could see something we couldn’t see with 10 years of data from the previous generation.”

Spotting this particle can 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 solves a 20-year-old mystery.

In 2002, physicists working on the SELEX experiment at the Fermi National Accelerator Laboratory in Illinois thought they had seen a particle very similar to Xi._cc⁺but with a much lower mass than predicted at only the 4.7 sigma confidence level. “Now we have found it, but it is in a mass similar to the partner (Xi_cc⁺⁺) that we found a few years ago, and not at the mass 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 of the Vrije University Amsterdam in the Netherlands. “There is no rule in quantum chromodynamics that prevents this hadron from existing, but now that we have measured that it exists, we are not particularly enlightened.”

Part of this, says Rojo, is because our current theories do not predict well how heavier quarks inside baryons will interact or what their masses will be. “The data is now ahead of the theory for this type of particle, but it may be that this measurement five years from now will be able to answer some very important theory questions,” says Rojo, such as what different combinations of quarks mean for particle masses.