A particle discovered at CERN solves a 20-year-old mystery

A new particle has been created at the Large Hadron Collider at CERN, a heavier proton particle that contains two magic 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 different so-called flavors. In the case of the proton, the particles are two “up” quarks and one “down” quark.

But heavier quarks, such as those known as charm quarks, can also combine to form baryons. However, because these unusual combinations of quarks are heavier and so unstable, they are often short-lived and quickly decay into other particles.

In 2017, physicists working on the LHCb experiment at CERN spotted one of these exotic baryons, memorably named Xi_CC⁺⁺which consisted of two charm quarks and an up quark. This particle lived only a trillionth of a second. Now physicists working on the LHCb experiment have spotted a sister particle Xi full of magic_CC⁺⁺called Xi_CC⁺a particle that contains a down quark instead of an up quark, making it a heavier analogue of the proton.

This particle had a predicted lifetime six times shorter than that of Xi_CC⁺⁺making it much more difficult to detect. It was only found after the LHCb experiment was upgraded to perform more sensitive searches for particles. The finding has a statistical significance of over 7 sigma, a measure physicists use to express how confident they are that a result is not a fluke, easily clearing the 5-sigma line needed 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 these LHC upgrades have,” he says Chris Parkes at the University of Manchester in the UK. “We were able to see something in one year’s worth of data that we couldn’t see with 10 years of data from the previous generation.”

Observing this particle could teach us about how the strong nuclear force, which describes how quarks bind together, glues quarks heavier than those we see in protons and neutrons, Parkes says. 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 that spotted a particle who looked very similar to Xi_CC⁺but with a much lower mass than predicted, at a confidence level of only 4.7 sigma. “Now we’ve found it, but it’s in matter that’s similar to its partner. [Xi_cc⁺⁺] that we found a few years ago, and not at the mass predicted by SELEX,” says Parkes. The power of the new discovery closes the door on the question of the particle’s mass.

“It’s a very interesting measurement, but it’s not clear what we’re going to learn from it,” he says Juan Rojo at 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, Rojo says, is because our current theories don’t predict well how the heavier quarks inside baryons should interact or what their mass should be. “The data is now ahead of the theory for these kinds of particles, but it could be that in five years this measurement will be able to answer some very important theoretical questions,” says Rojo, such as what different combinations of quarks mean for particle masses.

topics:

• Large Hadron Collider/
• particle physics