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

A novel particle has emerged at CERN’s Large Hadron Collider, resembling a heavier proton and comprising two charm quarks.

Baryons, such as protons and neutrons, are particles that consist of three fundamental subatomic particles known as quarks, which appear in different “flavours.” A proton, for instance, is composed of two “up” quarks and one “down” quark.

Heavier quarks, such as charm quarks, can also form baryons. These heavier combinations are more unstable, leading to very short lifetimes before they decay into other particles.

In 2017, scientists at CERN’s LHCb experiment observed an exotic baryon called Xi_cc⁺⁺, consisting of two charm quarks and one up quark, existing for just a trillionth of a second. Now, researchers have identified a related particle, Xi_cc⁺, which includes a down quark instead of an up quark, making it a heavier version of the proton.

This newly discovered particle has a predicted lifespan six times shorter than the Xi_cc⁺⁺, making detection challenging. Its discovery was made possible after enhancing the LHCb experiment to conduct more precise particle searches. The result boasts a statistical significance exceeding 7 sigma, surpassing the 5-sigma threshold required to declare a discovery.

“The discovery of the Xi_cc⁺ particle is noteworthy in itself, having been sought after for a long time. It also highlights the effectiveness of the recent LHC upgrades,” says Chris Parkes from the University of Manchester in the UK. “With just a year’s worth of data, we observed something that was previously hidden in ten years of data from the earlier setup.”

According to Parkes, examining this particle could provide insights into how the strong nuclear force binds heavier quarks compared to those in protons and neutrons. This finding also resolves a long-standing 20-year mystery.

In 2002, physicists from the SELEX experiment at the Fermi National Accelerator Laboratory in Illinois believed they detected a particle similar to Xi_cc⁺ but with a lower mass than anticipated, with only a 4.7 sigma confidence level. “We have now found it at a mass close to its counterpart [Xi_cc⁺⁺], discovered a few years ago, diverging from the SELEX prediction,” Parkes explains. This discovery conclusively answers the question of the particle’s mass.

“This measurement is intriguing, yet its implications remain unclear,” says Juan Rojo from Vrije University Amsterdam in the Netherlands. “Quantum chromodynamics does not rule out this hadron’s existence, but knowing it exists leaves us with limited insight.”

Rojo suggests this is partly because current theories do not accurately predict how heavier quarks within baryons should interact or determine their masses. “The data has outpaced theoretical understanding for these particles, but in five years, this measurement might answer significant theoretical questions,” Rojo adds, such as the impact of different quark combinations on particle masses.

CERN and Mont Blanc, dark and frozen matter: Switzerland and France

Prepare to have your mind blown by CERN, Europe’s particle physics centre, where researchers operate the famous Large Hadron Collider, nestled near the charming Swiss lakeside city of Geneva.