A groundbreaking discovery at CERN’s Large Hadron Collider (LHC) reveals a new, heavier proton-like particle composed of two charm quarks.

Protons and neutrons fall under the category of baryons, each containing three fundamental particles known as quarks, each with distinct “flavors.” For instance, a standard proton comprises two “up” quarks and one “down” quark.

Interestingly, heavier quarks, such as charm quarks, can also combine to create baryons, albeit these novel quark combinations are heavier and less stable, leading to shorter lifetimes before decaying into other particles.

In 2017, CERN’s LHCb experiment captured a glimpse of an exotic baryon named Xi._cc⁺⁺, which consists of two charm quarks and one up quark, possessing a lifetime of just one trillionth of a second. Recently, physicists found its intriguing counterpart, Xi_cc⁺, which contains a down quark instead of an up quark, making it a heavier analog of the proton.

This latest discovery, characterized by a predicted lifetime six times shorter than that of Xi_cc⁺⁺, posed significant detection challenges. It was confirmed only after substantial upgrades to the LHCb experiment enabled more sensitive particle searches, achieving a statistical significance exceeding 7 sigma—well above the 5 sigma threshold needed for a legitimate discovery.

“Uncovering the particle Xi._cc⁺ is not just remarkable—it’s a testament to the transformative power of the recent upgrades to the LHC,” stated Chris Parks from the University of Manchester, UK. “With just a one-year data sample, we’ve observed phenomena that eluded ten years of previous data gathering.”

The identification of this particle may enhance our understanding of the strong nuclear forces that bind quarks together while also affecting heavier quarks found in particles beyond protons and neutrons. This finding could resolve longstanding questions in particle physics.

In 2002, the SELEX experiment at Fermi National Accelerator Laboratory encountered a particle resembling Xi_cc⁺, yet it had a significantly lower mass than predicted, with a confidence level of only 4.7 sigma. “Now that we’ve validated its existence and confirmed the mass aligns with our predictions, we have effectively addressed this particle mass issue,” Parks remarked.

“While this measurement is fascinating, the implications remain uncertain,” noted Juan Rojo at Vrije Universiteit Amsterdam. “Quantum chromodynamics currently does not preclude the existence of this hadron, but we are still in the observation phase. The next five years could yield pivotal insights regarding how quark combinations impact particle mass,” Rojo added.