Heavier cousin of the proton discovered at the LHC

Researchers at the Large Hadron Collider (LHC) have made a groundbreaking discovery, identifying a new particle known as the Ξ cc ⁺, or "Xi cc plus." This particle is a heavier cousin of the proton, and its elusive nature had evaded detection for decades. However, the upgraded LHCb detector captured the fleeting particle in just one year of data, providing a significant breakthrough in understanding the forces that bind quarks together.

Quarks are the fundamental building blocks of protons and neutrons, which form the atomic nuclei. Protons themselves consist of two up quarks and one down quark, held together by the strong force. This interaction is described by quantum chromodynamics (QCD), a sophisticated theory that governs the behavior of quarks and gluons. The Ξ cc ⁺ is unique because it replaces the two up quarks with heavier charm quarks, retaining only one down quark.

"Up and down quarks are labels we use to distinguish the different types of quark," explained Tim Gershon of the University of Warwick. "In the Ξ cc ⁺, both up quarks are replaced by the heavier charm quark. Since the charm and up quarks differ only by their mass – and have the same charge – this provides an ideal way to test QCD," Gershon added, who is also the spokesperson-elect for LHCb.

This change in quark content results in the Ξ cc ⁺ being roughly four times heavier than a proton. Its extremely short lifetime, lasting less than a trillionth of a second, has historically made it undetectable, even though the particle is produced frequently in LHC collisions.

The crucial development that enabled this observation was the upgrade of the LHCb detector. "We were able to observe the Ξ cc ⁺ in one year of data-taking, while we had not been able to do so in a decade of data collected with the original LHCb detector," Gershon emphasized.

The Ξ cc ⁺ briefly appears in proton–proton collisions before decaying into three lighter particles: a Λ c ⁺ baryon, a K⁻ meson, and a π⁺ meson. These decay further into five final particles, including a proton, two K⁻ mesons, and two π⁺ mesons. By reconstructing the trajectories of these particles and analyzing the decay patterns, scientists were able to confirm the existence of the Ξ cc ⁺.

This discovery not only validates the predictions of QCD but also opens new avenues for studying the strong force and the behavior of heavy quarks. As the LHC continues to operate and the detectors undergo further improvements, researchers anticipate uncovering more particles and deepening their understanding of the fundamental building blocks of matter. The detection of the Ξ cc ⁺ is a testament to the power of collaboration, technological advancement, and the relentless pursuit of knowledge in particle physics.