Physicists on the LHCb experiment at CERN's Large Hadron Collider have detected the Xicc+ particle, a baryon containing two charm quarks and one down quark. This heavier analogue of the proton resolves a 20-year-old mystery from a previous experiment. The discovery, confirmed at over 7 sigma significance, highlights upgrades to the LHCb detector.
Protons and neutrons are baryons made of three quarks. Heavier versions, like those with charm quarks, are unstable and decay quickly. In 2017, the LHCb experiment spotted Xicc++, composed of two charm quarks and one up quark, lasting a trillionth of a second. Now, researchers have found its sister particle, Xicc+, which swaps the up quark for a down quark, making it heavier and with a predicted lifetime six times shorter than Xicc++'s. This detection required an upgraded LHCb for more sensitive searches, achieving over 7 sigma statistical significance, surpassing the 5-sigma discovery threshold, using just one year's data—something impossible with 10 years of prior data, according to Chris Parkes at the University of Manchester in the UK. Parkes noted: “Not only is it interesting discovering the particle in its own right – the Xicc+ has been searched for for a long time – but it also really shows the power that these upgrades to the LHC are having.” The finding sheds light on the strong nuclear force binding heavier quarks. It also addresses a puzzle from 2002, when the SELEX experiment at Fermilab reported a candidate Xicc+ at 4.7 sigma but with a mass lower than predicted. The new mass aligns with Xicc++'s, contradicting SELEX. Parkes said: “Now we’ve found it, but it’s at a mass which is similar to its partner [Xicc++] that we found a few years ago, and not at the mass that was predicted by SELEX.” Juan Rojo at Vrije University Amsterdam called it “a very interesting measurement,” but added uncertainty on insights gained, as theories lag data on heavy quark interactions in baryons.