New LHC hadron spotted
CERN researchers reported observing a new hadron called the Xi‑cc‑plus, described as a particle made of two charm quarks and one down quark and counted among the LHC’s expanding hadron roster. (x.com) That update sits alongside other LHC search activity this week, including missing‑energy studies aimed at signs of extra dimensions. (x.com)
Matter is built from quarks, and the Large Hadron Collider finds new combinations by smashing protons together and reconstructing the debris. On March 17, 2026, CERN said one of those combinations had finally shown up: the Xi-cc-plus. (home.cern) A proton contains two up quarks and one down quark. The new particle keeps the down quark but swaps in two much heavier charm quarks, giving it the makeup ccd and a mass about four times that of a proton. (lhcb-outreach.web.cern.ch) Physicists did not see the particle directly. The Large Hadron Collider beauty experiment, known as LHCb, inferred it from decay products in 2024 collision data at 13.6 teraelectronvolts, with a signal above seven standard deviations, past the five-sigma discovery threshold used in particle physics. (arxiv.org, home.cern) The measured mass was 3619.97 megaelectronvolts per c-squared, almost exactly where theorists expected the particle’s isospin partner to land. LHCb said this was the first new particle found with its upgraded Run 3 detector, which was completed in 2023. (arxiv.org, home.cern) Quarks are held together by the strong force, the same force that binds protons and neutrons inside atomic nuclei. CERN said the Xi-cc-plus gives theorists a new test case for quantum chromodynamics, the theory that describes that force, because baryons with two heavy quarks are rare and harder to model. (home.cern) The result also closes a long-running gap in the doubly charmed baryon family. LHCb had already discovered the Xi-cc-double-plus, made of two charm quarks and one up quark, in 2017, while earlier claims for the plus version from the SELEX experiment were not confirmed by FOCUS, BaBar, Belle, or earlier LHCb searches. (home.cern, lhcb-outreach.web.cern.ch, arxiv.org) The Xi-cc-plus was especially difficult to catch because theory predicts it lives far less time than its 2017 cousin. The LHCb paper says its lifetime is expected to be three to six times shorter than the Xi-cc-double-plus lifetime, which leaves fewer clean decays to reconstruct. (arxiv.org, home.cern) A separate line of Large Hadron Collider work looks for particles that do not leave tracks at all. The Compact Muon Solenoid experiment, known as CMS, searches for events with a single high-energy photon and missing transverse momentum, an energy imbalance that could point to dark matter or extra dimensions if an unseen particle escaped the detector. (cms.cern, cms.cern) That search has not produced a signal so far. CMS said its latest monophoton analysis found no significant excess above Standard Model predictions and instead set tighter limits on dark matter models and theories with extra dimensions. (cms.cern) So the Large Hadron Collider’s latest week produced two different kinds of result: one new particle added to the hadron count, now 80 at Large Hadron Collider experiments, and one more round of null results that narrowed where stranger physics could still be hiding. (home.cern, cms.cern)
