What is the Xi-cc-plus baryon?

The Xi-cc-plus baryon is a subatomic particle containing two charm quarks and one down quark, making it approximately four times heavier than a proton. It was detected by the LHCb experiment at CERN with 7-sigma statistical significance from 915 candidate events at a mass of 3620 MeV per c-squared. This particle is only the second baryon ever observed containing two heavy quarks, representing a major milestone in particle physics research.

Why is this discovery significant for physics?

This discovery is significant because it provides direct evidence for the existence of baryons with two heavy quarks, which tests quantum chromodynamics predictions about how quarks bind together. The Xi-cc-plus is the first new particle discovered using the upgraded LHCb detector during Run 3 of the Large Hadron Collider. Understanding these exotic hadrons helps physicists refine their models of the strong nuclear force and could reveal new physics beyond the Standard Model.

How was the Xi-cc-plus detected at CERN?

The Xi-cc-plus was detected by the LHCb (Large Hadron Collider beauty) experiment during Run 3 of the LHC. Scientists analyzed proton-proton collision data and identified 915 candidate events where the particle briefly formed before decaying into lighter particles. The detection achieved 7-sigma significance, far exceeding the 5-sigma threshold required to claim a discovery in particle physics. The upgraded detector's improved tracking and timing capabilities were crucial to identifying these extremely short-lived particles.

What is the LHCb detector upgrade?

The LHCb detector underwent a major upgrade between Run 2 and Run 3 of the Large Hadron Collider, completed in 2022. The upgrade replaced nearly all sub-detectors with more advanced versions capable of reading out data at the full 40 MHz collision rate, compared to the previous 1 MHz. This allows the detector to process 30 times more data per second, dramatically increasing its sensitivity to rare particle decays and enabling discoveries like the Xi-cc-plus baryon.

How long does the Xi-cc-plus exist?

The Xi-cc-plus baryon exists for an incredibly brief time, less than a trillionth of a second, before decaying into lighter and more stable particles. Despite this fleeting existence, the particle leaves detectable traces in the LHCb detector as its decay products travel through the instrument. Scientists reconstruct the original particle by analyzing these decay signatures and matching them against theoretical predictions for the Xi-cc-plus mass and quantum properties.

Particle Physics / CERN2026

CERN Discovers New Doubly Charmed Baryon at LHC

Published: March 19, 2026

CERN's LHCb experiment discovered the Xi-cc-plus baryon containing two charm quarks, the first new particle found with the upgraded detector at 7-sigma significance.

Photo: CERN / Artist impression of Xi-cc-plus particle

Mass

3,620

MeV/c²

Significance

sigma

Events

915

candidates

Quark Structure

Two charm quarks (c) and one down quark (d) bound by the strong nuclear force to form the Xi-cc-plus baryon.

The Discovery: A New Particle from the Large Hadron Collider

In a major breakthrough for particle physics, the LHCb experiment at CERN has discovered a new baryon containing two charm quarks and one down quark -- the Xi-cc-plus. With a mass of 3,620 MeV/c², this particle is approximately four times heavier than a proton, making it one of the heaviest baryons ever observed.

The detection achieved 7-sigma statistical significance, far surpassing the 5-sigma threshold -- the gold standard for claiming a discovery in particle physics. From 915 candidate events identified in proton-proton collision data, scientists reconstructed the particle by tracking its decay products through the upgraded LHCb detector.

This is the first new particle found with the upgraded LHCb detector from Run 3 data, and only the second baryon with two heavy quarks ever observed. The particle flashes in and out of existence in less than a trillionth of a second, yet leaves detectable traces as its decay products travel through the instrument.

Photo: Scientific American / Cartoon illustration of doubly charmed baryon

Why This Matters

Baryons with two heavy quarks are a unique laboratory for testing quantum chromodynamics (QCD) -- the theory describing the strong nuclear force that binds quarks together. While most familiar baryons like protons and neutrons contain only light quarks, the Xi-cc-plus offers a window into how heavy quarks behave when bound into matter.

The discovery also validates the capabilities of the upgraded LHCb detector. The new system can read out data at the full 40 MHz collision rate -- 40 times faster than before -- allowing it to process 30 times more data per second and dramatically increasing its sensitivity to rare particle decays.

Tests QCD

Provides new data on how the strong force binds heavy quarks

2nd Double-Heavy Baryon

Only the second particle of this class ever observed

Upgraded Detector

First discovery from the upgraded LHCb using Run 3 data

Beyond Standard Model?

Could reveal new physics when compared against theory

Photo: HotHardware / CERN particle collision event display

The Upgraded LHCb: The Instrument Behind the Discovery

The LHCb detector underwent a major upgrade between Run 2 and Run 3, completed in 2022. Nearly all sub-detectors were replaced with more advanced versions, transforming it into an essentially new instrument in terms of performance.

The most critical upgrade was the ability to read out data at the full 40 MHz collision rate, compared to the previous 1 MHz. This allows the system to process 30 times more data per second, opening the door to detecting extremely rare particles like the Xi-cc-plus that were previously lost in the data flood.

"This is the moment we have been waiting for since the detector upgrade. Finding a new particle at 7-sigma significance demonstrates that the upgraded detector is performing beyond expectations."
-- LHCb Collaboration, CERN, 2026

▸ This discovery validates the upgraded LHCb detector -- unlocking many more discoveries ahead

▸ Understanding heavy quarks brings physicists closer to physics beyond the Standard Model

▸ Related: Vera Rubin Observatory | NASA ESCAPADE

References

CERN — LHCb Collaboration discovers new proton-like particle, March 2026
Scientific American — Physicists discover a charmed new baryon, March 2026
HotHardware — CERN's upgraded LHC makes first new particle discovery, March 2026

Frequently Asked Questions

The most common questions about the Xi-cc-plus baryon discovery at CERN.

By Emma Reyes · Climate & Science Correspondent

Published: March 19, 2026 · Updated: March 25, 2026

science·CERN LHCb · doubly charmed baryon · Xi-cc-plus · particle physics 2026