CERN scientists discover new Xi-cc-plus particle at LHC accelerator with two charm quarks

The team of researchers from Organização Europeia to Pesquisa Nuclear announced the identification of a new subatomic particle through the LHCb experiment, located in Grande Colisor of Hádrons. The discovery of the structure, officially named Xi-cc-plus, represents a significant advance in understanding the fundamental matter that makes up the universe and the forces that operate on a quantum scale. The finding was announced during the scientific conference Rencontres of

The new particle is classified as a baryon and has a composition considered rare in nature, containing two charm quarks and one down quark. Rigorous measurements carried out by the team indicate that the mass of this particle is approximately 3620 MeV/c², which makes it approximately four times heavier than a common proton. Essa extremely high mass is a direct result of the presence of two heavy quarks in its nucleus, a characteristic that differentiates it from the vast majority of particles observed in everyday life.

The data leading to this historic confirmation was collected throughout the year 2024 using a newly updated version of the LHCb detector. The equipment underwent profound modifications to increase its sensitivity and processing capacity, allowing collision events to be recorded with an accuracy unprecedented in the history of European accelerators. Analysis of these collisions revealed the particle’s clear signature, confirming theoretical predictions established decades ago.

Structure and composition of new baryon

In particle physics, baryons are defined as particles composed of three quarks, with protons and neutrons being the best-known and most abundant examples in the composition of visible matter. However, while ordinary matter is made up of light up and down quarks, the particle Xi-cc-plus belongs to a much more exotic and complex family. The presence of two charm quarks, which belong to the second generation of fundamental particles, gives this baryon unique dynamic properties. Physicists describe this specific configuration as a miniature planetary system, where the two heavy quarks orbit very close to each other, forming a dense, compact core, while the substantially lighter down quark circles this central duo in a wider orbit.

Due to its high mass and inherent instability, the particle CERN’s detectors do not observe the particle directly, but rather the products resulting from its rapid decay. The research team identified the signature of Xi-cc-plus by tracking its transformation into other lighter particles, specifically a Λc⁺ baryon, a K⁻ meson, and a π⁺ meson. The exact reconstruction of the trajectory of this subatomic debris, using high-precision silicon sensors, allowed scientists to calculate the original mass and confirm the existence of the parent baryon with extreme clarity and minimal margin for error.

Technological update of the hadron collider

The success of this detection is directly linked to the engineering improvements implemented in the LHCb experiment before the start of the current data collection period, known as Run 3. The particle tracking system has been completely redesigned to support a significantly higher collision rate, operating at energies reaching 13.6 TeV.

One of the most critical innovations for this finding was the adoption of a trigger system based entirely on advanced software. Instead of relying on rigid hardware to filter initial events, the complex’s computers now process tens of millions of collisions per second in real time, instantly deciding what should be recorded.

This unprecedented processing power allowed the detector to identify specific decay patterns that would previously have gone unnoticed by the old system. The efficiency of the new technological arrangement resulted in the capture of a volume of clean data that made complex statistical analysis finally viable for researchers.

Understanding the strong force in particle physics

The discovery provides a natural, highly controlled laboratory for studying the strong nuclear force, one of the four fundamental forces of nature. Essa force is mainly responsible for keeping the quarks together inside baryons and mesons, preventing atomic nuclei from disintegrating.

The theory that describes this interaction at the subatomic level is known as Cromodinâmica Quântica. Although Embora is a mathematically robust theory, performing exact calculations on how quarks interact at certain energy scales is an extremely complex process that requires the use of dedicated supercomputers.

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Baryons that have two heavy quarks simplify some of this enormous mathematical complexity. The difference in mass between charm quarks and light quarks creates a clear separation of energy scales, allowing theoretical physicists to use more accurate approximation methods in their calculation models.

By measuring the mass and lifetime of the Xi-cc-plus particle with high experimental precision, scientists can test these theoretical predictions directly. Qualquer substantial deviation between the value calculated on paper and the value measured at the collider may indicate the need for adjustments to the standard theory.

Data analysis and international collaboration

The data validation process involved the joint effort of more than a thousand scientists, representing dozens of research institutes and universities in more than twenty countries. The analysis required filtering billions of background events to isolate the genuine signal from the new particle, using the following verification parameters:

– Reconstrução of trajectories from the primary collision point.
– Medição of the flight time of the particles resulting from the decay.
– Identificação needs the electrical charge of subatomic debris.
– Eliminação of statistical noise generated by common particles.

History of research with heavy quarks

The search for doubly heavy baryons has been a long-standing goal in CERN’s extensive physics program, motivating decades of planning and engineering. The previous milestone in this specific area occurred in 2017, when the same international LHCb collaboration announced the discovery of the sister particle, Xi-cc-plus-plus, which contains two charm quarks and an up quark in its internal structure. The current identification of Xi-cc-plus, which replaces the up quark with a down, completes an important isospin doublet predicted by the Modelo Padrão of particle physics. Direct comparison between the masses and lifetimes of these two sister particles provides extremely valuable information about how the electromagnetic force acts in conjunction with the strong force over almost unfathomable subatomic distances. The mass difference between them is tiny, but it carries crucial data about symmetry breaking in the early universe, just after Big Bang. The time required between the first discovery and the current one demonstrates the extreme difficulty of producing the state with the down quark, requiring a substantial increase in the accelerator’s luminosity and the accumulation of years of uninterrupted collisions to generate sufficient statistics to irrefutably prove its existence.

Next steps in the European experiment

With the accelerator operating at its maximum luminosity capacity, continued data collection promises to further expand the catalog of known particles. The researchers will now focus on measuring additional properties of Xi-cc-plus, such as its exact production rate and the different channels through which it can decay, consolidating understanding of its dynamics.

Technological impact and collision processing

The volume of information generated by these collisions requires a distributed computing infrastructure on a global scale. CERN’s network system sends fragments of data to processing centers on several continents, where machine learning algorithms help identify anomalies and new signals.

The final result of the Xi-cc-plus analysis showed an extremely clear mass peak, corresponding to around 915 independently recorded events. The statistical significance of the discovery surpassed the 7 standard deviation mark, a value that far exceeds the strict threshold required by the scientific community to declare an official discovery, eliminating the possibility of statistical fluctuation.