7 July 2026

Photo: University of A Coruña/Titus Mombächer

The CODEX-beta detector. Left to right: Juliette Alimena (DESY), Philip Ilten (University of Cincinnati), Carlos Vazquez Sierra (University of A Coruña).

A new prototype detector called CODEX-beta has now recorded its first data at CERN. It is designed to test a different way of searching for hidden particles: looking for signs of new physics that could travel some distance from an LHC collision before decaying in a quiet, shielded space nearby.

The main experiments at the Large Hadron Collider (LHC) at CERN are among the best tools scientists have for exploring why our universe is the way it is. The current theory of particle physics describes the known elementary particles with remarkable precision, but it still leaves major questions unanswered: What is dark matter? Why is there more matter than antimatter in the universe? Why does the Higgs boson have the properties it does? And could hidden particles or forces be part of the answer?

At the same time, the LHC’s main experiments are not optimized for every possible kind of signal. CODEX-b is built on a simple but powerful idea: new particles might already be produced in LHC collisions, but be difficult to spot near the busy collision point. Some could travel several metres before decaying back into ordinary matter, leaving traces in places where dedicated detectors like CODEX-b can be designed to look.

The CODEX-b collaboration has now taken an important step toward testing that idea. In May 2026, it recorded its first data with CODEX-beta, a prototype detector installed at the LHC.

CODEX-beta is about two metres on each side, roughly a 1:5 scale version of the full proposed CODEX-b detector. Its purpose is to test the detector concept in real LHC conditions and to provide measurements needed for the design of the full experiment. The aim is to create an almost background-free search region, where ordinary particles from LHC collisions are strongly suppressed, and even a few unusual decays could stand out. Data from CODEX-beta will help researchers understand these backgrounds and optimize the shielding for the full detector.

“Designing and building this prototype was a tremendous effort by a small group of dedicated high-energy physics experimentalists, theorists, and engineers, many of whom donated their spare time to the effort,” said Philip Ilten, Professor at the University of Cincinnati and Spokesperson for the experiment. “The data from this prototype will be essential to realizing this type of detector concept, showing that we really can search for new physics this way.”

The particles that CODEX-b is designed to search for are known as long-lived particles: particles that survive long enough to travel a measurable distance before decaying. They appear in many theories of physics beyond the Standard Model and could be connected to major open questions in particle physics and cosmology.

“CODEX-beta is an important step because it lets us study the detector concept in real LHC conditions,” said Juliette Alimena, a DESY scientist, Cluster of Excellence Quantum Universe key researcher, and Deputy Spokesperson for the experiment. “If we want to search for rare particle decays in a deliberately quiet environment, we first need to understand that environment extremely well. These first data are the beginning of that process.”

First proposed in 2017 by four scientists from Lawrence Berkeley National Laboratory, the University of Cincinnati, and Paris LPNHE, CODEX-b has grown into an international collaboration of around 60 members at about 20 institutions. If approved by CERN, the full detector would be about ten metres on each side and be located next to the LHCb detector in the LHC’s interaction point 8 cavern.

"Even though the long-lived particles CODEX-b will search for are quite light in mass compared to the Higgs boson, being near an LHC collision point allows the experiment to explore exotic interactions with the Higgs boson itself," said Dean Robinson, a theorist at Berkeley Lab and one of the authors of the CODEX-b physics case proposal. "Better understanding how the Higgs interacts could be at the core of many open problems in particle physics today, and passing this milestone for CODEX-beta is really exciting."

For the collaboration, the first CODEX-beta data mark the beginning of a new phase. The project is no longer only a proposal, a simulation, or a detector under construction. It is now taking real data at the LHC, testing whether a deliberately quiet detector, placed away from the collision point, can open a new window onto hidden particles.