The Large Hadron Collider Enters a New Era

Far below the border between France and Switzerland, the world’s most powerful particle accelerator has temporarily gone quiet. After years of sending proton beams around its 27-kilometre underground ring and colliding them at almost the speed of light, CERN’s Large Hadron Collider has entered an extended shutdown.

The silence, however, does not mean inactivity. Across the enormous underground complex, thousands of scientists, engineers and technicians are removing ageing components, installing advanced systems and carrying out one of the most complex scientific upgrades ever undertaken.

When the machine returns to operation around 2030, it will begin a new chapter as the High-Luminosity Large Hadron Collider, or HL-LHC. The upgraded accelerator is expected to generate approximately seven times more collision data than the version of the LHC that enabled the discovery of the Higgs boson.

For those who have worked on the project for many years, the shutdown represents far more than a technical pause. It is the transition between two generations of particle physics.

My involvement in the High-Luminosity programme began before the Higgs boson was discovered in 2012. Over almost two decades, I have had the opportunity to contribute to the development of the upgraded collider through work in both the United States and the United Kingdom.

In the US, I served as upgrade coordinator for the Compact Muon Solenoid, known as CMS, one of the principal experiments operating at the LHC. CMS is positioned around one of the locations where two proton beams collide. Its vast and highly sophisticated detector records the particles produced in those collisions, allowing physicists to reconstruct and analyse what occurred.

My role involved helping to coordinate the international effort to prepare CMS for the much more demanding environment of the High-Luminosity collider.

Today, at Oxford, I work with Atlas, another major LHC experiment. Atlas and CMS pursue many of the same scientific questions using independently designed detectors and separate research teams. This duplication is essential: an important discovery made by one experiment must be confirmed by the other before the scientific community can have full confidence in the result.

Our Oxford team is producing silicon pixel detector modules for the upgraded Atlas inner tracking system. These modules will sit close to the point where proton collisions occur and will help record the paths of newly created particles with exceptional accuracy.

Recently, I watched the first complete pixel ring being assembled in Oxford. It was both technically impressive and unexpectedly beautiful: a finely organised structure of silicon sensors, electronics and support materials, representing years of design work, testing, refinement and international cooperation.

For the first time, something that had existed mainly in technical drawings, simulations, prototypes and meeting presentations had become a complete physical object.

Yet our contribution is only one part of a much larger international effort. The upgraded Atlas detector will contain thousands of components designed and produced by institutions around the world. Every element must operate as part of a single system before the HL-LHC can begin exploring the next frontier of particle physics.

Beyond the Discovery of the Higgs Boson

The Large Hadron Collider has already changed our understanding of the universe. Its most famous achievement was the discovery of the Higgs boson, the particle associated with the mechanism through which elementary particles acquire mass.

The Higgs boson completed the Standard Model of particle physics, our most successful theory describing elementary particles and three of the four known fundamental forces. But the discovery did not bring the investigation to an end. Instead, it created an entirely new scientific programme.

The central question is no longer simply whether the Higgs boson exists. Physicists now want to know whether it behaves exactly as the Standard Model predicts.

Even a very small difference between theory and observation could provide evidence of previously unknown particles, interactions or forces. Such evidence might help explain some of the greatest unresolved mysteries in physics, including the nature of dark matter and the reason the observable universe contains much more matter than antimatter.

The difficulty is that any signs of new physics may be extraordinarily faint. Finding them does not necessarily require dramatically higher collision energies. It requires a much larger number of collisions and therefore far more data.

This is the purpose of the High-Luminosity upgrade.

Luminosity describes how frequently particles collide inside the accelerator. Over its operational lifetime, the HL-LHC will produce approximately seven times more collision data than the current machine.

The difference can be compared to replacing a camera that takes one image every second with one that takes seven. A single photograph may appear almost identical, but a much larger collection allows researchers to detect patterns and details that would otherwise remain hidden.

For Higgs research, this increase will be revolutionary.

Studying the Rarest Higgs Decays

The Higgs boson is difficult to produce and disappears almost immediately after it is created. Scientists therefore study it by examining the particles into which it decays.

Some Higgs decays occur relatively often and have already been measured with increasing precision. Others are extremely rare and remain close to the limits of what the existing LHC can detect.

One important example is the decay of a Higgs boson into two muons. Muons are unstable subatomic particles related to electrons, but significantly heavier. Measuring this decay allows physicists to test whether the Higgs interacts with second-generation leptons in the way predicted by the Standard Model.

Another major challenge is the decay of the Higgs into charm quarks. This process is particularly difficult to identify because its signal is buried beneath an enormous number of ordinary particle interactions that can produce similar experimental signatures.

Both measurements investigate one of the Higgs boson’s most fundamental characteristics: whether its interaction with lighter particles follows the precise pattern predicted by current theory.

A small deviation could suggest that unknown particles or forces are indirectly affecting the Higgs.

An even more ambitious objective is the observation of pairs of Higgs bosons. Detecting enough of these events would allow physicists to measure the Higgs self-coupling—the strength with which the Higgs field interacts with itself.

This property determines the form of the Higgs field that extends throughout the universe. It may also have influenced the evolution of the cosmos during the first moments after the Big Bang.

Such measurements were among the main reasons the HL-LHC was designed. But obtaining them requires major advances not only in the accelerator, but also in the experiments responsible for recording the collisions.

Separating Hundreds of Collisions

The upgraded collider will create an extraordinarily complex experimental environment. Every time the proton beams cross, as many as 200 proton-proton interactions may take place almost simultaneously.

This means that the detectors will be filled with dense streams of overlapping particle tracks. Identifying which particles belong to a rare Higgs event will be similar to trying to follow one quiet conversation in a crowded hall where hundreds of people are speaking at once.

To manage this challenge, Atlas and CMS are receiving entirely new silicon tracking systems.

These detectors must measure particle trajectories with exceptional precision while surviving radiation levels that would rapidly damage earlier generations of technology. Their development has required major progress in silicon sensors, high-speed electronics, advanced cooling, data processing and lightweight mechanical engineering.

One of the most significant innovations will be the introduction of highly precise timing detectors.

Atlas will use the High Granularity Timing Detector, while CMS is developing a comparable system. These technologies will measure the arrival time of particles with a precision of only a few tens of trillionths of a second.

Although hundreds of collisions may appear to occur at the same moment, they are separated by extremely small differences in time. Measuring those differences will allow physicists to connect each particle with the correct collision.

In effect, time will become a fourth dimension of particle tracking.

This capability will be crucial for reconstructing rare Higgs processes that would otherwise disappear inside the enormous background of overlapping interactions.

Preparing the Next Generation

One of the most inspiring aspects of the upgrade is the involvement of young scientists. Students and early-career researchers are helping to construct the detectors that will eventually produce the data on which much of their professional work may depend.

They are not simply assisting with today’s engineering programme. They are helping to build the scientific instruments that could define the next several decades of particle physics.

When the High-Luminosity Large Hadron Collider begins operating, it will do more than continue the work of the existing machine. It will open a new age of precision research.

It may reveal small but meaningful inconsistencies in the Standard Model, providing the first evidence of a deeper theory of nature. Alternatively, it may confirm the existing framework with a level of accuracy never previously achieved.

Either result would be scientifically important.

The LHC may currently be silent, but beneath the French-Swiss border, the future of particle physics is already being assembled.