For a long time, it was thought that all matter consists of atoms and that atoms are the smallest divisible building blocks. However, in the 19th century, scientists discovered that atoms are divisible: they contain electrons and a nucleus with protons and neutrons. Yet even these appear not to be the elementary particles from which everything is built. What exactly does matter consist of and how is this studied?

Particle physics, the field that studies the smallest particles, has developed a standard model with all known elementary (indivisible) particles. For example, it contains electrons, but no protons: protons consist of multiple quarks and are therefore not elementary particles.

The standard model doesn’t seem to be entirely complete. For example, it is still unknown what dark matter consists of. Jory Sonneveld, UvA assistant professor working at the National Institute for Subatomic Physics (Nikhef): ‘We want to measure whether the standard model of elementary particles is correct, and if we can find new particles, for example.’ They do this by performing experiments with extremely advanced technologies. 

Colliding particles

An example of such advanced technology is a particle accelerator: a device that accelerates small particles to extremely high speeds. Sonneveld is working on the largest particle accelerator in the world, the Large Hadron Collider (LHC) of CERN. She explains: ‘The LHC is a 27-kilometre-long ring that is located 100 metres underground. In this, we let protons collide with each other, and with detectors, we can then measure the released particles.’

One of the most important discoveries in the LHC was the Higgs boson in 2012. Sonneveld explains: ‘The standard model of particle physics could not explain the mass of particles. One theory to explain this was the existence of the Higgs particle. The fact that we have now demonstrated that this particle actually exists is very exciting.’

Increasingly better detection

Sonneveld is working on improving one of the LHC detectors, the ALICE detector. She focuses on the part that is closest to the proton beam. ‘That is the spot where we want to measure the location of a particle that passes through the detector very precisely,’ she explains. These measurements are carried out with many sensors made of silicon. Thanks to technological progress, these sensors have become increasingly faster and more accurate in recent years.

In addition, the sensors in ALICE are very thin, because the electronics are integrated directly into the sensor. This provides a good resolution of about five micrometres. However, a disadvantage is that these thin sensors are less resistant to the radiation released by the proton beam. Sonneveld: ‘We want to improve the sensors so that they are more resistant to radiation and can perform measurements even faster.’ 

Exciting research

The sensors are improved, for example, by redesigning the electronics. The optimized sensors are then sent all over the world to be tested. Sonneveld: ‘Someone from our team regularly travels to CERN to measure the sensor in a test beam. That is very exciting, because you usually have one week to test, and you don’t want to waste any time. You hope that everything works well quickly.’ 

The thin sensors appear to be more resistant to radiation than expected and have another advantage: they are flexible. The detector currently consists of many small sensors, each with its own cable. Flexible sensors can be bent around the proton beam, which makes much longer sensors possible. This saves a lot of material, so it’s the next planned upgrade for ALICE.

Sonneveld is excited: ‘It all seems to be working well now, so that is really good news. We are not there yet, but things are moving in the right direction.’ In the long term, she hopes that this new detector technology will enable much better measurements. ‘I think that the standard model is not the final answer and that we may be able to detect new particles with this.’

Unexpected applications

The LHC sensors can also be useful for other applications, such as in space travel or for medical imaging equipment. Technologies developed at CERN often turn out to have unexpected new applications. For example, the World Wide Web was invented at CERN.

Sonneveld explains: ‘That is because, technologically speaking, we are not quick to say “this is not possible”. Even if we are not going to make it ourselves, we work with companies to adapt it to our wishes, even if it does not yet exist.’  

However, these new applications are often impossible to predict in advance. Sonneveld therefore emphasises the importance of fundamental research for society. ‘Research is something beautiful and important that advances our society. But you cannot say in advance what exactly it will be that will move us forward.’