“Especially for searching for molecules with very weak interactions.”

Since its founding 70 years ago, the CERN research centre near Geneva has made many physics discoveries possible. Researchers continue to develop experiments and are also working on new ways to search for previously undiscovered particles. In the future, a new detector could make it possible to search for extremely weakly interacting particles. SHIP, short for Search for Hidden Particles, will be significantly different from CERN’s other large detectors – such as CMS or ATLAS. The new detector will also be used in one of the pre-accelerators of the Large Hadron Collider. In an interview with Physicist, Heiko Lacker from Humboldt University in Berlin talks about the physical background of the experiment and the current status of the project.

Physicist: Why do we need another big detector at CERN?

Heiko Lacker: There are already several large detectors at CERN. The most famous are CMS and ATLAS. You are standing in front of the Large Hadron Collider – the LHC for short, the most powerful particle accelerator in the world. These two detectors were used to detect the Higgs boson more than ten years ago. On the one hand, this was a century-long discovery in particle physics. If this is the Higgs boson in the Standard Model, which is currently being studied at the LHC, then the Standard Model is a complete theory. But on the other hand, despite the potential completion of the Standard Model, many questions remain unanswered, such as: Why do neutrinos have mass and why is their mass so small? Why is there almost only matter in the universe and almost no antimatter? What is the nature of the mysterious dark matter, which has a mass in the universe of about five times the mass of the visible matter we know?

How can I find answers to these questions?

We do not know how these questions will ultimately be answered. So in elementary particle physics we pursue different experimental approaches that complement each other. The SHIP experiment is one such experiment, and it complements the detectors at the LHC. The detector will not be built directly at the LHC, but in one of its primary accelerators, the SPS. It is currently being developed specifically to search for very weakly interacting particles that could provide answers to the remaining questions in the future.

What particles interact very weakly?

An important class of particles for which SHIP was designed are the so-called Majorana neutrinos. In 1937, shortly before his mysterious disappearance, the Italian theorist Ettore Majorana proposed a very interesting hypothesis: that there might be certain particles that are also their own antiparticles. These particles, named after him, have never been observed. But there are good arguments for why neutrinos, of all things, could be Majorana particles.

Elementary particles of the standard model

What special properties do neutrinos have and what does this mean for Majorana neutrinos?

Neutrinos are among the elementary particles in the Standard Model. The known neutrinos are exceptionally light, about a million times lighter than other elementary particles such as electrons. They carry no electrical charge and come into contact with other matter only through what is known as the weak interaction. This interaction is so weak that neutrinos can fly past planets and stars. However, we don’t know exactly how neutrino masses are created and why they are so small. Majorana neutrinos could theoretically answer these questions. That’s why we want to search for them using SHIP.

What properties would you expect from Majorana particles?

If there are particles that are also their own antiparticles, then there could be not just one type of Majorana particle, but two, three, or even more different types of Majorana neutrinos. If this is the case, it could explain the small masses of neutrinos, the asymmetry of matter and antimatter in the universe, and perhaps even dark matter.

Could such particles be theoretically detected using ShiP?

If the mass of Majorana neutrinos is not too large, a detector like SHIP can find them. This is where a special feature of SHIP comes in: Majorana neutrinos must be relatively long-lived compared to many other particles produced in particle accelerators. At the same time, they cannot be detected directly, but only through their decay products. Since they are moving away from the origin at high speed, a longer flight path is required, which the SHIP detector covers. This is where SHIP differs from the detectors at the LHC. In addition, it is built on a pre-accelerator.

Schematic diagram of the SHIP experiment

What advantages does this bring?

The protons delivered by the SPS have much lower energy than the protons accelerated in the LHC. But you can simply fire the proton beam from the SPS, which has more protons than the LHC, into a lump of matter. This creates a large number of high-energy particles – and with a bit of luck, every now and then a Majorana neutrino or some other exotic particle that could, for example, belong to dark matter. Known particles such as electrons, protons and photons can now be absorbed by this particle beam and muons can be deflected using strong magnetic fields. Only uncharged particles such as the neutrinos we already know about fly straight ahead, and perhaps also the Majorana neutrinos we are looking for.

Preparations for the experiment

How can this be proven?

Majorana neutrinos are not stable, but according to theory, they can decay into known particles. So if a pair of particles suddenly appears out of nowhere along the path in the detector, it could be the result of Majorana neutrino decay. But this can only be determined using sophisticated statistical methods and years of measurements. It is also possible that SHIP will not find Majorana neutrinos, but other exotic particles.

What could these be?

There are a number of candidates that have been proposed from a theoretical perspective. These include so-called axions or axion-like particles. Then there could be the dark photon, a heavy sister particle to photons, i.e. light particles. But previously unknown siblings of the Higgs boson are also possible. Searches for such particles are already underway. But SHIP will provide unique opportunities to do this that other experiments cannot.

What is the current planning status?

At the moment we are still in the project planning phase. This year CERN selected SHIP as a future flagship experiment for the SPS. To do this, the countries involved still have to clarify funding issues. The technical design report should be ready in three years, so that the experiment could be completed around 2031. Data collection will then begin, and is scheduled to last around 15 years. These time frames show that fundamental research in particle physics requires a lot of patience.