At CERN near Geneva, tiny particles with extremely high energies are blasted at each other to answer big questions about the universe. The detectors that observe the collisions of these particles require regular upgrades. Lea Caminada and her High Energy Particle Physics research group at the Paul Scherrer Institute PSI play an important role in this quest.
In a spacious laboratory on the third floor of a new building next to the Paul Scherrer Institute stands one half of the detector that traced the Higgs boson. This elementary particle, sought after for decades, was experimentally confirmed at CERN in 2012 – making history in particle physics.
The legendary particle was discovered using CERN’s Large Hadron Collider (LHC) – a 27-kilometrelong underground particle accelerator. In the LHC tunnel, highly accelerated protons are fired at each other so that the decay products of these explosive particle collisions can be measured. The LHC continues to be used to answer questions about the fundamental physics of our universe. And it must keep up with the times: “The giant detectors need to be upgraded every few years,” explains Lea Caminada, head of the High Energy Particle Physics group at the PSI Center for Neutron and Muon Sciences. “Among other things, this is because the high-energy particles that the detectors are designed to register inevitably damage their electronics over the years,” the physicist explains.
To precisely measure the tiny particles, colossal instruments are needed: Four giant detectors are in operation at the LHC. One of them is called Compact Muon Solenoid, or CMS for short. Built in onion-like layers, this detector has an overall diameter of 15 metres. Lea Caminada has been working on the construction of the CMS detector and the experimental results it delivers practically since its inception.
The innermost part of CMS, known as the barrel pixel detector, is shaped like a slightly oversized Swiss roll. About 50 centimetres long, it consists of three layers of gold-gleaming electronics and a profusion of cables. It was originally developed and built at PSI, where Lea Caminada, then a doctoral candidate, was already involved. In 2017, this first barrel pixel detector was dismantled and replaced by a four-layer successor, also designed and partly built by Caminada’s group. The original cylinder detector was split lengthwise into its two halves; one is now kept as an exhibit in Caminada’s laboratory at the newly built Switzerland Innovation Park Innovaare, located directly adjacent to PSI. CMS will remain in use in its current configuration until mid-2026, when a major upgrade of the entire LHC is set to begin. This will further increase the number of particle collisions, while at the same time providing an opportunity to replace many of the detectors’ components with new, technologically improved parts.
End-caps to cover the blind spot
The LHC is scheduled to start up again as the High-Luminosity LHC in 2030. Many research groups around the world are currently working on individual new components of the CMS detector that will be installed during the renovation phase from 2027 to 2030. This time, rather than being responsible for the next barrel pixel detector, Caminada’s group is working on disc-shaped components that will be mounted vertically in front of and behind the barrel detector.
“These discs form what we call the tracker endcap detector,” Caminada explains. “This will be a completely new part of the CMS detector. With it, we will be able to follow particle tracks that lie in the blind spot of the current detector.” Different particle decays that could arise from proton-proton collisions in the LHC are expected to occur in different solid angles. If you’re seeking new physics, you have to look where no one has been able to look before.
Each of the 16 circular discs that make up the tracker end-cap detector has a diameter of 50 centimetres, and each will be populated with silicon detector modules on both front and back.
“To thoroughly cover all these surfaces, we will need around 2,000 identically constructed detector modules,” Caminada explains. These rectangular modules are smaller than the palm of your hand. They are highly complex electronic components that must be manufactured with extreme precision to operate reliably. “After the upgrade, the entire detector will remain in operation for several years – and we won’t be able to remove or repair anything during this time.” It’s like sending a probe into space – during operation, you lose all physical access to the components. And every small probability of failure that you accept for a single component is multiplied by a factor of 2,000 for the entire detector – corresponding to the large number of modules.
Two years to produce the modules
Amrutha Samalan carefully places one module after another into the precisely fitting recesses of a white, wired box. “Over the past few years, we have examined and tested various module prototypes and continuously provided feedback to the module designers on potential issues, which are rare but important to address given the large number units,” explains the postdoctoral researcher, who has been working in Lea Caminada’s group for nearly two years. In the meantime, the design phase has been completed. Pre-production is under way.
Pre-production means that the researchers are testing a small number of modules to ensure that each production stage is working properly and that the results exactly meet expectations. During this phase, they can also estimate the time required for each production step, so that everything can be scaled up later. Of the approximately 2,000 modules required, nearly half will be assembled at PSI during the actual production phase. The remaining modules will be assembled by the other participants in a Europewide consortium, based on the concept developed at PSI. Altogether, they will have two years to produce the 2,000 modules.
“This morning, I used this machine to wire-bond some modules.” Samalan points to a unit, roughly the size of a person, in the middle of the lab. “It inscribes the electronic connections we have programmed onto the module – similarly to the way a sewing machine works with thread.”
This is followed by a thorough visual inspection, where high-resolution images of each module are carefully scrutinised on a screen for imperfections.
Then, eight modules are inserted side by side into a so-called cold box – the white box that Samalan is now loading. “We perform an important part of quality control in this cold box: We can precisely control the temperature and humidity inside the box and, at the same time, test whether the sensors, readout chips, and pixels are all fully functional,” the physicist explains. The researchers also calibrate all detector pixels and their readout channels in this cold box. Each individual module has more than half a million pixels. With a spatial resolution of only ten by fifteen micrometres, they will be able to precisely track particle trajectories in the CMS detector.
We have co-developed several generations of this detector and are familiar with the entire chain of steps, from chip design and installation to data analysis.
Planning the generation after the next
One floor up in the same building, Wolfram Erdmann has just been in discussion with colleagues. He too is a member of Caminada’s group. He is leading the international project to design, plan and build the tracker end-cap detector. “After the upgrade, this will be the largest part of the CMS pixel detector in terms of surface area,” he says with evident pride.
Erdmann maintains contact with the consortium’s other research groups at the universities of Zurich, Hamburg, Helsinki in Finland, Santander in Spain, Vilnius in Lithuania, and Zagreb in Croatia. “Here at PSI, we develop many components and many processes that are then replicated at these other locations,” says Erdmann. The cold boxes, for example. PSI has been a member of the CMS experiment since 1998. “This is a significant commitment and requires exceptional expertise,” says Caminada. As the world’s largest and most powerful accelerator, the LHC is essential for the field of particle physics. The geographical proximity is welcome: “For us, it’s very convenient that Switzerland is host country of CERN,” says Erdmann.
Caminada’s research group is also involved in analysing the data produced by the CMS detector. “We have co-developed several generations of this detector and are familiar with the entire chain of steps, from chip design and installation to data analysis,” Erdmann summarises. “Among the institutions involved, that’s quite exceptional.”
Analysis of data from the tracker end-cap detector, for which production is just ramping up, is expected to start in the early 2030s. In particle physics, planning extends over long periods of time. Accordingly, some researchers are already thinking about the next upgrade after this one. “In fact, that is exactly what I was discussing with my colleagues, just now,” Erdmann smiles. In that future upgrade, the researchers’ focus will be on enhancing the detector’s temporal resolution.
That Erdmann and other researchers are already receiving resources to think this far ahead does not strike the physicist as unusual. From his many years of experience, he knows that “thinking about something is relatively simple. Building it afterwards is where it gets complex.”
