Lord of the data

If you want to explore nature deeply, you can’t be afraid of big data. No one knows this better than Stefan Ritt. The head of the PSI Muon Physics research group deals with vast amounts of data: “In the second phase of the Mu3e experiment, we want to measure two billion muon decays per second. That’s 200 gigabytes per second, enough to fill a normal hard drive in less than a minute.”

Stefan Ritt and his team are searching through mountains of data for reactions that shouldn't exist at all – at least not according to the Standard Model of particle physics. But there are already strong indications that this model doesn’t tell the whole story. If researchers can prove that the “Mu3e” decay, the transition of a muon into two positrons and an electron, actually occurs, they would be opening a gateway to a shadowy realm of unknown physics. They might even come closer to answering the question of what makes up one-fourth of the matter in the universe, which we currently know only as “dark” matter.

Two hundred Cheops pyramids and a needle

But before researchers can observe muons decaying, they first have to produce the elusive particles – in large quantities and continuously. At PSI, this is done at the Swiss Muon Source SμS (pronounced: es-mu-es) and the Swiss Research Infrastructure for Particle Physics CHRISP: There protons, that is, hydrogen nuclei, collide at high force with so-called targets. This process produces muons, which are essentially heavier relatives of electrons. These elementary particles live just long enough to be guided with magnets into measuring devices, where physicists can stop them and observe their decays.

Currently the PSI facility produces around a hundred million muons per second for a single experiment. This makes it the world’s most powerful continuous muon source. However, among a quadrillion decays “permitted” by theory, only a single Mu3e decay should occur: “The image of a needle in a haystack no longer applies,” Ritt says, “unless you can imagine a haystack two hundred times the size of the Great Pyramid of Giza.”

MIDAS – one for all

For such large amounts of data, you need not only high-performance systems but also the right software – and Stefan Ritt, as a young physicist at PSI, developed it himself: MIDAS, the Maximum Integration Data Acquisition System. “With the software available to us back then, we sometimes had to run four or five systems in parallel,” Ritt recalls. “I didn't like that. I wanted something that combined all the data into a single database.”

One for all – that’s the idea behind MIDAS. Already by 1996, the first version was completed. It was controlled initially via command lines; today, everything works by way of modern graphical interfaces and even smartphones. MIDAS runs hardware-independently on all common operating systems. This is the main reason why, after 30 years, it remains in use far beyond PSI. The software is running at research centres in Europe, Japan, and America. “MIDAS is so stable that hundreds of megabytes per second can run through dozens of computers for months without the program ever crashing.”

In addition, the software is exceptionally flexible. It only takes around 300 lines of code to adapt MIDAS to a new experiment – ​​the remaining 300,000 lines stay the same.

“Being able to understand and calculate nature, I find that cool!”

Electronics and software are in Stefan Ritt’s blood. His father, an electrical technician, taught him to solder as a child. As a schoolboy, he and a friend developed a microcomputer control system for telescopes, long before that became standard practice among amateur astronomers. He explains his decision to study physics at the University of Karlsruhe instead of a career as a software engineer this way: “A teacher once showed us how to shoot a metal ball into a glass of water on the first try by calculating elastic force and parabolic trajectory. So you can understand nature and calculate things that then actually happen. I found that cool.”

His professor in Karlsruhe introduced him to particle physics. He became acquainted with PSI during an internship in 1988. That’s where he first got engaged with the kind of data acquisition a modern experiment requires. “That's where it all began: the combination of fundamental research and technology, electronics, and software development, all within experiments that are truly cutting-edge.”

The allure of “forbidden” physics

Like Mu3e, one of these experiments concerned a “forbidden” muon decay that, according to the Standard Model, shouldn’t even exist. In the MEG experiment, Ritt and his team searched for the exotic decay of a muon into an electron and a photon. They didn’t find it, but they did achieve the most precise measurement to date. “After 15 years, we can rule out many new theories intended to go beyond the Standard Model. So we’re not simply measuring zero, but rather an important lower threshold.”

The successor experiment Mu3e has been under construction since 2012. Ritt expects the first data to be available this fall. The experiment will then be shut down at the end of 2027 – at which point the major PSI project IMPACT will begin. Over the course of two years, two new target stations will be built, which will generate a beam of muons at a rate ten times higher.

There’s also the National Centre of Competence in Research (NCCR) Muoniverse, led by PSI, which was launched at the beginning of the year. Over the coming years, further infrastructure will be developed for this project, enabling the use of muons for applications including materials testing, the development of new quantum materials, and even archaeology. Thus IMPACT and Muoniverse will secure PSI’s leading role in muon physics for many years to come. And naturally, Stefan Ritt's MIDAS software will play a central role in all of this: “Many sub-projects within Muoniverse require data acquisition, so I am advising all the groups as well as providing direct support.”

When asked how, with the support of a small team, he’s been able to develop this unique software continuously for more than 30 years without losing his drive, the physicist has a clear answer ready: “Thanks to the experiments. Knowing that my software helps our team at PSI and colleagues worldwide do better physics and arrive at important insights – that’s the greatest reward!”