Making the invisible visible
Astroparticle physicist Laura Baudis searches for hidden matter
The search for dark matter is one of the 'hot topics' in current elementary particle physics. Recently, powerful detectors have been developed that may be able to experimentally detect the components of this 'dark', invisible matter. Laura Baudis, professor of physics at the University of Zurich, has made a significant contribution to the development of this research infrastructure over the past 20 years.
It was in 1988 when the world suddenly came to a standstill for Laura Baudis. The daughter of a teacher couple was 18 years old at the time and had just completed her baccalaureate in the Romanian city of Timisoara. Elsewhere, all doors would have been open to her, but in communist Romania she was neither allowed to study nor to work. Shortly before, her parents had applied for permission to leave the country for the West. In socialist Romania, such an application to leave the country was tantamount to the loss of a life perspective in one's own country: the parents were transferred from the teacher's desk to the administrative department of a mine, and their daughter Laura was banned from studying and working.
Barely two years later, in February 1990, the family left the country. Meanwhile, the Romanian people had risen up against the one-party state; on Christmas Day 1989, ruler Nicolae Ceausescu and his wife were executed. "At the time, my parents were considering whether to stay in Romania after the revolution, but I wanted to finally leave after two years of waiting, and so we did." The parents moved with Laura and her two younger siblings to Germany, where the mother's sister lived. After spending time in several camps, the "Aussiedler" found a new home near Heidelberg.
Via Standford and Aachen to Zurich
"I had a happy childhood," recalls Laura Baudis, who grew up in a literate home. With her German-speaking mother, she translated poems by Romanian poets such as Paul Celan or Ana Blandiana. Through her father, who taught French, she became acquainted with existentialists such as Sartre, Camus and Simone de Beauvoir. Alongside literature, a second passion germinated at that time: "Mathematics was my great love at school," remembers the now 51-year-old. As a pupil, she attended a grammar school with a focus on mathematics, physics and computer science. After moving to Germany, she caught up on her 13th year - and decided to study physics in Heidelberg.
From now on, nothing seemed to be able to slow down the scientist's career: After studying and earning her doctorate at Heidelberg University, she moved to Stanford University (California) as a postdoc in 2000. In 2003, she took on an assistant professorship at the University of Florida in Gainesville. When the physicist and her husband planned to return to Europe three years later, she was offered chairs at RWTH Aachen University, the University of Bonn and the University of Zurich. In 2007, after a stopover in Aachen, Laura Baudis became a physics professor at the University of Zurich (UZH), while her husband - a trained medical doctor with a PhD in molecular cytogenetics - started a research group in bioinformatics at the same university, where he is now a professor. "You are not a dual-career problem, you are a dual-career opportunity for us," said Michael O. Hengartner, later UZH Rector, welcoming the newcomers to Zurich.
Doctoral thesis on neutrino research
Nathan, their son, was now eight years old, daughter Naima four. "In Standford we had a day nursery on campus, back in Europe my parents supported us," says Laura Baudis. "My husband and I always shared the care and education work, just as I knew my parents did, who both worked. In Romania, after all, all women were employedoutside of the home, a facet of communist life that I only really became aware of later."
In 1999, at the age of 30, Laura Baudis had her first child and her doctorate, just as she had planned. This laid the foundation for the family as well as for her academic career. In her doctoral thesis, she had devoted herself to neutrino research. She investigated a hypothetical radioactive decay ('neutrinoless double beta decay'). Among other things, she developed the concept of a neutrino detector, as it was built a little later in modified form at the Gran Sasso Laboratory in Italy with the GERDA experiment.
A series of detectors developed
As a doctoral student, the astroparticle physicist was already working on dark matter - the form of matter that must fill the universe, but for whose detection humankind has so far lacked suitable equipment. In the 1990s, scientists were looking for new ways to detect this matter. In 2006, a small research collaboration involving Laura Baudis resulted in the XENON10 experiment in the Gran Sasso underground laboratory northeast of Rome. Under the names XENON100, XENON1T and XENONnT, a series of further experiments were constructed in the years that followed, becoming ever larger and ever more sensitive. XENONnT is starting measurements these days, with the first results expected at the end of the year.
"I am proud of the fact that I was able to help develop the technology of the XENON detectors. A good 20 years ago, we started small. Today, we build large detectors that run stably and reduce background noise like no other experiment of this kind," says Laura Baudis. Reducing the backgrounds - this is a central prerequisite for having any chance at all of detecting the hidden particles of dark matter. To achieve this, a lot of scientific excellence had to be put into the right design and the selection of suitable materials. One focus of Baudis' group was on the inner detector, a container filled with the noble gas xenon, in which the sought-after particles - it is hoped - leave light traces that can be detected by highly sensitive photodetectors. Baudis' research group is also at the forefront of the readout electronics and data analysis.
Waiting for the big find
Laura Baudis has dedicated a good twenty years of research into the search for dark matter candidates such as WIMPs and axions, so far without success. "When we commissioned the XENON100 detector in 2008, we thought we had a good chance - based on theoretical predictions - of using it to discover the building blocks of dark matter, but we were wrong." Laura Baudis is undeterred by the lack of direct evidence so far. She remains optimistic and is pinning her hopes on the next experiment. "Even if we haven't found the particles so far, that doesn't mean we haven't found anything. Rather, we know better and better where exactly to look for the new, dark species. In addition, our detectors provide important insights into other research fields such as neutrino physics."
There's no question about it: anyone who studies the laws of elementary particles needs a lot of patience. This is especially true for such a fundamental research question as the detection of dark matter, which Laura Baudis and her research collaboration are working towards. In the event that the XENONnT experiment does not detect any new particles, the follow-up experiment DARWIN is already being planned, which Laura Baudis helped to design and which she represents externally as spokesperson. DARWIN is the last chance, so to speak, after which the detector concept will be exhausted because the interference from neutrinos will be so large that it will no longer be possible to detect dark matter. These are constraints that a researcher like Laura Baudis sometimes has to cope with. "When work demands too much of me again, I like to read a poem. It's a wonderful way to switch off."
Author: Benedikt Vogel
Portrait #5 of Women in science in the fields of MAP (2021)