With its European particle physics laboratory CERN, Geneva attracts many researchers to Switzerland. This was also the case with François Drielsma (28). In a doctoral thesis supervised by Prof. Alain Blondel (University of Geneva), the Belgian-born scientist investigated a completely new way to build a particle accelerator.
On 5 February 2020, an article entitled 'Demonstration of cooling by the Muon Ionization Cooling Experiment' was published in the science magazine 'Nature'. At the core of the scientific publication is the idea of a novel accelerator ring for elementary particles. Up to now, physicists have mainly used protons and electrons in particle accelerators and related experiments. In principle, however, all charged particles are suitable for such experiments, because they can all be accelerated in a magnetic field. For example, muons could be used instead of electrons. Muons carry electric charge like electrons, but have a greater mass.
Circular accelerators filled with muons would be advantageous because muons emit less synchrotron radiation than electrons and can therefore be accelerated to higher energies. Muon accelerators could also be used to produce a very clean neutrino beam and thus build a so-called 'neutrino factory'. The fact that muons have not been used in circular accelerators to date is due to their short lifetime of just two millionths of a second. Muons can only be used for experiments during this extremely short time, while electrons or protons are stable particles, which greatly simplifies their use in experiments.
Fundamentals for a muon accelerator
Despite their short lifetime, muons could be used for accelerator experiments in the future. The prerequisite, however, is that a muon beam of high quality ('brightness') can be produced. This was precisely the aim of the Muon Ionization Cooling Experiment (MICE), which was conducted at the Rutherford Appleton Laboratory near Oxford (UK) and whose results were reported by Nature in February. The scientists involved in the experiment succeeded at improving the quality of the muon beam by using a specific method known as ionization cooling. The MICE researchers come to a positive conclusion in 'Nature': "The results presented here are an important step towards achieving the muon-beam quality required to search for phenomena at energy scales beyond the reach of the Large Hadron Collider at a facility of equivalent or reduced footprint (means: space occupied by the accelerator, BV).”
One of the hundred or so researchers in the MICE collaboration was François Drielsma. As a master's student at the University of Geneva, he helped to set up the so-called 'Electron-Muon Ranger', which was used in MICE to distinguish muons from electrons. Subsequently, he devoted himself to ionization cooling in his doctoral thesis, which he completed in 2018. He developed an analysis technique to demonstrate the usefulness and effectiveness of this cooling method. "François Drielsma works hard, is methodical and learns quickly; he is an extremely creative and precise scientist," says Prof. Alain Blondel, who supervised Drielsma's doctoral thesis at the University of Geneva and was spokesman for the MICE experiment for over ten years.
Neutrino researchers at SLAC
The young physicist, praised in high notes, was born in the Belgian city of Liège in 1991. He grew up there and obtained a Bachelor's degree in physics. Enriched by a language stay in China he then moved to the University of Geneva in order to study for a Master's degree with Prof. Alain Blondel. This was only a stopover, as he soon went to England for 18 months to work on MICE. No sooner had his doctoral thesis been completed in Geneva than the young researcher embarked on his next journey: In the early summer of 2019 he left the Rhone city for California. He became a postdoc in the neutrino research group at the SLAC National Accelerator Laboratory in Menlo Park south of San Francisco.
Neutrino research is a dynamic field of current particle physics. In Japan, the construction of a new, large neutrino experiment called 'Hyper Kamiokande' was recently announced, and in the north of the USA, a second large experiment in this field of research is currently being built: the 'Deep Underground Neutrino Experiment' (DUNE). François Drielsma is also active in neutrino research, but as a SLAC scientist he is mostly participating in a smaller experiment called ICARUS. He has made a conscious decision to do so, as he says: "I prefer to work on a smaller scale experiment; that way, an individual may have a greater impact and can contribute more," says Drielsma.
Is there a sterile neutrino?
ICARUS brings together around 100 scientists and is thus one of the smaller experiments in today's particle physics. At the centre of ICARUS (short for 'Imaging Cosmic And Rare Underground Signals') is a detector that has been used for neutrino research in Italy since 2001. In 2017, the research instrument was shipped to the USA and newly installed at Fermilab near Chicago (Illinois), where it will be used for neutrino experiments from 2020. One of several scientific goals is to study production of electrons (or positrons) and photons in low energy neutrino interactions, where nuclear effects lead to large uncertainties in event rates and topologies. Anomalies have been seen in this region, invoking a possible ‚sterile neutrino‘. Studies with a powerful detector like liquid argon are eagerly awaited.
The ICARUS detector is designed to observe neutrinos when they interact with liquid argon in a tank ('Liquid Argon Time Projection Chamber'/LArTPC). François Drielsma is working from California on the ICARUS experiment in the north of the USA. "An important part of my work is to develop machine learning techniques for all LArTPCs." Drielsma's activities also contribute to the preparation of the DUNE experiment, which is currently being set up at Fermilab with the participation of the University of Bern.
A global family
When a native Belgian works in the USA on an experiment with Swiss participation, it may sound unusual. But for particle physics, this high degree of internationality is not unusual, as Drielsma says: "We are a large, international family, and most of the scientists in our research group at SLAC do not come from the USA at all, but from abroad. It doesn't really matter where we conduct our experiments - except perhaps that the hours here in the USA are counted less than elsewhere.”
Author: Benedikt Vogel