The delta+ is a type of baryon, the family of sub-atomic particles to which the more well-known proton and neutron belong. A team of physics enthusiasts at the International School of Geneva is working intensively on these extremely unstable elementary particles. In September they will travel to Hamburg for an experiment at the renowned particle physics research institute DESY. They owe this extraordinary research trip to winning this year's “Beamline for Schools” (BL4S) competition at CERN.
The 'International School of Geneva' is a secondary school in Geneva with a distinctly international profile. The students are mainly children of staff of international organisations based in Geneva, such as the UN, the International Labour Organisation and the World Health Organisation. One of the school's three locations is called 'Campus des Nations' with about 1000 students.
Secondary school students practise particle physics
Every school has teachers as well as pupils. One of them on the 'Campus des Nations' of the International School of Geneva is Thomas Riis-Johannessen. The Anglo-Norwegian, who studied in Bristol, has been teaching science and physics in Geneva for seven years. About three years ago, the doctor of chemistry was confronted with a challenging request: A group of pupils had formed a STEM working group (science, technology, engineering, mathematics). They asked their teacher if they could take part in the 'Beamline for Schools' competition organised by CERN. The competition allows winning teams of students access to CERN, or another particle physics laboratory, to carry out an experiment they have developed themselves.
"As a trained chemist, I was a bit overwhelmed by this request, because I couldn't really imagine myself what kind of experiment we could do as a class using a high-energy particle beam," says the 39-year-old physics teacher. For some years now, particle physics has been part of the curriculum on the way to the International Baccalaureate Diploma, which many students endeavour to achieve at the International School. But the development of an experiment of their own in this domain of fundamental research in physics took the challenge to a whole new level.
Success at the second attempt
A lucky coincidence gave the project wings: Mark Rayner, who did his teaching internship at the International School, was a theoretical physicist and had worked at CERN before. This created the conditions for the STEM Club members, with the support of their two teachers, to devise an experiment and submit it to the “Beamline-for-Schools” expert committee. They came back empty-handed after their first submission. However, one year later, after they had revised some of the finer details of their experiment, the “Nations' Flying Foxes” team won, together with a team of students from Berlin.
“Nations' Flying Foxes” - the name by which the sports teams at the Campus des Nations go - is the brainchild of six young physics enthusiasts aged 16-17. Together with their teachers and two supporting experimental physicists from CERN, they will travel to the Deutsches Elektronen-Synchrotron (DESY) in Hamburg for a fortnight on 23 September 2020 to conduct their experiment. Normally the experiments of the “Beamline for Schools” competition take place at CERN, but as the particle accelerator there is currently being upgraded, the DESY facility was made available for this year’s winners.
Measuring delta+ decay
To understand the planned experiment, it is important to know that quarks - the constituents of protons - can be put into an excited state with higher energy by electron bombardment (comparable to excited atoms whose electrons circulate in a higher-energy orbit). For the proton, one of these excited states is known as the delta+ (i.e. delta-plus). Like ‘normal’ protons, they consist of two up quarks and one down quark. However, delta+ baryons differ from protons in a key quantum property known as spin – protons have a spin of 1/2, the delta+ has a spin of 3/2, when measured in units of the reduced Planck constant ℏ.
The 'Flying Foxes' want to prove the existence of the delta+ baryon at DESY. The particles cannot be observed directly, since they have an extremely short lifetime: about one billionth of a billionth of a second. After this time, the delta+ particle decays with equal probabilities into a neutron, emitting a positively charged pion or a proton, emitting a neutral pion. (In addition, in one of about 170 cases, a delta+ can also decay in a proton and a photon).
The 'Flying Foxes' want to measure these decay products at DESY in order to confirm the existence of the delta+ baryons. The team intends to observe the delta+ in the following way: First focus a beam of high energy (6.4 GeV) electrons onto a lead target (source of protons) to excite (some of) the protons into the highly unstable delta+ state. Then observe the decay of the delta+ into positive pions (which have a lifetime that is long enough to follow their trajectories) by (i) using a magnetic field to separate positive pions from the incoming and scattered electrons, and then (ii) characterizing the positive pions with micromegas detectors (for momentum) and lead-crystal calorimeter (for energy).
A realistic picture of research
"The 'Beamline for Schools' trip to DESY is a wonderful opportunity for our students to go beyond the school curriculum and see what modern science really is all about, namely hard experimental work, which requires massive dedication and persistence," says Thomas Riis-Johannessen, who himself spent ten years in research before becoming a teacher. Thanks to the competition, secondary school pupils learn how to use particle beams and detectors, they learn the Python programming language, and they analyse experimental data. Furthermore, if the experiment is successful, it could lead to a scientific publication.
It should be noted that a variation of this experiment has already been carried out in the past, so it might not lead to fundamentally new scientific findings. But is this necessary? There is a vested interest in seeing whether or not conclusive data for this experiment can be obtained from the DESY facility. And, thanks to the BL4S competition, the students will experience a very real encounter with the weird and wonderful world of modern particle physics. Anyone who works at this level at the age of 16 can go a very long way in a scientific career.
As Geneva has been classified by Germany as a COVID 19 risk area, the 'Flying Foxes' team will not be able to travel to Hamburg as planned. Instead, the students will go regularly to CERN and participate in the experiments via video conference.
Author: Benedikt Vogel