Focus on research: physicist Els Koffeman

Els Koffeman

Photo: Bob Bronshoff

At the National Institute for Nuclear Physics and High Energy Physics (NIKHEF), the days, hours and minutes are slowly ticking by until the moment that the first particle collision takes place in the Large Hadron Collider at CERN. The whole physics community is excitedly awaiting the first results of this experiment during which the Higgs particle, which has never been observed to date, will hopefully be found. Or not. Even if no Higgs particle is found, people will also be excited. Els Koffeman, who works at the NIKHEF and who has been professor of Instrumentation in particle physics at the UvA since last December, designs and builds detectors that record particle collisions very accurately. She is presently working on the ATLAS detector, which may be the first to find the Higgs particle.

Els Koffeman works at the NIKHEF, designing detectors that are able to determine very accurately what happens when two clouds of elementary particles collide at enormous speed. New particles are formed during such collisions, some of which disintegrate immediately and form clouds of new, compound particles, also called jets. From the composition of these jets it is possible to deduce which elementary particles were formed during the collision.

As well as designing them, Koffeman also actually works on the detectors herself, which is quite a feat. The enormous contraptions are built with the utmost precision. A dedicated detector is built for each experiment, and this entails more than thinking it out at a desk. You need to test which materials can best be used, how to make optimal use of the space and to check whether everything fits. In short, this entails fitting and measuring, welding and soldering. “Although I don’t do much soldering myself,” laughs Koffeman. “The chips we use have thousands of electronic connections on a couple of centimetres. Nearly all the soldering is computerized, and we also have two very good technical departments.” Koffeman is, however, often to be found in the lab, bent over a detector part, or discussing heatedly with her colleagues. “Luckily it’s still possible use the voltmeter to measure things now and then.”

The search for the Higgs particle

The ATLAS detector on which Koffeman is currently working so hard will eventually be used in the Large Hadron Collider, a ring-shaped particle accelerator with a circumference of 27 kilometres. Small clouds of protons travel around the ring at almost the speed of light, to collide in the detector with a cloud of other protons coming from the other direction at the same speed. And this takes place 40 million times a second. Hundreds of gigantic superconductive magnets ensure that the particles stay in the ring. The project costs about two billion euros and has been keeping thousands of scientists all over the world occupied for many years. And all this to find a Higgs particle.

Perhaps this is rather a lot of effort to find a small particle which we are not sure even exists. “Our experiments are true fundamental science. I am doing this purely for the sake of curiosity. Of course, our experiments also have economic and societal value, such as the advancement of knowledge or training of scientists but this is, for me, not the true aim of fundamental science. If that was the only intention, it would have been possible to organize things differently.”

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Scientists are also looking for the Higgs particle to prove one of their theories, the so-called Standard Model. This model describes the fundamental forces (strong force, electromagnetic force and weak force), with the exception of the force of gravity. It also describes the elementary particles which go together with these forces. “All in all, a very complete picture of elementary particles seems to have formed, and some people feel that the theory is complete. Measurements taken during the past ten years all seem to fit in with the theory. We have measured many wonderful things which hadn’t been seen before, but that were not improbable. Yet the theory is a bit of a squeeze here and there and, in the past, this turned out to be a point where something really new was about to happen,” says Koffeman enthusiastically.

The Higgs particle is the only particle predicted by the standard model that hasn’t yet been found. It only comes into existence at extremely high energy. Previous accelerators were unable to shoot the protons at each other with sufficient force to produce a particle with so much energy. According to the theory, the Higgs particle gives elementary particles their mass. “We should be able to find the particle with the new accelerator. If we are not successful, the theory will run into problems.”

“There are, by the way, other models which predict the existence of not only a Higgs particle, but a whole menagerie of other particles. For example, the Model for Supersymmetry predicts that all presently known particles have a partner. I’m not a fervent adherent of this theory, but it does indicate that anything can happen.”

Counting carefully

“The first big experiment I was involved in as a PhD student, the Large Electron Positron (LEP) accelerator, was a scientific success. The LEP accelerator always performed better than expected, which is why we got such good results. We wanted to make very accurate measurements of the probability that on collision a Z-particle - the carrier of the weak force - would come into being. To determine the probability, it is important to take an exact count of the number of Z-particles. You need to be sure that it actually is a Z-particle you’re counting, and that the next particle hasn’t been detected while you’re still counting. Moreover, the detector has a certain finite surface. You need to know whether or not you will detect a particle if it comes into being at an interface. And so in the long run it all comes down to counting carefully, and being sure that you never make a mistake!

Counting was very precise with the detector we built, so that eventually we managed to measure the probability of a Z-particle coming into being with an accuracy 10% better than previously. These measurements, to which I contributed, are written down in the books for posterity! It doesn’t matter what fantastic discoveries or changes to the theory follow, those measurements will always exist.”

Lost in thought

Although Koffeman studies problems others wouldn’t even think about, this hasn’t much influence on her day-to-day life. “I even think that it’s the opposite: your work is part of your daily life. I do the same type of things as many others at work: I carry out tests, have discussions, that sort of thing. But being able to wonder about things now and then is an escape from reality, and this is what keeps me motivated in the long run. Those moments of being lost in thought, when you’re far away wondering what it all means, only occur occasionally. I can’t spend eight hours a day doing that, my daily reality consists of experimenting with the detectors. I turn something on and it doesn’t work, and I wonder why. You don’t wonder about things at the level of the universe at such moments. My work is 90% reality, technology.”

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Published by  Faculty of Science

21 August 2012