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The Particle Detective


10:15 a.m. I enter the gigantic golden science dome at the heart of CERN, the European Laboratory for Particle Physics. Inside, hundreds of scientists in white coats are sitting in front of a huge screen that shows live images from the particle collisions of CERN's accelerator. "There!" one of the scientists suddenly shouts, "stop and rewind!" and indeed, on the event display, a green particle track clearly stands out from all the grey ones. "A new particle--we'll get the Nobel Prize!" Bottles of champagne are brought in and the scene turns into a party. ...

This is not what happens at CERN!

Right from my school days I was very much drawn to fundamental science, mainly physics and astronomy. In particular it was the questions that I didn't find an answer for in school, such as "Why is time the fourth dimension?" that finally forced me to study physics. Fifteen years later I still don't know why time is the fourth dimension, but at least it became clear to me that "why?" is a bad question to ask in this context.

I went to the Vienna University of Technology in 1988, and my desire to understand fundamental physics naturally brought me into theoretical physics--quantum field theory, general relativity, and the like. But although I was very excited about understanding cosmology and the world of subatomic particles and fundamental forces, I found that I was less interested in the scientific research of theoretical physics since the modern topics of physics theory felt very far from reality. What became clear to me is that I like a physics theory when it is proven to match reality and nicely written up in a textbook.

During my eighth semester I attended a lecture about CERN and particle detectors in general, and I knew immediately that this would be the right place for me. These gigantic machines and experiments exploring the bizarre world of subatomic particles, bringing the high tech real world together with the frontiers of fundamental physics--nothing could fit my interests better.

In the summer of 1994, I had the pleasure of spending 2 months at CERN, attending student lectures in the morning and working with an experimental group for the rest of the time. I enjoyed my time there so much that I decided to apply for a doctoral student position, and after finishing my studies in Vienna I returned to CERN as a Ph.D. student in the summer of 1995.

CERN, situated on the Swiss-French border about 10 km from Geneva, is the world's biggest particle physics research centre. Founded in 1954 as a collaboration of 12 European member states, CERN today employs slightly fewer than 3000 people, representing a wide range of skills: physicists, engineers, technicians, craftspeople, administrators, secretaries, workers. ... Some 6500 visiting scientists, half of the world's particle physicists, come to CERN for their research. They represent 500 universities and more than 80 nationalities.

CERN's current project is the Large Hadron Collider (LHC), a proton accelerator housed in a tunnel about 100 meters underground with a 27-km circumference. The protons will collide at four so-called "interaction points" where the collisions are measured by huge particle detectors. The LHC was conceived in 1991, approved in 1994, and will start in 2007 and run for about 10 years. This time scale suggests that the project is quite complex and sophisticated.

During the summer of 1994 and my doctoral studies from 1995 to 1997, I worked on the ATLAS experiment, one of the two "general-purpose" detectors that will study the particle collisions at the LHC. This experiment will be a huge detector 40 meters long and 22 m in diameter, consisting of many different layers of specific detector types positioned like "onion skins" around the particle collision point. A vertex detector very close to the collision point assigns all the particle tracks to the specific collisions. A tracking detector farther away measures the deflection of charged particles in the strong magnetic field provided by a superconducting coil, which gives the particles momentum. In the next detector layer, a 63,000-liter volume filled with liquid argon (at -183 degrees C) and thousands of sensors measures electron and photon energies. Farther out, a 2300-ton structure of steel and scintillators measures the energies of strongly interacting particles, and, finally, from a radius of 5 to 10 m, the so-called "muon spectrometer" measures the momentum of muons with 2000 m2 of high precision positioning detectors.

ATLAS is the largest collaborative effort ever attempted in the physical sciences. Two thousand physicists are participating from more than 150 universities and laboratories in 34 countries. At CERN itself, 130 staff members, about 30 of them physicists, are involved. Naturally, the detector is divided into many subsystems so that, in the end, one is actually working together with five to 10 people in a small group. At regular intervals of about 2 months, the collaborators meet at CERN in order to discuss progress.

It was the development of the ATLAS muon spectrometer that I worked on. It uses the so-called wire chambers that were invented by Georges Charpak at CERN in the late 1960s and that won him the 1992 Nobel Prize for physics.

Although the detector's technology is quite well known, the detector's performance has improved considerably over the years due to new developments in highly integrated readout electronics. The situation can be compared to that in astronomy, in which the telescope mirrors, despite their increasing size, have not changed for a long time, but the readout elements, starting from simply looking through the telescope, via print film to highly sensitive charge-coupled devices and other sensors, have improved the instrument dramatically.

Arriving at CERN in the summer 1995, I was not at all clear about what my thesis topic would be, so I simply started to participate in an ongoing test of a detector prototype in order to gain an understanding of the problems to be solved. One of the issues that arose was the strange shape of the signals from single muons with their multiple spikes and secondary pulses. Due to the high particle rates expected in the final detector, this issue of strange signal shape was of special concern. I took up this problem and started to think about what was really going on in this detector. I started detailed measurements of these signals and compared them with computer simulations of the detector physics processes, and after some time it became clear that this strange signal shape was not some bizarre electronics effect, but was the result of the actual physics processes happening in the detector when a muon passes through it. This was extremely fascinating to me. On the one hand, I had an "invisible" particle passing through a "real-world" detector resulting in an electronics signal on an oscilloscope screen. On the other hand, a computer program could simulate all the physics, from the muon passing through the detector to the generation of the signal. And the two things matched! I actually knew exactly what was going on in this device.

Knowing in detail what is going on in a detector turned out to be a very useful thing. It enables one to improve existing detector designs, pushing them to their limits, or even to find ways to develop completely new detectors to meet the ever-increasing requirements of experimental particle physicists. This field, "detector physics," became my expertise, and I have stayed with it, despite a brief taste of dealing with the "outputs" during my postdoc.

I finished my thesis in 1997 and got a postdoc position at Harvard University that involved developing the front-end electronics for the ATLAS muon detector. It was there that I also did some physics data analysis, the thing an experimental particle physicist is supposed to do. But I found out very soon that a large fraction of this work consists of very tedious fights with huge software packages. This experience was a confirmation of my chosen research path--I decided to let other people crunch the numbers and discover the final answer. In the end, the detector performance decides the outcome of an experiment, and even the most sophisticated data analysis cannot make up for a bad detector. Since everyone needs a justification for why his or her work is the most important, I decided that the detector folks are the heroes; the data analysis that comes afterwards is tedious, but easy, so I stayed with detectors.

It is certainly not the case that physicists decide on a detector, ask a company to build and run it, and just analyse the data in the end. The development and construction of a modern particle detector has become a highly sophisticated, and certainly a well recognised, task. This is evidenced by the fact that the detector physicists are part of the author lists of the final physics papers and certainly by the fact that Charpak got the Nobel Prize in physics for developing a detector. It is certainly very exciting to learn about all the sophisticated techniques that make it possible to make the "invisible" world of microphysics visible.

Although the university atmosphere was very inspiring to me, I decided to return to CERN, where the real action is happening, and I accepted a 6-year staff position in the LHCb experiment, another LHC experiment that investigates matter-antimatter asymmetries.

My time is shared between detector and electronics development for this experiment and research on fundamental detector physics processes. I also supervise a Ph.D. student in this area.

I certainly don't regret having gone this way since I have been able to stay in touch with fundamental physics while at the same time getting my hands dirty with real experimental equipment. It is also satisfying to develop techniques that will make it possible to probe even further into the fundamental laws of nature.

It is difficult to say whether it is easier to find a permanent job in a typical academic career at a university or in a research lab such as CERN. Both places are quite competitive and there are not too many positions available. However, I know that I have learned many of things in the course of my career that are useful and applicable in other fields, such as astrophysics, space science, medical science, and other kinds of high-tech fields.

Coming back to the beginning of the story--the "cliché" part of a physics experiment represents only a small fraction of the entire story. Designing, developing, constructing, and getting the detectors to work cover a major part of the life of an experiment, and there is a great deal of interesting and certainly well-recognised work for physicists at every stage of this process.