# How can I become a particle physicist?

## The tools of particle physics

So-called scattering experiments are carried out to investigate the smallest building blocks of matter. This means that a particle beam hits the object to be examined (the so-called. Target) is steered and the particles scattered on it are measured in a detection device (detector). So-called particle accelerators are used to generate the particle beams. In many experiments, these particle beams are directed towards one another instead of towards a stationary target. In such experiments, the beam is also the target.

Accelerators: the microscopes used by particle physicists

Normal microscopes work with visible light, i.e. with wavelengths of a few hundred nanometers. For example, crystals of 10-6m to be examined. In principle, structures that are smaller than the wavelength of the radiation used cannot be resolved with a microscope. A better resolution therefore requires smaller wavelengths. For example, the analysis of atoms succeeds if one uses X-rays instead of visible light.

Instead of shorter-wave light radiation, radiation from particles of matter can also be used. It is a basis of quantum physics that matter also has wave properties. The wavelength that can be assigned to an electron, for example, is smaller the greater its momentum (= mass x speed). For small particle masses, there is no need to differentiate between momentum and energy.

 10-6m 10-9m 1keV = 103eV (electron microscope) 10-10m 10keV = 104eV 10-14m 100MeV = 108eV 10-15m 1GeV = 109eV less than 10-18m 1TeV = 1012eV
Table showing the relationship between the size of the object to be examined and the energy of the particle beam used

For this reason, particle physicists build devices that can accelerate particle beams to ever higher energies. The table shows which energy is necessary to achieve the respective resolution. In order to reach these energies, electrons or protons are accelerated by electric fields. Almost everyone of us has such an accelerator at home: a television set uses the same principle and accelerates electrons, which then hit a screen and generate light.

 View into the LHC tunnel. Click on the image to enlarge it!

Many levels of electrical acceleration are used to achieve great energies. The easiest way to achieve this is when the particles always go through the same steps on a circular path and thus become faster and faster. The largest accelerator is in Geneva and is currently being converted from accelerating electrons to protons. It has a circumference of 26 km and is supposed to deliver proton collisions with an energy of 14 TeV. His name is LHC (Large Hadron Collider), and the Wuppertal working group is involved in the construction of one of the detectors for the LHC.

 Clicking on it starts an animation explaining scattering experiments.

The basic idea of ​​this Scattering experiments can be easily illustrated. The situation is comparable to the attempt to examine an object by throwing balls at it and inferring its shape from the type and direction of the reflection of the balls. The following animation (click on the picture!) Illustrates the investigation of a proton with increasingly shorter-wave radiation.

In fact, the high energy of the experiments in particle physics has a double meaning: It not only corresponds to small wavelengths for the most accurate possible analysis of the sample, but also acts as an "energy source" for the generation of (unstable) elementary particles. According to the relationship E = mc2 namely, energy can be converted into mass. The existence of particles can therefore remain hidden if their masses are too large to be produced by today's experiments. Conversely, this explains why the search for new physics is closely related to the construction of higher-energy accelerators. It is hoped that the LHC will provide solutions to some of the problems of the standard model.

The eyes of the particle physicist

Just as important as the accelerator, however, are obviously the detectors, which measure the direction in which the particles were deflected and which particles were newly created by the collision. At the beginning of experimental nuclear and particle physics from 1911 on, so-called fog and bubble chambers were used for this purpose. The particles leave traces in them, similar to aircraft contrails in the sky. For this purpose there is a gas-vapor mixture in a cloud chamber, bubble chambers are operated with a liquid close to the boiling point. These particle tracks are finally photographed. The figure below shows such a bubble chamber image together with its interpretation. Incidentally, the liquid in the bubble chamber served as the target in these experiments.

 Recording of particle tracks in a bubble chamber. Click on the image to enlarge it.

While this technique is still in use for specific applications, it obviously has serious drawbacks. On the one hand, the evaluation of the images is extremely complex; on the other hand, the experimenter cannot directly influence when the measurement should be carried out, since the state of overheating must be generated before the particles pass. After all, such detectors need a relatively long time before they are able to record again after a particle has passed through.

The development of electronic detection devices

For this reason, electronic verification procedures were also developed from the 1940s onwards. With these no photographic recordings of the particle tracks are made, but the trajectory of the particles is reconstructed from the electronic signals. Charged particles can generate electrical signals in a volume of gas (ionization), which can be read out electronically. This principle is based on lane detectors. If the detector is also in a magnetic field, the measurement of the track curvature enables the particle pulse to be determined. This measurement hardly affects the particle properties.

 Animation of how a particle detector works (click!)

Neutral particles such as neutrons cannot be detected in this way. They can only be measured if they generate charged particles through interaction with the detector material. The original object is lost. The energy of the original particle can be deduced from the number of particles created in this way. Detectors for energy measurement (so-called calorimeters) work according to this principle. By choosing suitable materials, they can be specialized in the detection of certain types of particles. The animation (click on the picture!) Illustrates how a typical detector works on an accelerator experiment.

The Wuppertal participation in the Atlas detector

The Wuppertal working group is involved in the construction of the Atlas Detector (Fig.), Which will measure the product of proton-proton collisions at the LHC (Large Hadron Collider). The technical requirements here are enormous: the particles must be able to be detected every 0.000000025 seconds, and their location must be measured with an accuracy of 0.02mm. This requires state-of-the-art electronics and high-precision devices. And just to give an idea of ​​the scale of these projects: The Atlas detector will weigh around 45,000 tons, and around 2,000 physicists are involved in its development.

 Sectional drawing of the ATLAS detector with size comparison.

This picture shows a sectional drawing of the ATLAS detector. You can see the four essential components of the detector - highlighted in color -: The inner oneTrack detector (in yellow) for measuring the momentum of charged particles that calorimeter (in green and orange) to measure the particle energy, the Muon system (in blue) to identify muons and together with the Magnet system (gray) to measure their impulses.

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