physics – Henning Raach

Richard Feynman

When I was doing particle physics, Richard Feynman was my hero. Not only did he introduce Feynman diagrams which made high energy physics so much clearer, but also did he have a lot of humour. During a hard time I read his book Surely you‘re joking, Mr. Feynman! and I found it wonderful what a curious, intelligent character can live to see. Later I read the biography by Jagdish Mehra The Beat of a Different Drum. The title refers to Feynman’s passion for drumming and his originality. In an epitaph he was called a physicist’s physicist. Some people may have thought he was arrogant. But let us be honest, by far most people overestimate their intelligence 😉

Farewell to Darmstadt

Another part of my life came to an end in October. Writing about it puts the question in which attitude I do it. I want to write thankfully. In 2009, I came to Darmstadt. First I worked at the Technical University, then at the accelerator GSI. I wrote a monography. For 10 years I took part in the workshop band of Jürgen Wuchner who died in 2020. Eight times I participated in the Jazz Conceptions, a one week summer workshop. I enjoyed the vibrant music scene, especially the concerts at Jazz Institute and at Knabenschule. The Frankfurt Radio Big Band played at Centralstation. Darmstadt also has a Staatstheater (public theatre) with opera. Each summer the Jürgen Wuchner Workshop Band gave a promenade concert in Orangerie Garden if it did not rain. I feel sad that I had to leave friends. Certainly will I return as a visitor.

Climate Change

At present the Corona crisis is very severe. Many people die from the virus. The damage to the economy is huge. Nonetheless, I want to write on another problem of mankind, the climate change. I have read a German book on this subject by two climatologists from Potsdam, Rahmstorf and Schellnhuber. They report on a large consensus in science that the global warming is caused by man. Doubling the amount of carbon-dioxide in the atmosphere would mean a temperature rise of 3 Kelvin (+/- 1 K at a confidence level of 95%.) Those who want to know more are advised to read the reports of the IPCC (Intergovernmental Panel on Climate Change.) I think their scientific arguments should be taken seriously, as well as their concern to use the knowledge of our time in a responsible way.

Black Hole

Recently scientists succeeded in taking a picture of a black hole. Therefore, I want to write a few lines about this phenomenon.

In a black hole, matter is so highly compressed that even light cannot escape from its vicinity. General relativity treats a black hole as a point, i.e. it has no extension. This is hard to imagine. However, take as an example the electron which is considered as a point particle, although it has a mass at rest.

A black hole is surrounded by an event horizon. Everything that passes it becomes invisible for the world outside. If a matter-antimatter pair is created from the vacuum near the event horizon, a particle may cross it. The other particle seems to come out. This is Hawking radiation.

In particle physics, there are collisions at high energies. Can a black hole be produced in a collider experiment? This question was subject of a court dispute. The physicists bring forward the argument that the cosmic radiation contains particles of much higher energy. Although the cosmic radiation hits the Earth, our world still exists.

Superconductivity

There are materials that have zero electrical resistance if the temperature is sufficiently low. Also the current density and the magnetic field should not be too high. In 1908 Heike Kamerlingh-Onnes and his laboratory accomplished to liquify helium whose boiling temperature is about 4.2 K under atmospheric pressure. Thus he could discover the superconductivity of mercury in 1911.

In the following years many other superconductors have been discovered, even high temperature superconductors (HTS). HTS has the advantage that liquid nitrogen (77 K) is sufficient.

Superconducting magnets often consist of niobium titanium (NbTi) that has a critical temperature of approximately 10 K.

The colder part of current leads is frequently built with HTS for very large currents.

Jets in QCD

What are jets? What does QCD mean?

The intuitive picture of a jet is a collimated spray of particles which is created by the collision of two particles in a high energy physics experiment. (There can be more than one jet.) Quantum chromodynamics (QCD) is the physical theory behind them. It states that a certain class of particles, hadrons, are built of quarks and gluons. If a quark or gluon is scattered, a jet is created. A quark or gluon has never been observed as a free elementary particle and QCD states that this is not possible. Instead after a scattering new particles are created very rapidly that can be observed. Regarded as a jet they have the momentum of the scattered quark or gluon. The exact definition of a jet is not trivial and there are sophisticated algorithms to find them.

Why are jets interesting to physicists?

With their aid conclusions can be made on the fundamental process without having to measure a free quark or gluon.

The Plug Calorimeter

I am going to summarise my diploma thesis in plain words. It was a simulation of the ZEUS detector. ZEUS was the name of a collaboration which ran an experiment at the storage ring HERA in Hamburg, Germany. There, electrons and protons of high energy were brought to collision. Around the interaction point was the ZEUS detector covering almost the whole solid angle with the exception of the openings for the beam pipes. The idea for the plug calorimeter was to make the opening in the proton direction smaller. Thus the acceptance of the detector was enlarged. A calorimeter is an energy measuring device. The additional calorimeter was kind of a plug.

My first step was to implement several versions into the ZEUS detector simulation program MOZART. Then I simulated a test beam, i.e. I used a beam of particles of a single type with uniform energy. Finally, I tested with two kinds of electron proton scattering events. It could be demonstrated that these two types of scattering events can be much more easily separated in an analysis with the aid of a plug calorimeter.

A plug calorimeter was eventually built und installed. Finishing this article I want to mention a publication:

Bamberger et al., The ZEUS Forward Plug Calorimeter with Lead-Scintillator Plates and WLS Fiber Readout, Nucl. Instr. Meth. A450 (2000) 235-252 ( http://arxiv.org/abs/hep-ex/9912045 )

Current Leads

Current leads are a cryo-electrical component of the circuit of a superconducting magnet, a pair for each electrical circuit. One end, the warm terminal, has ambient temperature, about 300 K (Kelvin). The other end, the cold terminal, has the temperature of liquid helium, approximately 4 K.

If the cross-section of the conductor is too large, a lot of heat comes from the warm to the cold terminal. On the other hand, if the cross-section is too small, the electrical current will cause a significant heating and the current leads may burn through. The designer has to find a compromise.

There are current leads which are only cooled at the cold terminal. This is the conduction cooled type. The vapour cooled current leads guide the helium vapour all along the whole length from the cold to the warm terminal. Sometimes the temperature is fixed in between with liquid nitrogen (77 K) by the employment of a thermal anchor.