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> Students entering particle physics Issue: 2010-1 Section: 14-16



Particle physics


Fascination in the world of science – the Particle Physics: What are Hadrons or Leptons? What is Dark Matter? How did the big bang develop? A lot of questions about the cosmos are still unsolved and lie in secrecy. Get insight into topics and methods of basic research of the fundaments of matter and forces with the Hands on Particle Physics Masterclasses.

From 15th February to 5th March 2010 the 6th international Hands on Particle Physics Masterclasses took place, organised at the TU Dresden in the framework of the EPPOG (European Particle Physics Outreach Group).

Every year more than 5.000 interested high school students and teachers worldwide are invited to about 90 scientific institutes and universities in 23 different countries to discover the secrets of our universe. For one day the young academics may leave their classrooms and work like real scientists, look over the shoulders of researchers and have the chance to perform measurements on real data from particle physics experiments at CERN (Centre Européen pour la Recherche Nucléaire).

Each day up to six out of about 90 institutes participate. The program conceived so that at the beginning of the event, active scientists give the students an insight into topics and methods of particle physics and establish a basis so that no special knowledge is required beforehand. The young people can also find out how particle physicists work at CERN. In the second part, the students have to prove their own competences. In groups of two they are to analyze real data on provided computers which were recorded during collisions of fundamental particles at CERN in Geneva. Research assistants provide the students with help regarding the PC exercises. The results are collected, interpreted and discussed by the students.

The highlight is the video conference in English at the end of the day where the participating institutes and universities in different countries exchange results and share their experiences.


CERN and LHC key to understand the universe

CERN is an international research laboratory near Geneva at the border between France and Switzerland. It is the biggest centre for fundamental and particle physics research worldwide and was founded in 1954 by twelve European States; by now eight more European states support CERN financially, with Germany, France and the United Kingdom accounting for approximately 50% of the total financing. Another 65 states participate in the execution of this project, especially by delegating scientists to Switzerland. Apart from the 2500 employees, CERN is currently hosting around 8000 particle physicists from 580 universities and institutes.

To answer the unsolved questions from above, the scientists develop, build, perform and evaluate experiments. The technical requirements are extremely high and the engineers are performing pioneer work for the construction of the giant particle accelerators and detectors. The latest accelerator, the Large Hadron Collider (LHC), is the biggest machine in the world and is constructed for the purpose of creating conditions which existed shortly after the Big Bang. With its six detectors the scientists are hoping for great advances in the understanding of the physical world.

The following milestones show how far the researchers at CERN got in their aim: In 1975, the first accelerator started up. Eleven years later, the detection was revolutionised by a CERN-scientist: The examining of photographs was replaced by an electronic-based process. In 1971, the world’s first proton-proton accelerator began operation. The W- and Z-Particles were discovered in 1983. The world’s biggest electron-positron collider, the LEP, started up in 1989. The first time antimatter was observed was in 1995. Antihydrogen was successfully trapped for the first time in 2002. In 2004 CERN celebrated its 50th anniversary.

CERN is also known for the invention of the World Wide Web by Tim Berners-Lee whose initial intention it was to distribute the huge amount of data recorded from the detectors at the accelerators. The rapidly expanding data masses from the LHC need a new concept of distribution; therefore CERN is in the process of creating the World Wide Grid which could one day be the successor of the WWW. But this was actually just invented as an unplanned by-product during answering the question what matter is.


The – not yet completed - Standard Model

The first answer was mentioned by Newton in 1687. He said that the mass can be calculated with the volume and density. But in the last years scientists do not try to explain how something works anymore, but why. The foundation of the particle physics, where we hope to get an answer to all these questions from, is the Standard Model. In contrast to an atom, the particles listed in the SM are not built up by even smaller components, they are elementary. In the universe sixteen different particles exist, seventeen if you count the unconfirmed Higgs-boson. Scientists suppose that this particle provides the other particles with mass.

Regarding the picture above, you might see that there are quarks and leptons. Our world is made of the first family of Fermions: The up- and down quark which the atomic nucleus is made of, the electron which is circuiting around the atomic nucleus according to the Bohr model, and an electron neutrino which for example is involved in the radioactive beta decay. There are two more families with similar particles, the second family is just heavier than the first, and the third is even heavier than the second. The two last families are so instable that they immediately decay into particles of the first family. The mass of the top-quark is 50,000 times bigger than the mass of an up-quark. Hence they must interact completely different with the Higgs-boson. Another characteristic is the electric charge and the colour charge. They are used to describe and pre-estimate another central component of the Standard Model, the interactions between the particles. There are four different forces in this model, the weak and the strong interaction, the gravity and the electromagnetic interaction. With the exception of gravity, the scientists use four more particles called bosons to explain the origin of these interactions. Gluons are used for the strong interaction which keeps the quarks together to build up an atomic nucleus. The Z and W boson exchange the weak interaction and the photon the electromagnetic interaction, for example light or other electromagnetic waves. These four particles have no mass and move with the speed of light. As already mentioned, all these particles have different characteristics. Throughout the measurements and the identification of the particles, the scientists at CERN take advantages of this.


Detectors - tracking the particles

To use these advantages the scientist need sole particles, which they can only obtain with large amounts of energy. The demand is so high that the LHC was built to simulate the circumstances of the time directly after the Big Bang. For that purpose, two particle beams collide with incredible force thus splitting into other particles. The particle physicists hope to find the Higgs-Boson among the created particles. This is such an unstable particle that would split into other particles, but there are only a few possible combinations of particles in the end from which the Higgs can be deduced.

For each collision, Physicists have to identify all particles that were produced. Therefore they use detectors which have many layers that each have a particular role in the reconstruction of a collision. From the collision point the formed particles fly through an inner tracker, which reveals the trails of electrically charged particles. The next layer is the electromagnetic calorimeter, which absorbs electrons and photons. Hadrons lose their energy and are stopped in the next layer, the hadronic calorimeter. Only muons are able to pass all calorimeters and leave a signal in the muon chambers.

The event display shows an electrically charged particle in the tracker. It causes a spots in the electromagnetic calorimeter and the hadronic calorimeter but it still passes them. Finally it leaves signals in the muon chambers. So it has to be a muon.


This is just an example of what was covered by the Physics Masterclasses workshop. We absolutely recommend this as a unique experience for pupils and invite you to take part in the Physics Masterclasses. Because of the large number of institutes in Europe there will most probably be a Masterclass near your town. More information can be found on the website of the programme:

Katrin Kröger (16), Isabel Zhang (17), Nick Pawlowski (17) and Lars Berscheid (14), visited the Physics Masterclasses in Dresden on the 1st March 2010. We are all in the 11th form and pupils of the Sächsiches Landesgymnasium Sankt Afra in Meißen, Germany.