Elementary Particle Physics - The CMS Experiment

View of the innermost endcap disc of the CMS tracker, Muon chambers of the CMS Detector CERN

Searching for a new physics at the Terascale, the CMS Experiment is poised to take us to a new level of understanding of our universe. Involved in the project are 179 institutes from 41 countries. One of the four participating German research institutions is RWTH Aachen University.

Three of the University's physics institutes contributed to the development and construction of the CMS tracker. They collect data, participate in number of analyses, and are involved on the further expansion and improvement of the experiment. About 130 articles from the CMS experiment have been published, as of May 2012, in scientific journals. The so-called "Research Focus FSP-102" is funded by the Federal Ministry for Education and Research (BMBF).


Particle Physics explores the fundamental structures of matter, space-time, and the forces in our universe. Despite enormous progress over the last century, important questions remain unanswered today.

The CMS experiment, which has been operated at the new Large Hadron Collider at CERN since 2009, was built to answer some of them, in particular to find the origin of mass and electroweak symmetry breaking.

Two groups of particles are of particular interest: The Higgs particle is related to a theory from British physicist, Peter Higgs. His theory attempts to explain why particles have mass. This fundamental building block could not be experimentally traced until now.

Furthermore, evidence of super symmetrical particles is being sought. These are highly coveted, because the dark matter in the universe, for which astrophysicists have found no explanation thus far, may be made up of such particles. With the help of supersymmetry, numerous other open questions in physics can be answered.

The CMS Detector

Graphic illustration of the detector Dirk Rathje/Welt der Physik (www.weltderphysik.de) Licence: CC by-nc-nd

The CMS tracker serves to provide proof of and precise measurements of the particles that originate from collisions. The tracker is made up of multiple components arranged in bowl shape.

The paths of the charged particles can be determined with the inner tracking device, which was developed and constructed to a large extent in Aachen. These are surrounded by the calorimeters to measure the energy of the particles.

The superconducting solenoid follows, which ensures a magnetic field in the entire tracker that forces electrically charged particles along a circular path.

On the exterior, muons can be further tracked in the muon chamber detectors. These heavy varieties of electrons are particularly interesting, since many highly charged processes can be reocognized through muons: With the drift chamber built in Aachen, researchers anticipate proof of new particles disintegrating in muons, such as the Higgs particle, supersymmetrical particles, heavy quarks, or new gauge bosons. Models with additional space dimensions or small black holes can be examined with analyses of muon events.

Data Collection

A gigantic amount of data has arisen from the CMS experiment: despite a computer-directed restriction to potentially interesting collisions during data collection, the tracker delivers about 10000 terabytes of data a year.

Billions of proton-proton collision occurrences were recorded by the end of 2011. A computer network distribute worldwide with over 100000 processors will be used to evaluate the data. The processors are interconnected on a so-called grid. There is also a large computer network in Aachen, which saves and processes data as part of this grid.

As a result, researchers all over the world have access to the data and are able to evaluate the millions of saved collision records.

CMS - Contributions from RWTH Physics Institutes

All of the institutes have significantly contributed to both the development and construction of the CMS tracker and work intensively on data reconstruction and analysis.

I. Physics B

The department has played a leading role in the construction of the CMS tracker, the largest silicon strip detector ever built. One complete endcap, shown in the picture to the right, was integrated in the clean rooms of our institute.

Our physics data analyses focus on searches for the Higgs boson and for supersymmetric particles.

With the upgrade of the LHC to a yet more powerful machine on the horizon we have started research and development for a CMS tracker at the "SuperLHC".

III. Physics A

Depiction of a Muon Event CERN

The group is building muon detectors for the CMS experiment. In total, in a four-year period, the group has constructed 70 of the 250 muon chambers, and installed them at CERN.

The picture on the right portrays a spectacular event with four muons (the red lines). The event is characteristic of a Higgs decay in the CMS tracker. However it is also compatible with background processes.

Researchers of the institutes are actively involved in various investigations for a new physics, for example for supersymmetrical particles, new heavy guage bosons, and evidence of additional spatial dimensions, as well as measurements of top quarks. Additionally, the expansion of the muon system is being worked on for future challenges of an enhanced LHC with higher energy and higher data rates.

III. Physics B

At the institute, two major groups are involved in the CMS experiment: the "tracker group" and the "analysis group."

The tracker group has developed the official testing system for components to be used for the inner silicon tracker and contributed to the assembly of components into larger sub-structures.

Currently this group is preparing the analysis of test data from the silicon trackers, gained from measurements with cosmic muons.

The analysis group is preparing the analysis of experimental data in top physics and lepton number violating decays using Monte Carlo events.

Prospects for Students

In addition to the methods and content of current experimental particle physics, students and doctoral candidates learn how to work with large trackers, such as the CMS, programming tasks using the CMS software, and complex software packages, such as C++.

They become familiar with work in an important international collaboration and may have the opportunity to complete a research stay at CERN.

Students have the possibility to write their final thesis within the context of the CMS experiment. Example topics from recent years include:

  • Model Independent Investigations for a New Physics with the CMS Detector
  • Investigation of a Spin Correlation in Top-Pair Decays
  • Search for New Heavy Gauge Bosons with the CMS Tracker
  • Reconstruction of Supersymmetric Particles with the CMS Tracker
  • Development and Construction of New Particle Trackers

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