HERMES

The HERMES experiment investigated the spin of protons. The research was carried out until 2007 at the HERA particle accelerator at the DESY research centre in Hamburg. HERA accelerated electrons and protons and allowed these to collide with considerable energy at several locations.

The spin of a particle is best compared with the rotation of a spinning top. At the HERMES experiment, physicists investigated how the spin of protons could be explained by the characteristics of its building blocks: quarks and gluons. Previous research had already revealed that the sum of the spins of the building blocks did not equal the total measured spin of the proton itself.

Spin is a characteristic property of particles, just like mass and electric charge. It was only discovered in 1925 by the Dutch physicists Goudsmit and Uhlenbeck. In 1987, scientists at CERN in Geneva discovered that only a small fraction of the spin of a proton was caused by the spin of the quarks from which the proton is built up. The HERMES experiment was set up to discover the missing quantity of spin, and the experiments started in 1995. Spin plays an increasingly important role in countless applications. For example, proton spin is used in MRI scanners.

The HERMES experiment (HERA measurement of spin). The photo shows that the entire experiment stands on rails and can be moved in and out of the particle accelerator ring (HERA). At the bottom left of the photo, a part of the accelerator tube through which the electrons move (to the right) can be seen.
Until recently, four experiments were in progress at the Large Electron Positron accelerator (LEP) at CERN, Geneva. With a circumference of 27 kilometres, LEP was the largest machine in the world used to allow electrons and their antiparticles, positrons, to collide. Four large detectors studied the physical aspects of the strong and weak forces down to the smallest detail.
During LEP's 12 years of service, the Standard Model was tested with the help of these experiments. The four LEP experiments were ALEPH, DELPHI, OPAL and L3. Nikhef participated in DELPHI and L3. The tunnel that housed LEP is now used for its successor, the Large Hadron Collider (LHC).