ATLAS
Hunting the Higgs particle
Mobile telephony, television and radio function by the grace of electromagnetic waves which can bridge enormous distances wothout interference. Extistence of these waves follows from Maxwell's theory. Maxwell showed in 1865 that electrical and magnetic forces -until then thought to be unrelated- were in fact based on one and the same electromagnetic force. Electromagnetic waves are the carriers of this force. In 1887 Hertz proved the existence of these waves and just over ten years later Marconi used them to send wireless signals over long distances. Quantum theory teaches us that the infinite reach of electromagnetic waves in vacuum implicates the existence of a massless particle: the photon.
Famous physicists have tried to consolidate all the other known forces (the weak- and strong-nuclear force and gravity) into one theory. Glashow, Weinberg and Salam are the only ones who partly succeeded. In 1968 they formulated one theory for electromagnetic- and weak-nuclear force: the theory of electro-weak interaction.
(2) The decay of a Higgs particle as predicted by theory is shown in a computer simulation. The four straight green tracks are muons which appear in the step-by-step decay of the Higgs particle.
Apart from the massless photon, their theory predicted two very heavy particles: the W- and the Z-particle. They are heavy since the reach of the weak nuclear force is incredibly short. The fact these particles have mass, while the photon is massless, is a crucial point in the model. It can be explained by postulating that the 'nothingness' is filled with something physicists call the Higgs field. This Higgs field also explains electrons and quarks - the building blocks of all matter. It can be compared to stirring in respectively a cup of tea and a cup of syrup. In the latter stirring is hard; it seems the spoon is heavier. The Higgs field works in the same way: it couples weakly to the electron, stronger to the muon (a 200 times heavier copy of the electron) and even stronger to the tau particle (a 3600 times heavier copy) and in the same way to quarks.
(3) The Higgs decay from figure 2, as registered by the muon spectrometer. Three tracks are alway measured.
Particle physicists are now searching for the Higgs particle which belongs to the Higgs field and will use the LHC proton-proton collider -which will be operational in 2005- to this extent. The ATLAS experiment at the LHC accelerator is optimised for this search (fig. 1). The best chance for discovery is offered by the step-by-step deacy of a Higgs particle into four electrons, into four muons or into an electron- and muon-pair (fig. 2). Showing the existence of the Higgs particle via such a decay is a tremendous task: LHC produces approximately 400.000.000 proton-proton collisions per second, in which only 10 to 100 Higgs particles per year will be produced!
(4) A prototype, designed and constructed by NIKHEF, of the ATLAS muon spectrometer wirechamber.
The decay of a Higgs particle into four muon is the easiest to find. Muons are easy to identify; in comparison with other particles they fly unhindered through thick layers of material and reach the outer 'shell' of ATLAS. Here their orbit is bend by a magnetic field. Next, in the muon spectrometer -which consists of three layers of wirechambers- the curvature of the muon track is measured. From this the energy of every muon is established. To precisely measure the shallow curvature the instruments have to meet high standards. Special wirechambers have been developed at NIKHEF where the largest prototype of such a chamber was also constructed. In addition, NIKHEF has developed the innovative RASNIK alignment system with which the respective positions of the wirechambers are continuously monitored. To reconstruct the tracks, these positions must be known with a precision of under 10 micrometers.
(5a) The three components of the RASNIK alignment system: a light source -partly shielded by a mask, a lens and a CCD-camera which measures light intensity in approximately 400.000 pixels.
In addition to the possible discovery of the Higgs particle, LHC and ATLAS offer other fascinating research possibilities such as CP-violation and supersymetry. Maybe in a far flung future a 'new' Marconi will find uses for LHC discoveries!
(5b) The mask as seen by the CCD-camera. As soon as the image moves, the signal strength in the pixels change. This enables the software to precisely determine how much the light source or the lens has moved with respect to the camera. By installing mask, lens and camera on different wirechambers, the relative positions of these chambers can be determined within one micrometer.
Back to the index.