The CMS detector

Assuming that the machine indeed produces the particles that theorists predicted, we are still blind up to this point of the description and cannot see them. The eyes of an experimental particle physicist are the detectors that are grouped together to large experiments. These experiments are placed at an interaction point of the collider. Newly created particles are absorbed by the detectors, thus producing measurable results. Each detector layer is responsible for sampling different types of particles. For achieving good spatial precision, the density of detector elements and the number of data channels are very high. Small time resolution is a result of fast reacting detectors and low latency readout. The experiment is connected to a data acquisition system that records, filters and stores the interesting events (see figure 7).

The experiment that forms the environment for our work is called Compact Muon Solenoid (CMS)[col95,Wul98]. It is a general purpose experiment, capable of detecting a variety of particles at different momenta. As the energies of the colliding proton bunches in LHC are very high, there are a lot of elementary interactions between the enclosed quarks. A predicted outcome of such high energetic collisions is the production of the Higgs boson [col95] [Den95]. The existence of such a particle has been calculated, but it has not been proven to exist yet. It cannot be detected directly. This is possible for charged particles that are absorbed by a calorimeter, and the deposited energy gives information about the particles energy. The Higgs particle will decay in several modes, the easiest to detect is H $\rightarrow$ ZZ $\rightarrow llll$ (the Z particle may decay as follows: Z $\rightarrow \mu^+\mu^-$, Z $\rightarrow e^+e^-$)[Ell98]. So, the resulting leptons can be detected and any combination of the four could contain the signature of the predicted particle. A good muon detection system in the outer detector regions eases the task of filtering out interesting events. Muons are about 200 times more massive than electrons and will therefore pass through a relatively large amount of concrete matter compared to other leptons (electrons, photons). Only then have they lost enough energy to be in the detectable range for sensors. As all other particles are absorbed in the inner detection layers, a clean signature can be expected in the muon chambers (see figure 8). This kind of particle may then be used as a trigger for an interesting event. Whenever a pair of muons is seen, we might choose to retrieve the data from all subdetectors and store them for later research. The event might contain a combination of muons or electrons as described before. All subdetector data allows then reconstruction of the collision in greater detail. A strong magnetic field (a 4 Tesla superconducting solenoid) forces the escaping muons into bended trajectories. This eases identification and separation of different muon projectiles. By examining the degree of bending the momentum can be reconstructed[Jac98].

Although the detector is built to provide good time and space resolution, there are still too many ``interesting'' events and checking if they shall be recorded or not takes more time than is available between the collisions (25 ns). Therefore hardwired programs in the electronics close to the detector elements are used to make a preselection of data using simple threshold cut-off and pattern recognition algorithms[Cit96]. Deployment of this first level in a multilevel trigger reduces the data rate from $10^9$ Hz to $10^5$ Hz. This is the input rate of the actual data acquisition system.