The FMD in addition to determining the total particle production in pp and PbPb collisions at the LHC, also studies elliptic flow and multiplicity fluctuations; quantities that are essential for understanding the properties of the Quark Gluon Plasma, and the phase transition from normal matter to quark gluon matter.
The FMD consists of five rings of Si semiconductor detectors with a total of 51200 individual strips.
The rings are of two types: the inner type consists of 10 wafers subdivided into 20 sectors with 1024 strips each. The outer type is subdivided into 40 sectors each with 512 strips. The Si wafers are 300 micrometer thick and are manufactured out of 6” diameter Si disks. The FMD consists of 3 groups of detectors called FMD1, FMD2 and FMD3. FMD1 consists of a ring of inner type Si sensors. Both FMD2 and FMD3 consist of a ring of inner type Si sensors and a ring of outer type Si sensors.
Read here the FMD Technical Design Report.
For central (head-on) collisions of two lead nuclei we expect that all strips will be hit by charged particles in each event (on average between 0.5 and 3 charged particles per strip). The number of particles hitting each strip can be determined by measuring the total energy deposited in each event and comparing that to the energy deposited in a strip by a single particle.
For peripheral collisions or for proton-proton collisions the overall number of charged particles is much smaller (by about a factor of 200) and many strips will be empty, so that the number of charged particles in the acceptance of the detector can be determined by counting strips that have a ‘hit’.
To determine the real number of charged particles produced in the collisions, one must however first correct for the number of ‘secondary’ charged particles, i.e. particles that are produced when a ‘primary particle’, produced in the collision hits a piece of material on the way to the detector.