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The General Concept

In the design of fixed target heavy ion experiments, one can distinguish two opposite stratagems:

• Emphasize quality of a given set of measurements (resolution, cleanliness of particle identification). Sacrifice broadness of scope in order to fully concentrate on the selected measurements.
• Emphasize broadness of scope (acceptance, variety of identifiable particles). Sacrifice quality of measurements (resolution, quality of particle identification).

A clear awareness of one's stratagem, readiness to counteract its drawbacks and fully utilize its strengths is an essential prerequisite of success. The choice of a stratagem (or, in a mixed case, choice of a proportion of the mix) is driven by the experiment's goals. The goals of the NA44 Experiment have been:

• Hanbury-Brown-Twiss (for short, HBT) interferometry[25] with pions, kaons and protons, reaching a formulation as complex as 3D.
• Single particle spectra of pions, kaons, protons, antiprotons, deuterons, antideuterons, and even tritons. In particular, a study of low $p_T$ features of the spectra around midrapidity.
• Measurements of the abundances of the above mentioned particles.

Thus ordered, the goals entail the design concept of an experiment, which follows the first stratagem: a focusing spectrometer. Focusing magnetic field provides for excellent momentum resolution, but can be effective only in a narrow region of phase space. Hence the smallness of track multiplicity that enables clean particle identification using the relatively simple techniques based on time-of-flight and Cherenkov radiation.

Figure 3.1: The NA44 spectrometer during the Pb beam running. The T0 and the Si pad array are in the target area and are too small to be seen on this scale. The target area is shown in Fig. 3.2