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Spectrometer ``optics'' and acceptance.

A magnet system of two dipoles and three superconductive focusing quadrupoles, together with a tracking complex (a pad chamber, three highly segmented scintillation hodoscopes H2, H3, H4 and two strip chambers, see Fig.3.1) provides momentum resolution of 0.2% or better for all spectrometer settings. The quadrupoles allow for two modes of focusing - vertical and horizontal. The distinction between them plays a role in HBT analysis; in the single particle analysis the horizontal mode has always been used. The spectrometer accepts charged particles of a single charge at a time. The magnet system is mounted on a rotating platform and was operated at two angular positions (44 and 131 mrad). The data analyzed are taken with two different field strengths. In the weak field mode, spectrometer accepts charged tracks in the momentum range of $3.3<p<5.1$ GeV$/c$, and of $6.3<p<9.7$ GeV$/c$ in the strong field mode. These two field modes are often called ``the 4 GeV/c'' and ``the 8 GeV/c'' settings, respectively. Location of the acceptance area in rapidity $y$ and transverse momentum $p_T$ varies with field strength, arm angle, and particle mass, as is shown on Fig. 3.3. For $K$ and $\pi$, the acceptances are located within one unit of $y$ around the center-of-mass rapidity of the SPS $PbPb$ system ( $y_{CM}=2.92$) .

Figure 3.3: Acceptance area of the NA44 spectrometer in the laboratory rapidity $y$ and transverse momentum $p_T$. Top: in the weak field mode; bottom: in the strong field mode.

The magnitude of the spectrometer's acceptance is largely restricted by the factor $\Delta\phi/2\pi$ (typically around 0.02) representing narrowness of the azimuthal coverage. Within the spectrometer's acceptance at $p_T=0.4$ GeV/c for a midrapidity pion, the remaining factors would amount to about 0.5. In the low angle setting during the $Pb$ run, a tungsten collimator (the so called ``jaws'') was introduced into the magnet system in order to reduce the multiplicity of tracks in the spectrometer. Its effect on the acceptance has been computed via Monte Carlo and corrected for in the analysis. A more detailed discussion of these issues will be given in Section 4.1.