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The double differential multiplicity distribution

Figure 6.10: Double differential multiplicity distributions of charged particles plotted as a function of azimuthal angle $\zeta$ (with different symbols representing different rings) and of pseudorapidity $\eta$ (with different symbols representing different sectors). The $\zeta$ and $\eta$ are in the aligned coordinates.

The double differential multiplicity data (Fig. 6.10) illustrate the quality of the detector operation, calibrations (Section 6.3), geometrical alignment and Jacobian correction (Section 6.4). The data set is composed of two pieces, obtained by switching the magnetic field polarity: run 3192 is used for sectors 9 to 24 (range of $\pi/2<\zeta<3\pi/2$); run 3151 is used for sectors 1 to 8 and 25 to 32 (range of $0<\zeta<\pi/2$ and $3\pi/2<\zeta<2\pi$). The reason to disregard one side of the detector is additional occupancy due to $\delta$-electrons, as was explained in section 6.2. Figure 6.10 demonstrates the quality of alignment as well, since the $\eta$ and $\zeta$ along the horizontal axes are the aligned coordinates. Any geometrical offset of the detector makes acceptances of different pads non-equal and dependent on the pad position. The acceptance of each pad has been calculated in the aligned coordinates, and the $\,d^2N/\,d\zeta\,d\eta$ uses the actual acceptances $\,d\zeta$. The shape of the $\zeta$ dependence of $\,d^2N/\,d\eta\,d\zeta$ (left panel of Fig. 6.10) is flat as it should be for an event ensemble with no reaction plane selection. The $\eta$ dependence (right panel of Fig. 6.10) shows increasing multiplicity towards midrapidity, as is expected. The absolute value of $\,d^2N/\,d\zeta/\,d\eta$ includes a correction for the channel cross-talk, discussed in Subsection 6.5.5.