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Role of T0

The fraction of the full azimuthal angle covered by T0 depends on the (pseudo)rapidity and varies between 0.84 for $\eta=3.7$ and 0.22 for $\eta=1.4$, being 0 outside this $\eta$-range. Due to geometrical reasons, a ``natural'' (no trigger bias) sample of collisions is dominated by peripheral ones. To what extent do the peripheral collisions with pronounced reaction plane (and fragmentation ?) effects ``emulate'' the T0 response typical for the central events, given the mirror-symmetric (rather than azimuthally symmetric) geometry of T0 ? Our analysis of the Si pad array multiplicity allows to say that the combination of T0 discrimination with the ``midrapidity'' trigger coming from the spectrometer (MUL1) selects a fairly homogeneous sample of events without exceptionally low Si multiplicities. In the normalization scheme being described, the T0 amplitude of the $\delta$-free side (and only it) is used to define samples of different centralities.