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Role of MUL1

Distribution of the number of tracks that hit the spectrometer's acceptance deviates from the Poissonian one due to the variation of the Poissonian mean event to event, and due to two-particle and multiparticle correlations. Realizing that, it is nevertheless useful to recall that in the Poissonian case with average $\mu$, $(n+1)P(n+1)/P(n) = \mu$. Upon comparison with the ratio of double to single track events we see in the central trigger runs of our experiment, it is clear that even in Pb+Pb, the spectrometer (even in the low angle setting, due to the ``jaws'') presents a target which is difficult to hit ($\mu \ll 1$). I use

$$P(0) \simeq \exp(-2P(2)/P(1))$$ (20)

as a measure of the probability that the spectrometer has zero tracks. (In case of the Poissonian law, the equation (4.18) would be exact. The current discussion however does not pursue more than qualitative understanding.) Table 4.2.3 summarizes the Poissonian estimates of $P(0)$.

Table: The estimated fraction of events that do not create tracks in the spectrometer. It has been obtained according to the Poissonian law, $P(0) \simeq \exp(-2P(2)/P(1)$, based on the DST information.

momentum	+ 4 GeV		+ 8 GeV		- 4 GeV		- 8 GeV
$p_T$ setting	low	high	low	high	low	high	low	high
$h$	0.76	0.91	0.71	0.95	0.77	0.92	0.72	0.96
$K/p$	0.96	0.97	0.93	0.97	$h^{-}$	0.99	0.97	0.99

Had it been easy to satisfy ($\mu \ge 1$), the MUL1 requirement would not have been considered important component of the centrality trigger. Therefore, MUL1 needs to be taken into account for determination of the trigger centralities.