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Cherenkov veto trigger ( $\bar{C1}\cdot \bar{C2}$ and $\bar{C2}$)

In the high energy nucleon collisions, the majority of emitted particles are pions. In NA44, the product of the valid beam, centrality and multiplicity triggers provides a sample of events, strongly dominated by single pions. Measurements of kaons and protons, especially the HBT-correlation studies, require a dedicated trigger. It is implemented by vetoing events with noticeable emission of Cherenkov light in the medium with specially selected refraction index. By vetoing such events, one is able to get rid of the light particles, because

1. the Cherenkov photons of frequency $\omega$ are emitted by the medium when the charged particle's velocity $v$ exceeds the phase velocity of electromagnetic waves with frequency $\omega$ in that medium: $v > c/n(\omega)$, where $n(\omega)$ is the refraction index;
2. momenta of the different particles in the detector's acceptance are made nearly equal by the spectrometer optics, so that the differences in the velocity arise solely because of the particle mass.

Table 3.1: Threshold momenta (GeV/c) of common charged particles for $C1$ and $C2$.

Detector	Gas	Pressure, atm.	$e$	$\pi$	$K$	p
$C1$	Freon 12 ($CCl_2F_2$)	2.7	6.6 $\times 10^{-3}$	1.8	6.4	12.1
$C2$	$N_2/Ne, 85\%/15\%$	1.3	19 $\times 10^{-3}$	5.2	18.3	34.8

Table 3.2: Functions of the Cherenkov detectors in the trigger for various momentum settings

$<p>$	$<(p-<p>)^2>^\frac{1}{2}$	function		Trigger
GeV/c	GeV/c	in the trigger		mode
		$C1$	$C2$
4.1	0.4	ignored	ignored	no Cherenkov bias
4.1	0.4	veto	veto	$\pi/e$ veto
7.7	0.6	ignored	ignored	no Cherenkov bias
7.8	0.7	ignored	veto	$\pi/e$ veto

NA44 has two Cherenkov threshold detectors, known as $C1$ and $C2$. The information on their thresholds and functions in the trigger, collected in tables 3.1 and 3.2, illustrates their use.