Electron Identification
The electrons, which are contaminating the muon sample, can by rejected on a
particle-by-particle base by employing a Threshold Cherenkov Detector (TCD)
downstream to the last TOF counter.
Simulation studies show that at this point, z=??, the lateral spread of both the muon
and electron beams can be described by a symmetric gaussian distribution with
sigmas: σx σy  60mm. According to these studies there is a significant difference, in
both momentum and directionality, between the two particle species as it is shown in
Figure 1.
Figure 1: The momentum distribution (upper plot) and the directionality
distribution (lower plot) of the electrons (histogram) and muons (shaded
histogram). The angle θ, is the angle between the particle momentum and the z
axis.
In order to quantify the
electron
rejection
capabilities of the TCD, a
wide aperture detector is
considered
and
the
Cherenkov-photon
emission and detection is
simulated.
The photon collection
efficiency is set to 80%
and a mean quantum
efficiency of 15% is used
for the photon detection.
An electron is considered
misidentified when results
to
less
than
three
photoelectrons.
Figure 2: The electron misidentification factor as a function of the
refractive index of the radiator: a) for a one-meter (dots) and b) for a
three meters (open circles) long detector.
The fraction of misidentified electrons is estimated for several refractive indices, well
below to the muon Cherenkov threshold. These results are shown in Figure 2, for two
different detector lengths .
A radiator with a refractive index of 1.0051 offers a rejection factor of the order of
1000. The remaining, low momentum, electrons start radiating at much higher
refractive indices as it is shown in Figure 2. However a further rejection can be
achieved by applying extra kinematical cuts.
The lateral dimensions of this detector will be precisely defined when more accurate
simulations will be available.
1
This, as an example, corresponds to Nitrogen at 17 bars.