Violation in Mixing
Violation in Mixing
Violation in Mixing
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1375<br />
68 The BABAR Experiment<br />
920<br />
38.2˚<br />
1555 2295<br />
1127<br />
1801<br />
558<br />
2359<br />
22.7˚<br />
External<br />
Support<br />
15.8˚<br />
26.8˚<br />
Interaction Po<strong>in</strong>t 1-2001<br />
1979<br />
8572A03<br />
Fiber Optical Cable<br />
to Light Pulser<br />
Alum<strong>in</strong>um<br />
Frame<br />
Silicon<br />
Photo-diodes<br />
TYVEK<br />
(Reflector)<br />
Alum<strong>in</strong>um<br />
Foil<br />
(R.F. Shield)<br />
Mylar<br />
(Electrical<br />
Insulation)<br />
CFC<br />
Compartments<br />
(Mechanical<br />
Support)<br />
CsI(Tl) Crystal<br />
Preamplifier<br />
Board<br />
Figure 2-14. The electromagnetic calorimeter layout <strong>in</strong> a longitud<strong>in</strong>al cross section and a schematic view<br />
of the wrapped CsI(Tl) crystal with the front-end readout package mounted on the rear face (not to scale).<br />
��<br />
� �<br />
�<br />
� Ô � ��Î<br />
where � and �� refer to the energy of a photon and its rms error, measured <strong>in</strong> ��Î. The energy dependent<br />
term � arises basically from the fluctuations <strong>in</strong> photon statistics, but also from the electronic noise of the<br />
photon detector and electronics and from the beam-generated background that leads to large numbers of<br />
additional photons. This first term dom<strong>in</strong>ates at low energy, while the constant term � is dom<strong>in</strong>ant at higher<br />
energies (� ��Î). It derives from non-uniformity <strong>in</strong> light collection, leakage or absorption <strong>in</strong> the material<br />
<strong>in</strong> front of the crystals and uncerta<strong>in</strong>ties <strong>in</strong> the calibration.<br />
The angular resolution is determ<strong>in</strong>ed by the transverse crystal size and the distance from the <strong>in</strong>teraction<br />
po<strong>in</strong>t: it can be empirically parameterized as a sum of an energy dependent and a constant term<br />
�� � �� � Ô � ��Î<br />
where � is measured <strong>in</strong> ��Î. In CsI(Tl), the <strong>in</strong>tr<strong>in</strong>sic efficiency for the detection of photons is close to<br />
down to a few Å�Î, but the m<strong>in</strong>imum measurable energy <strong>in</strong> collid<strong>in</strong>g beam data is about Å�Î<br />
for the ��: this limit is determ<strong>in</strong>ed by beam and event-related background and the amount of material <strong>in</strong><br />
front of the calorimeter. Because of the sensitivity of the � efficiency to the m<strong>in</strong>imum detectable photon<br />
energy, it is extremely important to keep the amount of material <strong>in</strong> front of the �� to the lowest possible<br />
level.<br />
Thallium-doped CsI has high light yield and small Molière radius <strong>in</strong> order to allow for excellent energy<br />
and angular resolution. It is also characterized by a short radiation length for shower conta<strong>in</strong>ment at BABAR<br />
MARCELLA BONA<br />
¨ �<br />
�<br />
Output<br />
Cable<br />
Diode<br />
Carrier<br />
Plate<br />
11-2000<br />
8572A02
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