Violation in Mixing

entries per mrad
2.2 The BABAR detector. 65
from the end of the bars. This distance from the bar end to the PMTs, together with the size of the bars and
PMTs, gives a geometric contribution to the single photonČerenkov angle resolution of about � ÑÖ��. This
is a bit larger than the resolution contribution fromČerenkov light production (mostly a ��� ÑÖ�� chromatic
term) and transmission dispersions. The overall single photon resolution expected is about � ÑÖ��.
40000
20000
0
-50 0 50
Δ θ C,γ (mrad)
B ABAR
Tracks
15000
10000
5000
0
B ABAR
-10 0 10
θ C, track (measured) - θ C (μ) (mrad)
Figure 2-12. From di-muon data events, left plot: single photon Čerenkov angle resolution. The distribution
is fitted with a double-Gaussian and the width of the narrow Gaussian is ��� ÑÖ��. Right plot: reconstructed
Čerenkov angle for single muons. The difference between the measured and expected Čerenkov angle is
plotted and the curve represents a Gaussian distribution fit to the data with a width of �� ÑÖ��.
The image from the Čerenkov photons on the sensitive part of the detector is a cone cross-section whose
opening angle is the Čerenkov angle modulo the refraction effects on the fused silica-water surface. In the
most general case, the image consists of two cone cross-sections out of phase one from the other by a value
related to an angle which is twice the particle incidence angle. In order to associate the photon signals with
a track traversing a bar, the vector pointing from the center of the bar end to the center of each PMT is taken
as a measure of the photon propagation angles «Ü, «Ý and «Þ. Since the track position and angles are known
from the tracking system, the three « angles can be used to determine the twoČerenkov angles �� and ��.
In addition, the arrival time of the signal provides an independent measurement of the propagation of the
photon and can be related to the propagation angles «. This over-constraint on the angles and the signal
timing are useful in dealing with ambiguities in the signal association and high background rates.
The expected number of photo-electrons (ÆÔ�) is� � for a ¬ � particle entering normal to the surface
at the center of a bar and increases by over a factor of of two in the forward and backward directions.
The time distribution of real Čerenkov photons from a single event is of the order of � Ò× wide and during
normal data taking they are accompanied by hundreds of random photons in a flat background distribution
within the trigger acceptance window. The Čerenkov angle has to be determined in an ambiguity that can be
up to 16-fold: the goal of the reconstruction program is to associate the correct track with the candidate PMT
THE BABAR EXPERIMENT