Last updated     by   Thomas Naumann

The BST reconstruction determines two types of efficiencies:
internal and  external ones.

1. Internal efficiencies

The internal efficiencies are determined from the BST alone.
They do not rely on any other detector, its reconstruction,
calibration and alignment. They are calculated in the
local coordinate system of one BST sector. Hence they also
do not depend on the global alignment of the BST.

Tracks of high chi2 probability (>10%) and with 3 hits with
high S/N are selected and their intercept with a fourth plane
is calculated. A detector is considered efficient if a hit is
found within a multiple of the resolution and internal alignment
uncertainties. Since both are below the pitch this corridor
is only some strips wide so that no noise is picked up.

The internal efficiencies are around 95% for r and u detectors.

Due to the high internal pointing precision the efficiencies of
both r and u detectors can also be measured APC wise.
 

2. External efficiencies

The external efficiencies are determined from the intercepts
of the connections between a high energetic SPACAL cluster and
a good primary vertex with the considered Si plane.

Both vertex and SPACAL only have a space resolution of several mm.
Also the SPACAL and the external BST alignment have to be known.
Due to these uncertainties the residual of the closest BST hit
to the SPACAL-vertex connection has a width of 1.3 mm in data
and 1.1 mm in MC:

This residual allows to use a corridor of 5 mm. The background under this distribution
is estimated from the distribution of one additional hit except the closest.

The external efficiencies are around 90% and shown together with the occupancies for the r detectors:

and for the u detectors:

The difference between internal and external efficiencies is due to readout problems:

For more information visit the full series of efficiency plots.