Universit`a degli studi Roma Tre Measurement of the KL meson ...

30 CHAPTER 3. THE KLOE DETECTOR
All signals S above are in ADC counts. S 0,i are the zero-offsets of the amplitude
scale. S mip, i is the response for a minimum ionizing particle crossing the calorimeter
center. Dividing by S mip, i Eq. 3.4 above accounts for PM response, fiber light yield
and electronics gain. kE gives the energy scale in MeV, and it is obtained from
showering particles of known energy.
In order to obtain a calorimeter response independent of the position, a correction
factor Ai A,B (z), due to the attenuation along the fiber length, is applied. The cell
energy, Ei, is taken as the mean of the determinations at both ends:
Ei(MeV) = Ei AAi A + Ei BAi B
. (3.5)
2
The determination of the absolute energy scale relies instead on the use of the
monochromatic source of 510 MeV photons: the e + e− → γγ sample. This calibration
is routinely done each 200-400 nb−1 of integrated luminosity (i.e., approximately
every 1-2 hours during normal data taking). For the timing, the relative time offsets
of each channel, ti0, related to cable lengths and electronic delays and the light
velocity in the fibers are evaluated every few days with high momentum cosmic
rays selected with DC information. An iterative procedure uses the extrapolation
of the tracks to the calorimeter to minimize the residuals between the expected and
measured times in each cell. A precision of few tens of picoseconds is obtained for
these offsets.
The clustering procedure associates different hits in the calorimeter cells in a
single cluster as due to the same particle. Each group collects hit cells which are
close to each other. Among these, the cell with highest energy release is found;
then, the nearest hit cells are associated to that highest one, in order to reconstruct
a cluster.
The cluster energy is obtained by adding the energy released in the nearest cells:
Eclu =
i
Ei, (3.6)
where i is the index of the cell number.
The cluster position is evaluated as the energy-weighted average of the cell coordinates:
Rclu =
i Ei Ri
i Ei
, (3.7)
where Ri = (xi, yi, zi), zi is the cell coordinate along the fiber (see Eq. 3.3), and
xi, yi are the nominal positions of the cell.
Finally, the cluster time is obtained in an analogous way:
i Eiti
Tclu = , (3.8)
i Ei
where ti are evaluated as in Eq. 3.2.
3.2.2 Time, energy and spatial resolution
After the description of the calibration and monitoring procedures, the resolution
of the calorimeter is here summarized.