The energy of particles can be measured using the tracks (for charged particles) and the calorimeter (for all particles). Optimal experimental resolution for particle momenta and kinematic variables is obtained by combination of the information from the tracking detectors and the calorimeter, using a track-cluster matching algorithm.
Tracks measured in the tracking chambers are associated with energy
depositions in the calorimeters in a way that avoids
double-counting (of charged particle energies) and/or the omission
of energy contributions (from neutral particles).
In the usual approach well measured tracks are used for the
measurement of the charged particles while neutral particles and
particles at large energy scales (
GeV)
where the calorimeter resolution is superior to
that of tracks, are measured using the calorimeter.
For non-compensating calorimeters (such as the LAr-calorimeter of the
H1 experiment) the treatment of calorimeter energy deposits
requires the classification as hadronic or electromagnetic shower
in order to define the energy scale at which the calorimeter signals
are evaluated and combined with the tracks.
The classification, often called `software compensation',
is based on the compactness of the shower in the
calorimeter. Fluctuations of the shower development and
limited cell granularity make the classification ambiguous to a certain
extent.
In the matching algorithm the tracks are extrapolated to the calorimeter and energy depositions behind the tracks in the calorimeter are masked out to avoid double-counting. The masking leads to the loss of neutral particles which are close to the charged particles, resulting in a systematically reduced experimental response by up to 10%. To minimize these losses matching and distance parameters are tuned in a detector specific way, taking resolutions, granularities and average particle multiplicities in physics processes into account. Remaining effects are generally well described by the Monte Carlo simulations and can in this way be removed from the results of the measurements.
The combined track-cluster objects are then used to calculate global kinematic event variables and to reconstruct jets and other components of the hadronic final state.