Electron Reconstruction

Electron classes, energy scale corrections and electron momentum estimation

Results of S. Baffioni et al., presented at Physics Meeting, 05 December 05

The electron energy measurement in the ECAL involves algorithmic corrections for the cluster containment as well as corrections to account for the large amount of energy radiated in the Tracker material. Such corrections are better done separating the observed track-cluster patterns into classes, which in turn can serve as basis for electron quality estimation. From the corrected ECAL measurement, the track momentum and their associated errors, a combined estimate is constructed to obtain the final electron momentum at the vertex.

The study has been performed using back-to-back e+e- pairs in absence of LHC pileup and with energies in the range 5-100 GeV/c.

Electron classes

In order to distinguish "well" and "badly" measured electrons for physics application and to apply specific energy scale corrections, the following classes are defined:

golden electrons. Single cluster in the ECAL well macthed in E and position to the track and with a ow bremsstrahlung fraction from the GSF track fit. The predominatly truely "low radiating" electrons.
big brem electrons. Single cluster in the ECAL well matched in E but with a very high bremsstrahlung fraction from the GSF track fit.
narrow electrons. Single cluster in the ECAL well matched in E and with intermediate bremsstrahlung fraction from the GSF track fit.
showering electrons. Multi cluster patterns and single cluster pattern with bad E and p matching

	Fraction of electron population in the different classes as a function of the true pseudorapidity for initial energies between 5 and 100 GeV.
GIF EPS

Energy scale corrections

Algorithmic energy corrections involve first a f(Ncry) correction followed by an eta dependant correction. This last accounts in particular for energy which is lost in the tracker material, i.e. which never reaches the ECAL.

		f(Ncry) correction: mean of the raw reconstructed supercluster energy divided by the generated electron energy as a function of the number of crystals in the seed cluster that ocntain an energy greater than 2 standard deviation of the electronic noise: (left) electrons in the ECAL barrel, (right) electron in the ECAL endcaps.
GIF EPS	GIF EPS

		On the left, distributions of the corrected energy normalised to the true energy for the different classes for electrons in the ECAL barrel. On the right, the same distributions for electrons in the ECAL encaps.
GIF EPS	GIF EPS

Electron momentum estimation

The ECAL and Tracker estimates are then combined to estimate the final electron momentum magnitude, the momentum direction being taken from the track fit. The combination involves the errors wich are taken on a track by track basis from the track fit for p and from a parametrisation of the resolution for each class for E.

	The resolution of the E measurement as a function of the true electron energy for the different classes and for electrons in the ECAL barrel. A fit of the resolution for the golden class using standard calorimetric energy resolution function gives values well compatible with the one obtained in the test beam.
GIF EPS

The combination is then performed as:

the weighted mean of ECAL corrected energy and Tracker momentum estimate at origin, with weights defined as the normalized inverse of the variance of each measurement, when both measurements are in agreement
the corrected energy ECAL energy or the momentum estimate for the other cases

	The resolutions as measured by the relative effective RMS of the corrected supercluster energy (downward triangles), of the reconstructed track momentum at origin (upward triangles) and of the combined el;ectron momentum estimate (circles) as a function of the electron incident energy for electrons in the ECAL barrel.
GIF EPS

Last update: 30 June 2005
Comments: please contact Chris Seez, Yves Sirois