758
D. Froidevaux
calorimeter. Nevertheless, for electrons of 50 GeV in the barrel region, the ATLAS
energy resolution varies between 1.3% (at η = 0.3) and 1.8% (at η = 1.1) without
any specific requirements on the performance of the tracker at the moment. In
contrast, the CMS effective resolution is estimated to be 2%, demonstrating that
it is harder to reconstruct electrons, with a performance in terms of efficiency and
energy resolution similar to that obtained in test beam, than photons.
Further performance figures of critical importance to the electromagnetic
calorimeters are those related to electron and photon identification in the context
of overwhelming backgrounds from QCD jets and of pile-up at the LHC design
luminosity, of γ /π 0 separation, of efficient reconstruction of photon conversions
and of measurements of the photon direction using the calorimeter alone wherever
the longitudinal segmentation provides a sufficiently accurate measurement. All
these aspects rely heavily on the details of the longitudinal and lateral segmentation
of the EM calorimetry and the reader is referred to the ATLAS and CMS detector
performance reports [13, 27] for more information.
Another important issue, especially for the EM calorimeters is the calibration in
situ, which will eventually provide the final calibration constants required e.g. for
searches for narrow states, such as H → γ γ decays. These can be divided into an
overall constant defining the absolute scale and a set of inter-calibration constants
between modules or cells:
• the ATLAS EM calorimeter has been shown to be uniform by construction to
about 0.4% in areas of 0.2 × 0.4 or larger in η × φ space. One will therefore
have to calibrate in situ only about 440 sectors of this size. The use of the Z
mass constraint alone without reference to the tracking should be sufficient to
achieve an inter-calibration to better than 0.3% over a few days at low luminosity.
If additional problems arise because of the material in the tracker, the use of
electrons from W decay to measure E/p will provide additional constraints.
• the CMS crystals could not be pre-calibrated in the laboratory with radioactive
sources to better than 4.5%. This inter-calibration spread has been brought
down to significantly smaller values using cosmic rays. Without an individual
calibration of the crystals in the test beam, one has to rely on in situ calibration
for further improvements. Using initially large samples of minimum bias events
(including explicit reconstruction of π 0 and η decays) and low E T jets at fixed
η, the inter-calibration could be improved to 1.5% within φ-rings of 360 crystals.
At a later stage, high statistics samples of W-boson decays to electrons will be
needed to reach the target constant term of 0.5%.
• a key issue for both ATLAS and CMS will be to keep the constant term
below the respective target values of 0.7 and 0.5% in the presence of the
unprecedented amount of material in the trackers. For ATLAS, other major
potential contributions to the constant term (each one of the order of 0.2 to 0.3%)
are mostly short-range (detector geometry, such as φ-modulations, variations of
the sampling fraction in the end-caps, absorber and gap thickness fluctuations,
fluctuations in the calibration chain, differences between calibration and physics
signal), but the more potentially worrisome one is long-range and is related
D. Froidevaux
calorimeter. Nevertheless, for electrons of 50 GeV in the barrel region, the ATLAS
energy resolution varies between 1.3% (at η = 0.3) and 1.8% (at η = 1.1) without
any specific requirements on the performance of the tracker at the moment. In
contrast, the CMS effective resolution is estimated to be 2%, demonstrating that
it is harder to reconstruct electrons, with a performance in terms of efficiency and
energy resolution similar to that obtained in test beam, than photons.
Further performance figures of critical importance to the electromagnetic
calorimeters are those related to electron and photon identification in the context
of overwhelming backgrounds from QCD jets and of pile-up at the LHC design
luminosity, of γ /π 0 separation, of efficient reconstruction of photon conversions
and of measurements of the photon direction using the calorimeter alone wherever
the longitudinal segmentation provides a sufficiently accurate measurement. All
these aspects rely heavily on the details of the longitudinal and lateral segmentation
of the EM calorimetry and the reader is referred to the ATLAS and CMS detector
performance reports [13, 27] for more information.
Another important issue, especially for the EM calorimeters is the calibration in
situ, which will eventually provide the final calibration constants required e.g. for
searches for narrow states, such as H → γ γ decays. These can be divided into an
overall constant defining the absolute scale and a set of inter-calibration constants
between modules or cells:
• the ATLAS EM calorimeter has been shown to be uniform by construction to
about 0.4% in areas of 0.2 × 0.4 or larger in η × φ space. One will therefore
have to calibrate in situ only about 440 sectors of this size. The use of the Z
mass constraint alone without reference to the tracking should be sufficient to
achieve an inter-calibration to better than 0.3% over a few days at low luminosity.
If additional problems arise because of the material in the tracker, the use of
electrons from W decay to measure E/p will provide additional constraints.
• the CMS crystals could not be pre-calibrated in the laboratory with radioactive
sources to better than 4.5%. This inter-calibration spread has been brought
down to significantly smaller values using cosmic rays. Without an individual
calibration of the crystals in the test beam, one has to rely on in situ calibration
for further improvements. Using initially large samples of minimum bias events
(including explicit reconstruction of π 0 and η decays) and low E T jets at fixed
η, the inter-calibration could be improved to 1.5% within φ-rings of 360 crystals.
At a later stage, high statistics samples of W-boson decays to electrons will be
needed to reach the target constant term of 0.5%.
• a key issue for both ATLAS and CMS will be to keep the constant term
below the respective target values of 0.7 and 0.5% in the presence of the
unprecedented amount of material in the trackers. For ATLAS, other major
potential contributions to the constant term (each one of the order of 0.2 to 0.3%)
are mostly short-range (detector geometry, such as φ-modulations, variations of
the sampling fraction in the end-caps, absorber and gap thickness fluctuations,
fluctuations in the calibration chain, differences between calibration and physics
signal), but the more potentially worrisome one is long-range and is related