756
D. Froidevaux
• for the first time, there will be the possibility to control the absolute scale of
hadronic jet energy measurements by using sufficiently abundant statistics from
W → j et − j et topologies occurring in top-quark decays.
• extensive test-beam measurements in configurations close to that of the real
experiment will have been performed at the time of first data-taking.
• it should be possible to constrain the absolute scale of the overall hadronic
calorimetry using the measured response to charged pions of energies between 1
and 300 GeV and controlling this scale in situ, using a variety of samples, from
single isolated tracks at the lower end of the range to e.g. clean samples of
τ → π ± ν decays.
During the past 15 years, a large-scale and steady software effort has been
maintained in the collaborations to simulate in detail calorimeters of this type well
before they begin their operation. The complex geometries and high granularities
described above and the high energies of the products of the collisions have naturally
augmented considerably the computing effort required to produce large-statistics
samples of fully simulated events. A few examples are shown below for photon,
electron, jet and missing transverse energy measurements.
16.8.2.1 Electromagnetic Calorimetry
Figure 16.19 shows an example of the expected precision with which photon energy
measurements will be performed in ATLAS (left) and CMS (right) over the energy
range of interest for H → γ γ decays. In the case of ATLAS, the results are shown
for all photons (unconverted and converted) and for three values of pseudorapidity.
In the case of CMS, the results are shown for dominantly unconverted photons in
the barrel crystal calorimeter. The selected photons are required in this latter case to
have deposited more than 94.3% of their energy in a 3 by 3 crystal matrix normalised
to the 5 by 5 crystal matrix used to compute the total energy. This basically selects
unconverted photons and some late conversions with a 70% overall efficiency. For
a photon energy of 100 GeV, the ATLAS energy resolution varies between 1.0
and 1.4%, depending on η. These numbers increase respectively to 1.2 and 1.6%
if one includes the global constant term of 0.7%. The overall expected CMS energy
resolution in the barrel crystal calorimeter is 0.75% for the well-measured photons
at that energy (Fig. 16.19 includes the global constant term of 0.5%). This example
shows that the intrinsic resolution of the CMS crystal calorimeter is harder to obtain
with the large amount of tracker material in front of the EM calorimeter and in
the 4T magnetic field: between 20 and 60% of photons in the barrel calorimeter
acceptance convert before reaching the front face of the crystals.
Similarly, Fig. 16.20 shows an example of the expected precision with which
electron energy measurements will be performed in ATLAS (left) and CMS (right).
In the case of ATLAS, the results are shown for electrons at η = 0.3 and 1.1 in the
energy range from 10 to 1700 GeV. The energy of the electrons is always collected
in a 3 by 7 cell matrix, which, as for the photons, is wider in the bending direction
D. Froidevaux
• for the first time, there will be the possibility to control the absolute scale of
hadronic jet energy measurements by using sufficiently abundant statistics from
W → j et − j et topologies occurring in top-quark decays.
• extensive test-beam measurements in configurations close to that of the real
experiment will have been performed at the time of first data-taking.
• it should be possible to constrain the absolute scale of the overall hadronic
calorimetry using the measured response to charged pions of energies between 1
and 300 GeV and controlling this scale in situ, using a variety of samples, from
single isolated tracks at the lower end of the range to e.g. clean samples of
τ → π ± ν decays.
During the past 15 years, a large-scale and steady software effort has been
maintained in the collaborations to simulate in detail calorimeters of this type well
before they begin their operation. The complex geometries and high granularities
described above and the high energies of the products of the collisions have naturally
augmented considerably the computing effort required to produce large-statistics
samples of fully simulated events. A few examples are shown below for photon,
electron, jet and missing transverse energy measurements.
16.8.2.1 Electromagnetic Calorimetry
Figure 16.19 shows an example of the expected precision with which photon energy
measurements will be performed in ATLAS (left) and CMS (right) over the energy
range of interest for H → γ γ decays. In the case of ATLAS, the results are shown
for all photons (unconverted and converted) and for three values of pseudorapidity.
In the case of CMS, the results are shown for dominantly unconverted photons in
the barrel crystal calorimeter. The selected photons are required in this latter case to
have deposited more than 94.3% of their energy in a 3 by 3 crystal matrix normalised
to the 5 by 5 crystal matrix used to compute the total energy. This basically selects
unconverted photons and some late conversions with a 70% overall efficiency. For
a photon energy of 100 GeV, the ATLAS energy resolution varies between 1.0
and 1.4%, depending on η. These numbers increase respectively to 1.2 and 1.6%
if one includes the global constant term of 0.7%. The overall expected CMS energy
resolution in the barrel crystal calorimeter is 0.75% for the well-measured photons
at that energy (Fig. 16.19 includes the global constant term of 0.5%). This example
shows that the intrinsic resolution of the CMS crystal calorimeter is harder to obtain
with the large amount of tracker material in front of the EM calorimeter and in
the 4T magnetic field: between 20 and 60% of photons in the barrel calorimeter
acceptance convert before reaching the front face of the crystals.
Similarly, Fig. 16.20 shows an example of the expected precision with which
electron energy measurements will be performed in ATLAS (left) and CMS (right).
In the case of ATLAS, the results are shown for electrons at η = 0.3 and 1.1 in the
energy range from 10 to 1700 GeV. The energy of the electrons is always collected
in a 3 by 7 cell matrix, which, as for the photons, is wider in the bending direction