16 Integration of Detectors into a Large Experiment: Examples from ATLAS. . .
755
in the trackers is reflected in the overall reconstruction efficiency for charged pions
of low transverse momentum, which is only slightly above 80%, to be compared
to 97% obtained for muons of the same transverse momentum. The electron
track reconstruction efficiency is even more affected by the tracker material and
the numbers shown in Table 16.12 for electrons of 5 GeV transverse momentum
are only indicative, since the efficiency obtained depends strongly on the criteria
used to define a reasonably well measured electron track. The somewhat lower
efficiencies obtained in the case of CMS are probably due to the higher magnetic
field, which enhances effects due to interactions in the detector material. The
combined performance of the tracker and electromagnetic calorimeter is discussed
in Sect. 16.8.2.
The higher and more uniform magnetic field and the better measurement
accuracy at large radius of the CMS tracker result in a momentum resolution on
single tracks, which is better than that of ATLAS by a factor of almost 3 over the
full kinematic range of the fiducial acceptance of the trackers. The impact parameter
resolution in the transverse plane is expected to be similar at high momenta for both
trackers, because the smaller pixel size in ATLAS is counter-balanced by the chargesharing between adjacent pixels and the analogue readout in the CMS pixel system.
In contrast, the smaller pixel size of the CMS tracker in the longitudinal dimension
leads to a significantly better impact parameter resolution in this direction at high
momenta.
In summary, the ATLAS and CMS trackers are expected to deliver the performances expected at the time of their design, despite the very harsh environment
in which they will operate for many years and the difficulty of the many technical
challenges encountered along the way. In contrast to most of the other systems,
however, they will not survive nor deliver the required performance if the LHC
luminosity is upgraded to 10 35 cm −2 s −1 . The ATLAS and CMS trackers will
therefore have to be replaced by detectors with finer granularity to meet the
challenges of the higher luminosity and with an order of magnitude higher resistance
to radiation. This will be the major upgrade challenge for both experiments and a
lively programme of research and development work has already been launched to
this end.
16.8.2 Calorimeter Performance
The performance to be expected in situ for the very large-scale calorimeter systems
of ATLAS and CMS is difficult to directly extrapolate from test-beam data. The
calibration of these complex electromagnetic and hadronic calorimeter systems
can indeed be to some extent ported with high precision from the test-beam
measurements to the actual experiment and, more importantly, performed in situ
using a set of benchmark physics processes such as Z → ee decays and W → j et −
j et decays. This situation is somewhat new because of the following reasons:
755
in the trackers is reflected in the overall reconstruction efficiency for charged pions
of low transverse momentum, which is only slightly above 80%, to be compared
to 97% obtained for muons of the same transverse momentum. The electron
track reconstruction efficiency is even more affected by the tracker material and
the numbers shown in Table 16.12 for electrons of 5 GeV transverse momentum
are only indicative, since the efficiency obtained depends strongly on the criteria
used to define a reasonably well measured electron track. The somewhat lower
efficiencies obtained in the case of CMS are probably due to the higher magnetic
field, which enhances effects due to interactions in the detector material. The
combined performance of the tracker and electromagnetic calorimeter is discussed
in Sect. 16.8.2.
The higher and more uniform magnetic field and the better measurement
accuracy at large radius of the CMS tracker result in a momentum resolution on
single tracks, which is better than that of ATLAS by a factor of almost 3 over the
full kinematic range of the fiducial acceptance of the trackers. The impact parameter
resolution in the transverse plane is expected to be similar at high momenta for both
trackers, because the smaller pixel size in ATLAS is counter-balanced by the chargesharing between adjacent pixels and the analogue readout in the CMS pixel system.
In contrast, the smaller pixel size of the CMS tracker in the longitudinal dimension
leads to a significantly better impact parameter resolution in this direction at high
momenta.
In summary, the ATLAS and CMS trackers are expected to deliver the performances expected at the time of their design, despite the very harsh environment
in which they will operate for many years and the difficulty of the many technical
challenges encountered along the way. In contrast to most of the other systems,
however, they will not survive nor deliver the required performance if the LHC
luminosity is upgraded to 10 35 cm −2 s −1 . The ATLAS and CMS trackers will
therefore have to be replaced by detectors with finer granularity to meet the
challenges of the higher luminosity and with an order of magnitude higher resistance
to radiation. This will be the major upgrade challenge for both experiments and a
lively programme of research and development work has already been launched to
this end.
16.8.2 Calorimeter Performance
The performance to be expected in situ for the very large-scale calorimeter systems
of ATLAS and CMS is difficult to directly extrapolate from test-beam data. The
calibration of these complex electromagnetic and hadronic calorimeter systems
can indeed be to some extent ported with high precision from the test-beam
measurements to the actual experiment and, more importantly, performed in situ
using a set of benchmark physics processes such as Z → ee decays and W → j et −
j et decays. This situation is somewhat new because of the following reasons: