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C. W. Fabjan and D. Fournier
possible value with the technology available or under development. One of the most
advanced projects is the High Granularity Calorimeter (HGCal) replacement of the
crystal system in the endcaps of CMS [96]. In the dense core of the early part
of the shower, the signal to noise ratio and the intrinsic shower fluctuations are
such that a ~20 ps resolution has been obtained with Si diodes. A similar precision
could possibly be reached for non-showering particles (mips) by using “low gain
avalanche diodes” as developed and tested by several groups [112, 113].
For hadronic showers, the time development of the energetic component of the
cascade is of the order of tenths of nanoseconds, whereas the thermal neutron
capture may extend up to 1 μs. Nevertheless, typical time resolutions are found to
be at the level of 1–2 ns/
√
E. As an example, with multiple digital sampling a time
resolution of σ =1.5 ns/
√
E is measured in the ATLAS Tile Calorimeter [114]. The
different time evolution of electromagnetic and hadronic showers offer interesting
possibilities for improved shower treatment, a feature likely to be exploited at future
facilities (see 6.7.6.2).
6.4.3 Electron and Photon Identification
Apart from certain final states easily identified, like Bhabha scattering at e + e −
machines, electrons and photons are in general buried inside the copious production of hadrons or jets. This is particularly true at hadron colliders where the
electron/hadron ratio ranges from 10 −3 to 10 −5 . Since electrons and photons are
often signatures of interesting physics, their identification at the trigger and analysis
level is crucial. The basic criterion for electromagnetic shower identification
is the transverse and longitudinal shower shape, restricting em showers to the
electromagnetic compartment, as opposed to hadrons and jets depositing energy in
the full calorimeter. This condition is easy to implement, already at the trigger level.
Comparing shower shape parameters in the electromagnetic compartment (width,
length) to pre-programmed patterns provides the needed additional discrimination.
Further discrimination is obtained by treating electrons and photons separately. An
electron is signed by a charged track pointing to the shower barycentre, with a
momentum p compatible with the calorimetric energy E. The rejection power of this
E/p test is however compromised when the electron starts to shower in the tracking
device in front of the calorimeter, distorting the momentum measurement and
possibly the calorimetric measurement. The remaining background is dominated by
π 0 s overlapping with a charged pion. A photon is identified through the absence of
a track pointing to its barycentre. At this stage the background for photons is often
dominated by a π 0 decaying into close-by photons. Very fine granularity in the first
~5 X 0 is one approach to reject these π 0 s. As an illustrative figure, simulations
made for the ATLAS experiment, give a rejection factor of jets of about 3000 (for
a photon acceptance of 80%), when studying the γ + jet final state as a possible
background to the γγ reaction, with photon energies around 50 GeV [115]. For
certain physics reactions an ‘isolation criterion’-absence of tracks above a certain
C. W. Fabjan and D. Fournier
possible value with the technology available or under development. One of the most
advanced projects is the High Granularity Calorimeter (HGCal) replacement of the
crystal system in the endcaps of CMS [96]. In the dense core of the early part
of the shower, the signal to noise ratio and the intrinsic shower fluctuations are
such that a ~20 ps resolution has been obtained with Si diodes. A similar precision
could possibly be reached for non-showering particles (mips) by using “low gain
avalanche diodes” as developed and tested by several groups [112, 113].
For hadronic showers, the time development of the energetic component of the
cascade is of the order of tenths of nanoseconds, whereas the thermal neutron
capture may extend up to 1 μs. Nevertheless, typical time resolutions are found to
be at the level of 1–2 ns/
√
E. As an example, with multiple digital sampling a time
resolution of σ =1.5 ns/
√
E is measured in the ATLAS Tile Calorimeter [114]. The
different time evolution of electromagnetic and hadronic showers offer interesting
possibilities for improved shower treatment, a feature likely to be exploited at future
facilities (see 6.7.6.2).
6.4.3 Electron and Photon Identification
Apart from certain final states easily identified, like Bhabha scattering at e + e −
machines, electrons and photons are in general buried inside the copious production of hadrons or jets. This is particularly true at hadron colliders where the
electron/hadron ratio ranges from 10 −3 to 10 −5 . Since electrons and photons are
often signatures of interesting physics, their identification at the trigger and analysis
level is crucial. The basic criterion for electromagnetic shower identification
is the transverse and longitudinal shower shape, restricting em showers to the
electromagnetic compartment, as opposed to hadrons and jets depositing energy in
the full calorimeter. This condition is easy to implement, already at the trigger level.
Comparing shower shape parameters in the electromagnetic compartment (width,
length) to pre-programmed patterns provides the needed additional discrimination.
Further discrimination is obtained by treating electrons and photons separately. An
electron is signed by a charged track pointing to the shower barycentre, with a
momentum p compatible with the calorimetric energy E. The rejection power of this
E/p test is however compromised when the electron starts to shower in the tracking
device in front of the calorimeter, distorting the momentum measurement and
possibly the calorimetric measurement. The remaining background is dominated by
π 0 s overlapping with a charged pion. A photon is identified through the absence of
a track pointing to its barycentre. At this stage the background for photons is often
dominated by a π 0 decaying into close-by photons. Very fine granularity in the first
~5 X 0 is one approach to reject these π 0 s. As an illustrative figure, simulations
made for the ATLAS experiment, give a rejection factor of jets of about 3000 (for
a photon acceptance of 80%), when studying the γ + jet final state as a possible
background to the γγ reaction, with photon energies around 50 GeV [115]. For
certain physics reactions an ‘isolation criterion’-absence of tracks above a certain