702
D. Froidevaux
– Both experiments rely on a versatile and multi-level trigger system to make
sure the events of interest can be selected in real time at the highest possible
efficiency.
• Efficient identification with excellent purity of the fundamental objects arising
from the hard-scattering processes of interest is as important as the accuracy
with which their four-momenta can be determined. Electrons and muons (and
to a lesser extent photons and τ -leptons with their decay products) provide
excellent tools to identify rare physics processes above the huge backgrounds
from hadronic jets. The requirements at the LHC are far more difficult to meet
than at the Fermilab Tevatron: for example, at a transverse momentum of 40 GeV,
the electron to jet production ratio decreases from almost 10 −3 at the Tevatron
to a few 10 −5 at the LHC, because of the much larger increase of the production
cross section for QCD hadronic jets than for W and Z bosons.
For reasons of size, cost and radiation hardness, both experiments have limited
the coverage of their lepton identification and measurements to the approximate
pseudorapidity range |η| < 2.5 (or a polar angle of 9.4 ◦ with respect to the
beams). The implementation of these requirements has also had a very large
impact on the design and technology choices of both experiments:
– the length of the ATLAS and CMS super-conducting solenoids has been
largely driven by the choices made for the lepton coverage;
– ATLAS has chosen a variety of techniques to identify electrons, based first
and foremost on the electromagnetic calorimeter with its fine segmentation
along both the lateral and longitudinal directions of shower development, then
on energy-momentum matching between the calorimeter energy measurement
and the inner tracker momentum measurement, but enhanced significantly
over most of the solid angle by the transition radiation tracker ability to
separate electrons from charged pions. In contrast, CMS relies on the fine
lateral granularity of its crystal calorimeter and on the energy-momentum
matching with the inner tracker;
– CMS has privileged the accuracy of the electron energy measurement with
respect to the identification power with their choice of crystal calorimetry.
The intrinsic resolution of the CMS electromagnetic (EM) calorimeter is
superb with a stochastic term of 3–5.5% (see Sect. 16.8.2.1 for quantitative
plots illustrating the performance) and the electron identification capabilities
are sufficient to extract the most difficult benchmark processes from the
background even at the LHC design luminosity.
• The overall trigger system of the experiments must provide a total event reduction
of about 10 7 at the LHC design luminosity, since the number of inelastic protonproton collisions will occur at a rate of about 10 9 Hz, whereas the storage
capabilities will correspond to approximately 100 Hz for an average event size
of 1–2 MBytes. Even today’s state-of-the art technology is however far from
approaching the performance required for taking a trigger decision in the very
small amount of time between successive bunch crossings (25 ns).
D. Froidevaux
– Both experiments rely on a versatile and multi-level trigger system to make
sure the events of interest can be selected in real time at the highest possible
efficiency.
• Efficient identification with excellent purity of the fundamental objects arising
from the hard-scattering processes of interest is as important as the accuracy
with which their four-momenta can be determined. Electrons and muons (and
to a lesser extent photons and τ -leptons with their decay products) provide
excellent tools to identify rare physics processes above the huge backgrounds
from hadronic jets. The requirements at the LHC are far more difficult to meet
than at the Fermilab Tevatron: for example, at a transverse momentum of 40 GeV,
the electron to jet production ratio decreases from almost 10 −3 at the Tevatron
to a few 10 −5 at the LHC, because of the much larger increase of the production
cross section for QCD hadronic jets than for W and Z bosons.
For reasons of size, cost and radiation hardness, both experiments have limited
the coverage of their lepton identification and measurements to the approximate
pseudorapidity range |η| < 2.5 (or a polar angle of 9.4 ◦ with respect to the
beams). The implementation of these requirements has also had a very large
impact on the design and technology choices of both experiments:
– the length of the ATLAS and CMS super-conducting solenoids has been
largely driven by the choices made for the lepton coverage;
– ATLAS has chosen a variety of techniques to identify electrons, based first
and foremost on the electromagnetic calorimeter with its fine segmentation
along both the lateral and longitudinal directions of shower development, then
on energy-momentum matching between the calorimeter energy measurement
and the inner tracker momentum measurement, but enhanced significantly
over most of the solid angle by the transition radiation tracker ability to
separate electrons from charged pions. In contrast, CMS relies on the fine
lateral granularity of its crystal calorimeter and on the energy-momentum
matching with the inner tracker;
– CMS has privileged the accuracy of the electron energy measurement with
respect to the identification power with their choice of crystal calorimetry.
The intrinsic resolution of the CMS electromagnetic (EM) calorimeter is
superb with a stochastic term of 3–5.5% (see Sect. 16.8.2.1 for quantitative
plots illustrating the performance) and the electron identification capabilities
are sufficient to extract the most difficult benchmark processes from the
background even at the LHC design luminosity.
• The overall trigger system of the experiments must provide a total event reduction
of about 10 7 at the LHC design luminosity, since the number of inelastic protonproton collisions will occur at a rate of about 10 9 Hz, whereas the storage
capabilities will correspond to approximately 100 Hz for an average event size
of 1–2 MBytes. Even today’s state-of-the art technology is however far from
approaching the performance required for taking a trigger decision in the very
small amount of time between successive bunch crossings (25 ns).