6 Calorimetry
259
p T , nor calorimeter energy in a cone around the electromagnetic shower can be
applied to sharpen photon or electron identification. This criterion does not apply
e.g. for electrons resulting from heavy quark decays inside a heavy quark jet.
The Higgs boson discovery in the di-photon mode was a brilliant demonstration
that the necessary jet rejection was achieved by both ATLAS and CMS experiments.
At an invariant mass of the Higgs boson of about 125 GeV, the di-photon continuous
background consists of about 75% prompt di-photons, 20% photon-jet background
and about 5% jet-jet background.
Samples of electrons-positrons with an invariant mass around the Z 0 mass allow
a clean measurement of the electron sample purity, as well as of the selection
efficiency, using the “tag and probe” method, see Refs. [107, 110] for details.
6.4.4 Muon Identification
The registration of muons in calorimeters contributes to their identification, provides
an important means of cross-calibration and in-situ monitoring of calorimeter cells
and is used to improve the quality of the muon spectroscopy for instruments located
behind the calorimeter.
Identification relies on the reconstruction of a penetrating, charged track behind
the hadron calorimeter and possibly on the measurement of an energy deposit in the
calorimeter cells along the path of the muon. Typical most probable energy deposits
in an electromagnetic calorimeter (e.g. the CMS PbWO 4 calorimeter or the ATLAS
Accordion) are of order 300 MeV, whereas in the hadronic calorimeters several
GeVs are deposited. Such values are in general large compared to electronic noise
and to energy deposits from particle background. In the ATLAS hadron calorimeter
muons deposit more than ten times the energy from particle background due to
average inelastic collisions, even in case of event pile-up at the highest collision
rates.
Identification and triggering on muons based on calorimeter information is an
essential complement to the main muon trigger using tracking chambers, for physics
reactions producing low-p T muons, e.g. tagging c- or b-jets, or detecting J/ψ or Y;
production.
Muons are abundantly produced in pp. collisions (see Fig. 6.41). At low p T
the rate is dominated by ‘punch-through’ particles, i.e. hadrons, which have not
interacted in the calorimeter. At high p T prompt muons (in particular from W decay)
become dominant. [116].
6.5 Jets and Missing Energy
Jet spectroscopy and the related signature of ‘Missing Transverse Energy’ (MET)
have contributed to major discoveries (gluon, W-boson, top quark, . . . ). At LHC,
259
p T , nor calorimeter energy in a cone around the electromagnetic shower can be
applied to sharpen photon or electron identification. This criterion does not apply
e.g. for electrons resulting from heavy quark decays inside a heavy quark jet.
The Higgs boson discovery in the di-photon mode was a brilliant demonstration
that the necessary jet rejection was achieved by both ATLAS and CMS experiments.
At an invariant mass of the Higgs boson of about 125 GeV, the di-photon continuous
background consists of about 75% prompt di-photons, 20% photon-jet background
and about 5% jet-jet background.
Samples of electrons-positrons with an invariant mass around the Z 0 mass allow
a clean measurement of the electron sample purity, as well as of the selection
efficiency, using the “tag and probe” method, see Refs. [107, 110] for details.
6.4.4 Muon Identification
The registration of muons in calorimeters contributes to their identification, provides
an important means of cross-calibration and in-situ monitoring of calorimeter cells
and is used to improve the quality of the muon spectroscopy for instruments located
behind the calorimeter.
Identification relies on the reconstruction of a penetrating, charged track behind
the hadron calorimeter and possibly on the measurement of an energy deposit in the
calorimeter cells along the path of the muon. Typical most probable energy deposits
in an electromagnetic calorimeter (e.g. the CMS PbWO 4 calorimeter or the ATLAS
Accordion) are of order 300 MeV, whereas in the hadronic calorimeters several
GeVs are deposited. Such values are in general large compared to electronic noise
and to energy deposits from particle background. In the ATLAS hadron calorimeter
muons deposit more than ten times the energy from particle background due to
average inelastic collisions, even in case of event pile-up at the highest collision
rates.
Identification and triggering on muons based on calorimeter information is an
essential complement to the main muon trigger using tracking chambers, for physics
reactions producing low-p T muons, e.g. tagging c- or b-jets, or detecting J/ψ or Y;
production.
Muons are abundantly produced in pp. collisions (see Fig. 6.41). At low p T
the rate is dominated by ‘punch-through’ particles, i.e. hadrons, which have not
interacted in the calorimeter. At high p T prompt muons (in particular from W decay)
become dominant. [116].
6.5 Jets and Missing Energy
Jet spectroscopy and the related signature of ‘Missing Transverse Energy’ (MET)
have contributed to major discoveries (gluon, W-boson, top quark, . . . ). At LHC,