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W. H. Smith
12.2.6 Selection of Hadronic Tau Decays
The Level-2 processing of tau jets relies only on calorimeter information. In
ATLAS, the tau finding uses the same algorithms used for electron and photon
candidates, but retuned for taus. The inputs are the Level-1 RoIs. A cluster
summed over the full resolution data for the electromagnetic and hadronic cells
is required to have E T > 20 GeV with at least 10 GeV required individually in
the electromagnetic and hadronic cells. The position of the candidate cluster is
required to be consistent with the Level-1 tau-jet candidate. Then shower shape
variables are used to discriminate tau jets from regular jets. An example of one
such variable is R 37 , defined as the ratio of E T contained in a 3 × 7 cell cluster to
the E T contained in a 7 × 7 cell cluster centred on the same seed cell calculated
for the second electromagnetic layer of the LAr calorimeter. In CMS, the Level1 tau jets are used as seeds for the Level-2 tau-jet reconstruction that employs
an iterative cone algorithm with a radius of R = 0.5. Level-2 tau candidates are
then these jets which have E T > 15 GeV and are tagged as isolated if the sum of
electromagnetic calorimeter deposits in an annulus 0.13 < R < 0.4 around the jet
direction, E T < 5 GeV.
The subsequent processing of tau candidates involves tracking. ATLAS requires
a track formed from the pixel and SCT detector space points in the RoI to be within
R < 0.3 of the Level-2 tau candidate cluster direction. At Level-3 a requirement is
made that the number of tracks within R < 0.3 be either one or three. Additional
detailed jet shape requirements also refine the identification. In CMS, at Level 2.5
(the higher level trigger processing following the initial Level-2 processing that uses
calorimeter and muon information alone), tau selection is based on tracks with a
p T > 5 GeV/c that are reconstructed from seeds from the pixel hits found in a
small rectangle (η = φ = 0.1) around the tau-candidate direction. At Level 3
the rectangle is expanded to 0.5 and the p T cut is reduced to 1 GeV/c. To save CPU
time, these tracks are terminated when seven hits in the silicon strip tracker are
acquired since the resolution with seven hits is close to final. Reconstructed tracks
are associated with the tau-jet candidate if they are within a radius R < 0.5 and
originate from the primary vertex as determined by the pixel tracks. Tracks within a
radius R < 0.1 of the tau-jet candidate direction are classed as tau tracks. The leading
tau track must have p T > 3 GeV and there must be no reconstructed tracks within an
annulus 0.07 < R < 0.3 around this track.
12.2.7 Selection of b-Jets
The b-jet selection is based on track reconstruction to tag displaced vertices
associated with the jet. In ATLAS at Level-2, b-tagging uses reconstructed tracks
from the silicon tracker within the Level-1 jet RoI. For each of these tracks
the significance of the transverse impact parameter is computed and its error is
W. H. Smith
12.2.6 Selection of Hadronic Tau Decays
The Level-2 processing of tau jets relies only on calorimeter information. In
ATLAS, the tau finding uses the same algorithms used for electron and photon
candidates, but retuned for taus. The inputs are the Level-1 RoIs. A cluster
summed over the full resolution data for the electromagnetic and hadronic cells
is required to have E T > 20 GeV with at least 10 GeV required individually in
the electromagnetic and hadronic cells. The position of the candidate cluster is
required to be consistent with the Level-1 tau-jet candidate. Then shower shape
variables are used to discriminate tau jets from regular jets. An example of one
such variable is R 37 , defined as the ratio of E T contained in a 3 × 7 cell cluster to
the E T contained in a 7 × 7 cell cluster centred on the same seed cell calculated
for the second electromagnetic layer of the LAr calorimeter. In CMS, the Level1 tau jets are used as seeds for the Level-2 tau-jet reconstruction that employs
an iterative cone algorithm with a radius of R = 0.5. Level-2 tau candidates are
then these jets which have E T > 15 GeV and are tagged as isolated if the sum of
electromagnetic calorimeter deposits in an annulus 0.13 < R < 0.4 around the jet
direction, E T < 5 GeV.
The subsequent processing of tau candidates involves tracking. ATLAS requires
a track formed from the pixel and SCT detector space points in the RoI to be within
R < 0.3 of the Level-2 tau candidate cluster direction. At Level-3 a requirement is
made that the number of tracks within R < 0.3 be either one or three. Additional
detailed jet shape requirements also refine the identification. In CMS, at Level 2.5
(the higher level trigger processing following the initial Level-2 processing that uses
calorimeter and muon information alone), tau selection is based on tracks with a
p T > 5 GeV/c that are reconstructed from seeds from the pixel hits found in a
small rectangle (η = φ = 0.1) around the tau-candidate direction. At Level 3
the rectangle is expanded to 0.5 and the p T cut is reduced to 1 GeV/c. To save CPU
time, these tracks are terminated when seven hits in the silicon strip tracker are
acquired since the resolution with seven hits is close to final. Reconstructed tracks
are associated with the tau-jet candidate if they are within a radius R < 0.5 and
originate from the primary vertex as determined by the pixel tracks. Tracks within a
radius R < 0.1 of the tau-jet candidate direction are classed as tau tracks. The leading
tau track must have p T > 3 GeV and there must be no reconstructed tracks within an
annulus 0.07 < R < 0.3 around this track.
12.2.7 Selection of b-Jets
The b-jet selection is based on track reconstruction to tag displaced vertices
associated with the jet. In ATLAS at Level-2, b-tagging uses reconstructed tracks
from the silicon tracker within the Level-1 jet RoI. For each of these tracks
the significance of the transverse impact parameter is computed and its error is