Triggering on B-jets at CDF II Page: 2 of 6
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to measure electromagnetic showers and jets from quark frag-
mentation and consist of projective towers with electromagnetic
and hadronic sections covering the region up to l/ < 3.6.
III. CDF II TRIGGER SYSTEM
The CDF II trigger system has a three level architecture de-
signed to reduce the amount of data from 2.53 MHz, the bunch
crossing rate, to approximately 150 Hz to be written on tape.
At Level-1 (L1) raw muons, tracks and calorimeter information
are processed to produce a Li decision. Li is a synchronous 40
stages pipeline and it is based on custom-designed hardware.
It can provide a trigger decision in 5.5 ps and during normal
data taking its output rate is typically below 30 kHz. When
an event is accepted at L1, subsets of detector information are
sent to the Level-2 (L2) system, where some limited event re-
construction is performed and a decision is taken. The L2 is
an asynchronous pipeline and it is based on a combination of
custom-designed hardware and commodity processors. Its av-
erage latency is 20 iLs and its maximum output rate is 1 kHz.
Upon L2 accept, the full detector data is readout and sent to
Level-3 (L3) processors farm for further processing. Events ac-
cepted at L3 are sent to mass storage.
Recently, in order to sustain higher trigger rates due to
Tevatron increasing luminosity, many subsystems of the CDF II
trigger system had to be upgraded. The upgrades involved the
Li tracking processor (XFT, eXtremely Fast Tracker) and the
Level 2 CALorimeter (L2CAL) cluster finder. The new system
provides new tools that can be used to design innovative trigger
algorithms and helps to keep trigger rates under control.
These upgrades, combined with the existing Silicon Vertex
Trigger (SVT), are used for the implementation of the online
A. XFT Upgrade
XFT ,  has been developed to reconstruct tracks in the
plane of the drift chamber transverse to the beam axis in time
for Li decision using hit data from the 4 axial superlayers of
the chamber. Track identification is performed searching and
combining track segments in the 4 axial superlayers of the drift
chamber. XFT measures transverse momentum and azimuthal
angle 0 of all the tracks with pT > 1.5 GeV/c with an efficiency
greater than 96% and a resolution aP /p ~ 2% (GeV-1) and
a0 ~ 6 mrad.
The upgraded XFT maintains the existing axial system and
new boards are added to find track segments also in the outer
stereo layers of the chamber. The upgraded system can now re-
ject at Li fake axial tracks by requiring the association with
stereo segments with a rejection factor of about 7. An axial track
associated to segments in the stereo layers is a stereo confirmed
track. Moreover, stereo segments can be sent to L2 and matched
to the axial tracks for 3D-track reconstruction which provides a
good resolution on cotO(acote = 0.11) and zo(r = 11 cm).
B. L2CAL Upgrade
The old L2 calorimeter trigger algorithm was based on clus-
ters formed by simply combining contiguous regions of trigger
towers with an energy deposition above a given threshold in the
electromagnetic and the hadronic calorimeters. At high lumi-
nosity, when multiple proton-antiproton interactions occur in
the same bunch crossing, calorimeter occupancy is increased
and it can happen that clusters produced by different particles
are merged together, yielding a high L2 accept rate due to fake
clusters above threshold. Moreover, due to intrinsic hardware
limitation, the old system used only 8-bit energy resolution even
if 10-bit trigger tower information was available.
Now the upgraded system ,  uses a fixed cone cluster
finding algorithm which prevents fake cluster formation and ex-
ploits the full trigger tower energy information. A jet is formed
starting from a seed tower above a threshold and adding all the
towers inside a fixed cone centered at the seed tower and having
a radius AR = A52 + Ar2 = 0.7 units in the azimuth-
pseudorapidity space. The jet position is calculated weighting
each tower inside the cone according to its transverse energy.
This upgrade has reduced L2 trigger rate and has provided at
L2 jets with a quality nearly equivalent to offline ones.
SVT ,  is a L2 trigger processor dedicated to the
reconstruction of charged particle trajectories in the plane
transverse to the beam line. SVT combines hits from silicon
detectors with tracks reconstructed by XFT. The association
is performed by an associative memory, a massive parallel
mechanism based on the search of coincidences between hits
in silicon detectors and XFT tracks. When such an association
is found, a track fitter performs quality cuts and estimates track
parameters using the full available spatial resolution in a lin-
earized fit. Overall SVT tracking efficiency is about 80%. SVT
provides precise measurement of track impact parameter (do),
curvature and azimuthal angle. Impact parameter is measured
with a resolution of 35 /cm for 2 GeV/c tracks, which is com-
parable to the resolution obtained for offline reconstruction.
IV. TRIGGER ARCHITECTURE
The XFT improved tracking capabilities described above and
the already available SVT tracker can be combined with im-
proved jet reconstruction in order to perform at L2 an efficient
track-jet matching, key element of our b-tagging algorithm.
The algorithm is optimized for H -- bb search but it can also
be used to collect any final states with b-jets and in particular
Z -- bb events. Since b-quark travels some millimeters before
decaying, few tracks in these jets will be displaced from the pri-
mary vertex. The idea at the basis of our algorithm is to exploit
the displacement of b-jet tracks while trying to keep the cut on
the jet energies as low as possible in order to limit the bias on
dijet invariant mass distribution.
The effect of the trigger on signal events is studied using
Monte Carlo generated events: SM Higgs produced via gluon
fusion and MSSM Higgs produced in association with a b-quark
(bH in the following). In both samples the Higgs has a mass
of MH = 120 GeV/c2 and it is forced to decay into a pair
of b-quarks. In addition a Z - bb sample has been used to
evaluate the effect on lower mass resonances. The rejection of
background events, mainly composed by light quark jets, and
the trigger bandwidth occupancy are estimated using data events
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Amerio, Silvia; /Padua U. /INFN, Padua; Casarsa, Massimo; /Fermilab; Cortiana, Giorgio; /Padua U. /INFN, Padua et al. Triggering on B-jets at CDF II, article, January 1, 2009; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc926596/m1/2/: accessed April 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.