12 Triggering and High-Level Data Selection
551
parameterized as a function of p T . A b-jet discriminator is constructed using the
likelihood ratio method to determine for each track in the jet the ratio of probability
densities for the track to come from a b-jet or a u-jet. In CMS, Level-2 starts with
events with 1, 2, 3 or 4 jets passing various thresholds or a high total E T for the
whole event. At Level 2.5 tracks are reconstructed using only pixel hits (at least three
required), which are used to reconstruct the primary vertex. The b-tag algorithm runs
on the four highest E T jets with E T > 35 GeV and uses the pixel tracks and primary
vertex to tag jets as b-jets if they have at least two tracks with a signed 3D impact
parameter with large significance. Events pass Level 2.5 if they have at least one
b-tagged jet. At Level 3, tracks of up to eight hits are reconstructed in a cone of size
R = 0.25 around the b-tagged jets. The level-3 filter selects events where there is
at least one jet having at least two tracks with large impact parameter significance.
12.3 Outlook
Trigger and DAQ requirements will further evolve in the next decade with large
increases in luminosity and the associated pile-up. ALICE will continuously read
out the majority of its detectors with different latencies, busy times and technologies,
differently optimized for pp, pA and AA running scenarios [26]. Triggered readout
will be used by some detectors and for commissioning and some calibration runs.
LHCb will run trigger-free at 30 MHz, reading every bunch crossing with inelastic
collisions [27].
A major upgrade to the LHC, the HL-LHC [28], is planned to start in the middle
of this decade and deliver a luminosity of 5–7 × 10 34 cm −2 s −1 at the LHC design
centre of mass energy of 14 TeV, which corresponds to a pile-up of 140–200 at 25 ns
bunch spacing. Present link technologies operable in the radiation and magnetic
field environments of their inner detectors do not allow ATLAS and CMS to adopt
a “triggerless” architecture with an acceptable detector power and material budget
for their tracking detectors. Therefore, at the HL-LHC, both ATLAS and CMS will
retain architectures with Level 1 triggers.
In order to maintain Run-2 physics sensitivity at the HL-LHC, ATLAS and CMS
will add L1 tracking triggers for identification of tracks associated with calorimeter
and muon trigger objects and will also feature a significant increase of L1 rate, L1
latency and HLT output rate. Additionally, ATLAS and CMS are also studying the
use of fast timing information in the L1T. The ATLAS experiment will divide its
L1T into two stages [29]. A L0 trigger with a rate of 1 MHz and latency of 6 μs will
use calorimeter and muon trigger information to produce seeds used with tracking
and more fine-grained calorimeter and muon trigger information in the L1 trigger
with an output rate of 400 kHz and latency of 30 μs. This is processed by the HLT
with an output storage rate of 5–10 kHz. The CMS L1T latency will increase to
12.5 μs with an output range of 500–750 kHz for pileup ranging between 140 and
200 [30]. It will use an un-seeded L1 Track trigger along with finer granularity
551
parameterized as a function of p T . A b-jet discriminator is constructed using the
likelihood ratio method to determine for each track in the jet the ratio of probability
densities for the track to come from a b-jet or a u-jet. In CMS, Level-2 starts with
events with 1, 2, 3 or 4 jets passing various thresholds or a high total E T for the
whole event. At Level 2.5 tracks are reconstructed using only pixel hits (at least three
required), which are used to reconstruct the primary vertex. The b-tag algorithm runs
on the four highest E T jets with E T > 35 GeV and uses the pixel tracks and primary
vertex to tag jets as b-jets if they have at least two tracks with a signed 3D impact
parameter with large significance. Events pass Level 2.5 if they have at least one
b-tagged jet. At Level 3, tracks of up to eight hits are reconstructed in a cone of size
R = 0.25 around the b-tagged jets. The level-3 filter selects events where there is
at least one jet having at least two tracks with large impact parameter significance.
12.3 Outlook
Trigger and DAQ requirements will further evolve in the next decade with large
increases in luminosity and the associated pile-up. ALICE will continuously read
out the majority of its detectors with different latencies, busy times and technologies,
differently optimized for pp, pA and AA running scenarios [26]. Triggered readout
will be used by some detectors and for commissioning and some calibration runs.
LHCb will run trigger-free at 30 MHz, reading every bunch crossing with inelastic
collisions [27].
A major upgrade to the LHC, the HL-LHC [28], is planned to start in the middle
of this decade and deliver a luminosity of 5–7 × 10 34 cm −2 s −1 at the LHC design
centre of mass energy of 14 TeV, which corresponds to a pile-up of 140–200 at 25 ns
bunch spacing. Present link technologies operable in the radiation and magnetic
field environments of their inner detectors do not allow ATLAS and CMS to adopt
a “triggerless” architecture with an acceptable detector power and material budget
for their tracking detectors. Therefore, at the HL-LHC, both ATLAS and CMS will
retain architectures with Level 1 triggers.
In order to maintain Run-2 physics sensitivity at the HL-LHC, ATLAS and CMS
will add L1 tracking triggers for identification of tracks associated with calorimeter
and muon trigger objects and will also feature a significant increase of L1 rate, L1
latency and HLT output rate. Additionally, ATLAS and CMS are also studying the
use of fast timing information in the L1T. The ATLAS experiment will divide its
L1T into two stages [29]. A L0 trigger with a rate of 1 MHz and latency of 6 μs will
use calorimeter and muon trigger information to produce seeds used with tracking
and more fine-grained calorimeter and muon trigger information in the L1 trigger
with an output rate of 400 kHz and latency of 30 μs. This is processed by the HLT
with an output storage rate of 5–10 kHz. The CMS L1T latency will increase to
12.5 μs with an output range of 500–750 kHz for pileup ranging between 140 and
200 [30]. It will use an un-seeded L1 Track trigger along with finer granularity