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Text
and some figures extracted from the HLT section 3.2 of the LHCb Upgrade LOI
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Contents: |
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HLT1 |
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Strategy |
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Extra tracks |
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·
Decision ·
Examples |
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The High Level Trigger (HLT) is the
second (and last) level of trigger of LHCb, running on events passing the L0 trigger.
It consists of a C++ application (called “Moore”)
that will run in several copies on every CPU of the Event Filter Farm (EFF),
which consists of approximatively 1000 multi-core computing nodes. Currently
there are 26110 copies of Moore running in the EFF. The HLT application has
access to all data in one event. The HLT is divided in two steps: HLT1 and HLT2. HLT1
reduces the rate from the 1.1 MHz output of L0 to ~50 kHz. HLT1 reconstructs
particles in the VELO and determines the position of the primary vertices
(PV) in the event. To limit the CPU consumption, a selection of VELO tracks
is made based on their smallest impact parameter (IP) to any PV, and their
quality. For these selected VELO tracks their track-segment in the T-stations
are sought to determine their momentum (p), so-called forward tracking. HLT1
selects events with at least one track which satisfies minimum requirements
in IP, p, pT and track quality.
(Transverse
is defined perpendicular to the beam-pipe, in the plane formed by the track
and the beam-pipe.) It reduces the rate to a sufficiently low level
to allow forward tracking of all VELO tracks. HLT1 should reduce the rate to a sufficiently low level to allow the
full pattern recognition on the remaining events. HLT2 searches for secondary vertices, and applies decay length and mass cuts to reduce the rate to the level at which the events can be written to storage and processed offline. Currently this is ~3 kHz. It first performs a complete pattern recognition to find all particle tracks in the event, using VELO tracks as seeds. Then, a set of different selections are applied. Some of them are inclusive, aiming for generic B decays or for resonances like J/Psi(->mm) or D*(->pKp), and some of them are exclusive, aiming to provide the highest possible efficiency on specific B decay channels. The schematic of the overall LHCb trigger
scheme is shown in the figure below:
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A small fraction of the events in LHCb have large detector
occupancies, especially in the OT. Some of these events take even seconds to
reconstruct in HLT1, and are rejected before any reconstruction to keep the
average processing time ~25 ms per event. Events are rejected if the OT
occupancy is larger than 20%. For the remaining events the reconstruction
strategy is determined by the following considerations: ·
All B meson decays studied at LHCb
contain at least two charged tracks in their final state; ·
B mesons are heavy, and their average
momentum in the LHCb acceptance is ~100
GeV/c, so their decay products will have a large
momentum (p) and transverse
momentum (pT) compared with light-quark
hadrons originating from the PV; ·
The average decay length of B mesons
produced in the LHCb acceptance is ~1
cm so that their decay products will have a large impact parameter (IP) with
respect to their PV. ·
The VELO reconstruction is fast enough to
allow a full 3D pattern
recognition and PV finding to be performed for all events entering the HLT. |
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Velo
reconstruction This last point is illustrated in the figure:
The timing of the VELO 3D pattern recognition as a
function of the off-line reconstructed number of PVs in the
event. The entry at 4 PVs contains all events with more than 3 PV. The timing increases linearly with the number of PV in the events, and
is low enough to allow sufficient time for the subsequent reconstruction and
event selection. The event reconstruction therefore begins with the VELO
pattern recognition and PV finding. Because of timing constraints, the
momentum can only be determined for a limited number of VELO tracks. It is
necessary to select those tracks which are most likely to come from a B decay. Three selection criteria are used: 1.
The IP of the VELO track with respect to the closest PV 2.
The number of hits assigned to the VELO track 3.
The difference between the number of hits assigned to the VELO track
and the number of hits expected given the track direction and the first
measured point on the track (missed hits) The following figures compare the distributions of the latter two
criteria for tracks from a minimum bias event and for the highest pT B
daughter track in Bs →
φφ decays.
It can be seen that both the number of hits on a VELO
track and the number of missed hits are good discriminants,
especially since they can be applied before any forward reconstruction, thus
saving CPU time by having to consider fewer VELO tracks. Requiring |IP| > 125 μm, and number of
VELO hits and missing hits > 9 and < 3 respectively, is very efficient
at selecting the highest pT daughter
from a Bs →
φφ decay. The figure below shows the
number of VELO tracks per event for which the momentum needs to be determined.
Number of
VELO tracks per event which enter the forward reconstruction to have their momentum
determined. |
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The VELO tracks thus selected are extrapolated to the
tracking stations using the Pat-Forward algorithm. Imposing a minimum
momentum and transverse momentum (p,
pT) in the forward tracking
significantly reduces the search windows which have to be opened in the IT
and OT tracking stations and consequently reduces the required CPU power.
This is illustrated in the figure below (left), which shows the
reconstruction time per event of the forward tracking as a function of the
minimum (p, pT)
cutoff imposed. The figure below (right) shows the transverse-momentum
distributions of Bs →
φφ decay products which have been
selected by the VELO IP and quality cuts.
Hence, imposing a minimum (p, pT) of (12, 1.2) GeV/c results in a negligible loss of signal, while the
CPU time per event is acceptable. The final track selection proceeds in two
stages. First (p, pT) cuts are used in order to
reduce the rate. The remaining tracks are fitted using a Kalman
filter with outlier removal, in order to obtain an offline-quality value for
the track χ2 as
well as an offline-quality covariance matrix at the first state of the track,
allowing a cut on the IP significance squared (IP χ2). The number of tracks which have to be fitted is low
enough by this stage that the contribution of the track fit to the overall
timing is negligible. The track χ2 is a powerful tool for ghost
rejection in the trigger; however, being particularly sensitive to the
detector performance, it needs to be verified with real data. The figure
below (left) shows the distribution of the online track χ2/ndf for minimum bias events
recorded in 2010 surviving to this stage of the trigger, as well as the
highest momentum daughter from offline selected real data D+ →
h+h+h− decays. The data demonstrate that LHCb achieved an
excellent reconstruction performance with very low online track χ2 values
for genuine signal tracks. The track χ2 and
IP χ2 are
therefore used to achieve the final required rate reduction. The figure below
(right) shows the fraction of events which are triggered by at least one
ghost track as a function of the OT occupancy, imposing a track χ2/ndf < 3 cut for minimum
bias events surviving to this stage of the trigger. On average 15% of the
events accepted by HLT1 are triggered due to a ghost track.
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For decays involving muons an additional cut
is available: the muon ID algorithm which is in itself a powerful
ghost and background rejection tool. This fact is exploited in a parallel
trigger line which is run only on those events passing the L0 muon trigger, and hence most of the above cuts can be
relaxed for muon candidates. For radiative B decays, e.g. Bs → φγ, the requirements of the offline
background rejection impose a tight LLT electromagnetic cut of 2.4 GeV, so that L0 electromagnetic triggers make up only a
small fraction of the total L0 rate. Therefore HLT1 contains another parallel
line only running on L0 electromagnetic triggered events for which the (p, pT)
cuts are significantly reduced. In addition, as this trigger line searches
for lower momentum tracks, the track χ2 cut
is loosened. |
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HLT2 |
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HLT2 is mainly based on three inclusive trigger lines, so-called
topological lines. These lines in principle cover all B decays with a
displaced vertex, and with at least two charged particles in the final state.
In addition HLT2 contains trigger lines which exploit the presence of muons, and a few lines which aim at exclusively
reconstructing golden B-decay modes. The topological lines are designed to
have: ·
high efficiency for any B decay with at least two charged daughters,
due to the inclusive nature of the trigger lines; ·
an excellent timing performance and background rejection, due to the
small number of trigger lines; ·
excellent data-mining properties due to their inclusive nature; ·
trigger redundancy for golden modes, for which a few more
exclusive selections will be deployed. In an inclusive trigger cuts must be avoided on quantities such as the
mass of the B candidate or how well the direction of its momentum agrees with
the direction defined by the primary and secondary vertices. To trigger
efficiently on B decays with long-lived resonances (such as D mesons), tight
cuts on the quality of the vertices must also be avoided. Instead, quantities
that preserve the inclusiveness of the trigger while
also providing large background rejection factors are used. All VELO tracks
are extrapolated to the tracking stations to have their momentum measured,
imposing a minimum (p, pT) of (5000, 500) MeV/c in the PatForward
algorithm to save CPU time. To reduce the background rate due to ghosts, all
tracks are required to have a track χ2/ndf value less than 5. To
reduce the background rate due to prompt particles, all tracks are required
to have an IP χ2 value greater than 16. Due to the
inclusive nature of the HLT2 topological lines, this does not mean that all
of the B daughters need to satisfy these criteria. The trigger is designed to
allow for the omission of one or more daughters when forming the trigger
candidate. Processing time is saved in the HLT2 topological lines by simply assigning
each input particle a kaon mass. The multibody candidates are built as
follows: two input particles are combined to form a two-body object; another
input particle is added to the two body object to form a three-body object,
and so on. An n-body candidate is thus formed by combining an (n−1)-body candidate and a particle,
not by directly combining n particles. The importance of this distinction is
in how the distance of closest approach (DOCA) cuts are made. When a 2-body
object is built, a DOCA< 0.15 mm cut is imposed for the object to either
become a 2-body candidate or to become the seed for a 3-body candidate. When
a 3-body object is made by combining a 2-body object and another particle,
another DOCA< 0.15 mm cut is imposed for the object to either become a 3-body candidate or input to a 4-body
candidate. This DOCA is of the 2-body object and the additional particle, not
the maximum DOCA of the three particles. This greatly enhances the efficiency
of the HLT2 topological lines on B → DX decays. A similar procedure is followed when making 4-body
candidates from 3-body objects and an additional particle. All n-body
candidates that pass these DOCA cuts are then filtered using a number of other
selection criteria. If a trigger candidate only contains a subset of the
daughter particles, then the mass of the candidate will be less than the mass
of the B. Thus, any cuts on the mass would need to be very loose if the
trigger is to be inclusive. Instead a cut is made on the corrected mass obtained as follows:
where p′Tmiss is
the missing momentum transverse to the direction of flight, obtained using
the primary
and secondary vertices, of the trigger candidate. The quantity mcorr would
be the mass
of the parent if a massless particle was omitted
from the trigger candidate, i.e. it is the minimum correction to the trigger-candidate mass if any
daughters are missing. The figure below demonstrates the performance of mcorr.
B candidate masses from B → K*μμ
decays: (a) HLT2 2-body topological trigger candidates; (b) HLT2 3-body
topological trigger candidates; (c) HLT2 4-body topological trigger candidates.
In each plot, both the measured mass of the n = 2, 3, 4 particles used in the
trigger candidate (shaded) and the corrected mass obtained using the equation
above the figure (unshaded) are shown. For cases where there are missing daughters, the mcorr distributions are fairly
narrow and peak near the B mass. When the trigger candidate is formed from
all of the daughters, the mcorr distributions are slightly wider
and shifted upwards by a small amount as compared with the mass
distributions, as expected. Thus, the performance of mcorr is
ideal for an inclusive trigger line. The HLT2 topological lines require 4 <
mcorr <
7 GeV/c2. The HLT2
topological lines further reduce the background retention rate by requiring the
pT of
the hardest daughter be greater than 1.5 GeV/c and
also that the sum of the daughter pT values
be greater than 4.0, 4.25 and 4.5 GeV/c for the
2-body, 3-body and 4-body lines, respectively. To further reduce the
background rate from candidates with ghost tracks the HLT2 topological lines
require that at least one daughter particle has a track χ2/ndf < 3. The trigger
candidate’s flight-distance significance value is required to be
greater than 8, and its vertex must be downstream of the closest PV. The sum
of the daughter IP χ2 values
should be greater than 100, 150 and 200 for the 2-body, 3-body and 4-body
lines, respectively. One of the larger background contributions to the HLT2
topological lines comes from prompt D mesons, which is suppressed by
requiring that all (n −
1)-body objects used
by an n-body line either have a mass greater than 2.5 GeV/c2 or
that they have an IP χ2 > 16. A complete list of the cuts
used in all three of the HLT2 topological lines is given in the table below.
In addition to the topological lines, HLT2 contains a set of lines
which exploit tracks which have been identified as muons.
Dimuon candidates are formed and, depending on
their mass, cuts are applied on the flight distance and pT of
the dimuon candidate. Single muon
candidates are accepted either requiring large pT,
or a combination of IP χ2 and
pT cuts.
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Each
HLT1 alley and HLT2 selection produces summary
information which is written to storage for the accepted events. This
summary contains the information of all tracks and vertexes which triggered the
event. It
is foreseen to reserve a significant fraction of the output bandwidth for
triggers on semi-leptonic
B-decays, hence a sample in which the trigger did not bias the decay of
the accompanying B-hadron. The summary information
is used to check if an event would have triggered, even if the B decay of
interest would not have participated in the trigger. It therefore allows
studying the trigger performance. The summary information is also sufficient
to guarantee that during the analysis the trigger source of an individual
event is known. To
assure that during off-line analysis the trigger conditions are known, the
combination of trigger algorithms with their selection parameters will be
assigned a unique key, the Trigger
Configuration Key (TCK). All trigger configurations with their associated
TCK are pre-loaded in the EFF before a fill. To change from one trigger
configuration to another one, for example to follow the decaying luminosity
in a fill, the operator will select a new TCK. This TCK is attached by the
Time and Fast Control system (TFC) to each event, and it steers the
configuration of the algorithms on the EFF and allows full traceability of
the used configuration. |
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