Transcript A Brief Review of the Trigger System

Efficiency of LHCb Trigger
Systems
Elizabeth Leicht
Supervisor: Frederic Teubert
Experiment: LHCb
The LHCb and CP Violation
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CP violation was originally in neutral kaon decays
in 1964.
 However, in the B-meson system there are many
more decay modes available than for neutral kaon
decay.
 When complete the LHC will be the largest source
of B mesons of all accelerators, therefore, the
LHCb detector is designed to exploit this large
source of b-hadrons.
A Brief Review of the Trigger System
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A trigger system is the gatekeeper of the experiment.
It determines which data will be saved for later
analysis and which will be thrown away.
For the LHCb experiment, only the first two trigger
levels have been implemented at this time.
L0 is a fast, but crude trigger that is implemented
using hardware and data from only a few detectors.
L1 the first software trigger and is a series of
algorithms. It utilizes more data than L0 and is of
course slower.
Preselection Criteria
Primary Vertex
Track
Impact parameter with
respect to the primary vertex
The
impact
parameter
(d0)with respect
to the primary
vertex must
have a value of
less than 5 mm.
The associated
error of the
impact
parameter
(d)must be be
less than 500
m.
The
normalized
impact
parameter (d0/
d ) must be
greater than 1.
Reconstructing the Secondary Vertex
A loop
Positively Charged
Track
Primary Vertex
Reconstructed
Secondary
Vertex
Negatively
Charged Track
is made
over all of the
tracks passing the
preselection
criteria, to separate
the tracks by
charge.
The next step is
to run through all
possible
combinations of
oppositely charged
tracks and
reconstruct
secondary vertices.
Data Cuts Made from Reconstructed
Secondary Vertex
A vertex
separation cut. The distance between the primary
and the reconstructed secondary vertices must be greater
than 0.5 mm.
A 2 cut. The value of 2 must be less then 5.
A Little More About 2
Track 1
Not exactly an
intersection in three
dimensional space.
Track 2
Closest approach
between the two
tracks.
More Data Cuts Made from
Reconstructed Secondary Vertex
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An invariant mass cut. The combined invariant mass of the two
candidate tracks must fall between 5.0 GeV. and 5.5 GeV.
The significance of impact parameter, d0/ d, for both candidate pions
must be greater than 3.
 The momentum vector of the reconstructed B must be consistent with
the flight path from the primary to the secondary vertex, such that
cosB> 0.95 , where B> is the opening angle between these two vectors.
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Secondary Vertex
Primary Vertex
B
Last of the Data Cuts for the
Reconstructed Secondary Vertex
The transverse momentum, of both pions must
exceed 1 GeV/c and at least one pion must have a
transverse momentum of greater than 3.5 GeV/c.
 For the reconstructed B, the transverse momentum
must exceed 3 GeV/c.
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Transverse Momentum:
PT = P•sin
where  is the angle between the P vector
and a unit vector in the z direction.
Some Results
Plot of the Combined Invariant Mass Values
Number of
Events
Selected
Combined Invariant Mass (MeV/c2)
Plot of the Combined Invariant Mass Values
with L0 and L1 Triggers
Number of
Events
Selected
Combined Invariant Mass (MeV/c2)
L1 Trigger Efficiency vs. L1 Variable Cut
L1 Cut
0.6
0.7
0.8
0.9
1
Efficiency
95.4%
95.4%
95.4%
92.7%
90.8%
A Summery of Results
Cut Performed
Vertex seperation greater than
0.5 mm, invariant mass window of
5.0-5.5 GeV, and chi squared
less than 5
Significance of impact parameter
for both pions greater than 3.
Momentum vector of the
reconstructed B to be consistent
with the flight path from the
primary to the secondary vertex.
Transverse momentum of both
pions to exceed 1 GeV/c &
Transversmomentum of at least
one pion to exceed 3.5 GeV/c.
Transverse momentum of the
reconstructed B, greater than
3 GeV/c
Percentage with Respect to 5000 Events Run
L1 Trigger
Offline
(without
L0 & L1
Selection L0 Trigger
L0)
Triggers
Percentage with Respect
Percentage with Respect to Events to Events Passing Offline
Passing Offline Selection
Selection & L1
L1 Trigger
(without
L0 & L1
L0 Trigger
L0)
Triggers
L1 Trigger
32.1%
26.8%
32.0%
10.9%
35.4%
59.1%
20.5%
58.0%
10.9%
26.8%
32.0%
10.9%
31.1%
93.2%
29.8%
95.9%
10.7%
26.8%
32.0%
10.9%
30.9%
93.4%
29.7%
96.0%
7.5%
26.8%
32.0%
10.9%
35.1%
94.6%
34.1%
97.1%
5.8%
26.8%
32.0%
10.9%
35.4%
95.5%
33.6%
96.1%
Conclusions
From the analysis I’ve done it appears that
the L1 trigger is selecting the desired events
with high efficiency.
 The issue is the L0 trigger.
 One culprit for the low L0 efficiency may be
the impact parameter.
 It may be possible to make the cut of the L1
trigger harder, therefore, suppressing more
background, while retaining high efficiency.
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Thanks To:
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The University of Michigan
Dr. Krisch, Dr. Neil, & Dr. Dershem
Frederic Teubert & Thomas Schietinger
The NSF
CERN
My fellow REU students.