The ATLAS B-trigger exploring a new strategy for J/(ee)
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A brief introduction to B-physics on ATLAS
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The ATLAS B-trigger – an overview
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A new trigger for Bd J/(e+e-)Ks
Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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B-physics on ATLAS
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LHC is expected to run at a 'low' luminosity during its first years,
~2 x 1033 cm-2 s-1  good opportunity to measure CP-violation
in the B-system with ATLAS before the higher design luminosity
is reached. A high lumi degrades the B-layers that are necessary
for B-physics.
The asymmetry in Bd J/ K0s can give us a clean measurement
of sin(2) in the unitarity triangle. It is expected that the
reconstructed background will be relatively low.
J/ (mm) decay is easy to reconstruct. J/(ee) is harder due to
bigger background and bremsstrahlung losses. Since rate of CPV
is the same , it is important to measure both decay modes, as a
cross check (to identify systematic errors in measurements, etc)
and to increase the statistics.
Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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The ATLAS trigger
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At 40 Mhz and low lumi,
~ 5 events / bunch crossing.
O(108 Hz)
Event size ~ 1 MB
Maximum output to tape ~
100 MB/s, O(102 Hz)
Trigger reduction factor of
106 needed.
Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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Level 1
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Level 1 (LVL1) – hardware trigger
Maximum output rate of 75 kHz.
Uses reduced granularity measurements from
calorimeters and muon stations to form decision.
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Output consists of Regions of Interest (RoI) to LVL2.
Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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High Level Trigger
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LVL2 – software trigger with output rate of ~ 1 kHz
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Uses software algorithms and full granularity in calorimeter
to validate LVL1 objects like muons and EM clusters.
Full-scan of Inner Detector (ID) for tracks.
OR
Use RoIs from LVL1. Scan in limited volume of ID.
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Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
Event Filter – output rate
of ~ 100 Hz.
Uses offline algorithms to
refine LVL2 selection.
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The trigger costs money...
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Previous trigger studies have already covered Bd →Je+e-)KS
with promising results.
But... due to recent developments (increased start-up lumi, less
money to DAQ, possibilites of reduced detector at start-up), a
new trigger strategy is needed.
Previous trigger strategy for Bd →Je+e-)KS :
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LVL1: m with pT > 6 GeV (MU6), |h|<2.4
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LVL2: perform a full-scan of the Inner Detector to find interesting
tracks.
Gives good efficency but requires a lot of computing power,
which costs money.
Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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A new ATLAS B-trigger strategy
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A RoI-guided trigger is suggested instead.
Different strategies are being considered,
one of them for Bd →Je+e-)KS is:
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LVL1: require MU6 (or MU8) + at least 1 EM cluster (RoI)
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LVL2: Only scan for tracks in regions around the RoIs.
 only ~10 % of the ID has to be searched =
less computing power needed
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If only one RoI is found in LVL1, an extended scan around
the RoI is needed to locate both electrons.
Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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Can this be efficient?
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In the new strategy, at least one of
the electrons in a signal event has to
be detected by calorimeter at LVL1.
Mean: 3.0656
Bd→J/(ee)KS
On the other hand, if the multiplicity
in background events is too high, a
lot of of resources are needed
anyway.
A look at the signal and bg RoImultiplicities give promising results.
bb  μX
Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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Know your enemy background events in LVL2
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At LVL2 we will (ideally) only deal with events of type
b bbar → mX. Events that pass LVL1 will satisfy a pT-cut and
an h-cut.
sbg= s ( b bbar → mX | pT(m) > 6 GeV, |h| < 2.4 ) ≃ 2.3 mb
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BR(b → J/ ) ≃ 0.01
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BR(J/ → ee) ≃ 0.06
 s ( b → J/ (ee) X) ≃ 2.3 mb · 0.01 · 0.06 = sbg · 6·10 -4
Of the events that pass LVL1, only a very tiny fraction (6·10 -4)
are events we're looking for, so the signal trigger efficiency has
to be maximised while the background retention has to be
minimised.
Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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How much can background is tolerated?
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The output of LVL2 is ~1 kHz, this is to be shared between all
decay channels that ATLAS is looking for.
If Bd →Je+e-)KS is allowed to occupy ~1% of this bandwith, i.e.
10 Hz, we can estimate the upper limit of the background retention.
With L = 2 x 1033 cm-2 s-1 we find that the LVL1 rate for b-events
will be 4.6 kHz. To get a LVL2 output of ~10 Hz we need to
suppress the background retention to the order of
eLVL2,bg ≃ 0.1 %
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Is this possible without 'killing' the signal efficiency?
Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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What can we do at LVL2?
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The main theme of the LVL2 trigger:
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Scan for tracks in a volume around the EM RoIs (e.g.
ΔhΔ=0.20.2)
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Select tracks that match certain criteria (cuts)
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Calculate invariant mass for combinations of tracks from
different RoIs.
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If the invariant mass is sufficiently close to the nominal mass
of particle we want to trigger on, the event is chosen to pass to
the next trigger level, the Event Filter.
In this trigger, the electrons from J are used for the
invariant mass plot. (Ks is reconstructed at a later stage)
Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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Trigger tools
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To reduce the number of tracks, one studies different
properties of the real J-electrons. This results in a list
of cuts applied to each event.
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PT-cut on RoIs and tracks
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Cut on angle between different tracks, i.e. , Δh, Δ or
ΔR=(Δh2+Δ2)
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Cut on ratio Ecal/Ptrack (me << pe Ep)
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Demand different charge for the tracks
Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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An example of E/p-cut
E/p  1 does not only indicate that
we are dealing with a light particle,
it also indicates that the a correct
match has been made between the
track and the EM-cluster.
● By for example applying some cuts
on E/p we see that a great deal of the
background tracks would be sorted
out, while keeping most of the
signal.
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Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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Correlations between J/ -electrons
J/ electrons are emitted in a such a
way that their mutual 'angular'
distance stays between 0 < ΔR < 1.5
● Combinations of background tracks
span the entire spectrum.
● By applying an appropriate cut,
there is a lot to gain:
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Combinations between
background tracks are
rejected
● Choosing ΔR > 0 we ensure
ourselves that no
combinations of the same
track occur.
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Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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Electron identification
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The transition radiation tracker (TRT)
is able to provide e/π-separation. TRTtracking is time and resource
consuming. Unclear if this can be used
in LVL2.
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Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
Electron identification can
be done instead by looking
at different shower shape
variables in the ECAL.
Harder to separate e/π
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Results so far
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Problems with ATLAS software have not yet permitted a proper
trigger study. Only offline algorithms (for tracking, calorimeter
etc) have been available so far.
Preliminary studies based on offline algorithms have been done,
indicating that this particular strategy gives quite a low efficiency
but also a low background retention:
eLVL2, signal ~ 25 %
eLVL2, background ~ 0.1 %
The low efficiency primarily comes from scanning for tracks in a
too small part of the ID. This is a problem, but it can hopefully be
solved anyway.
Aras Papadelis, Lund University
8th Nordic LHC Physics Workshop Nov. 27-29 2003, Lund
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