The ATLAS B physics trigger

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The ATLAS B physics trigger

• Natalia Panikashvili
• Technion Institute of Technology, Israel
• University of Michigan, USA
• On behalf of the
• B physics trigger group

Beauty 2005, 10th International Conference on
B-Physics at Hadron Machines, Assisi (Perugia),
Italy. June 20.-24. 2005
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LHC and ATLAS detector

• tracking
• Pixel
• Semiconductor Tracker (SCT)
• Transition Radiation Tracker (TRT)
• forming the Inner Detector (ID)

• calorimeters Electromagnetic Liquid Argon and
  Hadronic Tile detectors

• muons
• Monitored Drift Tubes (MDT)
• Cathode Strip Chambers (CSC)
• Resistive Plate Chambers (RPC)
• Thin Gap Chambers (TGC)

• Center of mass energy 14TeV
• Bunch crossing rate 40MHz
• Luminosity
• L 1033 cm-2 s-1 - most of B-physics will be
  measured here
• L 1034 cm-2 s-1 -
• for High pT discovery
• physics, however will be
• used also for rare B decays
• ppbar collision 109Hz
• Bunch crossing interval 25ns
• Pileup 23 ( L 1034 cm-2 s-1 )

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Events rate too high to collect all events

• 99 of bunch crossing end up with non-b events
• The selection of physics signals requires the
  identification of objects that can be isolated
  from the high particle density environment.

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The Trigger system and Region of Interest
mechanism
Reduces the high data rate by selecting
interesting events through 3 steps

• LVL1 decision made
• Muon Trigger Chambers and Calorimeter data to
  find e, g, t, jet, m candidates above
  thresholds
• Identifies Regions of Interest
• Processing time 2.5 ms
• LVL2 uses Region of Interest data
• Combines information from all detectors
• Performs fast rejection.
• Processing time 10 ms
• Output rate 2kHz
• Event Filter
• Can be seeded by LVL2 result
• potential full event access
• Processing time 1s
• Output rate 200Hz

hardware
software
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Level1 Muons

• Trigger Chambers
• Barrel region ( h lt 1.1)
• Resistive Plate Chambers (RPC)
• End-cap region ( 1lt h lt 2.7)
• Thin Gap Chambers (TGC)

• To indicate a m candidate, a hit must be
  accompanied by hits in the other detector layers,
  within the coincidence window.
• Low pT m - 3/4
• High pT m - 3/4 1/2 (2/3) for barrel (end-cap)
  
• Trigger efficiency
• 85 low pT 87 high pT

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Level1 Muon trigger

• To reduce background from decay in flight of p/K
  ? pT typical threshold in Level1 scheme for low
  luminosity (L 1033 cm-2s-1) is 6GeV

Trigger rate (kHz) expected in Muon Spectrometer
from LVL1 TDR. The low pT assume a luminosity of
1033cm-2s-1
Only 4 kHz of these are b events!
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Level-1 Calorimeter
Example of e/g trigger algorithm

• Calorimeter Trigger looking for
• e/g Jets t objects
• Using trigger towers of Hadronic
• and Electromagnetic calorimeters

• The requirement for a trigger object
• The RoI cluster local maximum
• The most energetic cluster gt ET
• Total ET in EM isolation lt EM Isolation Threshold
• Total ET in Hadron lt Hadronic isolation threshold

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Level1 Calorimeter
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ATLAS B-Physics Programme
See talk of C. Padilla Overview of ATLAS
performance for B-physics
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B-physics trigger strategy
How to be efficient for interesting B-physics
events?

• Different LVL1 trigger menus will be used at
  different luminosities
• Lower luminosity - L 1033 cm-2s-1 LVL1 1m (
  pT gt 6 8 GeV )
• Higher luminosity - L 1034 cm-2s-1 LVL1 2m (
  pT gt 6 GeV lower threshold? )

• LVL2
• LVL1 confirmation
• m tracking
• Di m trigger
• J/y, Bs,d trigger
• Event triggered by LVL1 m, the
• additional information from e/g Jet Region of
  Interest can be used
• EM RoI
• Jet RoI

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Level2 m Tracking

• mFast - Standalone m reconstruction
• Propose Confirm the LVL1 trigger with a more
  precise pT estimation within a RoI
• Global Pattern Recognition involving Trigger
  Chambers and Precision Chambers
• Track fit involving drift time measurements,
  performed for each MDT chamber
• Fast pT estimation via a look-up-table (LUT)
  with no use of time consuming fit methods

Output rates after LVL2 standalone
m reconstruction still dominated by ?/K decays
mComb - Combined m reconstruction (Using
reconstructed m and ID information) Propose
Rejection on the p/K?m
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Level2 m Tracking

• IDSCAN- track reconstruction in Inner Detector
• Input - Space Points (SP) found in Pixel and SCT
  Detectors
• Output track and SP associated with them.

Single m Rate estimation for barrel region (kHz)
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Level2 Di m , J/y?mm-, rare decay selection
We would like to be efficient for gold channels
- J/y !
LVL1 1m RoI pT (m) gt 6GeV
Efficiency for both mm- from J/y to be found in
enlarged m RoI

• LVL2
• Confirm m RoI using
• m stand alone reconstruction (mFast)
• combine m with Inner Detector track (mComb)
• Open the region (Dh x Df) around m in order to
  find J/y or Bs,d
• Find ID tracks in selected region (IdScan )
• M (mm-) gt 2.8 GeV
• Extrapolate track to MS
• Associate track with MS hits
• Create Di - m or J/y Bs,d

EF Refit ID tracks in Level2 RoI Vertexing
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J/y selection overview
LVL1 pT(m) gt 6GeV
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Level2 J/y ? ee- selection using EM RoI
LVL1 1m RoI pT (m) gt 6GeV 1 EM
RoI ET gt 2GeV
Bd? J/y (ee-) Ks(pp-)

• LVL2
• Confirm m RoI
• Confirm e RoI using
• Calorimeter (T2Calo)
• If only one e found at LVL1
• Open larger region for 2nd e
• Find ID tracks in selected region
• Mass Cuts
• Associate the track with EM Calorimeter
  information
• If both electrons found at LVL1
• confirmation at LVL2 inside small region about
  each e

e
e-
EF Refit ID tracks in Level-2 RoI Vertex
reconstruction Transverse Decay length cut
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Level-2 J/y selection
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Level2 J/y efficiency and background rate
output background rate, does not include rate
of J/y from non-b events
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Level2 Hadronic final state selection using Jet
RoI
How to be efficient for Bs ? Ds ( f ( KK- ) p )
p ?
LVL1 1m RoI pT (m) gt 6GeV 1 Jet
RoI ET gt 5GeV 2 Jet RoI per event

• LVL2
• Confirm m RoI
• Confirm Jet RoI
• Open the region (Dh x Df 1.5 1.5) around jet
  in order to find Bs decay products
• Find ID tracks in selected region (IdScan )
• M (KK-)

EF Refit ID tracks in Level-2 RoI Vertex
reconstruction Transverse Decay length cut
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Level2 f and Ds selection
Using 1008MeV lt M ( f ? K K ) lt 1033
MeV 1880MeV lt M ( Ds ? f p ) lt 2024 MeV
Study was done for events with pT ( Bs ) gt10 GeV
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Level2 rare radiative B-decays
LVL1 1m RoI pT (m) gt 6GeV 1 EM
RoI ET gt 5GeV Rate 1075Hz
Selection of Bd ? K0 g and Bs ? f g

• LVL2
• Confirm m RoI
• Confirm g RoI using
• Calorimeter
• Open the region around g
• Find ID tracks in selected region
• Reconstruct K0?Kp- or f ?K K-
• Opening angle cuts, Impact parameter cut
• Rate 23 Hz

For 30fb-1 we have 15000 Bd ? K0 g 4800 Bs
? f g
EF Refit ID tracks in Level-2 RoI Vertex
reconstruction Mass cuts Rate 0.6 Hz Bd ? K0
g 0.5 Hz Bs ? f g
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EF Rates of Rare decays in higher luminosity
LVL1 2m RoI pT (m) gt 6GeV

• LVL2
• Confirm each m RoI
• m stand alone reconstruction (mFast)
• combine m with Inner Detector track (mComb)
• Mass cut

EF Refit ID tracks in Level-2 RoI Decay vertex
reconstruction Transverse Decay length cut Lxy
gt 500mm Angular Distribution cut

• Efficiency estimation
• 70 of B? mm-
• 60 of B? K m m-
• Output rate lt 10 Hz
• bb?mm- both m pTgt6 GeV

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Estimates of Overall Efficiencies

• Make Rough Estimates of expected efficiencies for
  some example channels
• A lot of unknowns e.g. efficiency for LVL2
  combined m ID m efficiency in end-cap

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Conclusions

• Studies of the benefits of a RoI based B-trigger
  are still being investigated
• Have demonstrated a flexible strategy for
  B-physics studies from initial running to final
  luminosity
• We are looking towards first collisions in 2007
  when we hope to record B-physics data from ATLAS.