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Studies of Electroweak InteractionsStudies of Electroweak Interactionsand Searches for New Physicsand Searches for New Physics
Using Photonic Events with Missing Energy Using Photonic Events with Missing Energy at the Large Electron-Positron Colliderat the Large Electron-Positron Collider
Marat GataullinMarat GataullinNovember 22, 2005November 22, 2005
BGO Electromagnetic CalorimeterBGO Electromagnetic Calorimeter
Barrel: 7680 crystals,Barrel: 7680 crystals,
2x24 rings of 160 crystals.2x24 rings of 160 crystals.
Endcaps: 1527 crystals,Endcaps: 1527 crystals,
17 rings of 46-128 crystals each.17 rings of 46-128 crystals each.
BGO Energy ReconstructionBGO Energy Reconstruction
Narrow profile -- 94% in SNarrow profile -- 94% in S9 9
To correct for leakage define:To correct for leakage define:
BGO Performance at the Beginning of LEP2BGO Performance at the Beginning of LEP2
19971997
Data 4 events, MC 0.5Data 4 events, MC 0.5
Beam tests in 1987-88:Beam tests in 1987-88:
RFQ Calibration SystemRFQ Calibration System
1. 30 keV H1. 30 keV H-- ion source (output current 7.5 mA) ion source (output current 7.5 mA)
2. 1.85 MeV RFQ accelerator (transmission 75%)2. 1.85 MeV RFQ accelerator (transmission 75%)
3. Steering and focusing magnets3. Steering and focusing magnets
4. Nitrogen-cell beam neutralizer (efficiency 60%)4. Nitrogen-cell beam neutralizer (efficiency 60%)
Output HOutput H00 flux up to 4 mA flux up to 4 mA
RFQ SystemRFQ System
RFQ ConceptRFQ Concept
Radiative capture of protonsRadiative capture of protons
produces a flux of produces a flux of 17.6 MeV photons17.6 MeV photons
Simultaneous calibration of theSimultaneous calibration of the
entire BGO calorimeterentire BGO calorimeter
RFQ runs performed twice each year, no major hardwareRFQ runs performed twice each year, no major hardware
problems. BGO calorimeter in standby mode. Average problems. BGO calorimeter in standby mode. Average
trigger rate of 80 Hz, giving 20 million triggers after 4 trigger rate of 80 Hz, giving 20 million triggers after 4 days.days.
RFQ OccupancyRFQ Occupancy
RFQ CalibrationRFQ Calibration
HH+ defined as a point half-way belowHH+ defined as a point half-way below
and to the right of the calibration maximumand to the right of the calibration maximum
RFQ Calibration ConstantsRFQ Calibration Constants
RFQ-Only Calibration with 45 GeV Bhabha electrons RFQ-Only Calibration with 45 GeV Bhabha electrons
However, statistical precision was found to be about 1%.However, statistical precision was found to be about 1%.
Significant systematic effects from variations in the photonSignificant systematic effects from variations in the photon
incident angle and energy extrapolation.incident angle and energy extrapolation.
Solution: use Bhabha electrons to set the absolute scale.Solution: use Bhabha electrons to set the absolute scale.
Bhabha Calibration SampleBhabha Calibration Sample
Select back-to-back events. Shower-shape cuts also applied.Select back-to-back events. Shower-shape cuts also applied.
LEP calibration runs 15-20 days/year and L=4pb/year.LEP calibration runs 15-20 days/year and L=4pb/year.
50k showers in the barrel and 850k showers in the endcaps.50k showers in the barrel and 850k showers in the endcaps.
RFQ+Bhabha Calibration AlgorithmRFQ+Bhabha Calibration Algorithm
Starting set of constants provided by the RFQ calibration.Starting set of constants provided by the RFQ calibration. Typically only 4-5 iterations required to converge.Typically only 4-5 iterations required to converge. Robust, now used in CMS calibration studies.Robust, now used in CMS calibration studies. Easy to implement corrections for dead crystals.Easy to implement corrections for dead crystals.
Calibration ResultsCalibration Results
Also checked with Also checked with ππ and and ηη two-photon decays two-photon decays
Dead CrystalsDead Crystals
About 1.5% of all BGO channels were malfunctioningAbout 1.5% of all BGO channels were malfunctioning 12% of selected showers had a dead crystal in 3x3 matrix12% of selected showers had a dead crystal in 3x3 matrix 5% were on the edges between the barrel and endcaps5% were on the edges between the barrel and endcaps Energy loss taken into account both during the calibrationEnergy loss taken into account both during the calibration
and physics analysisand physics analysis
Dead CrystalsDead Crystals
BGO AgingBGO Aging
Checked using 45 GeV Bhabhas, source unknown. Checked using 45 GeV Bhabhas, source unknown.
Loss of about -0.5% per year.Loss of about -0.5% per year.
BGO Non-LinearityBGO Non-Linearity
Checked using Bhabhas with energies between 45 and 103 GeV.Checked using Bhabhas with energies between 45 and 103 GeV.
Going from 45 and 103 GeV increased the BGO response by about 0.4%.Going from 45 and 103 GeV increased the BGO response by about 0.4%.
Overall conclusion: the energy scale was known to better than 0.3%.Overall conclusion: the energy scale was known to better than 0.3%.