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%.