Energy loss correction for a crystal calorimeter

1. Energy loss correction for a crystal calorimeter He Miao Institute of High Energy Physics Beijing, P.R.China

2. Introduction to BESIII • The Beijing Spectrometer (BES) III is a multi-purpose detector to be operated at the Beijing electron-positron collider (BEPC) II currently under a major upgrade for physics at tau-charm energy region. • The Electromagnetic Calorimeter (EMC), made of CsI(Tl) crystals, is one of the most important component of the BESIII detector. Its primary function is to measure photons with a high detection efficiency, good energy and position resolution.

3. Layout of BESIII • Two sub-detectors in front of EMC: DC and TOF. • A Be beam pipe around the interaction point. Beam pipe z DC: Drift ChamberTOF: Time Of Flight Counter

4. Material effect to the EMC performances • There are totally 30% radiation length in front of EMC. TOF takes up the most proportion. • The total energy loss in TOF spreads up to several hundreds MeV. It will deteriorate the energy resolution and detection efficiency. Energy loss in TOF for 1GeV photons

5. TOF calibration • In order to combine the energy measurement of TOF and EMC, the absolute energy calibration for TOF is required. • Calibrate the z-coordinate of hit position. • Muon is a good candidate for the calibration • its energy loss is a well defined quantity = dE/dx times track length • the hit position can be determined by the drift chamber from a track extrapolation • For photons, the energy deposit and hit position can be determined by the time and charge channel with the calibration constants.

6. Calculate track length in TOF track length • A charged track reconstructed by DC is extrapolated to the surface of TOF. • Based on the TOF geometry, track length and hit position can be calculated. TOF ext position z position DC

7. TDC to Hit Position • ΔT means the difference of measured TDC at each end of the scintillator. • Fitted with a linear function through the origin.

8. ADC to Energy • Calibration of each end of ADC. • Fitted with exponential functions. • Take the weighted average as the real energy. West ADC East ADC

9. TOF electronics linearity • Dynamic range: 0.2V-5V(1.8MeV-45MeV at z=0) • Fitted curve: second order polynomial. • Fitted error: <2% TOF electronics response

10. Add TOF electronics response to MC data • Reconstruct TOF energy with calibration constants • Check the energy linearity and resolution Energy resolution Energy linearity

11. EMC i+1 i i-1 TOF j+1 j j-1 IP EMC and TOF Match - φ • According to the position of EMC shower, find the corresponding scintillator of each layer, called tof2x1. • The neighbors are also used, called tof2x3. • Tof2x3 contains more energy, but maybe includes more noise. • Use which lies on the noise level. • Matching efficiency: • Tof2x3 gives better efficiency. • The both decrease at low energies because the position resolution become worse as photon energy goes down.

12. EMC and TOF Match - θ • Compare the θ angle of TOF with EMC shower. • The matching window is |Δθ|<3σ. • Depends on the photon energy. cut=3σ thetaTOF – thetaEMC • Matching efficiency: • Uniform within the error. • Better than 96%

13. 0.1GeV photon 1GeV photon • At low energies, the energy deposit in TOF is relatively large and a tail can be seen in the EMC reconstructed energy. • At high energies, the long tail is suppressed and the TOF correction can narrow the width.

14. Performances • Energy resolution • Good photons selection efficiency Emc5x5: 15.1MeVEmc5x5+Tof2x1: 13.2MeV Emc5x5: 63.7MeVEmc5x5+Tof2x1: 52.9MeV Good photons selection efficiency definition: Invariant mass spectrum

15. Summary • Material effect of inner-detectors to the performances of the BESIII Electromagnetic Calorimeter (EMC) is investigated. • Calibration of TOF energy and hit position has been developed, as well as a matching algorithm of TOF and EMC. • Based on a Monte Carlo simulation, the energy resolution and detection efficiency for photons are improved with TOF correction. Thank you!