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December, 16 20081 Calibration of electromagnetic calorimeter of Hall A DVCS experiment Eric FUCHEY Ph.D Student @ Laboratoire de Physique Corpusculaire UMR 6533 CNRS/IN2P3 Université Blaise Pascal Clermont-Ferrand
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December, 16 20082 Outline 1- Motivations of calibration 2- Real data calibration –Method –Results 3- Simulated data calibration –Method –Results 4- Conclusion
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December, 16 20083 Outline 1- Motivations of calibration 2- Real data calibration –Method –Results 3- Simulated data calibration –Method –Results 4- Conclusion
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December, 16 20084 Experimental layout Calibration for ep->ep 0 Incident beam : E = 5.75 GeV during experiment –Scattered electron measured by HRS –2 photons issued from 0 measured by 132 PbF 2 blocks calorimeter –Recoil proton detected by proton array -> not used
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December, 16 20085 Looking at real and simulated 0 data sets We measure We can reconstruct for each event a missing mass: with and We can either reconstruct for one block a missing mass distribution for all events with one photon hitting this block.
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December, 16 20086 Mp2Mp2 Reconstructed missing mass distribution in each block depends on block position in calorimeter. Effects assumed to come only from calorimeter. => necessity to calibrate calorimeter to correct these effects.
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December, 16 20087 For analysis, we need to fit 0 data set with 0 simulated set to extract cross section unconstistency of resolutions for real and simulated. => necessity to calibrate and to smear simulated.
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December, 16 20088 Outline 1- Motivations of calibration 2- Real data calibration –Method –Results 3- Simulated data calibration –Method –Results 4- Conclusion
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December, 16 20089 Method of calibration (real) Goal of calibration: to find for each block of the calorimeter a coefficient in order to reconstruct in each block a missing mass squared in accordance with M p 2. Coefficient of the form: for block# ( = 0,…,89)
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December, 16 200810 Method of calibration (real) For an event i, a photon is in block #a, the other is in block #b. Correcting each photon energy: Recomputing M X 2 with these corrected energies, and filling #a and #b blocks distributions.
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December, 16 200811 Method of calibration (real) Fit distributions by a gaussian -> Mean of the gaussian => New missing mass squared value. New calibration coefficient Correct 2 photons to reconstruct missing mass -> calibration coefficients for each block are correlated. necessity to calibrate by steps. necessity to estimate calibration quality for each step.
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December, 16 200812 Corrections Before filling distributions, we apply some corrections: –By effects of resolution, missing mass squared and invariant mass ( ) are correlated –-> possibility to improve resolution. with R = 13 GeV –Additional cuts: 0.4 GeV 2 < M X 2 < 1.4 GeV 2 ; 0.105 GeV < m < 0.165 GeV. k’ = pHRS +/- 4.5% ; r-function > 0.005 Each photon is in a calibrated block, and E > 1.0 GeV -6.0 cm < z vtx < 7.5 cm
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December, 16 200813 Quality calibration control Once iteration completed : –Plot reconstructed missing mass squared value for each block over all blocks –Fit by a constant –Check for deviation from reference value, and dispersion (RMS).
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December, 16 200814 Results (real) Quick convergence from iteration 0 to 5-6 After, averaged M X 2 oscillates, RMS is stable. => Converged
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December, 16 200815 Results (real) Ex: Iteration #11 Dispersion is low, Calibration has converged Pion mass deviation less than 1 MeV
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December, 16 200816 Outline 1- Motivations of calibration –Experimental layout 2- Real data calibration –Method –Results 3- Simulated data calibration –Method –Results 4- Conclusion
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December, 16 200817 Method of calibration (simulated) Same principle, except we try to match simulated with data: –Searching for a calibration coefficient: –Searching to bring each block simulated resolution ( of gaussian fit) to real block resolution. => also need for each block a « smearing » coefficient
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December, 16 200818 Method of calibration (simulated) For an event i, a photon is in block #a, the other is in block #b. Correcting each photon energy: Smearing each photon energy: picking a random variable in a gaussian distribution. : corrected energy, : Recomputing M X 2 with these corrected energies, and filling #a and #b blocks distributions.
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December, 16 200819 Quality calibration control Once iteration completed : –Plot M X 2 | simu - M X 2 | real for each block over all blocks and (M X 2 | simu ) - (M X 2 | real ) for each block over all blocks –Fit each plot by a constant
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December, 16 200820 Results (simulated) Averaged M X 2 converges until iteration 15, then stable RMS does not stop decreasing: <0.001 since iteration #30
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December, 16 200821 Results (simulated) Since ~iteration #20, seems to be converged. Check out for resolution to conclude
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December, 16 200822 Results (simulated) Quick convergence averaged (M X 2 ) of from iteration 0 to 6-7, stable after. averaged (M X 2 ) RMS stable since Iteration #15
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December, 16 200823 Results (simulated) Invariant mass deviation due to calibration is still less than 1 MeV Since ~iteration #20, dispersion is low, calibration has converged. Only advantage to go further is to improve M X 2 dispersion.
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December, 16 200824 Outline 1- Motivations of calibration –Experimental layout –Consequences on measurements 2- Real data calibration –Method –Results 3- Simulated data calibration –Method –Results 4- Conclusion
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December, 16 200825 Conclusion Our goal was to calibrate the calorimeter with our set of Kin3 0 data and our set of Kin3 0 simulated to be able to make consistent fit for analysis. We have reached regime of stability for both real calibration and simulated calibration iteration processes => Calibration has been succesfully completed.
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