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Optimization of the Properties of Electromagnetic Calorimeter of CMS Detector and Possibility of Measuring the Higgs boson Spin and Parity

The test beam data of the CMS electromagnetic calorimeter are analyzed to find out how the overall performances of the electromagnetic calorimeter (energy resolution, the radiation hardness and the light monitoring system) depend on the individual properties of PbWO4 crystals and avalanche photodiode. This analysis showed that it is necessary to increase the crystal length, and ensure a proper longitudinal light collection curve. It was demonstrated that the radiation dose does not affect the scintillation mechanism, only the crystal transparency is reduced. Analysis of the test beam data showed that the precise light monitoring system can track the changes of crystal transparency at the level necessary for correction to keep the constant term at the level of the design requirements. The data obtained in the 1999 test beam with the final mechatronic design of the electromagnetic module prototype showed that the CMS design requirements are achieved. The expected features of dynamical algorithm method for e/photon reconstruction defined in a such way to take into account the spatial extension of the electromagnetic shower are demonstrated by analyzing test beam data taken in 1999. The main outcome of the dynamical algorithm method studies are: 7 crystals tower contains the 95% of the shower energy independently of the incident particle energy, the optimal energy threshold to stop the clustering process is three times the individual channel noise. Experimental studies of the electromagnetic photodetectors performed at the Technical University of Split (FESB) provided very useful information about APDs and VPTs properties. It was shown that APDs have excellent spatial uniformity of the quantum efficiency and of the gain at which APDs will operate in the CMS detector. Measurements of APDs spatial uniformity with very fine light spot showed that there are two regions near the APD edge (away form the photosensitive area) were the APD is light sensitive but with no gain. In the determination of the bias voltage for the given operating gain, one has to be aware how the APD is illuminated. When the full APD area is illuminated than the bias voltage for given gain is overestimated. The photosensitive regions near the APD edge contribute to the measured current with the same amount as the central active area when the APD operates at gain 1 while the contributions of these regions to the measured current is negligible when the APD operates at higher gains. The measurements of the spatial uniformity of the VPT photocathode quantum efficiency show that a simple visual inspection is sufficiently reliable to select VPTs with a level of non-uniformity acceptable for the CMS Collaboration. The results obtained in Split are accepted by Hamamatsu Photonics (APD manufacture) i.e. measurements at the Hamamatsu now take care about the APD illumination for the gain-bias voltage determination. The producer of VPTs accepted the visual inspection of the VPT as part of standard procedure for the VPT acceptance test by the CMS collaboration. The qualitative study of CMS potential to measure the Higgs spin and CP parity shows that CMS has a potential to establish the Higgs SCP, if its mass is in the 200-300 GeV range, through decay channel after one year of the LHC running at the nominal luminosity. The influence of the background on the signal should be investigated in more details and the analysis should be extended to include also the Higgs boson masses down to LEP limit of 114 GeV.

LHC; CMS; Higgs boson; avalanche photodiode; electromagnetic calorimeter

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