Studies Of The MPGD-based Digital Hadronic Calorimeter For The CEPC Detector

After the discovery of the Higgs boson,several Higgs factories have been proposed for precision measurements of the properities of the Higgs boson.Among them is the Circular Electron Positron Collider(CEPC)with a center of mass energy of 240 GeV that has been proposed by the Chinese particle physics community.The CEPC physics program requires precise measurement of hadronic jets by an energy resolution of～30%/(?).This places stringent demands on the performance of its detector.To meet this requirement,the particle flow algorithm(PFA)is adopted in the CEPC detector baseline conceptual design for event reconstruction.A highly granular calorimetry system composed of an electromagnetic calorimeter(ECAL)and a hadronic calorimeter(HCAL)is the key to realizing the PFA approach.There are two modes of reading out a PFA calorimeter:the digital and analog modes.A digital HCAL(DHCAL)is the baseline option for the CEPC PFA HCAL.Gaseous detectors are usually used as the sensitive detector of a DHCAL.Micro-pattern gaseous detectors(MPGD)are an important category of gaseous detectors that can be used for DHCALs due to their high gain,high rate capability,good position resolution,compact structure and availability of large-area fabrication.This thesis presents studies of a MPGD-based DHCAL for the CEPC detector including design optimization of the DHCAL and development of key technologies for fabricating large-area MPGDs for the CEPC DHCAL.Energy reconstruction algorithms are studied for the CEPC DHCAL and the DHCAL performance with different energy reconstruction algorithms are compared.Response of the CEPC DHCAL to single hadrons and hadronic jets is simulated for both the digital readout mode with a single threshold and semi-digital readout mode with multiple thresholds in the CEPC software framework.Compared with the single-threshold readout,the multi-threshold readout can recover the saturation in the high-energy response of the DHCAL.Thus,the energy linearity and energy resolution of the DHCAL are both improved with the multi-threshold readout.When applying the multi-threshold readout for the Higgs mass reconstruction in the Higgs decays to two gluons,there is almost no improvement in the Higgs mass resolution compared with the single threshold readout mode.This is due to that the final-state hadrons produced in the Higgs decay channel are mostly of low energy making the multi-threshold readout unnecessary.However,It is expected that the energy reconstruction with multi-threshold readout would result in better physics performance for processes with high energy hadronic jets.The Higgs boson mass resolution in the H→gg decay channel is also examined for different parameters of the CEPC DHCAL geometry,providing important data for the design optimization of the CEPC DHCAL.The results show dead area fraction and cell size of the gaseous detector have obvious impact on the Higgs boson mass resolution.A gaseous detector with a small dead area fraction and a small cell size is required to fullfil the performance requirements for the CEPC DHCAL.To reduce cost of the system,the gaseous detector with a compact structure is necessary.GEM is a mature MPGD that has been used in many nuclear and particle physics experiments.A key issue to be solved for its application to DHCALs is to realize a very compact structure.To this end,a 30 cm×30 cm double GEM prototype with a 3 mm drift gap,1 mm transfer gap and 1 mm induction gap was built with the self-stretching technique.The gain of the detector could reach 104 while operating with Ar-iC4H10(95%-5%)mixture gas,and its detection efficiency for MIPs(Minimum Ionization Particles)could exceed 95%.Performance of the detector prototype using a MICROROCbased electronics system was studied with cosmic-rays.Results of the studies show a hit multiplicity of about 1.2@95%detection efficiency.However,a GEM detector built with the self-streching technique requires extra mechanical support at the detector edges,leading to a large dead area in a DHCAL.So it is necessary to develop a large area and compact MPGD detector with a very small dead area.MPGDs of the WELL type have a simple and compact structure,and can obtain a high gain and stable working conditions when using a resistive layer.This makes the resistive WELL MPDG(RWELL detector)suitable for the CEPC DHCAL application.In addition,the core component of the RWELL detector-RWELL foil can be produced with a regular PCB process.This makes possible large-scale application of the RWELL detector.The key technologies for fabricating a large area RWELL detector were developed,and its production process was optimized including bonding of the RWELL foil and the readout PCB,a fast grounding structure,a separation structure of the readout PCB.After several R&D iterations,a RWELL detector prototype with an sensitive area of up to 50 cmx 100 cm was successfully developed.The dead area in it detection area was only 1%,and the total thickness was 5 mm.The RWELL prototype was tested with both Xrays and cosmic-rays.Results of the tests show that the gain uniformity of the detector is 14.0%@5200 gain,the detection efficiency is 95.9%at this gain,and the rate capability of the detector is greater than 100 kHz/cm2 of 8 keV X-rays.The performance of the detector can satisfy the requirements for the CEPC DHCAL.This work presented in this thesis provides important input to the design and optimization of the CEPC digital hadron calorimeter.The large area and compact RWELL detector technology developed in this work provides a promising solution for the sensitive detector of the CEPC digital hadron calorimeter.