The Circular Electron Positron Collider (CEPC)  is one of the future experiments aiming to study the Higgs boson’s properties precisely. For this purpose, excellent track reconstruction and particle identification (PID) performance are required. Such as the tracking efficiency should be around 100%, the momentum resolution should be less than 0.1%, and the Kaon and pion should have 2 sigma separation power for momentum below 20 GeV. To fulfill these requirements, a tracking system combining a silicon tracker and a drift chamber is proposed for the CEPC experiment. Here the drift chamber is not only used for improving track reconstruction performance but also used for excellent PID with cluster counting method. To evaluate the performance of this design carefully, the simulation should be close to the real situation as much as possible.
This contribution presents a refined drift chamber simulation by combining the Geant4 and the Garfield++  simulation in the CEPCSW . The Geant4 is used for traditional particle transportation and interaction simulation in the detector. The Garfield++ aims for detailed simulation in each drift chamber cell including precise ionization simulation, pulse simulation as well as waveform simulation. Due to the extremely time-consuming avalanche process simulation in Garfield++. It is not feasible to simulate the waveform of the whole drift chamber using Garfield++. To solve the barrier, a fast pulse simulation method based on the Normalizing Flow technology  is developed which can simulate the pulse’s time and amplitude according to the local position of an ionized electron. The result shows the fast simulation has very high fidelity and more than 2 magnitude speed up can be achieved. To further validate this method, simulating drift time is performed using real data from the BESIII experiment . It shows the simulated drift time distribution is consistent with real data. Last but not least, the track reconstruction performance is shown by using this more realistic drift chamber simulation.
 CEPC Study Group Collaboration, M. Dong et al., CEPC Conceptual Design Report: Volume 2 - Physics & Detector, arXiv:1811.10545
 Jean-Fran¸cois Caron, et al., Improved Particle Identification Using Cluster Counting in a Full-Length Drift Chamber Prototype, 10.1016/j.nima.2013.09.028
 Garfield++ Team, https://gitlab.cern.ch/garfield/garfieldpp, 2021. GitLab repository (2021).
 CEPCSW Team, CEPCSW prototype repository, https://github.com/cepc/CEPCSW, 2021. GitHub repository (2021)
 I. Kobyzev, S. J. D. Prince and M. A. Brubaker, “Normalizing Flows: An Introduction and Review of Current Methods,” in IEEE Transactions on Pattern Analysis and Machine Intelligence, vol. 43, no. 11, pp. 3964-3979, 1 Nov. 2021, doi: 10.1109/TPAMI.2020.2992934.
 BESIII Collaboration, Design and Construction of the BESIII Detector. Nucl.Instrum.Meth.A614:345-399,2010
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