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Fix: adjust magnet field simulation granularity to avoid momentum reconstruction bias;

Li Zhihao requested to merge lizhihao/CEPCSW:fix/precision-loss into master

When using Geant4 for simulations, the granularity of the magnetic field settings can affect the simulation results. For example, with Geant4's default parameters, placing a 2 mm thin layer at a location 10 mm away from the origin can result in abnormal PostStep coordinates. Different experiments have chosen granularity settings based on their specific requirements.

  • Mokka: A strategy is implemented to adjust granularity to finer values for true values or inner layers, while using default parameters for outer layers. The specific settings are:

    • DeltaIntersection = 3e-5 mm
    • DeltaOneStep = 3e-4 mm Mokka_parameter

  • ATLAS: The standard is based on ensuring a deviation of no more than 1.e-5 mm at a distance of 3 m. The chosen parameters are:

    • DeltaIntersection = 0.00001 mm
    • DeltaOneStep = 0.0001 mm
    • MaxEpsilon = 0.001
    • MinEpsilon = 0.00001 ATLAS_parameter

  • CMS: A segmented approach is used, with three sets of parameters tailored to different regions. CMS_parameter

  • Geant4 Default Parameters:

    • EpsilonMinDefault = 5.0e-5
    • EpsilonMaxDefault = 1.0e-3
    • Default_Delta_One_Step_Value = 0.01 mm
    • Default_Delta_Intersection_Val = 0.001 mm

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