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Question tuning the precision of field parameters 

Forum: Fields: Magnetic and Otherwise
Date: 03 Apr, 2017
From: Renee Fatemi <Renee Fatemi>

Dear GEANT4 experts,

I am trying to calculate the appropriate integration parameters, specifically SetMinimumEpsilonStep and SetMaximumEpsilonStep, for a simulation framework designed to track muon position, momentum and spin in B+E fields.

I read in the manual that epsilonMin/Max are defined as the min/max relative errors on position (and momentum). However it is not clear to me if I should calculate SetMinimumEpsilonStep as:

A) the minimum allowed relative error in the true path length.

B) the minimum allowed error in the determination of the final position divided by the true path length.

For example, if a muon travels 1 meter in path length along the x axis, from (x0,y0,z0) to (x1=x0+1m,y1,z1), and the calculated final position is not allowed to deviate from the true final position by more than 0.02m ( for example y1 = y0+0.02) then the calculation of the parameters would differ by two orders of magnitude depending on the method:

A) (1 - sqrt(1^2 + 0.02^2))/1 ~ 0.0002

B) 0.2/1 = 0.02

Is the correct method A or B (or neither!) Thank you for any guidance you can give on setting these parameters.

Renee Fatemi

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