Dear all,
May I ask you about particle tracking for the beam background study at the SuperKEKB:
We define beam-pipe aperture for each ring (LER/HER) as the following:

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(* Set aperture *)
(* global aperture setting *)
Scan[(LINE["AX",#]=0.045;LINE["AY",#]=0.045;LINE["DX",#]=Twiss["DX",#];LINE["DY",#]=Twiss["DY",#];)&,LINE["NAME","AP.*"]];
(* ZVQLC3RP: 26.23m upstream, ZVQLC3LP: 27.50m downstram from IP *)
Scan[(LINE["AX",#]=0.040;LINE["AY",#]=0.040;LINE["DX",#]=Twiss["DX",#];LINE["DY",#]=Twiss["DY",#];)&,
Select[LINE["NAME","AP.*"],(LINE["POSITION","ZVQLC3RP"]<LINE["POSITION",#]<LINE["POSITION","ZVQLC3LP"])&]];
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Here we use twiss parameters DX/DY for the center of the aperture ellipse. Do you know why?
As I understand, it must be 0. Because we need to apply aperture with respect to the design orbit which seats at DX=0 DY=0. Isn’t it? As far as I know Twiss[“DX”] returns an offset between design and closed orbits.
Thanks in advance.

Best regards,
Andrii