transform reflect

[Peter Kammel]

Dear Tom,

I asked ChatGPT about backtracking through a beamline in G4BL. I want to measure

the phase space with some instrumental uncertainty and then track is back to upstream elements.

ChatGPT gave this handy command below. Does this exist or is it freely invented by

the creative AI?

Thanks

Peter

explain transform reflect in G4BL

ChatGPT said:

Understanding transform reflect in G4beamline (G4BL)

The transform reflect command in G4beamline (G4BL) is used to mirror elements in a beamline across a given axis. It effectively inverts the positions, orientations, and momenta of all affected components while maintaining proper physics behavior.

---

Syntax of transform reflect

transform reflect <axis>

• <axis> can be x, y, or z, representing the axis of reflection.
• This applies to all subsequent elements in the beamline.
• The entire beamline (or selected components) is mirrored across the chosen axis.

How transform reflect Works

When applied, this command:

• Flips the sign of the chosen coordinate (e.g., if reflecting in Z, an element at z=1000mm moves to z=−1000mm).
• Adjusts momenta to match the new geometry.
• Reverses magnetic and electric field orientations where necessary.
• Does NOT reverse particle charge or spin automatically—fields must be adjusted accordingly.

[Tom Roberts]

This is complete and utter nonsense -- there is no 'transform' command in G4beamline, and the "command" that ChatGPT made up would not do what you want. I have no idea where ChatGPT got information to support its made-up "command", and there is nothing like that in G4beamline.

Unfortunately, there is no direct way to "backtrack" in G4beamline. But in an analysis program you could collect trackIDs at a downstream element and then determine how they are distributed in an upstream element.

Tom Roberts

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[Peter Kammel]

Dear Tom,

Thanks, I expected this response. Still I wanted to post it as a warning to the community about AI guidance.

As regards the technical part, the issue is that the downstream phase space gets modified by the instrumental

uncertainty, and the one-to-one correspondence to the initial phase space is lost.

Best

Peter

--

Peter Kammel / pka...@uw.edu
University of Washington, Department of Physics
Center for Experimental Nuclear Physics and Astrophysics
Box 351560
Seattle, WA 98195-1560 
Tel (206) 221-0770 CENPA
Tel (206) 685-2401 Physics

[Volodymyr Rodin]

Dear Peter, Tom,

But this idea in principle can work, this suggestion is reasonable.

You need to invert the relevant momentum component of the obtained distribution and launch the beam from the correct position.

Cheers,
Volodymyr

Senior Fellow at CERN

HL-LHC project

SY-STI-BMI section, 864/2-D13

Topic:
LHC collimation studies and FLUKA development

[Volodymyr Rodin]

* momentum components