adjust energy loss parameterisation (Bethe-Bloch etc)

[danieljeans]

BEGINRELEASENOTES

• • add missing factor 0.5 to density term of Bethe-Bloch parameterisation
    ENDRELEASENOTES

[gaede]

Well spotted ! We seem to have used the wrong formula for > 15 years ;-(
Did you make any checks on on how big the effect is ?
I assume it is rather small...

[danieljeans]

Well spotted ! We seem to have used the wrong formula for > 15 years ;-( Did you make any checks on on how big the effect is ? I assume it is rather small...

it's the difference between green and red at high beta-gamma:
compare.pdf
I suspect it has a rather small effect. including the 1/2 factor brings red onto green.

Actually, I am looking at the small beta-gamma region, where we currently use the red curve, which gets very incorrect below around 10^(-1.5), but that's a story for a future pull request!

[danieljeans]

commit ba574d7
implements the "Ziegler" parametrisation of energy loss, used when beta-gamma<0.1.
Essentially copied/adapted from G4 code.
Resulting dedx curves for muons in Fe are shown here:
compare_BB_Zieg.pdf

(curves for orig dedx code ; with the density term bug fixed; and with Ziegler at low beta-gamma)

also similarly for K in Ar gas:
compare_BB_Zieg_KAr.pdf

current BB param gives negative dedx for beta-gam < ~few 10-2

[tmadlener]

Hi @danieljeans, could you rebase this onto the current master branch to pick up the new CI configuration?

[danieljeans]

@tmadlener : rebased

[tmadlener]

Thanks a lot. Looks like the tests also passed (as expected). Is there anything from the physics side still blocking this for inclusion in the upcoming developers release?

[gaede]

We clearly want this to go in - and then study the effect w/ the developers release...