[Rivet] thrust

[Andy Buckley]

On 08/09/15 16:20, Andrii Verbytskyi wrote:
> Dear Andy,
> 1) recently after looking into the thrust routine in Rivet I've fount
> that objects are arranged by |p| and not p_T as in the referenced Pythia
> code (line 77228 of pythia6428.f).
> In some cases it produces unexpected outputs.
> 2) The line
> if (axis.x() < 0) axis = -axis;
> in the same code looks very suspicious. Is that a bug or a feature?

Hi Andrii,

Thrust is principally an e+e- observable, so it makes sense that pT is 
not prioritised. If you want transverse thrust then the input pz values 
need to be set to 0 (or approximately zero -- I forget if the numerics 
require some z component). I checked the header and documentation and 
didn't find any mention of pT... did I miss something?

I've not looked into the code, but what in particular seems suspicious 
about the axis inversion? This looks to me just like a way to 
consistently orient the resulting set of principle axes. No physics 
should depend on the absolute sign of the axis direction.

Andy

-- 
Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow