[Rivet] JIMMY parameters in AGILe

[Jonathan Butterworth]

Holger Schulz wrote:
> Hi Jonathan,
>>
>> JMRAD() is the array defining the area of the particles in units of 
>> GeV^-2 (inverse radius squared!)
> So it can be pictured as the geometrical cross-section of the particles 
> plane perpendicular to the
> beam direction, right?

That's correct; or at least as a parameter controlling that 
distribution. The distribution itself is gaussian for a proton.

>>
>> PRRAD in Agile is a shorthand used to set these parameters such that
>> JMRAD(73)=JMRAD(91)=PRRAD.
> Ok, so Arthur's setting of JMRAD(73) = 2.2 should translate to PRRAD = 
> 2.2, right?

yep

> What keeps me worried is that there is a file in AGILe, called 
> fherwig-atlas-default.params
> where the setting of PRRAD is 0.71, which is also the default in Jimmy.
> However, the MC08 setting is quoted by Arthur to be 1.8.
> If this is all correct, do you think that a setting of 2.2 is still 
> reasonable? What
> would is the meaningful parameter range?

yes, it's ok. The 0.71 is based on the proton's electromagnetic form 
factor, and is also roughly ok with the t-dependence of elastic 
scattering. However, all the evidence (esentially from tuning and from 
Tevatron double-parton scattering) seems to point to the fact that the 
MI don't see the whole proton, but just "hot spots". This would 
correspond to a smaller radius, i.e. a larger PRRAD.

See
http://www.slac.stanford.edu/spires/find/hep/www?eprint=arXiv:0806.2949
for some more discussion on this if you are interested. There are 
interesting connections to the total pp xsec.

I would say a reasonable range is 0.5-3.0. I would expect the best fit 
to be around 2 though.

> 
> 
>>
>> The offset by one is just from c++ starting arrays at 0 when fortran 
>> starts them at 1.
> D'oh!

:)

Cheers,
Jon

> 
> Holger

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Prof. Jonathan Butterworth,              http://www.hep.ucl.ac.uk/~jmb/
Physics and Astronomy Department                  Tel: +44 20 7679 3444
ATLAS, CERN                                       Tel: +41 22 76  72340
University College London                 Gower St, London WC1E 6BT, UK
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