[Rivet] Rivet 2.4 and xsec/event counter information

[Andy Buckley]

HepMC3 has a generic attributes system (I can't remember if that's the 
noun we used, but no matter), so hopefully is already flexible enough 
for this.

Would we need to pass an array of these, for multi-weight systematic 
variations etc.? Marek Schoenherr has pointed out to me that we'll need 
something like that for xsecs, which generally vary with scales, PDFs, etc.

Andy


On 06/11/15 22:31, David Grellscheid wrote:
> Hi Andy,
>
> Simon and I were discussing this at Les Houches. It will need support from
>
> - Generators
> - HepMC
> - Rivet
> (- YODA)
>
> to pass a new value 'attempts' through, just like xsec at the moment;
> but the benefit is that we can finally do the stats correctly.
>
> Passing 0-weight events along can be done as a workaround, but for the
> sake of future efficiency it would be good to have this value passed
> along explicitly.
>
>    David
>
>
> On 06/11/2015 12:40, Andy Buckley wrote:
>> On 06/11/15 10:16, Daniel Rauch wrote:
>>> Hi Andy,
>>>
>>> oh sorry, our bad - we were only looking at the individual histograms.
>>>
>>> So what we were/are looking for is the number of 'attempts' which would
>>> differ from the number of 'events' by the count of events with zero
>>> weight (that also contribute to the calculation of the cross section).
>>> Obviously this would have to be communicated to Rivet from the generator
>>> for which there currently is no existing infrastructure, right?
>>
>> That sounds correct. Generators will usually have some minimum weight
>> cut, below which they don't bother "sending" the event. If they output
>> events with zero weight this would work, of course, but would be
>> wasteful (unless they also made sure that the event was ~empty.)
>>
>>> The motivation for this is to calculate the statistical MC error also
>>> using the second part "-(sum of weights)^2/N" where N is the number of
>>> attempts. You can find a copy of my master's thesis at
>>> <http://www.itp.kit.edu/prep/diploma/PSFiles/ms-thesis-daniel-rauch.pdf>http://www.itp.kit.edu/prep/diploma/PSFiles/ms-thesis-daniel-rauch.pdf
>>>
>>>
>>> - the corresponding sections would be 4.2 & 4.3 for the equations for
>>> the combination of 1D histos and their MC errors, appendices B and C for
>>> the derivations and sec 5.4 for the implementation in the
>>> Herwig-Parallel toolset.
>>>
>>>> We're avoiding building any special interpretation of such variables
>>>> into YODA (and especially yodamerge), so that it remains a generic
>>>> data analysis package with capabilities useful for these studies,
>>>> rather than a one-trick package specifically for MC analysis, so at a
>>>> later point we plan to have a rivet-merge or similar script which will
>>>> use the logic of yodamerge with added MC-specific use of this
>>>> information.
>>> So I understand this is the reason for going with the sqrt( sum of (
>>> weights^2 ) ) error definition - we will then most probably try to check
>>> if we can somehow develop similar patches for Rivet 2.4 to do the
>>> communication of the number of attempts from Herwig to Rivet.
>>
>> Well, we use that because it is the standard definition of weighted
>> binomial error. If the generators could pass the number of failed
>> attempts in their output stream, we could certainly feed it through into
>> the yoda files... but it would need a bit of work to decide how to pass
>> it. I guess it's a bit like the cross-section, the estimate of which
>> evolves through the run, hence we write a "current xsec estimate" in
>> every event, and use the last one given when normalizing histograms etc.
>> (via the crossSection() method as called in Rivet's finalize()).
>>
>> Andy
>>


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