[Rivet] Rivet 2.4 and xsec/event counter information

[David Grellscheid]

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
>