[Rivet] NLOHisto1D

[Frank Siegert]

Dear all,

the correct statistical treatment of correlated MC-events is a very
common user request -- just yesterday I had two users asking me about
it (cc'ed). I have so far always only provided the workaround in
contrib/nlohisto, but it would probably be nice if we could put this
directly into Rivet/YODA. The necessary changes are very simple: each
histogram has to become a wrapper around a temporary histogram, which
is then filled for subsequent correlated events (signalled by the same
event number) and only upon a new event number the temporary histogram
contents are synced to the actual histogram using something like

for (int i=0; i<_tmphist->bins().size(); ++i)
  fillBin(i, _tmphist->bin(i).area());

(cf. https://rivet.hepforge.org/trac/browser/contrib/nlohisto/NLOHisto1D.cc )

>From my side there are only two questions:

1) Do we want to always use this in all analyses, i.e. do we accept
the (probably minor) overhead from this bookkeeping? I don't know
whether it would be an option to introduce a switch which dynamically
enables this functionality within the histogram class.

2) Should this live in a Rivet wrapper around YODA::Histo1D or is this
more a functionality of the histogram class that should be moved into
YODA? Of course, in the latter case we would not fill using
Rivet::Event but just event weight and event number.

What do you think?

Cheers,
Frank