updated ttbar cross section value @13TeV

[jabuschh]

I happened to noticed that the nominal cross section of ttbar @13TeV was updated in the corresponding twiki.

I was wondering how we make sure that we use the latest recommendations for cross section, branching fraction, etc. in general.
Maybe we can think of some kind of procedure to check regularly?
Although it seems to me that there's no way to avoid checking all the sources individually, which is quite tedious.
Perhaps it could help to add a date from the latest check as a first step? Just thinking out loud here...

What do people think on this?
(probably we should create a separate issue for this)

[MatthiesC]

@jabuschh You can open an issue of you like. The Twiki says that it is under construction; maybe we should wait a bit longer before merging this PR

[jabuschh]

Fine with me!
I opened issue #250 to gather thoughts/ideas/...

[anmalara]

We are talking about ~0.2% change, while the uncertainty is x10 larger. This will be relevant only for high-precision measurement in the top group.
I agree we should update these numbers when there is a new version, but I don't think this will affect anyone in this group.

Also, do you know if this has been officially released yet?

[jabuschh]

We are talking about ~0.2% change, while the uncertainty is x10 larger. This will be relevant only for high-precision measurement in the top group. I agree we should update these numbers when there is a new version, but I don't think this will affect anyone in this group.

I fully agree, I mainly wanted raise awareness that these values might be outdated ;)

Also, do you know if this has been officially released yet?

Well, I just stumbled upon the updated value in the twiki I mentioned above. I just checked with the latest Review of Particle Physics (page 817) and there the reference value is still 831.8 pb. So let's keep it as it is for now.

[MatthiesC]

I am also in favor of keeping the old value of 831 for numeric reasons (even if the actual difference is covered by uncertainties)

[finnlabe]

For me, the comment was given by the conveners hat I should switch to 833.9 pb. Did they accept the old value for you @jabuschh?

[finnlabe]

We discussed in a boosted tops meeting two weeks ago that we should merge this, as its more in line with the current Twiki recommendations so that any future analysis iterations have the most up to date values. So, unless there are any objections here, I'll merge this later today :)