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Top Physics at the Tevatron/D0 and ATLAS October 21, 2007

Posted by gordonwatts in Conference, physics, travel.

CIMG4622I spent last Thursday, Friday, and Saturday at a really great conference in Grenoble. The topic was top physics, and lessons and techniques at the Tevatron and at ATLAS. I wish I could put my finger on what made it great: it was a small group (less than 50 people, I think), and everyone was willing to talk. The first day we ran about 3 hours over, for example (yes, yes, a good 30 minutes of that was my fault). Both Lorenzo and Harrison gave great talks as well as the others. I took some pictures too.

I have pages of notes from the conference (all stored electronically, of course, despite my busted laptop – which is getting worse). Some tidbits (if you don’t care, skip over to the last paragraph – partly to help me remember):

  • At the Tevatron we produced about 8 tops per day, and 4 single tops per day. LHC will be one per second, and a single top 1 ever two seconds. This is a game changer – top becomes a calibration sample, for example (b-tagging, JES…). I can’t help but wonder if we haven’t explored all of its possible uses as a calibration sample yet. Heck, data for 9 hours at low luminosity (but full luminosity) will give you a top mass peak that you can see! And it is going to be exciting to measure that cross section at 14 TeV!! And they really are are going to do everything without b-tagging at the start of the run!
  • No one yet talks about doing analysis in such a way to make the eventual combination with CMS easy. I really wish we’d start thinking that way from the beginning.
  • I had not fully appreciated the history of the D0 JLIP tagging algorithm — it was nice to see it in action as far back as LEP and also (as I’ve seen) in ATLAS. I’m sure it is in CMS as well.
  • During my talk I made the sarcastic comment that one shouldn’t measure things twice – because you are bound to get different answers. I really did mean that sarcastically — but people kept mentioning that I mentioned it thought out the rest of the talks. Sheesh. 😉
  • There was a big difference between the ALTAS and D0 talks. In general, the D0 talks talked about one method, and then went into all the details and tricks and the general mess than a proton-proton collider causes. The ATLAS talks were often an overview of techniques. For example, for the Jet Energy Scale ATLAS showed results (but not details) from about 4 different methods. I’m sure this will change when data arrives.
  • Harrison made a rather provocative statement fairly early on in the meeting: we spend too much time trying to get the absolute scale of the Jet Energy correct. Instead we should just match the Monte Carlo to the Data and then move on; who cares about the absolute scale. I and others made the obvious point: in order to do the match don’t you end up with the absolute scale. After several goings around (over wine at lunch!), we figured out: he completely agreed that reducing the error on the jet energy was well worth the technique, but he still wonders if we could save time by not developing independent JES for MC and Data.
  • I need to start exploring TMVA.
  • In general, the D0 analyses are quite sophisticated — using multivariate techniques like decision trees. Most of the ATLAS analyses don’t yet. In fact, there was some scientism from the audience about our use of these techniques. M. Mangano came up to me at one point and asked how we knew that our decision tree analysis had actually seen single top and not something else (MC tests, the Matrix Method also sees it and it isn’t a machine learning, etc.). It sounds to me like this battle isn’t done yet.

There was only one thing I didn’t like about the conference: the agenda page is password protected. Unless you are a member of D0 or ATLAS you aren’t supposed to have the password. The reasoning is as follows: they wanted to have D0 people be able to talk about internal matters — really wanted to know the dirt behind what went wrong and what to try to avoid when working on ATLAS. And the ATLAS folks should talk about anything they wanted to as well – regardless of weather or not it had been approved. It didn’t work out that way: the ATLAS top group reacted saying that they didn’t want preliminary results shown. The result was the page was password protected. This is too bad, and I think, in the end, the password was not required. First of all, ATLAS is only showing Monte Carlo results (though the ATLAS philosophy is different than the D0 one – D0 is quite comfortable with MC results going out without nearly the cross-checks that data results get). Second, as one speaker put it, “new versions of these results will be shown shortly in ATLAS and will improve on these old results” — so the results we were seeing weren’t the preliminary ones in most cases. As for D0, I think all of the D0 speakers got an email from the top group reminding us that only publicly approved plots were allowed to be shown at this conference. Fortunately, it did not limit the discussion and all of us were quite frank in our revealing of faults and other problems. I am in general against external conferences being password protected. 🙂



1. jeff moss - November 7, 2007

fisusist……i can add too

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