Christmas comes early at the LHC

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I am sitting in the CERN review meeting where the ATLAS and CMS experiments have just reported that they have passed a major milestone six months early.
After the planning meeting at the beginning of the year, the people responsible for running the Large Hadron Collider announced that their target was to deliver one inverse femtobarn of integrated luminosity in 2011.
The integrated luminosity determines how many proton-proton collisions the experiments can record. Last night the tally of collisions collected both by my experiment (ATLAS) and our rivals (CMS) passed this marker.

ATLAS and LHC integrated luminosity for 2011, also showing the number for 2010, so the total is 1 inverse femtobarn (fb-1) and a cartoon bottle of champagne. From Beate Heinemann's talk to the LPCC.

This is in some ways just a psychological milestone. However, lots of the studies and projections done over the past year used an inverse femtobarn as their “baseline projection”, so we know very well how much physics can be done with this amount of data. And it’s a lot!
Even though we are ahead of target, of course we won’t be stopping. The Tevatron already has several inverse femtobarns of data anyway, though at much lower energies. And even in February we hoped that we might exceed this target by some way. Now those hopes are likely to be justified we’re very keen to make use of the maximum amount of data we can get.
By coincidence, the first ATLAS paper on the search for the Higgs was submitted today*. Still no Higgs, it would not be expected yet. But this paper uses just 0.04 inverse femtobarns of data. It gives an idea of the power of the data we are collecting.
Barns, femtobarns and inverse femtobarns
A barn is a unit of area. It was supposed to be something big and easy to hit (as in “barn door”) and is roughly the cross sectional area of a Uranium nucleus. It is small by everyday standards – 10-28 square metres, or one ten billion billion billionth of a metre. This is a nonsense number, I know, and it gets worse. A femtobarn is a million billionth of a barn. We just have to deal with the fact that nature operates on a variety of distance scales and not all of them are easy to imagine, I guess.
Inverse femtobarns are useful units for luminosity, because the cross section for a proton-proton scattering at the LHC to produce a Higgs, for example, is expected to be about 10,000 femtobarns. If you multiply femtobarns by inverse femtobarns you get the the number of collision events. So we might have made 10,000 Higgs bosons so far! Unfortunately most of them decay in ways impossible for us to distinguish from other kinds of events, but we’ll get enough of them soon if they are there.
Anyway, an inverse femtobarn is one, divided by a very very very small shed. Hope that helps.
There is a nice explanation of luminosity and how to increase it here (by Lauren Rugani).
*The first CMS paper on the Higgs came out a while ago, but used fewer Higgs decays in the search. This kind of leapfrogging will carry on for a while…

Written by physicsgg

June 15, 2011 at 8:53 am

Posted in High Energy Physics

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