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Seth Zenz | Imperial College London | UK

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Can the LHC Run Too Well?

For CMS data analysis, winter is a time of multitasking. On the one hand, we are rushing to finish our analyses for the winter conferences in February and March, or to finalize the papers on analyses we presented in December. On the other, we are working to prepare to take data in 2012. Although the final decisions about the LHC running conditions for 2012 haven’t been made yet, we have to be prepared both for an increase in beam energy and an increase in luminosity. For example, the energy might go to 8 TeV center-of-mass, up from last year’s 7. That will make all our events a little more exciting. But it’s the luminosity that determines how many events we get, and thus how much physics we can do in a year. For example, if the Higgs boson exists, the number of Higgs-like events we’ll see will go up, and so will the statistical power with which we can claim to have observed it. If the hints we saw at 125 GeV in December are right, our ability to be sure of its existence this year depends on collecting several times more events in 2012 than we got in 2011.

We’d many more events over 2012 if the LHC simply kept running the way it already was at the end of the year. That’s because for most of the year, the luminosity was increasing over and over as the LHC folks added more proton bunches and focused them better. But we expect that the LHC will do better, starting close to last year’s peak, and then pushing to ever-higher luminosities. The worst-case we are preparing for is perhaps twice as much luminosity as we had at the end of last year.

But wait, why did I say “worst-case”?

Well, actually, it will give us the most interesting events we can get and the best shot at officially finding the Higgs this year. But increased luminosity also gives more events in every bunch crossing, most of which are boring, and most of which get in the way. This makes it a real challenge to prepare for 2012 if you’re working on the trigger, because have to sift quickly through events with more and more extra stuff (called “pileup”). As it happens, that’s exactly what I’m working on.

Let me explain a bit more of the challenge. One of the triggers I’m becoming responsible for is trying to find collisions containing a Higgs decaying to a bottom quark and anti-bottom quark and a W boson decaying to an electron and neutrino. If we just look for an electron — the easiest thing to trigger on — then we get too many events. The easy choice is to ask only for higher-energy electrons, but beyond a certain points we start missing the events we’re looking for! So instead, we ask for the other things in the event: the two jets from the Higgs, and the missing energy from the invisible neutrino. But now, with more and more extra collisions, we have random jets added in, and random fluctuations that contribute to the missing energy. We are more and more likely to get the extra jets and missing energy we ask for even though there isn’t much missing energy or a “Higgs-like” pair of jets in the core event! As a result, the event rate for the trigger we want can become too high.

How do we deal with this? Well, there are a few choices:

1. Increase the amount of momentum required for the electron (again!)
2. Increase the amount of missing energy required
3. Increase the minimum energy of the jets being required
4. Get smarter about how you count jets, by trying to be sure that they come from the main collision rather than one of the extras
5. Check specifically if the jets come from bottom quarks
6. Find some way to allocate more bandwidth to the trigger

There’s a cost for every option. Increasing energies means we lose some events we might have wanted to collect — which means that even though the LHC has produced more Higgs bosons, it’s counterbalanced by us seeing fewer of the ones that were there. Being “smarter” about the jets means more time spent by our trigger processing software on this trigger, when it has lots of other things to look at. Asking for bottom quarks not only takes more processing, it also means the trigger can’t be shared with as many other analyses. And allocating more bandwidth means we’d have to delay processing or cut elsewhere.

And for all the options, there’s simply more work. But we have to deal with the potential for extra collisions as well as we can. In the end, the LHC collecting much more data is really the best-case scenerio.

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