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Aidan Randle-Conde | Université Libre de Bruxelles | Belgium

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Balancing the budget, one collision at a time

Imagine you’re in charge of a budget for a large organization of a few thousand people who are experts in their field.  Imagine that if you don’t spend some of the money in the budget that you can’t keep what you’ve saved- it will be lost forever.  Now imagine that there’s another group of a few thousand experts with exactly the same budget, right down the last penny.

That’s the kind of scenario that we face at the LHC, except the budget is in time and not money.  We count proton collisions and not dollars.  The LHC is delivering world record luminosities right now, and the different experiments are getting as much data as they can.  For LHCb and ALICE there is pressure to perform, but between ATLAS and CMS the competition is cut throat.  They’re literally looking at the same protons and racing for the same discoveries.  Any slight advantage one side can get in terms of data is crucial.

What does any of this have to do with my work at ATLAS?  Well I’m one of the trigger rates experts for pileup.  When we take data we can’t record every proton collision, there are simple too many.  Instead, we pick the interesting events out and save those.  To find the interesting events we use the trigger, and we only record events when the trigger fires.  Even when we exclude most of the uninteresting events we still have more data than we can handle!  To get around this problem we have prescales, which is where we only keep a certain fraction of events.  The trigger is composed of a range of trigger lines, which can be independent of one another, and each trigger line has its own prescale.

A high pileup event at ATLAS

High pileup scenarios. Can you count the vertices? (ATLAS Collaboration)

The term “pileup” refers to the number of proton collisions per bunch crossing (roughly how many interactions we can expect to see when we record an event.)  When I came to ATLAS from BaBar I had to get used to a whole new environment and terminology.  The huge lists of trigger lines alone made my head spin, and so far pileup has been the strangest concept I’ve had to deal with.  Why take a scenario that is already overwhelmingly complicated, with one of the most intricate machines the world, and make it even harder to understand, for the sake of a few more events?  Because we’re in competition with CMS, that’s why, and everything counts.  The image on the right shows a typical event with multiple interactions.  Even counting the number of vertices is difficult!

Balancing the different prescales is where things get interesting, because we have to decide how we’re going to prescale each trigger.  We have to make sure that we take as much data as possible, but also that we don’t over-burden our data taking system.  It’s a fine balancing act and it’s hard to predict.  Our choice of trigger prescales is informed by what the physicists want from the dataset, and what range of types of events will maximize our output.  The details of what kinds of events we want is a very hotly debated topic and one that is best left to a separate blog post!  For now, we’ll assume that the physicists can come up with a set of prescales that match the demands of their desired dataset.  What usually happens then is that the trigger menu experts ask what would happen if things were a little different, if we increased or decreased a certain prescale.

The effects of proton burning on luminosity.

The effects of proton burning on luminosity. (LHC)

We need to pick the right times to change the prescales, and it turns out that as we keep taking data, the luminosity decreases because we lose protons when they interact.  This is known as proton burning and you can see the small but noticeable effect of this the image above.  As we burn more protons we can change the prescales to keep the rate of data-taking high, and that’s where my work comes in.  The rates for different trigger lines depend on pileup in different ways, so understanding how they act in different scenarios allows us to change the prescales in just the right way.  We can make our trigger very versatile, picking up the slack by changing prescales on interesting trigger lines, and pushing our systems to the limit.  My job is to investigate the best way to make these predictions, and use the latest data to do this.  The pileup scenarios change quite rapidly, so keeping up to date is a full time job!  And every second spent working on this means more protons have been burned and more collisions have taken place.

It’s not an easy task, it forces me to think about things I’ve never considered before, and keeps the competition at the forefront of my mind.  I knew I’d be in a race for discovery when I joined ATLAS, but I never realized just how intense it would be.  It’s exciting and a little nerve-wracking.  I don’t want to think about how many protons pass by in the time it takes to write a blog post.  Did we record enough of them?  Probably.  Can we do better?  Almost certainly.  There’s always more space in this budget, and always pressure to stretch it that little bit further.


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