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Vivian O'Dell | USLHC | USA

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Beam is coming — Look busy!

A lot has been written about the new LHC schedule. Like everyone here, I was deeply disappointed by the “incident” last year and I’m eagerly anticipating first collisions this year. Because of the delay in the collider schedule, my family thinks I’m on an extended vacation, but somehow I’m almost busier now than if we really were taking data.

As we wait for the accelerator repairs, my major focus is on managing one of the data analysis working groups. Actually maybe “facilitating” is the right word. My job is to make sure that the folks in our working group are ready to look at the first data with the CMS detector with all their software (and youthful enthusiasm!) revved up and ready to go. We do this using what we call “Monte Carlo Data” — which is a bit oxymoronic considering “Monte Carlo” means simulations of what we think LHC collisions will look like in our detector and “Data” implies the real signals from real LHC collisions.

The Monte Carlo “Data” is made in several steps:

Step 1 – Event Generation: In this step we generate a list of particles that might be created from a particular proton-proton collision. To do this we use the theory of Quantum Chromo-Dynamics (or QCD) combined with results from all previous High Energy Physics experiments to predict all the end products (particles) in an LHC proton-proton collision. This is (only!) our best guess of what the collisions will look like. There are many different “generators” for generating the final particles in a collision as we can only approximate QCD predictions and there are different models for how and what to approximate. Also there are a lot of input parameters to QCD (like particle masses for example) that have to be put in by hand. The output from the generation step is a list of all the particles from the collision, their charge, their direction and how fast they are traveling (i.e. momentum). Because in each collision there is only a probability for a particular set of final particles, we “roll the dice” and repeat the collision millions of time in order to get, on average, an idea of what the collisions look like. Hence the origin of the name “Monte Carlo”. Unfortunately knowing how to use a QCD Monte Carlo generator does not help you win at the gambling tables.

Step 2 – Simulation: Once we have a set of particles from Step 1, we simulate how the particles would interact in our detector. To do this we have to have a very complete implementation in software of our detector, including the positions of all the components and exactly what the signal from each type of particle would look like in each component.  Each part of the detector is designed to collect complementary signals from a particle. Even parts like the cables that bring signals from the inside of the detector out to the electronics that register the data have to be in the simulation since there is some probability that a particle will interact in the cables!

Step 3 – Reconstruction: In this step, we now forget we ever knew anything about the original generated particles and we try to reconstruct them given the signals we simulated in Step 2. This step we also perform on real data once we get it.

Step 4 – Calibration: From the reconstructed Monte Carlo events of Step 3, we can ask ourselves how well the reconstructed particle matches the particle we generated in Step 1. This gives us an idea of how well our detector will perform when we reconstruct real data (where we don’t know what the generated particles).

The better our simulation and reconstruction steps (steps 2 and 3) the more like real data our Monte Carlo will look like. The better our calibration step (Step 4) the better our understanding of the particles coming out of our real proton-proton collision will be.

Step 5 – Analysis: this is where we take the final calibrated collisions and look for all the cool stuff we hope to find like the Higgs boson or mini black holes. But most of the data will come from more mundane QCD processes, so in order to claim we see something new and different, we will have to work hard to make sure we understand the data in terms of our QCD generators (from Step 1). Likely we will have to do some tweaking of the generators since will be looking at hadron collisions at a much higher energy than anyone has ever before studied and we will have to modify some of our assumptions and approximations.

Right now, as we wait for the repairs to the LHC, we are busy creating millions of Monte Carlo collisions using different models for Step 1 and ever more precise detector information for Steps 2, 3 and 4. It’s a lot of work, but it will pay off in faster data analysis time once the real collisions begin. And I should mention that even though we think we are planning for all the eventualities in how the detector will react to particles from collisions, there are almost always surprises. So while we will see nice pictures of collisions at the instant they happen, it will take us longer to understand and analyze them.

So, in short, this is not much of a winter vacation. And I left out all the work we are doing repairing andimproving our own CMS detector while we have the opportunity during this down time!

P.S. I’m new to this whole blog thing, so please feel free to critique/ask questions/boo me off the stage…

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5 Responses to “Beam is coming — Look busy!”

  1. I notice that almost half the US/LHC bloggers are women though only about 6% of physicists in the US are women. Is there greater gender equity at the LHC than in the field as a whole?

    In general what challenges do women face in the area of high energy physics? What are the benefits for women of choosing to pursue higher education and a career in high energy physics as opposed to a field that has a higher concentration of women?

    Colette Morrow
    Past President, National Women’s Studies Association
    Senior Fulbright Scholar

    Answer from Vivian: I see my colleague Steve has started a “Women in Physics” thread, so I will post further comments there. But you are right, the blog women/men ratiio is much higher than reality. Both as an undergraduate and a graduate student in physics, I was the only woman in my class. Certainly I believe that, in general, male dominated fields tend to be higher paying than female dominiated fields at the same educational level, and I would encourage women not to be afraid to infiltrate a male dominated field. Being the only woman in a class can be very intimidating. Luckily I did it when I was too young and stupid to realize how uncomfortable/unpleasant it really was.

  2. How accurate are the “in silico” or particle simulation estimates (based on previous experiments)? If you find that the simulation data closely matches the net result of physical testing, do you begin to try extreme cases with the simulation and iterate? Is this simulation process basically tying the theoretical stuff to a test?

    What sort of applications do mini black holes have in a new simulation? In other words, what kind of innovative simulations could be constructed from a mini black hole?

    Certainly very interesting! Would love to see some video of these tests.

    Answer from Vivian:
    Hi Thomas, thanks for the question! I’m not sure I completely understand it, but I’ll try to answer it. Please feel free to keep asking if I’m not answering the question your are asking.

    The simulations are more or less made to match previous experiments (so they are “tuned”
    in order to match various aspects of the events). For example, the Monte Carlo simulations we are using for the LHC were tuned to reproduce the data from the Fermilab proton-antiproton collider. In terms of accuracy, when we try to reproduce some of the “soft” or low energy particles there are differences in predictions by factors of 5 or even more. We won’t be able to tune our predictions of the full LHC spectrum of particles until we collect real LHC data. However, in general the new stuff we are looking for,
    like the Higgs or mini black holes, show up as large energy particles so the fact that we cannot predict very well the lower energy particles doesn’t mean we don’t have some idea of what “new physics” signal will look like.

    In a future blog, I promise to dig up a plot that shows Monte Carlo simulations vs. data for one of the Fermilab experiments so you can see how well we do.

  3. Harbles says:

    Thanks for blogging! This is exactly the sort of information that non-scientist science enthusiasts such as my self crave, more detailed than the usual ‘outreach’ articles on the LHC but not as dense as the official documentation and papers published.
    My question is do you have cosmic rays in your simulations and has the actual data from cosmic ray tracks correlated well with your simulations?

    Answer from Vivian:
    Great question, Harbles! We have spent the last few years studying cosmic rays with whatever parts of the detector were ready. Last spring and fall, in preparation for LHC beams, we collected millions of cosmic ray events both with our (4 tesla!) analysis magnet on and off in order to study how the detector responds to real particles. These types of events will become background to real proton-proton collisions when the LHC starts producing them. Instead of simulating them, we collect the real thing and “overlay” them on our simulations. In other words, we simulate a proton-proton event using the Monte Carlo, and then we add to it at random one of the real cosmic ray events we have collected in order to study how to eliminate the cosmic ray signal background from the event.

    I hope this answers your question. If not, let me know.

  4. Harbles says:

    Yes very good thanks Viv! I guess I had the typical layman impression that you take a snap shot of a higgs and go ‘oh ho’ there it is write the paper.

  5. Wombat says:

    And there was me thinking that it was to do with the queue for physics being shorter than the one for literature!!!! And now you have been able to provide a wonderful answer to the question of how it is that geckos can stick to walls and ceilings – I wasn’t asking you to take it up as a piece of research, and yet I am sure that the ramifications are exttrememly important!!! How much would your fee be to come to London to deliver a modest lecture?

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