It’s been a while since I’ve blogged and to my readers I apologize. I have been working on my leptoquark analysis, which is rolling right along. I’ll be sure to share that at a more appropriate time but I thought it would be fun to take note of what I do on an average work day. So why not today…
9:30-ish A.M. – arrive at Brookhaven National Lab. Usually I go to Stony Brook, but today I’m working with one of the post-docs at BNL on how to estimate QCD background.** Upon arriving, we get a quick coffee and get caught up on this weekend’s US Open tennis championships. Discussion of my analysis code ensues.
**ASIDE: QCD (Quantum ChromoDynamics) is the theory of strong interactions. Of the forces we study at the LHC, this is the least exactly understood. It relates to all quark/gluon (color charged) interactions. QCD has two peculiarities: confinement and asymptotic-freedom. In short, confinement means that as you move two quarks away from each other the force between them gets larger (like a rubber band). This explains why you never see single quarks, instead you see showers of quarks (called jets). As they move away from each other, more quarks pop out. Asymptotic-freedom means that high energy quarks and gluons interact less. At a hadron collider, the most prominent thing you get out are jets. They are a significant background that you have to remove to do an analysis. It’s also difficult to simulate just based on the sheer number of jets that come out. My advisor once told me that the least expensive way to simulate all the QCD background needed is to build a collider and take data. **
10:30 A.M. – After getting some unexpected results in my electron analysis, the anonymous post-doc confirms that there is indeed a bug in my analysis code. This is what I spend the vast majority of my time doing. Writing and debugging code to perform an analysis on data and simulated data. He has written a modified version on my electron selection code, so I work to combine the two.
11:15 A.M. – We get into a discussion about what the electron/photon (called e/gamma) group has defined as an appropriate selection criteria for the electrons I want to include in my analysis. I want to use a standard selection criteria but the standards vary in the early days of data-taking. We decide that the best approach is to use the selection criteria of my greatest background: W bosons produced with jets.
12:15 P.M. – Lunch time. We join the other ATLAS scientists at BNL on the walk over to the cafeteria. The cafeteria contains your standard fare. I usually go for an egg-salad sandwich. The days I go to BNL, I don’t make my own lunch. The discussion flows from the fate of the Tevatron to the post-doc’s baby girl and back to the US Open (I’m a big tennis fan… go figure).
1:00 P.M.: Head back from lunch, and start the discussion of QCD estimation. Unfortunately the bug in my code and discussion of selection criteria for electrons took up the morning, but that leaves the afternoon for some strong force fun. **So how do we do this ** The discussion takes a while, but once it’s done….
**Another Aside!: QCD estimation is done a couple of different ways. It gets pretty detailed, but here is a brief overview. You look at two different regions: an area that you believe to be signal poor and QCD background rich, and the other signal rich and QCD background poor. You can then pick selection variables that are uncorrelated: like a lepton ID variable, and a Missing Energy variable (from neutrinos). You can then plot the values of the events in terms of the two variables and come with four regions: an area that is signal for both variables, an area that passes signal cuts for one variable but not the other (2 of these), and an area that fails the signal cuts for both variables. From these numbers and ratios you can predict the amount of QCD you should get. –I’ll note again, this is simplified, but the general idea is there.**
2:45 P.M.: I now get to write the code that makes these plots 🙂 And spend the rest of the afternoon doing so.
6:45 P.M.: I get home in time to see the US Open men’s championship get rain delayed, so I pop open my laptop and start coding again. I have a farewell party for a friend who graduated and is moving to Germany tonight, so I can’t work too much longer.
-Regina
