Quantum Diaries

The  CMS collaboration is made up of about 3600 people coming from 183 institutes in 38 countries. Besides the United States, there are large representations from Western Europe, Eastern Europe, Russia, Asia and… well just about everywhere. In fact, it is probably easier to list the areas where we don’t have collaborators: notably Africa, Canada (the Canadians seem to prefer ATLAS) and Australia (also ATLAS).  As a result, my collaborators come from diverse cultures, religions and mother languages and the only thing we have in common is an unstoppable curiosity of how the universe works. We do have culture clashes, but we try to rise above them. I remember during a particularly conflictual meeting being chaired at CERN (pre CMS days) by a German colleague, one of our French colleagues got so angry he said “We threw you out of France before and we can do it again!” The room got very quiet as most participants looked uncomfortably at their shoes. These kind of direct cultural references are rare, though, and mostly you think of your colleagues as your fellow researcher and not as Russian, Turkish, Chinese or Georgian.

Many countries or set of countries have so many collaborators that they form their own mini-collaboration structure so that they can better deploy their own people in the larger context of CMS. For example, the US has a mini-collaboration: USCMS which comprises all the US institutions working on CMS. Last fall I was invited to the CMS RDMS collaboration meeting to give an overview of the particular physics I am interested in. RDMS stands for Russia-Dubna Member States, and is made up of the CMS collaborating institutions in Russia, Armenia, Belarus, Bulgaria, Czech Republic, Georgia, Slovak Republic, Ukraine and Uzbekistan. This past year, the RDMS collaboration meeting was held in Minsk, Belarus. Belarus is a tiny country between Poland and Russia and was part of the Soviet Union until 1991 when it declared its independence. It has its own language, Belarusian, although most people there also speak Russian. Not that that helped me at all — I don’t speak Russian or Belarusian.

The RDMS collaboration is very active in CMS and during this collaboration meeting I got to learn more about what they were doing as well as summarizing work in my particular area. Our hosts in Minsk were terrific and in addition to scientific meetings they gave us a tour of Minsk, took us to the opera, to the symphony and to museums. Our meeting was even featured on the national news along with film footage of the foreign scientists. I returned to CERN feeling much better connected with my RDMS colleagues and with plans to strengthen ties with them in my particular data analysis interests.

Months after the meeting ended, I received an email from the Chicago Field Office (I work for Fermilab, just outside of Chicago, Illinois), Office of Intelligence and Counterintelligence asking for details on my trip to Belarus. They were particularly interested in any connections I had made, especially on a personal level, with people I met at the conference. They also wanted to know if there was any chance my laptop computer had been tampered with or if my room had been searched or if people seemed to know more about me than would be normal. Not only did they want to know what hotel I stayed in, but also they wanted the room and floor number. I should mention that I can’t even remember what I ate yesterday; I certainly can’t remember the room number of the hotel I stayed in six months ago. Of course I understand why we worry about these things, and I am not naive enough to believe that there was no possibility that I was spied on (although I don’t think I know anything particularly interesting), but it does make me realize that the world I live and work in is very special place, where country boundaries are often not noticed, at least not by my fellow researchers.

Subsequently, I had the possibility to lecture at a summer school in Iran, the First IPM Meeting on LHC Physics. Unfortunately this trip didn’t even get off the ground: judging by the collective guffaw from everyone I mentioned it to, I decided to save myself the pain of filling in forms in triplicate just to amuse the State Department and/or the Department of Energy for the two seconds it would take them to say no.

One of the great things about this field, and about international scientific collaboration is the opportunity to work and visit scientists from around the world. In general we speak the same language (physics) and we work hard at not forcing our own culture, religion, politics or ideology on others. Our biggest arguments are about how to optimize our detector and how to define which data to collect and how to interpret it. Luckily there is plenty of material there to argue about without resorting to politics or religion.

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…