Dr. Kenneth Bloom is a postdoctoral researcher in experimental high-energy particle physics at the University of Michigan.
More photos from Ken Bloom
Particle-physics experiments are very complex, and a lot of thought and careful work is needed through the long process of recording the data, processing them, and then understanding what they tell you. Today I spent some time working on both the starting and finishing point of this process.
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StumbleUponIn our experiment, protons and antiprotons collide 7.5 million times per second. We need the collision rate to be that large because we are interested in very rare occurrences such as top-quark production; the more data, the better. But our detector, as sophisticated and fast as it is, can only record data from the collisions to computer disk at the rate of 75 times per second, which means that we must throw away the data from 99,999 out of every 100,000 collision events. It turns out that most of the 99,999 are not very interesting to us, and we can live without them. But any time you throw away data, you are taking the risk that you are throwing away something important, and in this particular case, we're throwing away 99.999 percent of our data before a human ever looks at them. We'd better know what we are doing.
If we were just to keep and toss events at random, we would never keep enough of the rare collisions that interest us. Instead we have developed specialized, high-speed electronics that can process a little bit of the data from a collision very quickly, decide if they are interesting, and then decide whether to keep the rest of them. I designed some of the electronics; my circuit boards measure the time at which a charged particle flew by particular points in our detector. This information is used to identify charged particles that carry a lot of energy, which means that they might be of interest to us, and therefore we should record the event for further study.
Designing the boards was one thing; making them work and keeping them working is another, and we've tended to run low on spare parts. This summer, one of our undergraduates has been trying to get some of our problem children working. Dave has made good progress, but some of the boards have been pretty tricky, so I spent some time working with him this morning. We check a board by sending test inputs to it, and making sure that the output electronic signals are what we expect them to be. If they aren't, we work through the board methodically, trying to find where things start going wrong. When fixing circuit boards, I tend to think about the scientific method; you must be careful about changing only one thing at a time, so that you can really understand the effect of each change. Dave and I had mixed luck today, as some of the problems seemed to come and go at random. All the scientific method in the world can't help you if you don't have a stable system to look at.
Meanwhile, I was still bothered by the problems that Nate was showing us yesterday. We definitely are having trouble understanding that data sample. But what does "understand" mean, anyway? To borrow an example from my thesis adviser (thanks, boss!): Imagine that your job is to determine how many diners in a restaurant are natural redheads. It sounds easy enough—go into the restaurant and count the number of people with red hair. But there are probably some true redheads who have gone bald or gray, and there may also be some apparent redheads who got their color out of a bottle. You might be undercounting or overcounting.
No, you're not allowed to ask people about their hair color—you'll have to develop some more sophisticated tools. Let's focus on the overcounting problem. How do you estimate the number of fake redheads? How many people who come to this restaurant dye their hair red? To answer that, you could spend some time at the beauty parlor and see how many people come through for a dye job and use that number to make an estimate of your "fake" rate. But is the beauty parlor a good model of the restaurant? What if people who get their hair done there don't like Ethiopian food?
This is an unrealistic analogy, but it shows the basic problem of experimental particle physics. You want to isolate a sample of collision events that include a particular kind of particle (redheads) for study, but you never have a perfectly pure sample, and you always miss some of the desired events. You have to compensate by estimating the fakes (the dyed redheads) and the losses (the bald ones), and your estimates are based on studies in other samples (the beauty parlor). Counting redheads in a restaurant is easy; it's finding the other samples and figuring out how to get good information out of them that takes the greatest effort.
I was musing on this problem with Dave, Nate's thesis adviser. (Not the same Dave as above; it's a common name in particle physics, for some reason.) Nate seems to have a problem accounting for the fake redheads in his sample. At first it looked to us like he was having trouble counting the customers at the beauty parlor, but now we are starting to think that it's worse—he may actually be counting the cats at the pet store. We spent some time looking at what other people have done with this problem in the past and coming up with ways to apply those solutions to our particular data sample. Now we have a list of things for Nate to work on, some of which might answer our questions—we'll have to try them and see what works! I'm going to Fermilab tomorrow, so I'll be able to tell Nate about this when I get there.
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Notes From The Fray Editor:
Discussions of people doing remarkably complex things tend to move in two directions: toward the people and toward the remarkably complex things. In this case, Air Vent began a discussion of physicists that sounds like almost every group-of-physicists-yukking-it-up I have ever been around while andy finished off (more or less) a discussion of the relationship between the complicated math and the reality of the reality it speaks to (previous posts in that thread are less daunting).
Remarks From The Fray:
I recommend against counting 'Daves' at Fermi, as the statement 'Dave is a common name in particle physics' is an unsupported statement. Fermilab is I think unrepresentative of particle physic generally. I think CERN must be sampled too and two universities also. I think Cal Tech and the U. of Michigan must be sampled as well. At CERN there may be some Dave's who go by Pierre for example to fit in and at Fermilab there may be some Pierre's who go by Dave to fit in at Fermilab. I suggest checking birth certificates. So just in conversation you may have picked up both some false negatives and some false positives on this score. Hope this is helpful to further research.
--Air Vent
(To reply, click here.)
the retrospective realism comment is perceptive.
we talk about the various flavors of quarks as if they're real (also the leptons and gauge bosons); to the extent that we can measure quantities that appear to correspond to each of the particles, we should consider to be real. but there are ambiguities.
first, even field theorists generally do not focus on the elementary particles as the fundamental concepts of reality. rather, the main concept in particle physics is the so called SU(3) X SU(2) X U(1) local gauge symmetry. it's the symmetry principle that takes precedence; the particles are looked on as the quantum excitations of the vacuum induced by the field operators present in the theory. i suspect that many theorists would say that what's real is the symmetry principle, and the observations of the various particles and the manner in which they interact with each other confirm the prediction of the symmetry-based gauge theory.
second, field theory may not be "fundamental". quantum field theories are often referred to by theorists as "effective field theories" these days. the reason is that most people doing theory nowadays believe that QED/flavordynamics and QCD are only approximations of an underlying level of complexity at higher energies (corresponding to shorter distances.)
one of the many reasons for the prevalence of this belief is the existence of infinities in various calculations. theorists tend to think that such infinities indicate the theories are incomplete; this explains the interests that theorists have in strings, M-theory etc. this brings us to a third ambiguity in the concept of the top quark:
one of the field theory calculations that give infinite result concerns the mass of the quarks. obviously, the infinity present in the calculation is not observed, so we try to "sweep" it under the rug by replacing it with some arbitrary parameter which is determined by measurements made at a specific energy scale [this trick is generally known as 'renormalization' and one of the most popular schemes for doing so is called 'modified minimal subtraction']. however, from QCD theoretical calculations, it turns out that the mass of a quark varies depending upon the energy scale at which we perform the renormalization. thus the "mass" of the top quark is a somewhat ambiguous concept.
Finally, as you already know, we can't observe free quarks even at the energies that will be reached by the future LHC collider at CERN. this is due to the nature of QCD which dictates that the strong force increases in strength at lower energies (larger distances); the concept of six quarks organized into three 'families' is postulated from the fact that such an arrangement would consistently describe other particles as quark composites...otherwise there'd be hundreds of known "elementary particles". so to a certain extent, a quark is a convenient construct.
-- andy
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