Kenneth Bloom, particle physicist - By Kenneth Bloom

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Posted Tuesday, Aug. 6, 2002, at 1:50 PM ET

Dr. Kenneth Bloom is a postdoctoral researcher in experimental high-energy particle physics at the University of Michigan.

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I'm a research fellow in the department of physics at the University of Michigan and a member of a research group in particle physics. I live in Ann Arbor, a very nice university town, but we don't do our experiments right here—a particle accelerator takes up so much space and is sufficiently expensive that very few universities, even big ones like Michigan, can have one on campus. Instead, my group does its research work at the Fermi National Accelerator Laboratory, a Department of Energy facility located 30 miles west of Chicago, and a four-and-a-half hour drive west of Ann Arbor. (I'll be there later this week.) There, the Tevatron accelerator collides protons (the nuclei of hydrogen atoms) and antiprotons (the proton's antimatter partner—yes, antimatter really exists) at extremely high energies. Einstein's famous equation, E=mc², says that energy can be converted into mass, and vice versa. We convert the energies of the protons and antiprotons into new, massive particles that are never seen in our day-to-day lives, but that were abundant in the high-energy environment of the early universe.

The experiment that I work on, the Collider Detector at Fermilab, seeks to discover and understand these massive particles. As I mentioned yesterday, there are 500 physicists from around the world working on CDF, and we all share the data and use it to explore our own particular physics interests, much as social-science researchers share data from the U.S. census to study their interests. I myself, (and most of my group at Michigan) am interested in the properties of the top quark, the most massive elementary particle yet discovered, 175 times more massive than a hydrogen atom. The top quark was first definitively observed at Fermilab in 1995, but we still don't know too much about it, and its huge mass is quite a puzzle. And this is an exciting time for us—we've spent the last six years refurbishing CDF, putting in improved instrumentation and preparing to take 20 times the amount of data we had in 1995, maybe enough data to solve this and other puzzles.

And I wonder why science moves slowly!

The heartbeat of a university is the pounding of construction equipment building new research facilities, and today it pounded its way into my own office, taking over my computer. No real work for today, I guess. But there are other things to do anyway, such as our weekly group meeting. We're lucky to have a relatively large research group; having more people means that we can work on more interesting problems. Our group is comprised of four professors, six postdoctoral researchers (including myself), seven graduate students, and a handful of undergrads. Three of our group members are women, and we represent four different countries. Most of us live in Ann Arbor, but right now one postdoc and five grad students are stationed at Fermilab, so they join our meeting through video conferencing.

Group meeting, with Fermilab contingent on television

People at different stages of their careers have different responsibilities in the group. The professors set the intellectual direction for the group and decide what major projects we will work on, but they teach classes and handle the management issues and don't have a lot of time for hands-on work. (Management includes applying for research grants so that the postdocs and students can get paid; most of our grant money goes toward the education of the younger people in the group.) The postdocs run the hands-on part—we have the experience to know how to realize the plans of the professors and turn them into measurements that we can publish. The graduate students are learning their way around the field, and working on their Ph.D. theses, with assistance from the professors and postdocs. Despite this hierarchy, we all educate each other; everyone knows something that someone else can learn from. By the time a student has finished a thesis, he or she typically knows much more about the topic than the professors!

We don't understand the data yet

At our group meeting today, one of our students showed some of his recent work. Nate is trying to understand the characteristics of a particular subsample of our recent collision data. He doesn't quite yet; he can't get plots made from simulations of the data to match up to the plots from the real data. But that's OK; we're all just starting out with this, and we have a lot to learn, and by the time we're done Nate will understand it better than the rest of us. The professors and postdocs offered suggestions of other plots to make to help understand the matter better, and all of us tried to understand what physical processes led to the data in this sample. We know that we have to explain these data before we can get on to our real goal, studying top quarks, which are produced much more rarely than Nate's events. We spent some time talking about what we can do to get that going; we've been recording data at a slower rate than we had hoped, but we are starting to convince ourselves that soon we will have enough to learn about top-quark physics.

By the time we're done with the meeting, my desk has been liberated. Now I can continue on the quest for knowledge.

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Entry 2

Posted Tuesday, Aug. 6, 2002, at 1:50 PM ET

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Dr. Kenneth Bloom is a postdoctoral researcher in experimental high-energy particle physics at the University of Michigan.

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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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