Carl Hagen

Carl Hagen

Name: Carl Hagen

Title: Professor of particle physics

Current institution: University of Rochester

Home state: Illinois

Alma mater: MIT

Reason for studying physics: I attended a small high school in Chicago, and I really did not get much exposure to physics, nor did I have a real appreciation that one could actually do physics as a career. When I went to MIT (ostensibly as an engineering student) I found my physics courses fascinating. I also saw that some of the best students were physics majors. So I essentially said to myself that I think I'm as smart as they are, so why not become a physics major. And so it happened.

Paper published on theory of mass-giving mechanism: Global Conservation Laws and Massless Particles (Physical Review Letters, published November 1964, co-authored with Carl Hagen and Tom Kibble)

Contribution to theory: Gerald Guralnik, Tom Kibble and I showed (unlike Brout/Englert/Higgs) that the massless Goldstone boson (the particle thought to appear when symmetry breaks) need not be a problem since it occurs only in the so-called gauge sector of the theory. That is to say, it does not emerge as a physically observable particle. We developed the theory in a certain gauge (the radiation gauge) that does not have gauge excitations and consequently no massless particle.

What were some obstacles you faced developing your theory? Chiefly, there was the issue of showing (and even more important convincing ourselves) that there was indisputably an "out" from the Goldstone theorem. Our obsession with this goal is what caused us to delay by several crucial weeks our submission to the journal. I might also mention that our drive toward that goal also led us to realize the rather startling fact that local charge conservation does not necessarily imply a globally conserved charge. (Stated somewhat less abstrusely, this just means that there does not necessarily have to exist a conserved total charge operator even though the formal theory makes it easy to jump to that conclusion.

How was the paper received in the scientific community? It didn't have any noticeable impact at first. But Steven Weinberg saw a few years later that it would allow him to circumvent the problem of zero-mass particles. It was a major problem in his attempt to construct a unified theory of the weak and electromagnetic interactions. This led eventually to the discovery of the W and Z bosons and the general recognition that this was indeed the way to construct a coherent view of the world of elementary particles. It also got him a Nobel prize!

What did you think about the July 4, 2012 announcement made at CERN about a newly discovered particle that looked a lot like the Higgs boson? It resolved any remaining doubts I ever had that I chose a career in particle physics theory. Imagine: 50 years later some of my work had motivated a $10 billion investment in the LHC and motivated thousands of physicists to toil in the effort to find a particle that we had conjectured! It doesn't get much better than that.

What are you up to now? My most recent work has been on the application of group theoretical methods to solving problems such as the derivation of the Aharonov-Bohm phase shifts.