July 7, 2012
The standard model of nuclear and particle physics has been around for a while. When I started doing physics, the model was in a construction phase, some pieces, in retrospect, were on the table; we were talking about quarks, although not all physicists viewed them as real objects. The unification of the weak and electromagnetic interactions followed soon and, by the early seventies, the theory of the strong interactions had appeared. We often speak of the beautiful symmetries, which the theorists seek when looking for unifying themes. However, in many ways we needed to understand the imperfections, the ways the symmetries were broken so that not all the entities of the model were massless. And this applied to both the quarks and to the intermediate vector boson partners of the photon in electroweak interactions, the W boson and the Z boson. The way these theories worked, we were looking at indications in the world we could see of the world we couldn’t see, that world at higher energies than our existing machines.
These indications were tantalizing and compelling. The charm quark was found soon after papers suggesting the existence of multiple generations. The bottom quark was found and machines were built with the express purpose of finding its partner. The first such machine, Tristan in Japan, found no such particle, but did see the hint of the Z boson. The SpbarpS proton-antiproton collider at CERN did not find the top quark, but did find the W and the Z bosons, which were then studied with the Large Electron Positron collider (LEP). Meanwhile, the Tevatron, a proton antiproton collider at Fermilab, was built to look beyond the W and Z, and, indeed, did find the top quark, but with a much higher mass than originally envisaged. Although, again, there were hints from lower energies, from the data at LEP, that the top mass would be high.
As you can see, a pretty full hand of quarks and their force carriers had been observed, but something was missing at the turn of the millennium. Already at the beginning of the story, a mechanism had been dreamed of to generate these masses, the so-called Higgs mechanism. In fact, the work was that of at least five contemporaries of Higgs, but the name has stuck like few others.
The idea of a search for the Higgs, and other new phenomena predicted to go with it, had motivated the development of the Superconducting Super Collider, started but not finished in Texas. It also motivated the construction of the Large Hadron Collider in CERN, which was eventually constructed; it also was recognized that LEP could possibly find the state and some felt they had just missed it at 114 GeV. It was recognized that the Tevatron could have sensitivity to the Higgs, and the last 10 years of Tevatron operations were dominated by that search. The Tevatron finished with what some might call evidence for a signal in the region 115 to 135 GeV. But the LHC appears to have nailed it at 125 GeV with two experiments, each with statistical strong signals at the same mass. The story is not fully over for the Higgs; its characteristics are not known. Is it really “standard” and, as with most things in physics, a discovery that will turn out rather to be a first word than a last word.
This is great news for physics from many points of view.
We at Jefferson Lab are part of the big science world. To the person reading the New York Times the work we do, and the work done at the LHC are more similar than they are different. The names of the people suggest in both cases that they come from all over the world to work there. Neither makes much use of a Bunsen burner or test tubes, but both use accelerators and when we talk about projects, the scale is hundreds of millions of dollars. So, it’s incredibly good news when a sister project to our own not only succeeds with the construction and operations, but also succeeds with a discovery.
So if the Higgs is very important, what can Jefferson Lab add?
There are many points that can be made. I will choose two.
First, in telling the story above, I suggested that the effects of high-energy phenomena can sometimes appear at lower energies. That was used to try to look ahead and to predict the masses, in turn, of the top quark, and of the Higgs. But now having found what we think is the Higgs, we may be able to make predictions, testable predictions, about what we might find in say, a parity violation experiment like Q-weak, which ran in Hall C for a couple of years, or like Moller being developed for Hall A.
Second, the masses generated for the light quarks, the up and the down quarks, which are the ones which dominate the makeup of the protons and neutrons, are extremely small. The only way we know for these specks to generate the 938 MeV of observed proton mass and, subsequently, the nuclei, like lead and gold, is through the strong interaction. Somehow, the quarks are dressed into what we call constituent quarks and then they bind together to make the proton. This is the piece of this mystery of mass that we can and do attempt to understand with both our experiments and our calculations at Jefferson Lab.
So in 2012, July 4, the Higgs discovery announcement brought an extra bonus of good news - a beautiful piece of science and, in lots of ways, a justification for the thrust of our own endeavors.