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The End of Accelerator Physics?

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In summer 2012, a dramatic news conference was held at the CERN laboratory in Geneva. There it was announced that two independent experiments at the lab's Large Hadron Collider (LHC) had confirmed the existence of the Higgs boson. This was a great triumph for the standard model of elementary particle physics that had been developed in the 1970s. In 1964, three different papers had predicted the existence of the Higgs particle, which then became an integral part of the standard model. In the model, the field associated with the Higgs particle provides a mechanism for generating the masses of other elementary particles.

However, the LHC, which cost ten billion dollars, was expected to do much more than simply confirm a model that, since its inception, had already agreed with every experiment designed to test it. The collider was supposed to find signposts for whatever physics lies beyond the standard model, at higher energies. In particular, it was anticipated that even before the LHC saw the Higgs it would produce a long-awaited set of new particles and other evidence for the theory called supersymmetry (SUSY).

A whole generation of theoretical physicists had devoted their lives to SUSY and its related theories. SUSY produced what these theorists believed was the key to the eventual unification of gravity with the three subatomic forces: the strong, weak and electromagnetic interactions, as well as providing the long-awaited quantum theory of gravity. It also potentially solved a number of technical problems with the standard model. Furthermore, a generic supersymmetric particle termed the "neutralino" was the favorite candidate for the dark matter that accounts for 27 percent of the mass of the universe. SUSY seemed to be inevitable--everything the doctor ordered.

But, so far, the LHC as well as searches elsewhere for neutralinos have come up empty. While many technical and popular articles have declared, "supersymmetry is dead," this is not yet quite the case. SUSY is still alive, but on life support. Now being upgraded to double its energy, the collider is scheduled to go back into operation in early 2015. The future of particle physics--at least the kind done at accelerators--may depend on what is found.

The standard model had unified the electromagnetic and weak interactions into a single "electroweak force." Shortly after it was developed, attempts were made to unify all three subnuclear forces in what was called the "grand unified theory" (GUT). Hopes were high since the simplest GUT predicted that protons are unstable with a very long but still measurable lifetime. In the 1980s, several large underground experiments capable of such a measurement were constructed and came up negative, falsifying the minimal GUT. None of the more complicated GUTs provided for practical tests with the technology of that day.

Some physicists at the time conjectured that grand unification occurs at a very high energy unreachable by any conceivable particle accelerator and that only existed in the very early universe. This seemed to be confirmed when a minimal supersymmetric model projected that the strengths of the three forces of the standard model, which are quite different at current accelerator energies, would come together at a single point nine orders of magnitude above the energy of the LHC. Although this energy is far out of the range of any accelerator, SUSY effects were still expected to appear at the LHC.

So we will have to wait and see what the new data show. If they show nothing--no SUSY or any other hint of new physics--then physicists will have a difficult sell to build the new, higher energy, and even more expensive generation of particle colliders now on drawing boards. This could mark the end of particle accelerator physics.

Of course, it won't mean the end of other branches of physics, which continue to thrive, scientifically if not financially. And it won't mark the end of particle physics either. There are other ways than higher and higher energy colliders to probe the physics at the grand unification scale--most notably, proton decay.

Although protons were not observed to decay with the lifetime predicted by the minimal GUT, they almost certainly must decay at some point. The standard model contains a principle called "baryon number conservation" which says that the total number of baryons in a reaction must remain constant. The proton is a baryon and the antiproton is an anti-baryon. Baryon conservation would say that the number of protons and antiprotons in the universe should be equal. If that were the case in the early universe, then by now there would be none left since they would all have annihilated with one another and we wouldn't be here.

Rather, the protons in the universe outnumber the antiprotons by a billion-to-one. So baryon number conservation had to be broken in the early universe, which also means that the standard model eventually has to give way to some new model, such as a GUT, that allows the violation of baryon number conservation. And this means protons must ultimately be unstable.

Since the proton is heavy compared to all the other non-baryonic particles into which it can decay, such as photons, electrons, muons, neutrinos, pions, kaons, Higgs bosons, . . . there are dozens of possible decay modes. The various grand-unified theories predict different rates for these decay channels, so measuring them would enable physicists to determine the exact nature of the physics that must exist beyond the standard model.

The largest proton-decay experiment currently running is Super-Kamiokande (Super-K) in Japan, which I worked on in the late 90s. It is on the verge of reaching sufficient sensitivity to observe some of the predicted decay modes. A new version 20 times larger called Hyper-K has been proposed by a Japanese collaboration. This is not getting the attention it deserves, with no visible effort by Americans or other nationalities to join in what could turn out to be the most important physics project of the next decade. With the current antiscience mood of Congress there is not much hope it would approve the needed funding.

Besides proton decay, another way to probe the high energies where unified theories apply is with the cosmic microwave background (CMB). Observations of the anisotropy and polarization of the CMB, which have received a lot of attention in recent years, explore the physics of the very early universe. They already seem to be telling us about gravitational waves and the nature of physical processes that resulted in inflation, the exponential expansion of the universe by many orders of magnitude that took place in the first tiny fraction of a second. Even optical astronomy, which looks at only one half of one percent of the mass of the universe, provides information about the other 99.5 percent as well as helping to probe the early universe.

So, even without accelerators, we can still look forward to significant advances in our knowledge of fundamental physics.