Questions at the TeV Scale		Back to the Beginning

Storming Heaven

Heavenly secrets will benefit from studies conducted by accelerators and other instruments like telescopes and satellites. Some supersymmetric particles are prime candidates for the very weakly interacting particles Murayama thinks are the best candidates for the dark matter, but there are other candidates as well. Accelerators can measure the masses of the candidates and see how frequently they interact with other particles; this will allow their cosmic abundance to be calculated, a number that can be directly compared with telescope and satellite observations.

	Dark energy is the most mysterious of the "heavenly problems," not least because it accounts for almost three-fourths of the universe's total density.

Dark energy is the most mysterious of the heavenly problems, and like dark matter, calculating its density in the universe will crucially involve the TeV scale. No one knows what dark energy is. Two mutually exclusive proposals, one called the cosmological constant, the other quintessence, have gotten most of the press.

The cosmological constant derives its name from a purely mathematical term that Einstein inserted in early versions of General Relativity (but later withdrew) to keep his equations from predicting that the universe would collapse under its own gravity. Today's cosmological constant, if it is real, is thought to be some puzzlingly weak form of the quantum-mechanical energy of the vacuum. The cosmological constant is indeed constant; quintessence is quite different, an unknown force whose "pressure" on the contents of the universe changes over time.

Murayama favors a third alternative, which he calls a "frustrated network of domain walls." The idea is that the universe is composed of many domains that have never been in communication with one another, even at the big bang. The edges or walls of such domains would exert pressure opposite that of gravity. Just as the tiny magnetic domains in iron tend to line up and order themselves as the hot iron slowly cools, cosmic domains tend to join together as the universe expands. But if this process is frustrated, and the domain walls themselves expand, negative pressure builds and causes expansion to accelerate.

To decide among these alternatives — the cosmological constant, quintessence, frustrated domain walls, or other proposals — will require measurements of distant supernovae with telescopes on the ground and in space, to determine the ratio of the dark energy's pressure to its density. This number is called the equation of state. "Once we know the equation of state, we will get the first glimpse of the nature of dark energy," Murayama says. "Where we go from there will depend on what we find."

Looking all ways

Of all the scattered pieces of physics that the TeV scale will serve to illuminate, neutrino physics is one of the most remarkable — not least because it shows up in so many different contexts. The recent discovery that neutrinos have mass is particularly significant for "vertical" questions like whether and how the forces of nature may be aspects of the same force.

The mass of the neutrino would be too tiny to measure were it not for the fact that there are three flavors of neutrinos — the electron's neutrino, the muon's neutrino, and the tau particle's neutrino — each with a different mass. Neutrinos appear to oscillate from one flavor to the next as they travel. Exact masses for each type have yet to be determined. Even more important will be to determine the differences among the masses.

Experiments at neutrino detectors like SNO and Super-Kamiokande (pictured here) have established that neutrinos oscillate among various flavors, each with a different tiny mass.

Tiny and elusive as neutrinos are, it's possible to study them because they are produced by many sources, including nuclear reactors, cosmic rays, and our own sun, and because we can measure their oscillations over long baselines like the diameter of the earth. Most of them pass easily through such barriers, but a few can be caught in the heavy water or mineral oil of detectors like the Sudbury Neutrino Observatory, which measures neutrinos from the sun, or KamLAND, which measures neutrinos (and antineutrinos) from nuclear reactors and from radioactive decay within the earth.

It may be possible to use these oscillations — what Murayama calls "neutrino interferometry" — to probe energy scales well beyond the TeV scale. Neutrino mass and neutrino oscillations may be a key to grand unified theories, theories that regard the electromagnetic, weak, and strong forces as aspects of a single kind of interaction.

The still-mysterious nature of neutrinos may even hold the answer to the origin of all matter in the universe. The problem is one of Murayama's "horizontal" kind, having to do with the flavors that distinguish the basic families of quarks and leptons, which despite their very different masses share the same quantum numbers.

"If two states share the same quantum numbers, you expect them to have the same energy level," he says. "That's not the case among families of quarks and leptons. I and other theorists think there must be a hidden quantum number, a flavor number, by which the families are distinguished."

Several models for flavor quantum numbers are possible, and TeV-scale physics is one of the key ways to identify the candidates and perhaps choose among them. Such physics may turn out to be supersymmetry, or hidden dimensions, or some hybrid. Whichever model takes the lead, it will necessarily involve the basic questions of the origin of matter and what neutrinos may have to do with it.

Murayama favors a mechanism known as leptogenesis. "Leptogenesis is an attempt to answer the question of why we don't see an appreciable amount of antimatter in the universe," he says. "Equal amounts must have been formed in the early universe, but it all annihilated. Lucky for us, just one part in a billion of ordinary matter was left over."

While there are small and measurable differences in the production of particles of matter and antimatter in some processes — due to a phenomenon known as charge-parity violation — these processes are unable to account for the absence of antimatter in the universe as a whole.

	If neutrinos and antineutrinos are the same particle, when an ordinary neutrino (blue) collides with a Higgs boson, it transforms into a short-lived, very massive antineutrino (red), which soon converts back to an ordinary neutrino. This so-called seesaw mechanism may mean that neutrinos and antineutrinos are the ultimate source of all matter in the universe.

What's needed is some additional mechanism for converting antimatter to matter, "but no one has ever seen one turn into the other," says Murayama. "Neutrinos may offer that possibility." The reason is that, unlike electrons or other familiar particles of matter, neutrinos may be their own antiparticles. If so, tell-tale charge-parity violation may be observable in neutrino oscillations.

"We've never seen a right-handed neutrino, but we have seen right-handed antineutrinos," says Murayama. "If neutrinos are their own antiparticles, this could be just a matter of our point of view."

If neutrinos really are their own antiparticles, there exist mechanisms, called seesaw models, by which they can convert ordinary matter to antimatter and back again; in this view, the more antineutrinos there are, the more ordinary matter like electrons there are to balance them. And herein lies the genesis of all matter.

Coming Soon to an Accelerator Near You

"When the LHC switches on in 2007, it could open up a realm of physics that's easy to interpret," says Murayama. "Much more exciting would be physics that's rich and complex, one that allows many explanations, many models, including supersymmetry, extra dimensions, and a host of others — even ideas that nobody has come up with yet.

"Maybe the answers will scream, 'I'm here,'" Murayama says. "But I suspect it won't be that easy, and we'll have a lot of fun trying to figure them out."

Additional information

• More about supersymmetry
• More about neutrino experiments at SNO (the Sudbury Neutrino Observatory) and KamLAND (the Kamioka Liquid Scintillator Antineutrino Detector)
  
• More about neutrino mass (pdf)
• More about symmetries and antimatter
• More about antimatter and leptogenesis

Return to "Looking Toward TeV" Part 1