|Looking Toward TeV
Where Cosmology and Particle Physics Meet
|Contact: Paul Preuss, [email protected]|
Theoretical physicist Hitoshi Murayama is an optimist. Contemplating a discipline that many think is fragmenting into ever more pieces, like a smashed vase with shards labeled "dark energy," "antineutrinos," "hidden dimensions," "charge-parity violation," "supersymmetry," and the like, Murayama sees physics coming together instead.
It may not be a serenely classical coming-together, restoring the fractured vessel of physics to a seamless work of art. Nevertheless Murayama, who is a member of Berkeley Lab's Physics Division and a professor of physics at the University of California at Berkeley, foresees an approaching "synergy at the TeV scale," the experimental and theoretical regime where energies and masses are measured in trillions of electron volts. (The mass of subatomic particles is routinely expressed in electron volts, a number understood to be divided by the speed of light squared. The mass of the proton is given as .938 GeV, for example, just under a billion electron volts.)
"Any of these interesting physics topics must go through the TeV scale before they can reach their own destinations," Murayama says. Experiments at the TeV scale, along with the theories that motivate them and will hopefully explain their outcomes, are "a hub where everybody has to transfer to another flight."
Up, Down, and Sideways and Heaven and Hell
To talk about physics at the TeV scale, Murayama arranges myriad distinct problems into four groups. "Horizontal" problems deal with relationships among the three families of quarks and leptons, particles known as fermions. The families are similar except for their masses and are distinguished only by their so-called "flavors." Questions include what determines the masses of these particles; why neutrinos have any mass at all (yet so little!); and how the disparity between matter and antimatter in the universe originated.
"Vertical" problems deal largely with the particles known as bosons, which carry forces like electromagnetism and the weak force. For example, why does electric charge reduce to the same unit, that of the electron or proton? (True, quark charges are measured in one-third or two-thirds of that unit, but quarks are never found in isolation.) Can all these forces the electromagnetic force carried by photons, the strong force carried by gluons, the weak force carried by Z and W bosons, and much weaker gravity, carried by the yet-to-be-found graviton ever be described by a unified system?
Many (not all) horizontal and vertical problems have been a mainstay of physics since the promulgation of the Standard Model over 30 years ago. Especially within the last decade, however, new kinds of questions have arisen, what Murayama calls "questions from heaven" and "questions from hell."
Questions from heaven are cosmological: what is dark matter? What is dark energy? It's odd that the density of dark energy in the universe is only about twice the density of ordinary and dark matter plus ordinary energy; by cosmological standards that makes them almost equal. Why do we live at a time when the opposing pressures of matter and dark energy balance so neatly?
Questions from hell are just as "dark" but different: why is there anything at all in the universe? If mass and energy are equivalent, asks Murayama, "Why do particles have energy when they are just sitting around?"
Particles derive their mass (thus their energy) from a dark field that fills the universe. Murayama emphasizes that, while we have given it a name we call it the Higgs boson we don't know what it is. (Physicists often use the name of a field and its characteristic boson interchangeably, as in the Higgs field and the Higgs boson or bosons). When the curious potential of the Higgs field is plotted on a graph, it resembles the inside of the bottom of a wine bottle. "That's the hell we are in," says Murayama, "the bottom of the bottle."
Murayama points out that because of its strange potential, the Higgs field appears to behave like a quantum-mechanical curiosity known as a Bose-Einstein Condensate, or BEC. "A BEC is what you get when you cool a collection of particles, such as atoms, below some critical temperature," he explains. At this point they begin to behave as a single wavelike entity, a "quantum liquid" with peculiar properties. Liquid helium is such a condensate, which climbs out of any container it's put into, defying gravity, "because it somehow 'knows' that outside the container it can reach a lower potential energy."
Superconducting materials represent another form of condensate, in which cooled electrons form pairs that slide through the atoms in a crystal without resistance. Superconductors repel magnetic fields beyond a shallow penetration depth, which is why a superconductor levitates in a magnetic field. The Higgs field has an analogous effect on the weak force: the weak force, unlike the long-range forces of gravity and electromagnetism, is carried by massive bosons and can act only over short distances, as if it were being repelled by a BEC.
"Apparently we are living in a Bose-Einstein Condensate in which all particles are immersed. If we turned it off, mass would vanish and everything would fly apart in a nanosecond," Murayama says. "With atoms, you know what you started with to create a BEC, but the BEC in the universe, the Higgs field, is already there. We'll be able to use the next generation of accelerators to pump in energy, in hopes of seeing what the Higgs is made of."
Although Murayama refers to questions about the Higgs field as a "hell" problem, he concedes that "it could come from the heavens. Our keen interest is to understand the origin of the universe, with which the Higgs field is intimately related. New clues may come from learning more about dark matter and dark energy."
Whether from heaven or hell, the next steps in the inquiry require investigations at the TeV scale. The most attractive candidates for dark matter are weakly interacting particles whose estimated masses range widely, from at least a 30th of a TeV to a 10th of a TeV or more. The mass of the Higgs boson or bosons, depending on their nature, has been estimated to be a 10th of a TeV at minimum.
To create particles this heavy will take the most powerful accelerators ever built. After CERN's Large Hadron Collider (LHC) switches on in 2007, it will soon collide two beams of protons accelerated to 7 TeV, adding up to a center-of-mass energy of 14 TeV.
Colliding protons amounts to colliding bags of quarks, so the exact energy of any given quark-quark collision is inherently uncertain. Physicists hope to follow the LHC within a few years with an International Linear Collider (ILC) capable of colliding beams of electrons and positrons at a center-of-mass energy of 1 TeV; because these are leptons, point particles, the ILC will be able to explore many phenomena with a precision unobtainable by the LHC.
At first glance, hell that is, the Higgs field might seem to be inaccessible to study. If the Higgs boson is an ordinary particle whose mass is some fraction of TeV, it will repel itself just as the electron does. This means that the interaction of the Higgs with other particles couldn't be studied at anywhere near the short distances at which particles interacted in the early universe, the intimacy required for a Bose-Einstein Condensate.
But just as the theoretical proposal and subsequent discovery of antimatter specifically the electron's antiparticle, the positron made it possible to understand how electrons were able to come close together despite charge repulsion, the notion of supersymmetry may make the Higgs field easier to understand. Just as antimatter did for ordinary matter, supersymmetry doubles the number of fundamental particles in the game.
The quantum-mechanical process by which virtual pairs of antiparticles, electrons and positrons, constantly wink in and out of existence causes annihilations among these particles, which cancel much of the electron's self-repelling energy; thus the extra energy needed to study electron interactions is easily reached by today's accelerators.
Likewise, supersymmetry suggests there is a partner for every known ordinary particle, including the Higgs boson. If the self-repelling energy of the Higgs is cancelled by its superpartners, the Higgs interactions, including self-interactions, will be accessible to TeV-scale accelerators like the LHC and the ILC.
Supersymmetry is not the only theory to be tested by the LHC. Instead of extra particles, there may be extra "large" dimensions of space, which have been proposed to explain why gravity is so much weaker than electromagnetism or even the short-range weak force. The idea is that gravity only appears weak. All the other forces are confined to our familiar three dimensions of space, while the strength of gravity is diluted by acting in four or more dimensions.
Extra dimensions would reveal themselves in TeV-scale particle collisions because not all the particles that should be there will be; some, namely gravitons, will have escaped into the extra dimensions. If so, scientists will be able to find out how many extra dimensions there are simply by measuring the rates of such events in the ILC.
For a discussion of "problems from heaven" and the strange role neutrinos may have played in the origin of all matter in the universe, go to "Looking Toward TeV" part 2.