String Phenomenology Revs Up in Anticipation Of LHC Turn on

“String Phenomenology” program organizers, Joe Lykken, (l) of Fermilab, and Shamit Kachru and Eva Silverstein of Stanford. Photo by Charmien Carrier

String theory, initially conceived in the late 1960s to explain the strong force that traps quark triplets in protons and neutrons, is no longer all that “new.”

With the discovery of the correct theory of the strong interactions in 1973, string theory became a backwater of particle physics for a decade until Michael Green of Cambridge University and John Schwarz of the California Institute of Technology discovered that string theory could incorporate both quantum mechanics and general relativity. That discovery set off a sustained exploration of the dazzling intellectual terrain opened by the string/brane approach to a theory of fundamental physical reality, which has now gone on for more than 20 years.

To some observers, both lay and scientist, the intellectual terrain opened by the theory has seemed to stretch out in directions and dimensions ever further from ordinary observations of reality. And, accordingly, string critics have, on and off for almost two decades, pointed to how developments have taken place solely in the heads of theorists whose feet have strayed from the path of experiment.

That disjunction between theory and experiment is what makes the program “String Phenomenology,” which ran at the KITP from Aug. 7 to Dec. 15, 2006, seem to some an incarnate oxymoron.

“‘Phenomenology,’” explained program organizer Gordon Kane, “is a term that applies in physics only to particle physics. In the other fields of physics,” said Kane, “there are experimentalists and there are theorists. Only in particle physics is there a further distinction among theorists who incline either towards phenomenology or towards formalism.”

Accelerator facilities collide particles, and the aftermath of the collisions is recorded via detectors. In order to “read” that data, phenomenologists first have to convert the theory of, say, a quark into its signature in the detector. So phenomenologists are, in particle physics, the middlemen between experiment and theory.

Kane characterizes the newly emerging role of phenomenology in string theory as the attempt to connect the theory to nature.

“A few years ago, almost nobody would allow that there was something called ‘string phenomenology,’” said Kane. “What people said then was, ‘Until you figure out string theory, you can’t do phenomenology.’ Now I think that is exactly opposite of what will happen. I think we’ll figure out string theory by doing phenomenology.”

After more than two decades of theory development, finally there are nearing completion facilities whose experimental results can be used to guide the theory — the Large Hadron Collider (LHC), set to turn on soon at CERN, in Geneva, Switzerland, and the Planck Satellite soon to be launched by the European Science Agency (ESA) to explore further the cosmic microwave background radiation, and the newly discovered changes in the expansion rate of the universe. Interestingly, both these grand efforts to probe deeper into fundamental reality are based, not in the United States, but in Europe.

The KITP “String Phenomenology” program was designed to enable participants to anticipate the results of these experiments and ask how what is seen might relate to string theory and vice versa.


Data to issue from experiments conducted at the LHC are expected to determine whether low energy supersymmetry (SUSY) exists or not. “Low energy” is relative here to the colossal energies that pertained in the early universe because the TeV (trillion electron volt) energy-scale of the particles accelerated at the LHC will exceed by an order of magnitude energies achieved in previous accelerator experiments 

Almost as old as string theory, supersymmetry is an idea that emerged in the early 1970s from hints coming from early efforts at string theory. It requires extra, quantum dimensions of space (not to be confused with the extra spatial dimensions of string theory). These quantum dimensions are characterized by anti-commuting numbers (i.e., ab is not equal to ba, but rather -ba), whereby every fermion has a bosonic superpartner and vice versa. Or, to put it another way, for every one of our known particles, such as the quark and electron and photon and neutrino, there exists a more massive superpartner (respectively, “squark,” “selectron,” “photino,” and “sneutrino”).

According to supersymmetric theories, half the particles have so far been discovered. And, notably, the lightest supersymmetric particle(s) to which all the others would decay at the low energies of our world and present universe — the neutralino — is one of the strongest candidates for the dark matter that astrophysicists have discovered makes up most of the matter in the universe.

Supersymmetry also enables the unification of the three forces of quantum mechanics — the electromagnetic and the weak (already unified in the electroweak theory) and the strong — with each other and with gravity. At high enough energies and at short enough distances, gravity, which is (relative to the other forces in our everyday world) extraordinarily weak, becomes as strong as the other three forces. Each of the forces then is a low energy manifestation of one force.

If Kane is an expert on and long-time enthusiast for supersymmetric theories, another “String Phenomenology” program organizer, Eva Silverstein, has a career-long interest in the complicated problem of supersymmetry breaking.

The world as we know it is not supersymmetric, so for supersymmetry to have existed, it had to have been broken somehow in relation to some energy scale. The question is whether that scale is as low as the TeV scale of the Large Hadron Collider.

Silverstein, whose research has focused, among other areas, on understanding the compactifications of the extra spatial dimensions that string theory requires, is “eagerly awaiting the LHC results on physics at the TeV scale.”

So far, efforts to understand how the extra dimensions of string theory are curled up or compactified have mostly centered on Calabi-Yau manifolds, which accord with low-energy supersymmetry. Silverstein has been working on a less supersymmetric class of compactifications.

Whatever the verdict rendered by experiment at the LHC on low-energy SUSY, string theorists such as Silverstein benefit because they acquire a sense of direction for the theory. It is almost as if she and kindred string theorists have said, “If low-energy SUSY is discovered, that’s great! But if it isn’t, then what space-shape could account for those extra dimensions?” In other words, the anticipation of experimental findings — whatever the outcomes — itself spurs theoretical research, and that impetus is the glory of this new string phenomenology.

Other space-shapes

Seated in her office during the program, Silverstein said, “I am enjoying the KITP now because of the presence of a lot of expertise in the study of more generic compactification manifolds than people normally study. People have almost exclusively focused on Calabi-Yau manifolds, which involve turning off the leading terms in the potential energy that you obtain from a more generic starting point in string theory."

“But looking from a so-called top-down point of view, we try to see if string theory in itself leads to a preference for any particle kind of low energy physics. If we take that point of view, then we need not make too many assumptions from the start, and most spaces that could be shapes for the extra dimensions of string theory are not Calabi-Yau, but spaces with more curvature, and,” she adds, “those with some negative curvature are by far the most generic among the possible geometries. This study is really interesting both for the potential application to low energy physics obtained from such compactifications, and for the description of space and time that is different from naive general relativity or point particle geometry."

“This interpretation of a string theory in extra dimensions is what’s new,” she said, and admits that, “It is not yet translating into an observable signature, but these kinds of things have a way of doing so after you understand them.”

Participants in the “String Phenomenology” program frequently used a metaphor to describe the state of string theory research. They talked about “living near the lamppost or not” — i.e., working where there is enough light to enable seeing what has mostly already been seen, but being kept, by the safety of the lighted way, from getting lost in the greater, dark surrounding terrain, where likely the answers lie.

Kane added an old twist to the new metaphor, “There is a joke about the drunk who looks under the lamppost for the keys to his car. He knows he dropped the keys elsewhere, but prefers to look where there’s light.”

In addition to Kane of the University of Michigan and Silverstein of Stanford University, other organizers of the program included Michael Dine of the University of California at Santa Cruz, Shamit Kachru of Stanford, Joe Lykken of Fermilab, and Fernando Quevedo of Cambridge.


KITP Newsletter, Spring 2007