The standard model (SM) of elementary particle physics provides a remarkably successful framework for explaining three of the four known forces of nature: strong, weak, and electromagnetic. It is well known, however, that the SM as developed by Weinberg, Glashow, and Salaam leaves unanswered a number of basic questions: Why is there more matter than antimatter in the universe (see Baryogenesis) ? Why are the masses of neutrinos so tiny? Why is electric charge quantized? Why is the scale of electroweak symmetry breaking near the W-boson mass rather than the Planck scale? How is quantum gravity incorporated in a self-consistent way?

A number of scenarios for a "new standard model" have been proposed that address one or more of these questions. NPAC theorists are focusing on three: supersymmetry (SUSY), grand unified theories (GUTs), and models with additional spacetime dimensions. Each scenario predicts the existence of new particles that could be discovered through direct production in pp collisions at the Large Hadron Collider or e+e- annihilation at a possible linear collider. In addition, these particles can modify processes involving only the known particles of the standard model through virtual quantum effects. A major focus of our work involves analyzing these possible virtual effects in order to identify the particular "footprints" one expects in SUSY, GUTs, or extra dimension scenarios. Perhaps, a well-known example of such a footprint is the anomalous magnetic moment of the muon, where the present 3.4σ difference between the experimental value and standard model prediction hints strongly at SUSY in the large tanβ regime.

During the coming decade, a host of low-energy, high precision measurements will be performed, seeking to uncover additional footprints - a pattern of deviations from, or agreements with, standard model expectations -- that may point to a particular candidate for the new standard model. These measurements will involve studies of parity-violating asymmetries in electron scattering, weak decays of the neutron, pion, muon, and atomic nuclei; and various properties of the muon, such as its anomalous magnetic moment and possibly its lepton flavor-violating decay into final states containing an electron.

Recently, NPAC theorists have completed extensive studies of the possible effects of SUSY on these processes (for a comprehensive review, see the article by Ramsey-Musolf and Su). SUSY is a symmetry involving the interchange of fermionic and bosonic degrees of freedom. For each particle in the standard model, it predicts the existence of a "superpartner" with complementarity spin-statistics (see the charts on the Research main page). Many superstring theories suggest the existence of low-energy SUSY. If present, it would stabilize the electroweak scale in an elegant way, explain the breakdown of electroweak symmetry through radiative corrections, and lead to coupling unification at high scales. In addition, the presence of superpartner interactions could generate the cosmic baryon asymmetry (see Baryogenesis) and provide a candidate for cold dark matter.

To date, no superpartner has been observed. Consequently, if SUSY exits, it must be a broken symmetry, making the superpartners much heavier than the SM particles. Nevertheless, we may already be seeing the footprints of superpartners due to their fleeting, virtual impact on SM processes. The virtual effects of superpartners can arise from loop effects, such as those shown below, where they modify the interaction of the W-boson with second generation leptons.

We are now studying analogous effects that would arise from new particles predicted by GUTs or new particle-like modes arising in models with extra dimensions. GUTs provide a symmetry-based framework for understanding how the known forces of nature may have arisen from a single "superforce" at the birth of the cosmos and they can help us explain the unusually tiny scale of neutrino masses in a rather natural way (see Neutrino Properties). The larger gauge symmetries of GUTs imply the existence of additional particles, such as new Z and W-bosons, Higgs boson like scalars (see Higgs Boson), or additional fermions. If these particles are sufficiently light, they may be observed at the LHC and their virtual effects may modify SM processes, as in the case of SUSY.

Models with additional spacetime dimensions provide an alternate way to explain the stability of the electroweak scale. From our view within four spacetime dimensions, the presence of additional dimensions appear in the guise of new particles, called Kaluza-Klein (KK) modes. As with the new particles of GUTs or with superpartners, KK modes can either be seen through direct production in colliders or through their virtual effects on SM electroweak observables. NPAC theorists have recently initiated a program to study these virtual effects on a variety of electroweak precision observables.

In addition to computing the possible contributions from new particles, NPAC theorists are also working to refine our standard model predictions for electroweak precision observables. Many of these observables involve the low-energy interactions of light quarks and gluons, thereby introducing complications associated with the non-perturbative character of Quantum Chromodynamics. For example, the weak mixing angle, θ_W, that characterizes the way the SU(2)_L and U(1)_Y gauge bosons combine to form the photon and Z-boson is one of the most important parameters of the standard model. The value of sin²θ_W depends on the energy scale Q of a given process, as shown in the figure below (courtesy J. Erler):

Recently, NPAC theorists and their collaborators have reduced the QCD-related theoretical uncertainties in the "running" of sin²θ_W at low Q by a factor of eight compared to earlier computations. This advance makes the measurement of parity-violating asymmetries in elastic electron scattering at the Jefferson Laboratory and elsewhere a significantly more powerful probe of virtual effects of new particles because the standard model predictions are better known. We are currently working to refine the standard model predictions for other observables, such as parity-violating deep inelastic electron scattering.