The Standard Model (SM) of elementary-particle physics provides a unified description of all phenomena in fundamental physics discovered to date. It is a triumph of 20th century science. The SM describes, at a microscopic level, the interactions between the elementary constituents of matter (leptons and quarks). While it lacks a consistent quantum theory for the gravitational force, there are currently no compelling hints of any discrepancies with SM predictions from particle-physics experiments at energies ranging into the 100 GeV (= 100 billion electron-volt) region.

Despite its many successes, the SM leaves open many fundamental questions, such as what exactly creates the masses of elementary particles (i.e., the structure of the Higgs sector), why there are three generations of elementary fermions, why there are four forces, why there is more matter than antimatter in the universe, and what is the nature of the dark matter. The modern view is that the SM is an effective field theory valid at those energies and length scales accessible to present experiments. At some higher energies or shorter distances, this theory will have to be extended to a more fundamental description of nature.

The search for physics beyond the SM (often referred to as “New Physics”) is almost as old as the SM itself. Much of the model building efforts in the past 20 years have been driven by attempts to solve the hierarchy problem – the question how the electroweak scale M_{w} c^{2} ≅ 100 GeV can be stabilized at a value so much less than the scale of grand unification or the Planck scale. The most popular extensions of the SM addressing this problem include supersymmetry (a symmetry between bosons and fermions, which predicts a yet undiscovered “superpartner” for each SM particle), technicolor (which postulates a new strong force), and extra (spatial) dimensions (which explain the weakness of gravity through geometrical effects).

Searches for physics beyond the SM can be divided into two distinct approaches: those that aim at producing new, heavy particles using particle colliders at the highest accessible energies, and those looking for indirect hints (“virtual effects”) of new particles or new interactions in high-luminosity low-energy measurements. A broad program in flavor physics, studying the properties of B and D mesons, kaons, neutrinos, or charged leptons, follows the second route of approach. “Flavor” is the quantum number distinguishing the three generations of quarks and leptons. The study of rare decay properties of quarks and leptons in processes such as flavor-changing neutral current decays or neutrino oscillations can be used to probe for new particles or interactions at energy scales into the range of 10^{3}-10^{4} GeV, far extending beyond the reach of high-energy particle colliders. These studies may help to explain the matter-antimatter asymmetry in the universe and provide hints of physics beyond the SM.

Our group pursues a broad research program in particle-physics phenomenology centered around these questions.

**Flavor physics and heavy quarks**

A major experimental effort is presently underway to study the weak decays of B mesons at several dedicated facilities (“B factories”). The aim is to test the flavor sector of the SM, to explore the phenomenon of CP violation, and to search for new particle and interactions at higher energies. We are developing theoretical tools that make it possible to interpret the data collected at the B factories and extract from them the fundamental parameters of the underlying theory. The main challenge is to control the strong-interaction QCD effects in weak decays using tools such as effective field theories, factorization theorems, symmetries, and heavy-quark expansions.

**Beyond the Standard Model**

Our group is also involved in studies of extensions of the SM such as supersymmetry and theories with extra dimensions. A particularly attractive scenario for solving the hierarchy problem consists of theories with a warped extra dimension (Randall-Sundrum models), in which the weakness of gravity is explained by a position-dependent scaling factor in the metric, which varies along the extra dimension. The AdS/CFT correspondence implies that 5-dimensional Randall-Sundrum models are dual to 4-dimensional, strongly coupled theories of electroweak symmetry breaking. We are especially interested in aspects of flavor physics in warped extra dimensions. The presence of the warp factor provides a new mechanism for explaining exponential hierarchies in flavor physics in terms of localization of fermion wave functions in the extra dimension.

**Collider physics**

The search for new phenomena at high-energy colliders such as the Tevatron at Fermilab, the Large Hadron Collider (LHC) at CERN, or the planned International Linear Collider (ILC) requires precise calculations of a variety of processes, including the production of new heavy particles as well as many SM background processes. Precise calculations of cross sections in perturbative QCD often suffer from the presence of large logarithmic corrections, which invalidate a fixed-order perturbative expansion. These logarithms typically result from a complicated interplay of soft and collinear emissions of massless partons. We have developed a novel approach based on effective field theory – the so-called soft-collinear effective theory – which allows for an efficient and transparent factorization and resummation of such large logarithms to all orders in QCD perturbation theory for a variety of kinematical situations. This technique has already been applied to deep-inelastic scattering, heavy-quark fragmentation, the Drell-Yan process, and the production of Higgs bosons and top-quark pairs.