This is a not-so-brief retrospective of my research path. It became too long, so I broke it up into a series of sub-pages. These extra pages also provide links to some of my favorite projects.

**Quark models and hadron spectroscopy**

When I first entered the field, I was told that even the inventor of the idea of quarks, Murray Gell-Mann, considered them as useful mathematical abstractions rather than real physical objects. However, after the discovery of the J/psi in 1974, this rapidly changed. One of the lasting triumphs of particle physics soon after that was the understanding that all the strongly interacting particles are made of real, dynamical quarks. This goes beyond just the overall quantum numbers – many of the dynamical properties are manifestations of the quark substructure also. Read more.

**Weak hadronic matrix elements
**

The other major lasting development of the 70’s and 80’s was the demonstration that the SU(3)xSU(2)xU(1) gauge theory – the Standard Model – was in fact what underlies all of the physical world. Even after the theory was fully proposed this conclusion was not obvious. There initially seemed to be many phenomena that led people to propose elaborations that tried to explain supposedly anomalous results. One of these areas was in weak decays involving only strongly interacting particles – non-leptonic decays. Read more.

**Parity violation in nuclear physics
**

The field of nuclear parity violation existed well before the Standard Model. My first single authored paper (written while Golowich and Holstein were away for the summer) was the first to apply the Standard Model to this topic, following crucial work by Altarelli, Ellis, Maiani and Petronzio. . Read more.

**Effective field theory and chiral perturbation theory
**

The development of chiral perturbation theory was a significant step in the study of the strong interactions. It produces rigorous results about the low energy behavior of QCD, vwhich have as much validity as perturbative results at high energy. Moreover, chiral perturbation theory provides one of the best examples of an effective field theory, and so was also important in the development of EFT methods. Read more

**Weak decays**

The weak interactions have a particular interest because they have a very rich phenomenology, and these topics reveal a great deal of the fundamental theory. When writing our book, we became aware of how each separate reaction has its own literature and indeed forms its own little sub-field. It was impossible to describe each in the detail that it deserves, yet each contributes to the richness of our understanding of the Standard Model. Read more

**CP violation**

The violation of CP symmetry has long been especially intriguing. The effect is small and could be a very sensitive indicator of physics beyond the Standard Model. Recently experiment seems to be indicating results consistent with the notion that the Standard Model is the only source of CP violation, but this conclusion was far from obvious in the past. Read more

**Heavy quarks
**

Surprisingly, when dealing with heavy quarks, some aspects of the Standard Model become simpler. Decay of these quarks are then are sometimes especially good probes of the Standard Model. Read more

**Dynamics of the Standard Model
**

Our book, *Dynamics of the Standard Model* (JFD with Eugene Golowich and Barry Holstein) was published in 1992 by Cambridge University Press. This book provides a summary of how one connects the Lagrangian of the Standard Model to the physics of our world. We feel that it has a somewhat unique place in the literature of the Standard Model, and we have been pleased with the reception of the book.

While the book still is quite relevant, many advances have happened since its initial publication. Experimentally, the top quark was discovered and B Physics was extensively explored, but we are still waiting for the discovery of the Higgs boson. Barry, Gene and I are presently working on a second edition which will provide an updated treatment of all topics.

After the publication of our book, my research branched out into several new areas.

**General Relativity as an effective field theory
**

The conventional lore is that quantum mechanics and gravity are not compatible, that there is a clash between these two fundamental ingredients of our world. This is considered one of the crucial problems in physics, and has spawned the field of Quantum Gravity. However, the conventional view is wrong in some ways. Gravity and quantum mechanics in fact work fine together at ordinary energies. There are signs of trouble are extremely high energies – the Planck scale – but we can treat the two together in situations of low energy and low curvature such as our present world. This insight comes from effective field theory. Read more

**Anthropics and the multiverse
**

For many years anthropic reasoning had a bad smell in particle physics. It seemed to imply the arrow of causality in the wrong direction – that our existence determines something about fundamental physics. For me, this changed with Weinberg’s discussion of the cosmological constant as an anthropic constraint. This redirected the point of view. If there can be multiple domains in the universe (the so-called multiverse or landscape) then it is natural that we find ourselves in an anthropically compatible domain. Since there is presently no other sensible theory of the cosmological constant, this becomes a strong motivation for the consideration of these types of theories. Read more

**Baryons and Nuclei**

Neutrons and protons form the stable baryons – these are the most direct manifestations of the multiple facets of the Standard Model, with important strong, electromagnetic and weak properties. Understanding baryons then goes a long way towards understanding how the Standard Model leads to the properties of our world. Read more.

**Physics in the early universe**

The temperature fluctuations in the cosmic microwave background (CMB) are in principle dependent on all the scalar fields that are light at the time of inflation. Models of new interactions predict many possible light scalars at the time of inflation. I am interested in the multiple (significant but not yet compelling) anomalies in the CMB power spectrum as potential indicators of the physics at the time of inflation. Read more.

**Emergence
**

I explored the possibility that the Standard Model is emergent, in the sense that the true fundamental theory is constructed with other degrees of freedom, and the the fields of the SM only emerge at low energy. An analogy is how water waves can be treated as objects that satisfy a well defined wave equation, even though the fundamental physics at short scales are atoms bumping into each other, which is described by very different laws. Can photons similarly be the macroscopic description of a very different fundamental theories, where the idea of a photon is not even the right degrees of freedom? Read more.

**Quantum gravity as a renormalizeable quantum field theory** At present I am interested in understanding whether there is the possibly the ultimate theory of quantum gravity is a regular renormalizeable field theory. We already know that the low energy limit is described by quantum field theory in the form of an effective field theory. But effective field theory has limitations at high energy. If the theory is renormalizeable, then it can be valid at all energy scales. However this possibility requires features that are a bit unusual. For example the fundamental propagators must have terms quartic in the momentum. There are thought to be problemmatic issues of such theories, and the challenge is to see if this could possibly work. Readers are invited to look at my recent papers for descriptions of this work.