And now for the conclusion of “My Research,” a description of particle phenomenology in six acts; Part 1 (Act 1-3), Part 2 (Intermission).
Act 4: Particle Theory
Theoretical particle physics focuses on ways we can understand nature beyond the Standard Model. There are roughly two kinds of particle theory phenomenology and formal theory. Phenomenologists attempt to study the next level of effective theory by looking for signals of physics beyond the Standard Model in experiments and constructing new models. Formal theorists attempt to answer the bigger question of finding a fundamental “theory of everything” that is a complete theory that describes nature down to the smallest length scales (i.e. not just an effective theory). Most formal theory today focuses on string theory.
Since the characteristic scale of gravity is well beyond anything that is experimentally accessible in our lifetimes, formal theory often comes up against the barrier of experimental assessment. Much of the motivation for string theory comes from the hope that it can be a self-consistent theory of quantum gravity.
My interests are on the phenomenological side, where theory and experiment engage in a back-and-forth dance.
Act 5: Particle Phenomenology
Particle phenomenology often used as a blanket term used to describe theoretical particle physics that is not string theory. This generally refers to particle theory that is more closely related to experiment, with theory and experiment each suggesting new research directions to the other. It is an exciting time to be in this subfield since the Large Hadron Collider (LHC) will open up a new sectors of nature to scientific inquiry.
Some phenomenologists study finer details of the Standard Model, these include on-going studies of CP violation (such as Japan’s BELLE experiment and SLAC’s BaBar experiment) and neutrino physics (SuperK in Japan, various experiments in the US). There is also a subgroup of phenomenologists who work on the theory of strong interactions (i.e. quarks and gluons), called quantum chromodynamics (QCD) which is notorious for being nonperturbative. Most QCD research involves applying new mathematics (such as twistor methods) or computer simulations on discretized space (lattice QCD) to extract more accurate predictions from the theory.
While these are both very promising directions, my primary research interest is what happens when our current effective theory breaks down. The answer is almost certainly that it is replaced by another effective theory, perhaps motivated by string theory, that sheds further light on the structure of nature.
Act 6: Beyond the Standard Model
This is often called “beyond the Standard Model” phenomenology. It deals with ways to extend the Standard Model past its range of validity and, hopefully, include any new physics we discover at the Large Hadron Collider. There are several sources of data for particle physics, including astrophysics and cosmology, but colliders still represent our best controlled experiments.
There are good reasons to believe that there should be physics “beyond the Standard Model” within the reach of the LHC even though quantum gravity is well beyond that range. For one, from astrophysical observations we know that there is a class of massive particles called “dark matter” that is reponsible for the clustering of galaxies. Within reasonable assumptions, such a particle should be produced at the LHC. Another reason is the mass of the Higgs boson, which seems to suggest a “UV completion” at the TeV scale.
The two most prominent ideas in BSM phenomenology are supersymmetry and extra dimensions. Supersymmetry (SUSY) adds extra quantum dimensions to spacetime that lead to each particle having a “supersymmetric partner.” This is analogous to each particle having an antiparticle. Extra dimensional scenarios extend our spacetime with classical dimensions, allowing our known particles to resonate in these extra directions to produce new “Kaluza-Klein” particles.
For the past ten or twenty years, BSM phenomenology has been centered around model building, i.e. developing new theories or reworking old theories that can solve the problems of the Standard Model. With the LHC turning on, however, the BSM community has shifted towards developing bottom-up data-driven approaches to new physics. The big question when the LHC turns on will be whether we can identify signals that are beyond the Standard Model. This is not a trivial thing since piecing together experimental signatures at a particle collider is very much a detective mystery in its own right; luckily this task is shared by experimental particle physicists.
The BSM phenomenology community has been waiting patiently for new data and trying to squeeze the most that it can out of old sources of data. We hope to see new and unexpected things at the LHC that we can then spend another couple of decades thinking about.
