Particle Phenomenology: What I Study

09Jul06

It occurs to me that it might be prudent to write a post describing, to a general audience, what it is that I study. Here’s my best attempt, starting from the general and going into more detail.

Act 1: Science

Science is a branch of human knowledge associated with the rational, objective, and empirical study of the natural world. The primary mode of generating knowledge is the scientific method, by which ideas are checked against experiments. It differs from the humanities in its subject and from the arts (including philosophy) in its method.

The only facts in science are directly based on observation. Causal explanations or unifying models for these facts are theories that can be rigorously checked against experiment. It is noteworthy that a “theory,” in the scientific sense, explains observed phenomena while also predicting phenomena that can later be observed. In this way scientific theories are falsifiable (i.e. subject to the scientific method) and differ from the common use of the word “theory” that implies opinion of speculation. The phrase “just a theory” cannot reasonably be used to refer to scientific theories as there is no level of scientific truth beyond a theory whose characteristic predictions have been rigorously checked.

Act 2: Physics

Physics is the branch of science that is concerned with nature on a fundamental level. Branches of physics study atoms (and all things subatomic), materials in different phases (condensed matter), motion in different systems (such as geophysics, general relativity), outer space (astrophysics and cosmology), and applications to other sciences and applied sciences (biophysics, physical chemistry, optics). A unifying theme would be an explanation of natural phenomena at their most basic scientific level. In some sense physics is the “purest” science since it is an interface between fundamental models of nature and experiments.

Unlike the other sciences, physicists can roughly be divided into theorists and experimentalists. Theorists are primarily concerned with models of nature that can be used to explain experimental data. Experimentalists are primarily concerned with testing theories and acquiring new data that may point to science beyond current theories. This divide occurs because of the high degree of specialization required to study nature at the level of physics. Theorists must be fluent in advanced mathematical methods while experimentalists must be talented in building apparati and interpreting data.

Act 3: Particle Physics (or “High Energy” Physics)

Particle physics is the branch of physics that asks what are the smallest building blocks of nature. In the past century, the “particles” physicists considered “smallest” have gone from atoms, to nuclei, to protons, to quarks (as well as electrons and their cousins). We have also learned how to think of the fundamental forces of nature in terms of force-mediating particles such as the photon.

Why do we study these particles? One reason is that we hope that by studying the basic building blocks of the universe we can understand composite objects better. There is also a philosophical (i.e. non-scientific) aspect associated with the question of what the ultimate basic building blocks of the universe should look like.

The current canon of particle physics is called “The Standard Model” and was mostly completed in the 1970s. It is a kind of quantum field theory called a non-abelian gauge theory (this means it is based on certain kinds of symmetries) and explains the strong and weak nuclear forces as well as electromagnetism. It has passed every experimental test (up to some recent modifications in the neutrio sector) and is regarded as a stunning success.

Intermission: Effective Theories

We know, however, that the Standard Model is incomplete. This is not to say that it is wrong, but that it is an effective theory for the distance scales that we have probed. In the same sense, Maxwell’s equations are an effective theory for electromagnetism above the atomic scale, where quantum effects become relevant (and another theory is effective: quantum electrodynamics).

The reason why effective theories are reasonable is that nature tends to only care about physics at the scale you are probing. For example, when a chef bakes a cake, there are several chemical reactions that occur as the batter bakes. At the heart of these chemical reactions are statistical and quantum effects which are ultimately explained by the Standard Model, which, in turn, may ultimately be explained my a more fundamental theory such as string theory. The chef, however, does not need to know particle physics, quantum mechanics, or even chemistry to bake the cake; the chef has an “effective theory” of how to bake cakes that is based on measuring cupfuls of ingredients.

In the same way the Standard Model is an effective theory for physics at the length scales we have probed. (Particle physicists measure scales in electron volts, which are inversely proportional to length; we have probed scales up to around the hundreds of giga-electron volt range.) There must be more to the story at smaller scales, but they don’t have an appreciable effect on the scale that we’ve currently been able to study. One of the major “missing pieces” in the Standard Model is a quantum theory of gravity.

Act 4: Particle Theory

Theoretical particle physics focuses on ways we can understand nature beyond the Standard Model. There are two kinds of particle theorists: phenomenologists and formal theorists. 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). The most studied formal theory today is string theory, though a smaller group of researchers (primarily outside the US) study another approach called loop quantum gravity.

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. From the point of view of Stanford Mathematics Department chair Yasha Eliashberg, string theory has turned the scientific paradigm upside down since instead of using experiments to confirm mathematically-motivated theories, string theorists use mathematics as experimental verification of ideas (this is a bit of a joke about physicists not knowing how to do math).

Act 5: Particle Phenomenology

I am not a string theorist. Particle phenomenology is somewhat of a blanket term used to describe theoretical particle physics that is not string theory or some other form of formal theory. While phenomenology gave way to string theory in popularity in the past 10 years or so (due in large part to the dearth of experimental data pointing to new physics), it is an exciting time to be in this subfield since the Large Hadron Collider (LHC)–the most powerful particle collider ever built–will be fully operational in just a few years.

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). Also, there is a subgroup of phenomenologists who work on the theory of strong interactions (i.e. quarks and gluons), called quantum chromodynamics (QCD). QCD is part of the Standard Model which is well formulated, but which is notorious for its difficult calculations (slightly mathematically, its calculations deal with Taylor series that don’t converge well). 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.

Act 6: Beyond the Standard Model (BSM) Phenomenology

My particular research interests are in “beyond the Standard Model” phenomenology which deals with ways to extend the Standard Model to address some of the problems that we already know about and, hopefully, any new physics we discover at the Large Hadron Collider. (To be fair there are several sources of data for particle physics, including astrophysics and cosmology, but colliders 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, we know that there is a class of massive particles called “dark matter” that is reponsible for the clustering of galaxies (this is due to astrophysical obsevations); such a particle should be produced at a machine like the LHC. Another reason is that there are instabilities in the mass of the Higgs boson, the last unobserved particle in the Standard Model, that can be cured by new physics at the LHC scales. Additionally, the strengths of the 3 forces described by the Standard Model come close to one another at a high energy scale. New physics could allow these three forces to unify into a single force that ‘broke’ into component forces in the early universe.

The two most prominent ideas in BSM phenomenology are supersymmetry (which was born in the context of physics rougly when I was) and extra dimensions (which was born a little less than 10 years ago), the former of which is currently capturing my attention.

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. On the eve of the LHC, however, many phenomenologists are realizing that adding new features to models that have been studied to exhaustion may not be the best use of our time. This is roughly analogous to someone who was trying to design a better VHS tape player just as DVDs were catching on. Since we hope to be awash in new data in just a few years, some phenomenologists have turned their efforts back to understanding the “Standard Model background” that we expect to see at the LHC. These are signals and data from the Standard Model that we expect to see but, since we haven’t really thought about what they look like at the LHC scale, that we don’t necessarily fully understand. 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.

By and large 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.

Filed under: Physics   |  8 Comments