Research of James Wells

Standard Model of Elementary Particles

The Standard Model of elementary particle physics is the theory of how the known elementary particles interact. Elementary particles are divided into two groups: matter particles and force carriers. Matter particles are the stuff that make up our bodies, for example. They include electrons, neutrinos and quarks that make up the proton and neutron. The second kind of particles are the force carriers. They include the photon, which mediates the electromagnetic force, the gluon, which keeps the quarks inside the proton and neutron, and the weak gauge bosons W+, W- and Z, which enables neutron decays and other radioactive processes.

Problems with the Standard Model

The Standard Model has been a very successful theory in describing the basics of elementary particle interactions. However, it is surely incomplete. There are many questions it cannot answer. These questions come in three categories: experimental questions, unification questions, and philosophical questions.

Experimental questions: The Standard Model does not have allow for dark matter, whereas we know convincingly from astrophysical and cosmological experiments that dark matter must exist. Furthermore, the Standard Model cannot explain why there is much more matter in the universe than antimatter. New physics beyond the Standard Model must be invoked to address those questions. There are many ideas. Although some of these ideas have been ruled out by experiment, no one idea has been confirmed yet by experiment. 

Another experimental question of the Standard Model is confirmation of the only speculative idea within the theory. Each fermion of the theory, whether it be the electron or a quark, comes with two different spins. The left-handed spin fermion always interacts differently than the right-handed spin fermion. These two different spins of the fermion must marry to obtain mass, but when they interact differently this is impossible to accomplish on their own. A new particle must bring the two together. This new particle is called the Higgs boson, and it is an hypothesis of the Standard Model that a single Higgs boson ultimately gives mass to all the elementary particles.

The speculative Higgs boson idea has not been directly confirmed by experiment, although the idea remains compatible with all previous data as long as its mass is between 114 GeV and about 200 GeV. The lower bound is obtained by lack of direct discovery of the Higgs boson at the LEP2 Collider at CERN, and the upper bound is obtained by requiring that the quantum loop corrections that the Higgs boson contributes to particle interactions is not too high.

Unification questions: The Standard Model is a complex theory with many independently moving parts to it. To start with there are three independent forces and a plethora of elementary particles. Can they be organized in a more efficient or unified manner? Grand Unified theories of all different stripes and colors have been postulated to answer these questions. The results are mixed. For every clever idea that unifies the forces, additional problems often surface. Furthermore, no idea has been directly confirmed by experiment, nor would one likely be confirmed in the near term due to the very high energy scales that are assumed to be required for unification. Nevertheless, subtle inferences may be able to made on low-energy data to gain confidence in a unification theory, and there is much effort on this. For example, supersymmetric theories often play a palliative role in unification ideas, and the finding of superpartners at the LHC would likely give more confidence in unification theories.

Philosophical questions: There are additional concerns regarding the Standard Model that are best described as philosophical in nature. One such question is why the elementary particle masses are so much smaller than the scale of gravity, the Planck mass. This is often called the hierarchy problem or the finetuning problem. Quantum theory, as we presently understand it, is extraordinarily finetuned if we allow the Higgs boson to be the answer for elementary particle masses. Perhaps there are different underlying considerations, such as a string landscape of solutions, that changes the calculus on finetuning. Or, perhaps, as some claim, we do not understand quantum field theory properly, and the problem is merely the creation of our false understanding of quantum corrections. Nevertheless, ameliorating the finetuning problem, or equivalently the hierarchy problem, is taken very seriously and is one of the most important considerations in theoretical physics today. Supersymmetric theories and extra-dimensional ideas are usually invoked to help solve this problem.

Another related philosophical question is whether we need to simultaneously solve the cosmological problem with the hierarchy problem. The cosmological constant is also many, many orders of magnitude below its would-be value characterized by the Planck Scale, or even the electroweak scale. Both problems involve our confusions about how serious to take apparent destabilizing influences of the gravitational Planck scale to our low-energy physics theories.

Yet another philosophical problem is the question of how well can we break the big problem into smaller pieces. What is the big problem? It is The Theory of Everything, as it is sometimes called. Combining quantum mechanics with gravity, understanding mass generation, explaining the masses and coupling constants from some principle, etc. To develop a theory that has the smallest number of parameters possible, perhaps even down to zero, is the ultimate goal. So far, progress has come in stages. Most recent revolutions in understanding involved taking a mass of confusion, such as the collection of hadrons, and understanding it within the context of renormalizable effective field theories. How much further can we go with renormalizable effective field theories? Should it be considered a guide to future theories on the way to the Theory of Everything (e.g., Supersymmetry)? Or, should we hold that renormalizable theories have had their day and now is the time to pursue non-renormalizable effective theories (e.g., Randall-Sundrum theories). Furthermore, is it now necessary to address the full scope of theory from low scale to the Planck scale in order to make the next step in theory progress (e.g., String Model Building)? 

These are philosophical problems today for researchers choosing what to pursue, but they do not necessarily stay philosophical problems. We expect them to be clarified and perhaps even resolved in the future by data, after which new philosophical problems surely will arise.

My Research Activity

The long introduction serves to help define my research scope. I contemplate the questions above and pursue ideas of physics beyond the Standard Model theory.  My activities and goals are many-fold: exploring ideas, finding new ways to discover theories by experiment, developing and refining theories to answer as many questions as I can while generating the fewest additional problems, using experimental results to guide theory, and motivating future experimental work. Many of these activities are pursued with valued collaborators, including postdoctoral researchers and students.

The best way to understand what comes out of all these considerations and goals that I have described above is to consult my publication list. One will find themes in my research that include studies of more general Higgs boson theories, supersymmetric theories of unification, extra dimensional theories, collider physics probes of beyond the Standard Model ideas, and dark matter theories.