Why do I work on dark matter and supersymmetry?

 

I'm a phenomenologist, which means I take my cues from experimental data rather than from first-principles theory. In my opinion, the two most robust pieces of data we have to guide particle physics research are the existence of dark matter and the mass of the Higgs boson. Read on for some background on the implications of these measurements, and check out the other pages under "Research Topics" for particular projects addressing these issues.

Dark matter - beyond WIMPs

Of all the mass energy* in the universe, the particles comprising everything we've ever measured or observed directly -- stars, planets, interstellar gas, all life on Earth -- make up only 18% of the cosmic pie chart. The vast majority of the mass of the universe has only been observed indirectly, and has been dubbed dark matter. Despite this, the gravitational effects of dark matter are numerous and profound, and many independent measurements have all converged on a remarkably consistent story:

  • Rotation curves. By observing how fast stars rotate about the centers of their galaxies, we can infer how much total mass exists inside the galaxy. This is a fun calculation which uses only first-year physics and Newton's Law of Gravity, and is the same calculation which determines the orbital radius of satellites traveling at a given speed, knowing the mass of the Earth. Lo and behold, the calculation tells us there is much more matter than we can observe through electromagnetic radiation (microwave, infrared, visible light, or X-rays), and amazingly, this "halo" of dark matter extends far beyond the last stars of the galaxy.
  • Structure formation. The universe has been expanding since the Big Bang, and hence all atoms have been flying apart from one another since they were created. So how did enough matter collect in one place to form galaxies, stars, and planets? Gravity makes things clump, and the more stuff there is in one place, the more other stuff tends to clump around it. Dark matter can provide the "seeds" for structure formation by clumping first, after which ordinary matter will be gravitationally attracted, eventually forming stars and galaxies. Without dark matter, the primordial nuclei would have been moving too fast to have time to clump. This is entirely consistent with the observed "halos" described above.
  • CMB power spectrum. The universe took a selfie at the tender young age of 380,000 years (that is, almost 14 billion years ago), and we now have the technology to develop the image. In the early universe, charged particles were so hot and dense that light couldn't travel any appreciable distance before being deflected or absorbed. But once the universe had expanded and cooled sufficiently, light could travel unimpeded, and the "surface of last scattering" 380,000 years after the Big Bang is a snapshot formed by light escaping from this cosmic soup which we see today as the cosmic microwave background (CMB). Tiny temperature variations in these photons from different parts of the sky track density variations at that point in cosmic evolution: hotter photons means more stuff was there.** Dark matter's distinctive property is that it carries mass but doesn't interact with photons, so it contributes differently to the behavior of the CMB. Measurements of correlations between CMB temperatures at different points in the sky (the power spectrum) agree precisely with the presence of a significant component of dark matter, and allows us to determine that it makes up 82% of the mass of the universe.

In summary, dark matter is not an arbitrary fudge factor physicists have added to their equations to tweak some puzzling observations. It is a single ingredient which, in one fell swoop, explains vastly disparate properties of the universe from the earliest times until today, through independent measurements, in a consistent picture. This is the best kind of result in science: a theory that keeps on giving! Of course, not every observation agrees perfectly with the theory and work is still ongoing, but the broad picture is firmly in place.

The problem is, we know nothing  about dark matter apart from the fact that it's there. How much does it weigh? Does it interact with itself, or weakly with ordinary particles? Are there multiple species of dark matter, perhaps analogous to the proton, neutron, and electron which make up ordinary matter? Much effort has focused on a particular scenario: dark matter weighing about as much as an atomic nucleus, interacting with atomic nuclei via the weak nuclear force, and consisting of just a single particle species. This kind of dark matter is known as a WIMP, or "weakly-interacting massive particle," and it's a beautiful theory: with a minimal number of moving parts, it predicts exactly the right amount of dark matter, and is tied to both the Standard Model and supersymmetry (see below). Alas, the simplest explanation is not guaranteed to be the correct one.  Dozens of experiments have searched for this particle -- passing through the Earth, annihilating at the center of the galaxy, or being produced in particle colliders -- to no avail. The WIMP scenario is certainly not ruled out, but it is becoming highly constrained.

I'm interested in exploring theories of dark matter beyond the WIMP: lighter particles (MeV-scale or sub-eV scale, in particle physics units), particles with different interactions (a dark photon rather than the weak nuclear force, or with electrons rather than nuclei), or multiple species of particles. Each of these theories would give different experimental signatures, and my research is focused on proposing experiments to look for these different kinds of dark matter. I'm especially drawn to the creativity and open-mindedness required to find just the right experimental avenue to detect these particles, and also to the possibility of collaboration between different fields of science (neutrino physics, condensed matter, physical chemistry, plasma physics, materials science) to develop the right equipment to build the experiment. I believe discovering the identity of dark matter is the most pressing question in particle physics which is likely to be resolved on a 50-year timescale, and I want to help cover all the bases in case dark matter is hiding in a place we least expected it.

 

Supersymmetry and a 125 GeV Higgs

The Standard Model of particle physics, whose experimental confirmation culminated in the discovery of the Higgs boson, has remarkable internal consistency. By this I mean that the theory is valid up to spectacularly high energy scales, comparable to the Planck scale where quantum gravity becomes important. Unlike the situation in the 1930's, where Fermi's theory of muon decay required  an additional particle (the W boson) to keep the theory from predicting probabilities nonsensically greater than 1, no additional particles are required to keep the Standard Model humming like a well-oiled machine. There's nothing wrong with adding additional particles, like dark matter, or ingredients, like neutrino masses, but by the same token, nothing determines what properties these particles or ingredients must have.

If you buy this story, then you also have to buy an exceptional numerical coincidence: the Higgs mass of 125 GeV (about the mass of a xenon nucleus) is 1000000000000000 times smaller than expected. This is like throwing a dart at a dartboard and not only hitting the bullseye, but hitting a specific proton inside a specific nucleus inside a specific molecule...you get the point. (More fun analogies on the "Supersymmetry" page.) This coincidence is known as the hierarchy problem. Physicists don't like coincidences, they make us suspicious.  So let's entertain the idea, motivated purely by theory rather than experiment, that some extra mechanism is required to keep the Higgs boson so light.

One of the best candidates for a solution is supersymmetry, which postulates that each type of particle has a partner particle called a superpartner, and these pairs of particles conspire to keep the Higgs boson light. This may sound a little contrived, but just like dark matter, supersymmetry is a theory where you get out more than you put in: predictions include the unification of gauge couplings and the WIMP candidate for dark matter. In addition, for string theory to describe our universe, supersymmetry is a necessary component. In unbroken supersymmetry, the superpartners have the same mass and charge as their matter partners, but different spin. This is wildly excluded by experimental data since, for example, we have never seen a spin-0 electron. So, in the parlance of high-energy physics, supersymmetry must be spontaneously broken. Practically speaking, this means that the desired aspects of supersymmetry can still work even if the superpartners are too heavy to be detected.

Can the superpartners be arbitrarily  heavy? Interestingly, the measured value of the Higgs boson mass says probably not. The heavier the superpartners get, the more they push the Higgs boson mass upwards (reintroducing a milder version of the numerical coincidence we tried so hard to avoid!), and in most models of supersymmetry, the maximum mass of the superpartners is about a PeV, or about 100 times heavier than the energy of the Large Hadron Collider. This presents an appealing target for future particle colliders and other searches; while it may be true that the simplest models of supersymmetry have now been ruled out, it is by no means true that every model of supersymmetry consistent with the measured Higgs mass has been tested.

My interest in supersymmetry is two-fold: (a) how do we build theoretical models for spontaneously-broken supersymmetry that get the right Higgs mass while preserving other desirable features like gauge coupling unification? And (b) if superpartners are indeed too heavy to ever detect directly, is it possible to diagnose the presence of supersymmetry in our universe by some other means? Practically speaking, supersymmetry is a framework, not a theory, and there are just too many theories of supersymmetry breaking to definitively rule them all out. And it could just be that the Higgs is exceptionally light for no particularly good reason. But the measured Higgs mass is tantalizing: light enough to require supersymmetry, but heavy enough to potentially put the superpartners just out of reach until the next collider is built. Stay tuned.


*for the experts, I'm deliberately leaving out dark energy. My perspective is that the dark energy problem is not "what is dark energy?" but "why dark energy?" All observations are consistent with dark energy behaving like a cosmological constant, so its identity is not really in doubt. The harder questions are why the observed dark energy density is so fantastically small compared with theoretical expectations (the cosmological constant problem), and why it's an order-1 fraction of the total energy density of the universe today, rather than being vanishingly small or completely dominant (the coincidence problem). I will leave these fascinating questions to the cosmologists, for the time being, and eagerly follow their progress.

**also for the experts, there are competing effects on the photon temperature: adiabatic perturbations work as advertised, but there is also the Sachs-Wolfe effect, where photons in overdense regions have to climb out of a deeper potential well and hence appear colder. Which effect dominates depends on the angular scale of the fluctuations. CMB physics is beautifully complicated!


Further reading