Moira Gresham - Research

Links to Papers

Research

I am a theoretical particle physicist and cosmologist. Particle physicists try to understand the most fundamental physical entities.¹ Cosmologists study the Universe as a whole, including its large-scale features and evolution through time.

As a theorist, I approach these questions by exploring models that might better explain observed phenomena. I also work out the implications of models or new ideas in order to make predictions or compare to experimental results. Such activities often involve interesting mathematics and complex computations. Most of my professional work is best classified as "phenomenology"—so named for making direct connections between theory and physical phenomena.

My current and past interests center on:

Dark matter, whose identity we don't know in detail by virtue of its very weak interactions with ordinary matter, but whose existence is demonstrated by astrophysical observations.
Beyond the Standard Model particle phenomenology, including especially phenomenology involving the top quark—the heaviest, and among the least well studied Standard Model particles.
Cosmic inflation, the primordial period of rapid expansion postulated in order to explain, among other features, the flatness and large-scale homogeneity of the observed Universe.

First, What is the "Standard Model"?

The so-called "Standard Model" of particle physics can be viewed as a sort of periodic table for fundamental (irreducible, non-composite) particles, including, for example, electrons and quarks—the building-blocks of atoms. In addition to cataloging fundamental particles, it specifies how these particles interact with one another. The Standard Model does a fantastic job of characterizing virtually all of the subatomic phenomena that we've been able to study in Earth-bound laboratories. (For an introduction to the Standard Model, I suggest starting with The Particle Adventure.)

The Standard Model, along with the basic rules of quantum mechanics and general relativity, can more than sufficiently account for everyday physical phenomena that we encounter on Earth.² This is not to say that we understand in every case how to move from the fundamental physics description to the detailed phenomena, but we're confident that we understand the building blocks. (For more on this point, see for example this blog post.) However, for a variety of reasons, we know that the Standard Model provides an incomplete description of Nature in more extreme circumstances, well beyond our everyday experience. Like my particle physics colleagues, I'm driven to fully understand what basic laws underlie all possible physical phenomena. So one of my most immediate questions is, "What lies beyond the Standard Model?"

Dark Matter

Astrophysical (non-Earth-bound) measurements indicate that about 25% of the energy density (think "contents" if you prefer) of the Universe is dark matter and about 70% is dark energy. The other 5% is normal matter—the stuff you learn about in chemistry. We don't know the identities of dark matter or dark energy, and the Standard Model cannot account for their known properties. In a literal sense we currently know very little about most of the content of the Universe.

Understanding the identity of dark matter is one of the biggest and yet most approachable outstanding problems in physics. Uncovering the nature of dark matter may help us to move beyond the Standard Model; at the very least, dark matter represents an obvious hole in our knowledge of the fundamental nature of matter. Recently I have contributed to the quest to discover the identity of dark matter by analyzing the potential of direct detection experiments, which are designed to "directly" detect the non-gravitational scattering of dark matter particles off of atomic nuclei.³

Currently, direct detection experiments have ruled out many previously attractive models of WIMP (Weakly Interacting Massive Particle) dark matter. At the same time, the DAMA direct detection experiment claims a high-significance detection of dark matter, which is in severe tension with the null results of many other experiments, given certain assumptions. A few other direct detection experiments have seen dark matter "anomalies"—possible evidence of dark matter, though at only marginally significant levels and again in severe tension with null results from other experiments.⁴ Such tensions, along with the absence of a clear dark matter signal at the sensitivity of current experiments, have rightly driven the particle astrophysics community to revisit its assumptions. In particular, efforts have been made to (1) consider a larger swath of nonstandard WIMP scenarios that might relax some of the current tensions and (2) develop a more assumption-independent approach to dark matter direct detection. My three most recent publications synthesize ideas relating to both points. See

V. Gluscevic, M.I. Gresham, S.D. McDermott, A.H.G. Peter and K.M. Zurek,
Identifying the Theory of Dark Matter with Direct Detection,
JCAP 1512, no. 12, 057 (2015),
arXiv:1506.04454 [hep-ph],
M.I. Gresham and K.M. Zurek,
Effect of Nuclear Response Functions in Dark Matter Direct Detection,
Physical Review D 89, 123521 (2014),
arXiv:1401.3739 [hep-ph],
M.I. Gresham and K.M. Zurek,
Light Dark Matter Anomalies After LUX,
Physical Review D 89, 016017 (2014),
arXiv:1311.2082 [hep-ph],

Beyond the Standard Model Phenomenology

As stated above, the Standard Model provides an incomplete description of the fundamental constituents of matter and their interactions. With each new particle physics experiment we hope to reveal shortcomings of the Standard Model that will serve as clues to a more complete description of particle physics.

The newest and biggest such experiment is the Large Hadron Collider (LHC). LHC scientists have already discovered the linchpin particle in the Standard Model: the Higgs boson. The LHC also produces unprecedentedly many top quarks. The only other (and the first) experiment to produce top quarks is the Tevatron collider at Fermilab. In fact, before its shutdown in 2011, some Tevatron scientists thought they had uncovered anomalous behavior in top quark pair production events.

The CDF and D0 collaborations at Fermilab reported larger-than-expected top quark pair forward-backward asymmetries (denoted A_FB). It's a big deal when a result appears to conflict with the Standard Model. When such conflicts appear, experimentalists must check and double check their results, poring over possible systematic errors that they may have somehow missed. At a certain point, theorists should jump in and try to find sensible explanations for the measurement. Having an idea of what previously unknown physics could underlie the observation helps in discerning what other observations and/or analyses might clarify the situation, and to make the reality of the anomaly more or less plausible.

During the height of excitement over the possible top quark anomaly, I helped to determine whether any new additions to the Standard Model might possibly explain the top A_FB while being consistent with numerous other observations, especially those made at the LHC.⁵ With collaborators I have published the following series of papers, which have contributed significantly to the narrowing of possible theoretical explanations of the top A_FB:

M.I. Gresham, J. Shelton and K.M. Zurek,
Open windows for a light axigluon explanation of the top forward-backward asymmetry,
Journal of High Energy Physics 1303, 008 (2013),
arXiv:1212.1718 [hep-ph].
M.I. Gresham, I.-W. Kim, S. Tulin and K.M. Zurek,
Confronting Top A_FB with Parity Violation Constraints,
Physical Review D 86, 034029 (2012),
arXiv:1203.1320 [hep-ph].
M.I. Gresham, I.-W. Kim and K.M. Zurek,
Tevatron Top A_FB Versus LHC Top Physics,
Physical Review D 85, 014022 (2012),
arXiv:1107.4364 [hep-ph],
M.I. Gresham, I.-W. Kim and K.M. Zurek,
On Models of New Physics for the Tevatron Top A_FB ,
Physical Review D 83, 114027 (2011),
arXiv:1103.3501 [hep-ph].
M.I. Gresham, I.-W. Kim and K.M. Zurek,
Searching for Top Flavor Violating Resonances,
Physical Review D 84, 034025 (2011),
arXiv:1102.0018 [hep-ph].

While I have put this line of inquiry aside for now, I eagerly await further analyses from the LHC top physics groups.

Cosmic Inflation

Earlier in my career, I spent substantial time investigating the evolution of the universe just after its birth. I'm especially interested in using new, impressively precise measurements of the cosmic microwave background radiation to probe models of cosmic inflation—the primordial period of rapid expansion postulated in order to explain, among other features, the flatness and large-scale homogeneity of the observed Universe.

For example, in

T.R. Dulaney and M.I. Gresham,
Primordial Power Spectra from Anisotropic Inflation,
Physical Review D 81, 103532 (2010),
arXiv:1001.2301 [astro-ph.CO],

with a younger graduate student colleague I investigated the signature of an inflation model in which isotropy, or symmetry under rotations—the local symmetry that for example leads to conservation of angular momentum and consequently the characterization of particles by their spin—is broken. Though there are ways around it, which we explored, it turns out that inflation typically does a very good job of restoring symmetry under spatial rotations—i.e. in washing out anisotropy. Indeed, the isotropy of the early Universe is often taken to be a prediction from inflation. Our main motivation was to revisit this prediction that is usually taken for granted.

Footnotes

Are they one or many? Particles or fields? Something else entirely? How do they behave? ↩
The Standard Model goes beyond everyday experience to also account for exotic stuff like cosmic rays and high-energy collisions in particle accelerators.↩
"Directly" detecting dark matter means observing the nucleus of an atom within your detector recoiling due to some particle flying in and hitting it. You infer that the particle is dark matter after painstakingly eliminating other possible explanations for the recoil. The "detector" could be, e.g., a huge vat of liquid xenon, surrounded by sensitive instruments. ↩
See, for example, this blog post and links/references within. ↩
Recently, more sophisticated calculations of the Standard Model expectation and analysis of the full D0 top quark data set indicate that the D0 data agrees with Standard Model predictions. Furthermore, with the revised Standard Model calculation, the CDF results are in only mild conflict with expectation, so one might argue whether we should still refer to the CDF measurement as "anomalous." Though the LHC cannot make exactly the same measurement (in short, because it produces proton-proton rather than antiproton-proton collisions), other LHC measurements will help to clarify the situation. ↩