Adam Ritz

Research Interests

My research interests cover a number of formal, and more phenomenological, questions relating to the origin and structure of matter on small scales. Some recent research directions are outlined below. For a full list of publications, see INSPIRE.

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Gauge theories form the structural foundation of the Standard Model, one that has now been extremely well tested in the perturbative sector. However, we still have much to learn about the dynamics of these theories particularly in strongly interacting regimes, such as low energy QCD which leads to the spectrum and interactions of the hadronic states we observe. Beyond QCD, this aspect of gauge dynamics may prove very important for new high scale physics, such as supersymmetry breaking, or even electroweak symmetry breaking, and with the advent of the AdS/CFT correspondence actually provides us with a window into the quantum dynamics of gravity in the context of string theory. I have interests in all these aspects of the formal structure of gauge theories.

Progress in this area relies in part on the remarkable fact that strongly interacting regimes are often `dual' to a different weakly interacting theory. Supersymmetry can often assist in making these dualities manifest, as it provides a number of important calculational tools, while still allowing for theories with realistic dynamics. Supersymmetric gauge theories are thus the primary laboratories for understanding the full phase structure and dynamics. Recent work (some in collaboration with M. Shifman and A. Vainshtein) has focussed on the study of a special class of states in supersymmetric gauge theories, known as BPS states, which are protected by supersymmetry and thus tractable, but are nonethless dynamically often the most important. Most recently, I have been studying a special subset of these whose mass or tension scales in a way that naturally identifies them with D-branes in a putative string dual. The basic example corresponds to domain walls in N=1 super Yang-Mills. See e.g. hep-th/0308144 and hep-th/0405175. The ``strings'' in these theories are realized as vortices and they themselves also have interesting dynamics particularly in situations where the worldsheet theory on the vortex exhibits conformal invariance, see hep-th/0612077.

The long-standing qualitative relation between strongly interacting gauge theories and string theory has in the last decade been brought to a quantitative level with the AdS/CFT correspondence and its generalizations. In a suitable limit this invites us to interpret gauge theories as theories of gravity in higher dimensions and also provides a number of new calculational tools. This remarkable dual viewpoint on the foundation stones of the Standard Model is driving much of the current research in this area. Recently, the utility of the AdS/CFT duality has been extended to study other strongly-interacting theories for which alternative techniques are often lacking, from the quark-gluon plasma at RHIC, to aspects of nuclear and condensed matter physics. These physical examples are of course unconstrained by supersymmetry and an important question is then how close do the theories which are accessible via this duality come to the real physics? Recent work (with P. Kovtun, also at UVic) has involved trying to understand the class of theories to which AdS/CFT is applicable in its most tractable large-N form, and whether this defines a new notion of universality, see 0801.2785, 0806.0110 and 0811.4195.
Beyond the foundational questions, at the phenomenological level there are a number of crucial questions about the observed matter content in the universe which remain unanswered. In particular, the universe we observe is asymmetric with regard to the abundance of matter vs anti-matter, albeit at an apparently very small level at early times of one part in 10^10. The (Sakharov) conditions that allow for this asymmetry to develop dynamically are well-known, but whether and precisely how these conditions are satisfied in nature remains a puzzle within particle physics - that of baryogenesis - but also constitutes one of our strongest pieces of evidence for physics beyond the Standard Model. It is tantalizing that the resolution of this question may be linked with the physics of the electroweak scale. After earlier work on aspects of the electroweak phase transition hep-ph/9710271, I have primarily focused on one aspect of this problem, the need for new sources of CP-violation. In this regard precision searches are often highly complementary to direct collider probes such as the LHC, and the CP-odd sector is most effectively probed by searches for electric dipole moments (EDMs).

Recent work (in collaboration with M. Pospelov, also at UVic) has involved currently the most precise calculations of hadronic matrix elements relevant for EDMs, and also a systematic study of the sensitivity of existing and future searches within models such as the MSSM. See e.g. hep-ph/0504231 and hep-ph/0510254.

There are also intriguing hints that versions of electroweak baryogenesis may be viable with a modified Higgs sector providing both a sufficiently strong 1st-order phase transition, and new sources of CP-violation. Such models are highly testable, both with the anticipated exploration of the Higgs sector at the LHC, and through searches for EDMs, see e.g. hep-ph/0610003.
While baryogenesis can explain the imbalance in baryonic matter vs anti-matter, astrophysics and cosmology have provided us with compelling evidence that the universe also contains a much larger component of non-baryonic dark matter, about which the Standard Model has little to say. At this point it is a compelling problem to find either direct or indirect means of detecting dark matter in the galactic halo, and precision searches play an important role here.

Other than the inferred relic density of dark matter, there are relatively few intrinsic handles currently available to discern its interaction with the rest of the Standard Model. One simple assumption, inspired in part by the success of big bang nucleosynthesis in explaining the abundance of light elements, is that dark matter should be a thermal relic with a density fixed by the time at which its self-annihilation rate froze out due to the expansion of the universe. This assumption leads to a weak-scale annihilation cross-section and the notion of WIMP dark matter. However, even if this assumption is correct, there are many other parameters which determine its detectability: its scattering cross-section, which determines its thermal decoupling scale (i.e. how `cold' it is), and its annihilation and scattering cross-sections in the galaxy today. The latter two in particular will determine the chances for direct and indirect detection.

Recent work (with M. Pospelov, M. Voloshin & B. Batell) has explored the possibility for more complex signatures from variants of the prevailing WIMP paradigm for dark matter, some of which are motivated by aspects of the little hierarchy problem in particle physics, see e.g. hep-ph/0703128. and 0803.2251. One of our motivations was in part to explore the level to which direct and indirect (i.e. astrophysical) detection techniques can probe complementary parts of the parameter space for different models. In this regard, the intuitive notion that dark matter is part of a more complex hidden sector with additional light states has provided a useful scenario within which to study the various detection strategies. Renormalizable interactions are remarkably constrained, occuring only through `portal' couplings such as kinetic mixing with hypercharge, which allows for a rather generic study of the various observable signatures. We have recently studied this `secluded dark matter' scenario 0711.4866 in some detail, exploring both indirect 0810.1502 and various collider 0903.0363 and fixed target 0906.5614 sensitivity.