Executive summary of the MITP Scientific Program
"Effective Theories and Dark Matter"
16-27 March 2015,
Mainz Institute for Theoretical Physics,
Johannes Gutenberg University
The MITP Scientific Program “Effective Theories and Dark Matter” brought together physicists working on a broad range of subjects ranging from theoretical nuclear physics and QCD to model-building, plus a few experimentalists, all with interest in applying the technology of these fields toward understanding the particle nature of the mysterious dark matter which comprises most of the mass of the Universe. Given the broad array of subjects, there was great need for this kind of multi-disciplinary conference to bring together the diverse interests and help integrate cutting edge results between them. From this point of view, the workshop was a great success, with talks scheduled in the morning and more free form discussions in the afternoon.
The questions ranged from the very basic to the technically advanced, illustrating the fact that the speakers and discussion leaders did an excellent job of keeping the discussion basic enough for those outside of the primary topic to follow, while still allowing the experts to have useful discussions to advance the state of the art. Several new results, both experimental and theoretical, were presented for the first time at the workshop. These included new SuperCDMS constraints on effective WIMP-nucleon interactions; a new analysis of nucleon sigma terms in two flavor chiral perturbation theory; new results for two-body currents in chiral effective theory; and new results on composite dark matter from the lattice LSD collaboration.
"Effective Theories and Dark Matter"
16-27 March 2015,
Mainz Institute for Theoretical Physics,
Johannes Gutenberg University
The MITP Scientific Program “Effective Theories and Dark Matter” brought together physicists working on a broad range of subjects ranging from theoretical nuclear physics and QCD to model-building, plus a few experimentalists, all with interest in applying the technology of these fields toward understanding the particle nature of the mysterious dark matter which comprises most of the mass of the Universe. Given the broad array of subjects, there was great need for this kind of multi-disciplinary conference to bring together the diverse interests and help integrate cutting edge results between them. From this point of view, the workshop was a great success, with talks scheduled in the morning and more free form discussions in the afternoon.
The questions ranged from the very basic to the technically advanced, illustrating the fact that the speakers and discussion leaders did an excellent job of keeping the discussion basic enough for those outside of the primary topic to follow, while still allowing the experts to have useful discussions to advance the state of the art. Several new results, both experimental and theoretical, were presented for the first time at the workshop. These included new SuperCDMS constraints on effective WIMP-nucleon interactions; a new analysis of nucleon sigma terms in two flavor chiral perturbation theory; new results for two-body currents in chiral effective theory; and new results on composite dark matter from the lattice LSD collaboration.
Some of the major topics of discussion included:
- What are the cutting edge tools to describe nuclei and their response to being scattered by dark matter particles? How can we make progress in improving and vetting them?
- How does one relate a given theoretical model at the electroweak scale to predictions for observables, including the low energy effective theory at the scale of the strong interactions?
- Given constraints from the wide array of experiments, what are currently the best theories of dark matter? Are there corners of theory space left unexplored?
Dark matter and nuclei
Direct detection searches use heavy nuclei as targets. The workshop discussed current status of nuclear models, along with prospects and needs for future improvement. The status of spin-dependent cross sections computations, and the impact of two-body and other nuclear effects was reviewed. Particle attention was paid to the discussing the impact of two-body currents deriving from scalar versus tensor quark-level operators.
The participants engaged in a vigorous discussion on the merits of using quark-level versus nucleon-level parameterizations for presenting the observations/constraints of direct detection searches. The former is required in order to connect to UV models and collider bounds, while the latter provides a more immediate connection to direct detection experiments. It is likely that both approaches will play an important role in understanding future data, and the workshop featured several discussions on the work needed in connecting the different descriptions.
QCD and nucleon matrix elements
The calculation of WIMP-nucleus scattering cross sections from WIMP-quark and WIMP-gluon effective operators requires interfacing particle and nuclear physics. The workshop brought together dark matter practitioners with theorists in the perturbative QCD community, and featured talks presenting recent work using heavy particle and soft-collinear effective theories, and applications to direct detection, indirect detection, and collider searches.
The scalar matrix elements of the strange (and charm) quarks in the nucleon are particularly important for a large class of WIMP candidates. The status and some new results for these and other nucleon-level matrix elements from lattice QCD and chiral perturbation theory were presented.
UV theories
At the workshop, specific discussion assessed the viability of novel ideas for the nature of dark matter, such as the possibility that it is a bound state composed of charged constituents held together by a dark analogue of the strong force, or that the dark matter possesses a notion of flavor analogous to that present in the quarks and leptons of the Standard Model. In both cases, viable parameter space results with implications for how searches for dark matter should be interpreted if these ideas prove to be realized.
Theoretical models aimed at predicting the mass of the dark matter were explored, and it was demonstrated that there are principles which can suggest a value around the electroweak scale, either based on the observed abundance of dark matter or by tying the scale of the dark matter mass to the breaking of scale invariance.
A major area of discussion was the use of effective field theories to characterize dark matter interactions with quarks and gluons, and their use to interpret LHC searches for dark matter. These effective theories are powerful because they capture the low energy behavior of a large class of models, but care is needed when the masses of the mediators are comparable to the energies present in typical LHC reactions. Progress was made as to how to characterize this feature, but how to optimally present the analysis remains an open question.
Collider constraints on more concrete models of dark matter, such as having a minimally electroweak-charged particle, or in the context of the pMSSM were discussed, and the state of the art bounds were derived from existing data. These point the way to the most likely regions of parameter space. An open question is whether or not these bounds could be improved by additional analyses.
Finally, there was much discussion of an excess of gamma rays from the direction of the galactic center observed by the Fermi LAT. This excess is consistent with simple models for dark matter annihilation, but is difficult to interpret because of largely unknown astrophysical backgrounds. The discussion resulted in the most precise characterizations of this signal to date and explored the class of models capable of explaining it while remaining free from constraint by collider and direct searches. An open question is how one may make progress on better modeling of the astrophysical backgrounds.
Summary
Ultimately, the nature of dark matter is a question that must be settled experimentally. The synthesis of these diverse theoretical areas ensures that we make the most of existing experimental searches and understand where in “theory space” dark matter could hide from them.
Vincenzo Cirigliano (Los Alamos),
Richard J. Hill (Univ. Chicago),
Achim Schwenk (TU Darmstadt),
Tim M.P. Tait (UC Irvine)
