Moderated by Rocky Kolb
Panelists: Dan Bauer, Dan Hooper, Rouven Essig
The panel was impressed with the breadth of ideas presented for both experimental searches and theories concerning sub-GeV dark matter. The field feels almost as exciting as it did when WIMP searches began more than 20 years ago, but now has plans to cover a much bigger range of dark matter masses (1 keV to 1 GeV), experimental techniques and theoretical models. The light dark matter theory framework is now very well developed and flourishing with models. The ‘freeze-out / freeze-in’ paradigm provides a useful way to think about the parameter space. While there is no clear particle target, as there used to be with Supersymmetry and the neutralino, that is probably a good thing because it forces us to cover the whole parameter space. Model builders will have a field day with any hints of signals and there will be multiple experimental techniques able to follow up and confirm or deny such hints.
There were at least a dozen new experimental ideas presented at the workshop, many that spring from recent materials science research, with most of them focused on searching for electron recoils from keV to MeV mass dark matter particles. These include techniques and materials such as: color defects, magnons, scintillating GaAs, carbon nanotubes, Dirac materials, nanowires, diatomic molecules, and paleo detectors. Most of these are ideas that are very much in the R&D stage. Additionally, there are a dozen well-established techniques that are developing new legs, mostly focused on MeV to GeV dark matter masses, and searching for both nuclear and electron recoils. These include: CCDs, HV semiconductor detectors, single-photon cryogenic detectors, new bubble chambers, and doped LXe. Most of these have prototype detectors that have already shown promising performance. In a funding-limited national climate, many of these ideas will die on the vine. But the community needs to ensure that enough survive to cover the very broad parameter space. As choices are being made, we need to take a hard look at promised sensitivities compared with those actually achieved.
The panel considered a series of provocative questions as a way to summarize the workshop:
1) What experimental techniques have you heard about in the last three days that you believe are most likely to achieve at least an order-of-magnitude improvement in current sensitivity to light dark matter?
The clearest path to such improvements in the short term seems to be with CCDs, HV semiconductors and CRESST-style detectors. These technologies are already leap-frogging each other with results. But they have to solve the limiting problem of leakage currents to continue such rapid progress. We expect that to happen within the next couple of years. It also seems likely that bubble chambers with hydrogenated targets might be able to get x10 improvement in spin-dependent cross section reach with hydrogenated targets in a few years as well. The other techniques are likely to take longer to mature.
2) What theoretical models have you heard about in the last three days that seem most likely to be tested by experiments in the next few years?
It looks likely that the generic freeze-out dark matter abundances will be testable over much of the parameter space in the next three years. It will likely take much longer to fully-probe freeze-in scenarios.
3) Are WIMPs (10-100 GeV thermal freeze-out relics interacting with Standard Model particles through weak-scale mediators)
a) Dead
b) In critical condition
c) In serious condition
d) Coughing and running a fever
e) As healthy as they have ever been
The consensus of the panel is somewhere between d) and e). We all agreed on the need to complete the upcoming G2 WIMP experiments and also see whether anything new comes from the LHC in the next few years.
4) What would you recommend to the US Secretary of Energy for the dark-matter search funding priority
a) a broad suite of light dark matter direct detection experiments
b) a G3 experiment to reach the neutrino floor at high mass
c) build more coal-fired power plants
The consensus was definitely a). There is no good reason to pursue a large and expensive G3 experiment until we get well into G2 running. Meanwhile there is a great opportunity to probe a lot of the low-mass region with significantly less funding than required for a G3 experiment (or for coal-fired power plants).
5) Where is the “sweet spot” in mass and interactions to search for dark matter?
There is no real consensus on this, but maybe because there really isn’t a sweet spot. Experimentalists have prejudices based on what they think their experiment can do, while theorists have prejudices based on their favorite models. Fixating on sweet spots as the field did with the neutralino may actually be detrimental.
6) Future New York Times front-page news above the fold: “Dark Matter Discovered … (choose one)
a) by an underground experiment”
b) at a high-energy collider”
c) at a fixed-target experiment”
d) via an indirect signal”
e) in an axion-like experiment”
f) by a technique we haven’t dreamed of yet”
h) IT WILL NEVER BE DETECTED
Everybody attending the workshop fervently believes that particle dark matter will eventually be detected. Direct detection, although necessarily underground experiments, and axion experiments are still the most likely ways to convincingly demonstrate that particle dark matter has been discovered. Signals from indirect detection experiments, colliders and accelerators will always be harder to definitively ascribe to dark matter.