Scientific highlights from 2023 so far

This year 2023 has been a busy time for the DAMIC group at UW. Here are the scientific highlights so far:

  • The excess of ionization events in the bulk of our CCDs, first observed in our 2020 WIMP search, was confirmed with much more sensitive skipper CCDs that we installed in DAMIC at SNOLAB. The preprint is on the arXiv. The origin of the excess remains a mystery. There is no good "known-physics" explanation and the most straightforward dark matter interpretation is in tension with results from other experiments.
  • In spite of the presence of the excess, measurements of the spectrum at lower energies provided world-leading constraints on the existence of dark matter particles with masses in the MeV scale. For this measurement,  we operated two CCDs very similar to those at SNOLAB in the Low Background Chamber (LBC) at the Modane Underground Laboratory. The first spectral analysis was published in PRL, while a second analysis that considers the time evolution of the possible signal to extend the sensitivity to smaller dark matter masses is on the arXiv.
  • In an effort led by graduate student Kellie McGuire, we developed a technique to discriminate between nuclear and electronic recoils in the CCD target. The preprint is on the arXiv. This technique has huge potential since it may allow future CCD dark matter detectors to separately study the different interaction channels (i.e., with either nuclei or electrons) of dark matter particles.
  • Prof. Chavarria's proceedings from Low Radioactivity Techniques 2022 conference were published. The paper succinctly summarizes the background suppression capabilities of CCDs and the hybrid CMOS imagers that we develop for Selena.

Left: Exclusion limit from DAMIC-M's LBC for dark matter particles interacting with electrons via an ultralight mediator. In this scenario, we have world-leading sensitivity for a wide range of dark matter particle masses. Right: Comparison between the exponential event excess observed in DAMIC at SNOLAB in the original experiment from 2020 and the upgraded detector in 2023. The y and x axes provide the decay and amplitude of the exponential spectrum, respectively.


Research group updates

The following two announcements are long overdue:

  • In March 2023, Alex Piers received his Ph.D., the first from our group. We are super proud of him. Alex was central to the DAMIC program (both hardware and analysis) and Selena R&D.
  • Later in March, Dr. Heng Lin, who received his Ph.D. from Shanghai Jiao Tong University working on the PandaX-III experiment, joined our group as a postdoc.

Left: Group photo before Alex's defense in March 2023. Harry Ni, Marcel Conde, Kellie McGuire, Ryan Roehnelt, Michelangelo Traina, Alex Piers, Prof. Chavarria. Right: Alex and Prof. Chavarria celebrating with champs and a CCD cake!


Michelangelo Traina joins UW!

Dr. Michelangelo Traina started as our new postdoc on DAMIC-M on January 3rd, 2023!

Michelangelo received his Ph.D. from Sorbonne University in Paris and has been a DAMIC(-M) collaborator for the past three years.



Postdoctoral position open!

We seek a creative, conscientious, and highly motivated candidate to spearhead the activities in the construction and commissioning of DAMIC-M. In addition, the appointee will have the opportunity to participate in the data analysis of the prototype detectors, and the R&D program at the UW to develop next-generation skipper CCD detectors (Oscura) and CMOS/selenium detectors for neutrino physics (Selena).

Read the full advertisement and apply here.


Results from a 11 kg-day exposure of DAMIC at SNOLAB!

DAMIC's results on the search for low-mass weakly interacting massive particles (WIMPs) was posted yesterday on the arXiv! Link.

We probed with the same nuclear target, and with an order-of-magnitude improvement in energy threshold, the parameter space that corresponds to the WIMP-signal interpretation of the excess of nuclear-recoil events recorded by the CDMS silicon experiment in 2013.

We observed an excess of ionization events above threshold that is inconsistent with the standard WIMP-signal interpretation of the CDMS result and demands further investigation. Although this excess leads to a weaker-than-expected exclusion limit at small mχ, our data still provide the most stringent exclusion limit from a silicon target experiment for WIMPs with mχ < 9 GeV c−2.


Upper limit (90% CL) on the spin-independent WIMP-nucleon elastic scattering cross section σχ−n as a function of WIMP mass mχ obtained from the 11 kg-day exposure of DAMIC (solid red line). The expectation ±1σ band if only known backgrounds are present in our data set is shown by the red band. For comparison, we also include 90% CL exclusion limits from our 2016 result with a 0.6 kg-day exposure, other experiments, and the 90% CL contours for the WIMP-signal interpretation of the CDMS silicon result.


Cosmogenic activation of silicon measured!

We recently posted on the arXiv our measurement of the cosmogenic activation of tritium in silicon.

Tritium (3H) produced in the bulk silicon of CCDs by cosmic rays can be a problem for silicon dark matter experiments because the background from the low-energy beta particles can interfere with the dark matter signal. Tritium will accumulate in the silicon crystal starting when the ingot is pulled, throughout wafering and CCD fabrication, until the final devices are brought underground for the dark matter search. Thus, we measured the activation rate of cosmogenic tritium in silicon to determine the maximum time that the CCD silicon can spend on the surface before tritium decays become a limiting background for the DAMIC-M experiment.

For this measurement, we irradiated three 8-megapixel CCDs at Los Alamos Neutron Science Center (LANSCE). The neutron flux mimics the cosmic-ray energy spectrum but with much higher intensity. We acquired dark images with the irradiated CCDs to perform spectroscopy of the beta decays detected in the bulk silicon of the devices and measured the tritium decay rate. By scaling the quantity of tritium activated in the CCD by the relative neutron fluence, we obtained a cosmogenic neutron activation rate of (112±24) atoms/kg/day.

Fit to the energy spectrum of ionization signals in the irradiated CCD with Monte Carlo templates that include a realistic model of the CCD response. The hump of events below 18 keV from the radioactive decay of tritium is evident.


Single electron response achieved!

We have recently demonstrated, for the first time, single-electron response with a 675 um-thick skipper CCD fabricated on high-resistivity silicon. This is the first techonolgical milestone of the DAMIC-M program.

The extremely low noise of the skipper CCD will allow us to reach unprecedently low energy thresholds with DAMIC-M for an increase in sensitivity by several orders of magnitude in the search for light dark matter particles.

The prototype 1kx6k CCD was designed by Berkeley Lab and packaged at the University of Washington (UW). The data acquisition system and interface electronics were also developed at UW with the support of CENPA technical staff.

Left: 1kx6k DAMIC-M development CCD packaged at the University of Washington. Right: Distribution of pixel values in a CCD image, demonstrating a noise level of 0.07 e- RMS, which allows the device to resolve single charges per pixel.


Dark matter-electron scattering result from SNOLAB!

DAMIC's latest results on the search for light dark matter particles (MeV-scale masses) was posted today on the arXiv! Link.

These are the best exclusion limits on the existence of dark matter particles with masses in the 0.5–5.0 MeV range:

The 90 % C.L upper limits on the DM-electron free scattering cross section (σe) as a function of DM mass (mχ) for three different dark-matter form factors (FDM) obtained by DAMIC at SNOLAB (solid blue line).​ Other lines present results from competing experiments.


Wire bonder works!

Yesterday we completed the first DAMIC 4k x 2k CCD package at the University of Washington. The device is half the area of the CCDs currently taking data at SNOLAB but otherwise identical. This CCD is a candidate to go to Los Alamos over the summer for activation by a neturon beam. The goal is to perform an experimental estimate of the tritium that is activated in silicon by cosmic neutrons.

Left: The first 4k x 2k DAMIC CCD to be wire bonded at the University of Washington. Thanks to Bret from Luna Tech Sales for his assistance. The package is now complete. Right: The resulting wire bonds as seen through a microscope: 78/78 bonded in automatic mode, 100% success rate!