Unexplained low-energy excess in sensitive detectors

5 problems

• What microscopic process produces the excess in cryogenic calorimeters?

  Contested

  Crystals cooled near absolute zero record many more small heat pulses below a few hundred electron volts than expected, and the rate fades over weeks after cooling. The question is what inside the detector releases this energy.

• What sets the residual single-electron event rate in skipper CCDs?

  Open

  Skipper CCDs can count single electrons, but they register stray single electrons at a low steady rate that mimics the faintest dark matter signals. The question is where these stray electrons come from and how low the rate can go.

• What causes the bulk ionization excess in DAMIC CCDs at SNOLAB?

  Open

  Silicon camera chips used to search for dark matter deep underground see extra small charge signals from inside the silicon, beyond all known radioactivity. The cause is unknown.

• Do single-sensor and shared excess events have different origins?

  Partially resolved

  When two sensors are mounted on the same crystal, some excess events show up in only one sensor and others in both. Deciding whether these come from the sensor or from the crystal narrows the cause.

• What law governs the decay and reset of the excess rate?

  Open

  The excess fades in the weeks and months after a detector is cooled and partly returns if it is warmed and cooled again. The precise shape of this fading is a clue to its cause.