The DAMA collaboration, an experimental team searching for dark matter via direct detection, is poised to report this week that they have discovered Dark Matter. And I’m here to pre-empt that by bringing you the truth: no, they haven’t.
They take some very cold (cryogenic) atoms,
look for nuclear recoils resulting from dark matter collisions, subtract the background, and draw conclusions based on whatever’s left over. Their expectations are based on the following:
- Neutrons, neutrinos, and other standard model particles from the Earth, the Sun, and the Milky Way galaxy will collide with the detector, producing a background.
- Dark matter in a halo around our galaxy will also collide with the detector, producing a signal.
- The dark matter will be modulated annually; when the Earth moves faster with respect to the galactic center, we’ll see a greater signal; this signal will be periodic over the course of a year, every year.
The problem with this reasoning is that a whole number of things happen over the course of a year, with a one-year period, including:
- The Earth gets closer and farther from the Sun during the year, with aphelion occurring around January 4th every year at a distance of 147.5 million km and perihelion occurring around July 4th at a distance of 152.6 million km.
- The amount of solar radiation striking Gran Sasso, Italy, where the experiment is performed peaks during the summer solstice every year, June 22, and is minimized during the winter solstice, December 21st.
- The Earth has a motion towards the center of the galaxy and away from the center of the galaxy that is also periodic over the course of the year; any background that is modulated annually will look just like the dark matter signal you expect.
So the DAMA experiment ran for a number of years, and saw this annual modulation in signal, and declared that they had found Dark Matter, and released this graph:
And it’s plain as day: whatever they’re seeing is changing in amplitude from about 102% of the average signal from sometime in June/July to about 98% of average in December/January. The problem is that you don’t know that you’re seeing any dark matter! This could all be normal stuff that’s just more abundant in the summer than the winter.
So other dark matter searches looked for dark matter where DAMA predicted it. CDMS and Edelweiss were the two major ones. Here’s what they found when they did their experiments, where they could discriminate between background events and dark matter events. The CDMS results exclude everything above the red line, the Edelweiss results exclude everything above the dark green line, and the DAMA results predict a detection in the shaded red zone:
Oh! Guess what, DAMA? You didn’t see any Dark Matter! So when the DAMA-LIBRA collaboration announces at the APS meeting this month that they found Dark Matter, or, perhaps, on Wednesday at the NO-VE international workshop in Italy (at R. Bernabei’s talk: first results from DAMA-LIBRA), you’ll be among the first to know that they saw noise that gets louder and softer over the course of a year, and nothing more.