Dark-matter hunters may need to check their calendars. The sun鈥檚 gravity could change the time when dark matter signals are detected on Earth, which could help sharpen the search for the elusive substance.
Invisible dark matter is thought to make up most of the matter in the universe. Physicists hope to detect it in the form of weakly interacting massive particles (WIMPs) when they collide with ordinary matter in underground detectors.
Some have argued that the rate of such interactions should vary with the seasons, as Earth鈥檚 orbit brings it ploughing through the cloud of dark matter suffusing the galaxy. When the planet heads into this 鈥淲IMP wind鈥�, around 1 June, we should see鈥痬ore dark matter strikes; in December, when Earth is moving downwind, we should see fewer.
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Previously, two experiments, including the at Gran Sasso, Italy, and the detector in Soudan, Minnesota, reported observing just this sort of seasonal signal. But these claims have attracted scepticism because more-sensitive detectors have come up empty.
Warp factor
Now, Benjamin Safdi of Princeton University and his colleagues note something that all experiments have neglected: the sun. As WIMPs stream through the solar system, the sun鈥檚 gravity bends their trajectories, focusing the streaming particles on a鈥痯articular location in Earth鈥檚 orbit. This effect can shift the date of the maximum number of collisions by anything from a few days up to several months, depending on the WIMPs鈥� mass and speed. 鈥淭his force warps the dark matter 鈥榳ind鈥� in a way that had not previously been noticed,鈥� Safdi says.
The fact that the date of maximum WIMP collisions should change depending on their energy could lend future searches a sharper scalpel to scrape true dark matter signals away from background noise, he adds.
鈥淥ur result gives dark matter direct-detection experiments an excellent way of distinguishing real interactions with the galactic dark matter halo from background,鈥� Safdi says. 鈥淚t is hard to imagine a background source which could mimic this energy-dependent modulation.鈥�
Punchline coming
The work does not explain DAMA鈥檚 possible dark-matter signal, but re-analysing the data using the new approach could help support or refute their results, Safdi says.
鈥淭here is already a slight trend in the data consistent with our prediction for the gravitational focusing effect 鈥� that is, the date of the maximum moves further away from June 1 at lower energies,鈥� says team member , also of Princeton. 鈥淥ne punchline of our study is that accounting for the gravitational focusing effect can perhaps rule out or confirm the dark-matter interpretation of the DAMA annual modulation.鈥�
of Brown University in Providence, Rhode Island, who works on a direct-detection experiment in South Dakota called LUX, says that the new work could be important for helping design future experimental set-ups. 鈥淭hese researchers have clearly demonstrated just how potentially interesting data from a direct-detection experiment can be,鈥� he says.
Journal reference: , DOI:10.1103/PhysRevLett.112.011301
