Dark Matter Group

Faculty: Masahiro Morii
Postdoc: Michał Własenko
Graduate students: We are accepting new graduate students

The goal of our research is to detect the constituent particle of the Dark Matter in the laboratory. We currently participate in the LUX experiment, which uses 300 kg of liquid xenon as the target material. The experiment is being prepared for installation at the Sanford Underground Laboratory at Homestake, South Dakota. The Harvard group is building a part of the readout electronics for the experiment.

What is Dark Matter?

There is much more than meets the eye. In fact, most (83% in the latest estimate) of the mass of the universe is made of invisible matter, which we call the Dark Matter. We have known this since the mid-20th century because, although the Dark Matter is invisible, it exerts gravity to the visible matter and affects their behavior. Recent studies of gravitational lensing, in which the light from very far galaxies are bent by the gravity of the intervening matter, have left virtually no doubt about the existence of the Dark Matter.

But then, what really is the Dark Matter?

We don't know. What we know is that it's not made of ordinary particles such as electrons and protons. The basic consituent of the Dark Matter is a heavy particle with very weak interactions with ordinary matter. They are called Weakly-Interacting Massive Particles, or WIMPs.

Although there must be a lot of WIMPs to make up 83% of the universe's mass, they are very hard to detect because their interaction is weak. Even when an WIMP does bounce off an atomic nucleus in your experiment, the recoil energy is so small that the detector will have difficult time distinguishing it from background due to natural radioactivity. A large and highly sensitive detector, made of material free from radioactive elements and located deep underground to avoid cosmic rays, is needed to detect such an event.

LUX Experiment

The LUX Experiment uses liquid xenon as the target material for Dark Matter detection. If a WIMP hits a xenon nucleus, the recoiling nucleus produces two detectible signals: a faint light pulse and a number of free electrons. Combined, they allow us to detect nuclear recoils with kinetic energies as small as 5 keV, and distinguish them from electron recoils, which are much more often produced by natural radiactivity.

Liquid xenon is an attractive material for a Dark Matter detector. It has a relatively large probability (or “cross section”) of collision with WIMPs. It's dense (3 g/cm³), and therefore makes a compact detector for a given target mass. Since it is a liquid, it can be recirculated to remove impurities continuously. Although it is a cryogenic liquid, the relatively high boiling point (165 K, warmer than liquid nitrogen) makes it easy to handle.

The LUX experiment uses 300 kg of liquid xenon, of which about 170 kg forms the sensitive mass. This is more than 10-fold increase over the previous generation of liquid xenon experiments.

Masahiro Morii, November 17, 2009