Jim Sullivan

 Big Bang Theory  Dark Matter and Dark Energy  Detecting Dark Matter  Radiation and Pulse Shape Discrimination  SNOLAB  Summary

 Saw Dark Matter by effect on galaxies and speed of stars  Even the arrangement of galaxies controlled by Dark Matter  But how can we measure it on Earth?

V star F=m s V 2 /r F=G m g m s /r 2 r

 May be type of sub-atomic particle – ◦ WIMPs (Weakly Interacting Massive Particles) ◦ Axion (small mass, no electric charge, weak interaction)  Current effort focused on WIMPs  Properties of WIMPs ◦ Weakly interacting –  Surrounded by it and don’t notice ◦ Massive –  Enough gravitational force to hold a galaxy together

 Standard solid state radiation detectors – been tried, not successful yet, expensive and difficult to make large enough  Liquid argon detector ◦ Collision between WIMP and argon nucleus like billiard balls ◦ WIMP has lots of kinetic energy (earth moving through space) causes Ar recoil ◦ Enough energy to excite electrons in recoil ◦ Burst of light energy as atoms return to ground state, detected as photons

 Noble gas  M.P K ( o C)  B.P K ( o C)

Ar WIMP

Ar

 When atom returns to ground state, gives off photon  Use photomultiplier to detect photon

 Probability of WIMP hitting an Ar atom very small ◦ Weakly interacting  The more Ar atoms, the bigger the overall probability  The bigger the tank, the bigger the probability

 Don’t detect WIMP directly  WIMP excites argon atom  Atom gives off photon  Measure the photon  Do we detect anything else?  Yes, radiation  So…..

 …Radiation and Pulse Shape Discrimination