Dark Matter and the DRIFT Experiment What is Dark Matter? Why do we believe in it? How shall we identify it? C J Martoff, Professor of Physics Physics C056, Astronomy Temple University Spring '03
What is Dark Matter? Dark = not luminous (like stars) = invisible Matter = mass = gravity source
What's the Evidence for Dark Matter? Kepler's Law allows us to "see" distant mass (even invisible mass) R 3 = T 2 (M 1 + M 2 ) This works perfectly for solar system, Jovian moons, binary stars, anything small to Jupiter
Another Way to Look at Kepler's Law Velocity is easier to measure than orbital period (via spectroscopy and Doppler shift), especially for very distant objects We can write v = 2π R/T for orbiting object Then Kepler's Law can be rewritten using algebra: R 3 =T 2 (M 1 +M 2 ) (R/T) 2 =(1/R)(M 1 +M 2 ) (divide both sides by T 2, then by R) (2π) 2 (R/T) 2 = (1/R)(M 1 +M 2 )(2π) 2 (multiply both sides by (2π) 2 ) Finally get v 2 = (1/R) (M 1+M 2)(2π) 2
Kepler's Law on the Grand Scale Galaxies are a million times bigger than solar system (01 light year vs 10,000 light year) Galaxies have up to a million million (10 12 ) times more mass than the solar system It is interesting to ask, does Kepler's law still work for galaxies? IT DOESN'T!! Measured "Rotation Curve" for a galaxy
What does Kepler Law violation (?) Mean? The velocity is much too large for objects far outside the visible parts of the galaxy! The whole shape of the graph is wrong! To explain this, either: Kepler (and Newton) laws fail for large systems OR The visible parts of galaxies lie about where the mass is AND how much of it there is!
A Clue The rising part of the rotation curve is NOT unexpected This part of the curve is inside the visible part of the galaxy (as if a planet orbited inside the Sun) Kepler's Law explains it reasonably well when you do all the math
Just What is Measured for a Rotation Curve Objects out to r 0 are within the visible mass distribution; these give the initial rising part of the v 2 vs R curve, agreeing qualitatively with Kepler Objects beyond r 0 are outside visible mass; their v 2 vs R curve should fall as 1/R It doesn't Anyhow the total mass indicated by the curve is much too large A large "halo" of unseen mass would explain all this r 0 Dark Matter Halo
"Seeing" the Dark Matter Directly Rotation curves only show existence of Dark Matter indirectly To be sure it exists, we want to detect the stuff itself, not just its gravitational influence and apparent invisibility We must know (or have a guess) what the Dark Matter is made of in order to plan how to detect it This is where theorists come in handy
It IS a "gas" of Weakly Interacting Massive Particles (WIMPS) left over from the Big Bang It IS NOT normal matter (protons+neutrons+electrons) However, there are very good reasons to believe: The theories of "what is the Dark Matter" are beyond the scope of this talk (maybe later in chapters on Cosmology) It Is What It IsBut It Isn't Normal
Direct Detection of WIMPs If WIMPs exist and make up the Dark Matter, they will scatter from ordinary matter in a detector The detector atoms will recoil; "standard" nuclear radiation detectors can detect these Identifying a WIMP signal against ordinary radioactive backgrounds is VERY difficult (That's why we work in a deep mine)
What is the WIMP Signature? (There's that crazy picture again!) WIMPs have a distinctive signature because of the cloud-collapses-to-disk phenomenon discovered by Laplace, studying formation of Solar System During galaxy formation, ordinary matter cloud collapses in size and spins faster due to conservation of energy and angular momentum WIMPS are Weakly Interacting- they cannot easily lose energy WIMP cloud cannot collapse- it stays large (consistent with rotation curves) and spherical and non-rotating The Earth and Solar System rotate with the galaxy This gives distinctive directional signatures for WIMP scattering!
Direction-Sensitive WIMP Detection with the DRIFT Detector For technical info on DRIFT/NITPC: http://euclidmathtempleedu/~martoff/drift/talks Technical discussion of direction-sensitive nuclear radiation detectors is beyond the scope of this talk I invented the DRIFT concept (Negative Ion TPC) to detect WIMP directional signature after about 7 years of work on other methods It can determine the direction of very low energy recoil atoms with acceptable accuracy, size, and power consumption for mine labs
DRIFT is Hunting WIMPs Now Another three years work with collaborators (Occidental College, UK Dark Matter Collaboration) and ~$1M (US National Science Foundation, UK PPARC) led to a real WIMP detector deep underground This is DRIFT Running for a year and a half, still learning about the detector- but no WIMPs yet
DRIFT Data This is a background alpha-particle track from radioactivity in vacuum vessel 11,771 atoms of DRIFT gas ionized Approx 220,000 electron volts of energy deposited Track length greater than 25 mm Easy to tell it is not a WIMP
DRIFT Data This is a neutron recoil track We produce these with a neutron radioactive source for calibration This is EXACTLY what a WIMP event would look like (except the directional signature) About 2000 DRIFT gas atoms ionized Track length less than 4 mm About 100,000 ev of energy deposited We don't see these when the neutron source is taken away!
DRIFT Data This is another kind of calibration event, produced by an x-ray radioactive source 300 DRIFT gas atoms were ionized 5,900 ev of energy deposited Two wires report- the neighbors are dead quiet These wires are 2 mm apart (~1/16 inch) and there are 1024 of them in DRIFT (and 1024 amplifiers, 1024 waveform recorders and one LINUX computer to handle it all!)
DRIFT Data Acquisition (Temple's Job)