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Does Dark Matter?
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DOES DARK MATTER?
by Dr. John Roberts,
project manager for the UK Dark Matter Collaboration, May 2001
Imagine holding a black velvet bag full of ball bearings. You can feel their shape and put them on the scales to weigh them. But if you couldn't see or reach inside the bag, how would you be able to know for sure how many ball-bearings were in there?
As if that wasn't difficult enough, imagine how confused you'd be if you did finally manage to examine them, only to find that there were far fewer than you originally thought. But not only that, but when you put them on the scales again they only weigh a tiny fraction of their former mass!
This is similar to the conundrum facing physicists looking for dark matter - they are faced with two conflicting pieces of evidence when it comes to studying galaxies. There are two scientifically sound methods of weighing a galaxy. The problem is that they both give completely conflicting answers.
 
Firstly, if we consider a spiral galaxy, such as our own Milky Way, or our neighbour, Andromeda, we can measure its mass by measuring how fast it spins. The faster the galaxy turns, the more mass it contains. The second way is to calculate the mass of all the observable luminous parts within the galaxy.
But when physicists first made these measurements for spiral galaxies in the 1930s, they discovered that the two answers were not the same. And it wasn't a matter of a slight difference - the Milky Way was behaving as if its mass was TEN times that of the luminous component. The startling conclusion was that there must be stuff inside these galaxies that we couldn't see - 'dark matter'.
The race was on to discover what this elusive dark matter actually was. If we consider the ball bearings in the bag again, the analogy of the dark matter quest is like trying to measure the ball bearings inside the bag without being able to see them directly. So how do we know what we're looking for?
It's thought that the majority of dark matter is sub-atomic particles. This prediction is based on a particle physics theory called the Standard Model. This is a new theory that is attempting to unify the four fundamental forces in the Universe - the electromagnetic force (which keeps electrons bound to nuclei), the weak force (causing radioactive decay), the strong force (binding the atomic nucleus together) and gravity (which sticks everything that has mass together).
The Standard Model predicts that a group of particles exists called WIMPs or Weakly Interacting Massive Particles. They tend not to interact with everyday objects, which is why we haven't spotted them so far. The rest of the dark matter could be made up of MACHOs (Massive Astronomical Compact Halo Objects). Recent data indicates that MACHOs constitute no more than 20% of the missing mass.
A small percentage of the mass could also be made up from neutrinos which are now known to have mass. Or another candidate is an even more exotic particle called the axion. At the moment though, WIMPS, in particular a certain type called 'neutralinos', seem to have the best chance of filling the mass deficit.
How do we look for dark matter? There are several searches taking place around the world for neutralinos using different techniques. An experiment in a laboratory near Rome, called CRESST has a small crystal of sappire supercooled to milliKelvin temperatures. When an interaction between the neutralino and the crystal takes place there should be a measurable increase in temperature. An experiment based in the United States, called CDMS, uses a combination of temperature rise and ionisation of silicon or germanium. Another experiment, DAMA in the same Gran Sasso laboratory as CRESST uses scintillation detectors. It is this type of detector that we are currently using here in the UK, as part of the UK Dark Matter Collaboration, UKDMC.
All these experiments have one thing in common - they are all based underground! Our facility is based 1100m under the surface in Boulby Mine, approximately 10 miles north of Whitby on the North Yorkshire coast. The reason we conduct our experiments inside the deepest mine in Europe is to stop the detectors being bombarded by cosmic rays. These would completely mask the dark matter signal. The detectors are also constructed from the purest materials possible, as again any radiation emitted from the detector materials would swamp the signal.
The scintillation detector, in this case a crystal of sodium iodide doped with thallium, emits a number of photons in direct proportion to the amount of energy deposited in the crystal by the dark matter particle. Light is also emitted when background radiation such as gamma rays and neutrons interact with the crystal. Some noise signals can be rejected online whilst others are rejected during offline analysis.
 Identification of the different energetic particles, including dark matter, is done by pulse shape discrimination. This is possible due to the different way the pulses interact with the target. Gamma rays will give rise to electron recoil whereas neutrons and dark matter particles will cause the nucleus to recoil. Each of the interactions gives different pulse shapes.
Another way to look for neutralinos is by measuring the annual modulation of the signals from the detectors. As the earth rotates around the Sun, the Earth is either moving in the same direction as the dark matter flux (in December) or against it (in June). This is the technique utilised by the DAMA group. Preliminary results have been published which claim to show this effect, revealing the presence of neutralinos, but greater statistics are necessary to convince the scientific community that it is not just due to errors within the experiment.
Here at UKDMC we have recently installed a new type of scintillation detector based on liquid xenon, called ZEPLIN I. This should improve our results considerably, as liquid xenon has a higher nuclear mass than sodium iodide (which is used inside our current detectors), making it better suited as a target for neutralinos. The problem with using xenon as a target material is that it is only liquid over a very small temperature range of four degrees, which has provided us with some interesting challenges. But controlling the target between these temperatures has been achieved and a working detector is now operational in Boulby Mine.
The next stage in the ZEPLIN development is to improve the discrimination between signal (the neutralinos) and noise (other radiation). Two new detectors have been proposed and we are awaiting the birth of ZEPLIN II and III. Another detector currently under construction for deployment at Boulby uses a low-pressure gas and is called DRIFT. The beauty of this system is that it has almost 100% background noise discrimination and will be able to tell from which direction the particles are coming - the background noise should be homogeneous whilst the dark matter signal should show the annual modulation. With these advances in detector development we should soon be able to tell exactly what this elusive dark matter is and, in doing so, help solve one of the most perplexing problems in the Universe.
Dr. John Roberts is project manager for UK Dark Matter Collaboration and lectures in the Department of Physics at Sheffield University.
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