What if you don’t find dark matter interactions with LZ?
It is certainly possible that we may be looking in the wrong place: dark matter could be made of lighter particles than we can detect, or they could interact much more rarely than we think they do – or they may not interact at all except via gravity! We think that the WIMP paradigm is well motivated, but it is not certain to be true. However, the absence of evidence is not the same as the evidence of absence. It may simply mean that our experiment is not sensitivity enough. It has been the case in other rare event searches in physics that we did not find a signal until our detectors became both large enough and sensitive enough to probe at the right level. In any case, a null result from the leading experiment is also important: entire theories can be ruled out if they predict concrete observations which turn out not to materialise. This also makes physics go forward – although it is not as exciting as a discovery!
Are you also looking for dark energy?
No; dark energy and dark energy are the two ‘dark fluids’ in our standard model of Cosmology to explain how the universe behaves on large scales, but they seem to have a very different nature. Dark energy is the even more mysterious force which is making the expansion of the universe accelerate. That the universe is expanding is no surprise, but that everything is moving away from everything else ever more rapidly was a major shock to physicists a few years ago. Although we can ‘see’ the effects of dark energy, we have still little idea about its nature. Dark matter, on the other hand, behaves like any other type of matter: it interacts with gravity like ordinary matter, and it is slowing down the effects of dark energy. Although there are several types of astronomical observations that will help us understand more about dark energy, we have not been able to pin down what is causing it from a fundamental point of view.
Why does it take so long to build these experiments?
There are two main reasons for this. Firstly, we must reduce the background from the trace radioactivity which is present all around us before we manufacture our detector. This means selecting the best candidate materials and components and screen them for radioactivity – each sample taking weeks to assay in very sensitive low-background systems. For example, it took two years to find the right titanium for the LZ cryostat, and even longer to achieve the right radioactivity for our photons sensors. When LZ is finally built we will have screened nearly 1,000 samples in total!
When we finally detect a dark matter signal, this will probably come in the form of a handful of extremely faint interactions in our detector. Its design must be incredibly precise and every corner must be precisely understood – we must be confident that those interactions are bona fide scatterings of WIMPs from xenon atoms and not some artefact of a poorly designed detector. This is not difficult to do if you can accept that every once in a while, say 1% of the time, something happens which creates a fake signal. We cannot afford such luxury – we need to avoid fake signals to a level well below that…
Why do you need to work underground?
Cosmic rays bombard the earth in great numbers and they create all manner of particles that can interact in our detectors to fake a signal; neutrons produced by high-energy muons produced by cosmic-rays interacting in the upper atmosphere are the main concern. The only way to avoid these backgrounds is to operate deep underground. Interesting, some high energy muons still make it to the 1 mile depth of the Sanford Lab, but their flux is smaller by a factor of 10 million. Modern underground laboratories are well equipped and provide a good and safe working environment.
Are you going to create a black hole that swallows up the whole world?
Could it be that we simply don’t understand gravity and so dark matter is not needed?
Although this has been a possibility for many years, we now think that this is unlikely. It is quite possible that we don’t know well enough how gravity behaves in unusual situations (for example when it is very weak) and several theories of gravity beyond Newton and Einstein have been developed to explore this. On the other hand, there are some types of astrophysical evidence for dark matter which are difficult to explain in this way (you can look up the Bullet Cluster observations); particle dark matter can also explain our observations of the early universe, in a very different regime of gravity; our standard theory of cosmology works very well assuming dark matter particles, and we do not have a coherent alternative that explains all the evidence by modifying gravity.
What can dark matter do for us?
Why should we care about dark matter? It is unlikely we can harness the power of dark matter in the next few centuries. But it is hardly a nuisance either, despite us being right in the middle of it… There may be a million dark matter particles going through each square centimetre of our bodies every second. Of these, very few indeed are predicted to interact with ‘our’ atoms in our lifetime. If they do, they are far less intrusive than the cosmic-ray muons bombarding us all the time, as charged particles interact much more readily; and they are far less numerous than the neutrinos emitted by the sun, billions of them crossing each of our fingernails every second! Whatever benefit future generations may derive from harnessing dark matter is unclear – although these things tend to be remarkably unpredictable.
One clear benefit to us all is that the technical challenges we face lead to innovation – our experiments constantly push the boundaries of today’s technology. We help develop new photon sensors which have wide application in many industries. We improve radioassay techniques with applications in the food and pharmaceutical industries. We develop data analysis algorithms which can be used in nuclear medicine. In doing so, we train young people and give them skills to crack hard problems – that’s one important thing that dark matter can do for us.
But perhaps the greatest benefit for mankind would be that of just knowing. We are a curious species, we seem to be programmed to want to know how the world around us works. This we have in common with other animals, but we are far better at it. Knowing what the universe is made of and figuring out our role in it would be a great achievement. As a species, we have always tried to do that and have been better for it.