The total amount of visible matter that we observe forms only about five per cent of the total universe. The rest of it is dark matter and dark energy. Dark matter interacts weakly, and is made up of a huge number of massive particles, so much so that the earth and sun and all the galaxies are always ploughing through them like cars through the rain. Yet, detecting these particles is mighty difficult. During a recent visit to Chennai,Prof.Rupak Mahapatrafrom Department of Physics and Astronomy, Texas A&M University discussed withShubashree Desikanthe Cryogenic Dark Matter Search experiments, the search for the Weakly Interacting Massive Particle (WIMP) and about the labs that are slated to come up in Jaduguda mines and Bodi Hills in Tamil Nadu.
How many experiments are there looking for the WIMPs?
There are over 30 different experiments around the world, and they are all underground. The reason being that the signal you are trying to find is very small and on the earth’s surface you have strong cosmic ray effects. That’s not the end of the story, you have to do smart detector technology and shielding to get a clean space where you really know there are no fakes.
About your experiment, the Cryogenic Dark Matter Search…
This is a collaboration between major institutions. Our detectors have led the world in terms of how sensitive they are. They held the record for a decade… Recently they have been overtaken by the competing experiment called liquid Xenon in Gran Sasso lab [Italy].
Our detectors are made up of germanium sensors. There are more than 4,000 extremely sensitive sensors on each detector.
When the interaction happens we see a rise in temperature measured by superconducting films. The detectors are so sensitive… if you walk at a distance of 30 metres away it can sense the vibrations, that’s how sensitive they are.
How do you detect the WIMP?
When a WIMP hits the detector, we see many electron–hole pairs. So you apply an electric field and collect the electrons and you can see the signal. We have seen two events in 2010, but we cannot say we have discovered the WIMP because it is not statistically significant. We need five such events to be sure it is not something else. So we do not say we have seen the WIMP, rather we say that we have excluded the cross-section [which is a measure of the reaction strength], and it cannot be higher than a certain cross-section … So we have excluded a lot of the parameter space. Almost half of what is theoretically predicted.
We measure both the ionisation energy and the phonon energy. When a particle hits the detector it produces electron–hole pairs as well as vibrations or phonons. We measure both of these and that helps us decide whether it is a background or a WIMP.
Where are the detectors made?
Stanford University was making these detectors. But it was very expensive...we cannot do a one-tonne experiment
When I joined Texas A&M we set up a dedicated fabrication unit [in 2008]. This worked very well because these were well-controlled instruments and produced far superior detectors than earlier… It’s truly a uniform detector which means that it is much more sensitive for WIMP search purposes. Also they cut down on the cost from $3,50,000 to $50,000 per kg, due to the much higher success rate of more than 90 per cent.
These are not just cheaper but fundamentally better — more sensitive, better connections, better signal. We now have to make use of it.
You were talking about the next generation of detector and setting up labs in India. Please elaborate.
In the next generation experiment in Canada, we are going to put a 200-400 kg detector payload made of germanium, with each detector weighing 1.5 kg. The experiment will be in a mine in Canada.
I am hoping that the generation III experiment — a tonne-scale experiment — which will come up towards the end of the decade, will be set up in India — in the INO [India-based Neutrino Observatory] lab. It’s called Dark Matter at INO (DINO) part of the INO at Bodi Hills, in Tamil Nadu. It’s a large lab and that’s what we are missing in the U.S. So we’ll come here, build a major experiment and it could be a world-leading experiment, possibly the world’s most sensitive experiment — because we will have a tonne of detector material. The initial collaboration is between the institutions — Saha Institute of Nuclear Physics, Bhabha Atomic Research Centre, IIT Bombay, Physical Research Laboratory, Tata Institute of Fundamental Research and Variable Energy Cyclotron Centre, Kolkata.
We do not want to jump into a one-tonne experiment to start with, so the plan is to build a smaller experiment — a 30-kg experiment. The experiment will be set up in the Jaduguda mines near Jamshedpur. We have already started doing the groundwork for that. So there will be a dark matter experiment in India within a year or year and a half.
What is the connection between WIMPs and supersymmetry?
It’s like this — dark matter already exists, we have evidence for this from astronomy experiments. But in the standard model, which is the theory of elementary particles, there is no description of these dark matter particles. We need to go beyond the standard model, and supersymmetric theories may hold the theoretical description of these particles.
CERN results have not found evidence of supersymmetry. What does this portend for the WIMP?
CERN results have not found evidence of supersymmetry within a certain energy scale; this does not mean supersymmetry does not exist. In the Large Hadron Collider, particles are made to collide at certain energy — the experiment is limited by the energy — 300 GeV. In our search for the WIMP, the universe is the source, and we aren’t limited by the production of these massive particles. So supersymmetry may exist at higher energy.
It’s still possible that it [supersymmetry] does not hold true but dark matter exists so even if the theory that describes it is not supersymmetry, it must be something else. The search for the WIMP reminds one of the search for neutrinos. Fifty years ago, it was unclear when we would see the neutrino, but after a search of twenty years it was located. So may be [the case] with the WIMP.
