Locating the dark matter's particles

The nature of the dark matter that is believed to constitute most of the mass of galaxies, large clusters and super clusters of galaxies as well as other cosmic structures has been a perennial puzzle to astronomers for the past few decades. Ninety percent of the mass of Milky Way is dark matter. This is also true in the case of other galaxies. It is currently an accepted paradigm of modern astronomy that dark matter in the universe outweighs the ‘visible’ matter in the form of luminous objects such as stars and glowing nebulae by a large number. It is estimated that more than a quarter of the universe is made of dark matter that does not radiate or emit light. The total baryonic matter, that is the kind of matter made up of atoms and molecules of the various chemical elements, is only a sixth of the dark matter.

The essence
What this dark matter is made up is still a puzzle. The prevailing consensus — after ruling out black holes and other ‘extinct’ stellar objects, among others — is that it is some new kind of particle, stable and not coupled to radiation and is perhaps very massive. A lot of laboratory experiments are currently underway, searching for evidence for such kind of weakly interacting massive particles that are dubbed as WIMPs. Many of these experiments have been going on for several years. So, it is now of much interest that the most sensitive search so far has yielded null results after 20 months. This is the Large Underground Xenon (LUX) experiment, using a dark matter detector, located nearly two kilometres underground below the black hills of South Dakota, USA.

Recently, researchers presented the results in Sheffield, UK, at a gathering of experts of dark matter. LUX is designed to search for WIMPs of around ten to hundred times the proton mass, which interact very weakly with the Xenon nuclei in the detector. Their interaction cross-section — which determines their rate of interaction — is trillions of times smaller than that for photons of electro-magnetic radiation which make stars and everything else shine. Most particles such as cosmic rays that are constantly streaming down to Earth from all over the sky would be stopped by the rock and water around the detector, but the WIMPs are expected to pass through. If one of these collides into one of the densely packed Xenon atoms in the detector, it would release a light signal in a unique way.

After examining a huge amount of data from carefully calibrated devices over a 20-month period, the latest results reveal that nothing with the right properties to excite the Xenon nuclei made it through. It would have been great if the ‘improved sensitivity’ of the new experiment, which ran for nearly two years had indicated a clear dark matter particle signal. A less sensitive experiment that ended in 2013 also ended with a negative result. This upgraded LUX detector has given the best sensitivity signal so far, since the earlier 2013 result.

The 2014-2016 LUX collaboration has pushed the sensitivity of the instruments to final performance limits, at least five times better. Suspended in a huge tank containing three lakh litres of purified water, a two-metre-high titanium tank holds more than 300 kg of frigid liquid Xenon.

Xenon atom can light up with a jolt of electric charge and a faint flash of light caught by surrounding sensors when a dark matter particle collides with one of the Xenon nuclei. Lakhs of gallons of water and mile of rock stops anything else from getting in a disturbing the Xenon liquid. This gives it the research, around 10 signals per century per kilogram of Xenon. The whole set up is located in the space called Davis Cavern (named after Raymond Davis Jr, discoverer and grand pioneer of the solar-neutrino problem) in the underground research facility.

Currently, with this negative result, it is of interest that NASA’s Fermi gamma ray telescope has also put some of the strongest limits yet on hypothetical dark matter particles. Apparently, there are no WIMPs in space, decaying into gamma rays of the expected energy. The recent null results as announced in Identification of Dark Matter Conference 2016 in Sheffield, UK narrows down the search of future dark matter experiments.

Search for WIMPs
Next in the line is the LUX-Zeppelin, which has 70 times the sensitivity of LUX. It uses the same techniques of heavy shielding of material and waiting patiently for a naturally occurring dark matter particle to pass through. Other experiments like COUP- 60, super cryogenic cold dark matter (Super CDMS) search, CdTe experiment have given similar results. Only the DAMA experiment claimed the detection of a WIMP, but the team is yet to confirm the results.

After the recent negative result of the LUX collaborator, the focus is now on the XENON1T experiment at the Gran Sasso National Laboratory in Italy. It is now the largest and most sensitive search for WIMPs. It uses 10 times as much Xenon as the LUX experiment. It houses a cylindrical vat filled with 3,500 kg of liquid Xenon. It is buried in a cave 1,400 metres underground It will look for rare occasions when a dark matter particle collides with a Xenon nucleus; the impact gives off photons of unique energy. In other words, the ubiquitously present dark matter particles would give off light when hitting the Xenon nuclei. In principle, it should detect 300 events per year, with its large mass of liquid Xenon detector. The experiment is planned for two years and detection of even 10 particles that match the predicted properties of the expected dark matter is considered enough to claim a discovery.

Is this a last call for the ubiquitously present dark matter particles to show their face? If we don’t see the expected events, many experts feel that our ideas could be wrong, or not on the right track and more exotic explanations many be required as to why there seems to the much more gravitating matter than the visible matter. Perhaps, keeping the Vulcan, a small hypothetical planet that was proposed to exist in an orbit between Mercury and the Sun in mind, a modification of Newtonian gravity or Einstein’s general relativity over cosmic scales may offer a solution.

In another context, the Large Hadron Collider (LHC) now on its new run should be powerful enough to create WIMPS in some energy ranges. These energy ranges are similar to that of WIMPs detectable at Xenon IT. So, LHC experiment has chances of creating dark matter particles and detecting them as well.

If in the next two or three years, if neither the XENON1T or LHC can produce or detect these particles, the time may be really ripe for alternative theories. While all the large, expensive deep and dark experiments with their interesting concepts are going on, there is still no hint about dark matter. This really is an exciting time for basic science.