Dark matter: Let there be light

Now what if dark matter particles were really much lighter,mere wisps of neutralinos? And what if they had the same asymmetry–excess of matter over antimatter–as ordinary matter? In a paper in the July 2 issue of Physical Review Letters,Subir Sarkar and Mads Frandsen at the Rudolf Peierls Centre for Theoretical Physics,Oxford University,point out that if dark matter particles are as light as five times the mass of a proton,they will be continuously swept up by the Sun as it orbits around the centre of the galaxy and their number will grow since they have no antiparticles to annihilate with.

Proposing dark matter particles that are only about five times heavier than the proton (and thus five times as abundant) but more weakly interacting,Sarkar,who is also adjunct professor at the Tata Institute of Fundamental Research (TIFR),Mumbai,and at the Saha Institute of Nuclear Physics in Kolkata,says,“Neutralinos have no relation at all to the ordinary baryonic matter (protons and heavier nuclei) that we are made of. Instead,we propose that dark matter particles have the same matter-antimatter asymmetry as baryons do. We still do not exactly know how the observed excess of baryonic matter over antimatter was created,but we can conceive of the dark matter being related to protons and getting a similar excess through the same mechanism.” The origin of the “ordinary” matter we are made of,he says,is just as much of a puzzle as the nature of dark matter: “We are suggesting that these mysteries may be linked.”

Imagine looking for something called dark matter in a ball of light. But it makes perfect sense,says Sarkar,an alumnus of IIT Kharagpur and founder of the Oxford-India network for theoretical sciences which is now funded by the UK-India Education & Research Initiative. “There will be only one dark matter particle for every one thousand billion protons in the Sun and these will interact weakly with the protons and a little more strongly with each other. We have calculated that this is however enough for such particles to transport heat outwards through occasional collisions and this can cool the solar core a little. This is enough to alter the flux of neutrinos being emitted by nuclear reactions in the Sun by about 10-20 per cent,” he says. Experiments like the Borexino in the Gran Sasso Laboratory in Italy,which will soon be underway,are sensitive enough to measure the neutrino flux and can test the theory,which also explains the solar neutrino puzzle–the difference between neutrinos detected on Earth and the flux expected by theory.

“Finding this will be exciting but conclusive proof can come only from the nuclear recoil detectors,if these are redesigned to look for such relatively lighter particles. Observations of neutrinos from the Sun can complement laboratory searches for dark matter particles,” says Sarkar.