March 27, 2014
R. Ramachandran
The unusual events, which were detected in a long, 2.3 km deep tunnel, occurred during both the phases — 1960s-70s, and 1980s
Important finding:The proton decay detector used in the second phase proton decay experiments at KGF. — photo: special arrangement
The handful of unusual events observed in the underground experiments at the Kolar Gold Field (KGF) mines during the 1960-70s and the 1980s, which have remained unexplained to this day, may have been due to the decays of hitherto unseen Dark Matter (DM) particles.
This interesting hypothesis has been put forward by Profs. G. Rajasekaran and M. V. N. Murthy of the Institute of Mathematical Sciences (IMSc), Chennai, in a paper published in the latest issue of the physics journal Pramana.
While at that time the events were interpreted to be perhaps due to the decay of a massive unknown particle, subsequent accelerator experiments at CERN in Europe and Fermilab in the U.S. did not find evidence for any such massive particle. Also, the currently highly successful Standard Model of elementary particles, bolstered by the discovery of the Higgs particle in 2012, cannot accommodate such a massive particle.
The postulate of DM was put forward to account for the extreme velocities with which galaxies and clusters of galaxies are observed to be rotating that the gravity generated by their observable matter alone cannot explain. At such speeds they should have been torn apart long ago. It is believed that something that cannot be seen directly with light (electromagnetic radiation, in general) — and hence the name — is providing that extra mass, generating the extra gravity, needed to hold them together.
DM dominates the matter in the universe, outweighing all the visible matter by nearly six times, but its existence can be inferred only from the gravitational effect it seems to have on visible matter. Though existence of DM is now accepted, and it is all around us with varying densities, its nature has remained a mystery and various candidate DM particles have been proposed.
However, DM as a possible source of the ‘Kolar events’ was never considered until now perhaps because the concept of DM was yet to become of mainstream physics discussions at that time. The recent claim by the DM search experiment called CDMSII of possible evidence of a DM particle with a mass of 5-10 GeV (in energy units) has provided the motivation for the IMSc scientists to revisit the ‘Kolar events’ from a DM perspective.
The KGF experiments, which were sponsored by the Tata Institute of Fundamental Research (TIFR), were carried out in two phases. The first phase experiments, during the 1960s-70s, studied cosmic ray neutrino interactions and the second, during the 1980s, studied proton decay and set limits on proton’s lifetime. The unusual events, which were detected in a long tunnel at a depth of 2.3 km, were seen during both the phases.
While, in principle, such events could be produced by cosmic ray neutrinos (or antineutrinos) interacting with air molecules in the gap between the rock wall and the detectors, the large number seen could not be explained by known processes. Standard processes due to neutrinos or muons would only produce such events with a probability of less than one in 100 years. Here one was seeing eight such events (5 in the first phase and 3 in the second) in about as many years.
Instead of the early interpretation of cosmic ray neutrinos interacting with the surrounding rock and producing a massive particle which subsequently decayed to give rise to these anomalous events, the authors interpret the events to have been caused by the decay of a neutral DM particle with a mass of about 5-10 GeV and with a lifetime of the order of the lifetime of the universe (about 1010 years or 10 billion years).
The CDMSII experiment recently observed three events, which have been interpreted to be due to a DM particle with a mass of 8.2 GeV. This falls within the range that the IMSc scientists require for their interpretation. It may, however, be pointed out that the jury on this claim is still out as another DM search experiment LUX has not seen any evidence so far.
Arguing that not much attention has been paid to decaying DM particles, they choose a model that has both stable and unstable DM particles.
This gives them a DM particle lifetime of 1010 years as against the generally accepted value of greater than 1011 years. Using an appropriate detection volume of 1010 cm3 (from the known dimensions of the tunnel) and a DM density of 1 particle/cm3, they get a value for the rate of events that matches with the rate observed at Kolar.
“It is miraculous that such a crude estimate gives remarkable agreement. So there could be some truth in our speculation,” points out Rajasekaran. They, however, recognise that, if the lifetime is greater, or if the density is an order of magnitude smaller, DM could not have caused any Kolar event, as some critics of this work have also noted.
“Quibbling about these values does not make much sense when we know nothing about the nature of DM. All estimates are, after all, guesstimates. All we are saying is that, if our speculation is correct, it solves two problems in one stroke: explaining the anomalous Kolar events and observation of DM,” he adds.
“Independent of the Kolar events and their interpretation, any large underground detector must be in a position to see the decays of an unstable DM particle,” says the paper.
Neutrino experiments such as OPERA and MINOS, where the detector is similar to KGF, are well suited to look for such decays, they note. But, more importantly, the paper highlights the importance of considering DM decays in the analyses of experimental data. “Non-observation of decays may be used to set limits on [DM particle] lifetime,” the paper observes.
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