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Gold mine closures in southern India have also put paid to the world’s
longest running search for evidence that protons decay. Bahrat Gold Mines,
the government company which operates the Kolar Gold Fields (KGF), has been
declared ‘a sick unit’ – for the past few years, the cost of extracting
the precious metal has outstripped the profits from gold sales. As a result,
the world’s deepest physics laboratory, 2.3 kilometres underground, will
close this month.

The experiment started in 1980, funded by the Tata Institute of Fundamental
Research (TIFR) in Bombay and Osaka City University, Japan. It was set up
to find particles that theory says should be generated if protons decay.
To do this, detectors ‘keep watch’ over two massive matrices of iron plates,
one of 140 tonnes and other of 350 tonnes.

The laboratory’s position, deep underground, was chosen to minimise
interference by cosmic rays which can mask or even mimic the expected signs
of proton decay. Cosmic rays from space batter the upper atmosphere but
do not penetrate deep underground. Only neutrinos, generated by cosmic rays
colliding with particles in the atmosphere, can reach the experiment.

‘There were about half a dozen candidate proton-decay events in KGF,’
says Vemuri Syamala Narsimham, a physicist at the TIFR. But they could have
been caused by neutrinos. ‘We are in the process of stopping these studies
at KGF and will try to understand what we have so far,’ he says.

The search for proton decay was sparked off in the 1970s by the grand
unified theories which attempt to explain at a single stroke the common
origins of the strong and weak nuclear forces and electromagnetism. According
to these theories, the proton – and consequently all matter – cannot be
immortal. The half-life of the proton was originally calculated at 1030
years.

Candidate events have been announced by laboratories all over the globe,
but ‘there is no gold-plated example anywhere nor a statistically significant
sample of candidate events’, says Narsimham. ‘The experiments have certainly
pushed up the life of protons by about a factor of one hundred. This is
a very significant achievement leading to abandonment of several theoretical
models,’ he says.