The number of neutrinos moving freely throughout the Universe since the Big Bang is staggering, about 300 million in every cubic meter. The Sun and supernovae (exploding stars that have reached the end of their life) are the primary sources of neutrinos reaching us. Every second over 500 trillion solar neutrinos enter every square meter of the Earth's surface exposed to the Sun. By the time you finish reading this paragraph, about trillion neutrinos will pass through your body. But don't panic, they are inert and harmless; just “little bits of spin-energy that have got detached,” according to astrophysicist Arthur Eddington.
Why should we care about these furtive particles? Strange as it may seem, the intrinsic properties of neutrinos make them an influential player in the field of cosmology. They carry a wealth of information of great cosmic significance, about the high-energy events that produced them, deep secrets of the cosmos from distant galaxies and the remote past. They are essential in understanding what happened in the briefest moments after the Big Bang. They can tell us why the Universe contains more matter than antimatter. If they have mass, even if it is miniscule, it will be the dominant form of matter in the Universe.
Neutrinos act as telescopes and allow us to take a peek into the interior of the Sun. They are critical in understanding stellar evolution and dynamics of supernovae where heavy elements are produced. They are living witnesses of star birth and their violent death. Just like red shift and cosmic microwave background radiation, neutrinos have a tale to tell about the Universe of which they are part. Detection of neutrinos, therefore, is crucial if we want to have a handle on all of these and other aspects of cosmology.
It is notoriously difficult to detect these snippets; it is like looking for a needle in a giant haystack. Absence of charge makes them unresponsive to electromagnetic interaction. Lack of internal structure (no quarks) prohibits them from taking part in strong interaction. They cut through the Earth unhindered and move off to outer space. Only a slab of lead several light years thick will be able to stop these ephemeral particles. They announce their presence only through weak interaction associated with the radioactive break-up of a nucleus.
Occasionally neutrinos do interact with matter making it possible to capture a few of them if the detectors are large enough. Also known as “neutrino telescope,” the detector is a huge tank filled with large amount of liquid such as chlorine-rich cleaning fluid or heavy water. The liquid increases the chances for particle interaction. The detectors must be placed deep underground, beneath a mountain or inside a mine, so that only neutrinos can reach them. All other particles coming from outer space that could befuddle the detectors are blocked by the thick layers of overlying rocks.
The earliest detector built in 1967 is buried nearly a mile deep in a gold mine at Homestake, South Dakota. It was expected that out of the trillions of neutrinos that will reach the detector, the capture rate would be at least one per day. After more than two decades, it was found that on the average it trapped only one neutrino every three days. A more recent detector buried in a nickel mine in Sudbury, Canada revealed that the neutrinos on their way to the Earth can change flavor. The Homestake detector was tuned for only one flavor, the electron neutrino; hence the low count rate.
Clearly, the problem of detecting neutrinos is of immense proportions. We still have “miles to go” before we can say we are on the verge of solving the neutrino puzzle. However, the detection in 1987 of more than the expected number of neutrinos from a supernova explosion 168,000 light years away by underground telescopes in Japan and Ohio has infused new life into neutrino physics. Physicists are now more than ever inspired to build some of the most sophisticated detectors to find and unmask the guise of this gate keeper of the Universe.
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