Phantom Particles 

A mysterious force drives the 
expansion of the Universe, reports Biplab Das 

As you're reading this story, zillions of a particular kind of ghostly particles are piercing your body, and going out of it - yes, without leaving any trace at all. These harmless things, called neutrinos, are always streaming through the earth. They are one of the most-wanted objects in particle physics today. Why? "Because, neutrinos are one of the fundamental particles that make up the Universe," said Prof. Guruswamy Rajasekaran of Institute of Mathematical Sciences, Chennai. "They hold the key to many secrets of the cosmos are waiting to be discovered." Rajasekaran was delivering the 4th Chanchal Kumar Majumdar Memorial Lecture on 'Recent Discoveries in Neutrino Physics' at the S. N. Bose National Centre for Basic Sciences (SNBNCBS). 

"Neutrinos are churned out by the reaction that fuels the stars," said Rajasekaran. Inside an ordinary star, hydrogen atoms fuse to form a helium atom, a process known as nuclear fusion, which throws off neutrinos. "Being chargeless, they remain unaffected by the electromagnetic force that acts on electrons." The only force that disturbs neutrinos is the so-called 'weak force' that acts at the atomic level. "This also explains why physicists took nearly 30 years to detect these particles since Wolfgang Pauli predicted their existence in 1930," Rajasekaran said. "In 1959, a team led by Friederick Reines, a physicist from the University of California, Irvine, first detected neutrinos." 

The neutrinos detected by Reines didn't come from outer space. They were generated at a nuclear plant in South Carolina owned by Dupont. "Reines and his colleagues used a pool of heavy water (which has deuterium instead of hydrogen in it)," Rajasekaran said. "They detected low-energy neutrinos related to electrons." 

Till date, detectors laid out deep inside the earth have detected three varieties of neutrinos. Each variety is related to a charged particle which gives the corresponding neutrino its name. Except the electron-neutrinos (related to electrons), there are two other neutrinos which are associated with heavier versions of the electron called the muon and the tau. The muon neutrinos and tau neutrinos possess more energy than the electron-neutrinos. 

But physicists are still clueless about neutrinos' mass. "They had to wait until 1998 when the Super-KamioKande detector in Japan found that neutrinos do have mass," Rajasekaran said. "But they are yet to pin down the exact mass of three types of neutrinos." Besides nuclear power plants, they emanate from the core of a star like our sun. "At the core of this star, where the temperature hovers around 15 million degrees, nuclear fusion yields electron-neutrinos in copious amounts," said Rajasekaran. "Defying the sun's gravity, these neutrinos escape into space." The earth is literally bathed in solar neutrinos. 

To capture solar neutrinos, Raymond Davis, a physicist from the University of Pennsylvania and Brookhaven National Laboratory, built a swimming pool-sized tank filled with cleaning fluid and buried it deep inside the Homestake gold mine in South Dakota, US. Davis's detector found only one-third of the expected mass of solar neutrinos. This finding has been the first direct evidence that a star's inferno is fuelled by nuclear reaction. 

According to Rajasekaran, Davis's research on solar neutrinos has spawned a new field called neutrino astrophysics. 

As neutrinos stream through empty space and solid matter at the speed of light, unscathed by any external influence, they carry messages from the deep recesses of the Universe. Which is why scientists believe neutrinos may solve the mystery of the so-called dark matter (basically, a form of matter that gives a hint of its existence but defies all attempts to detect its form) that makes up 90 per cent of the Universe. 

"It is thought that there is a sea of neutrinos, each contributing its mass to dark matter," said Rajasekaran, adding that in a recent breakthrough, a team of physicists from the University of Washington has linked neutrinos with the mysterious 'dark energy', which is thought to have given a sudden spurt to the regular expansion of the Universe. The findings will appear in a forthcoming issue of the journal Physical Review Letters.