"Neutrinos: they are very small / they have no charge; they have no mass; / they do not interact at all." These verses, written by the well-known American author John Updike, may now have to be revised in light of a recent discovery by physicists in Japan.

Filling the Void

The Universe May Never Be the Same.

That was the front-page headline in the New York Times and hundreds of other newspapers around the world in early June.

The same level of excitement took place in scientific centres and laboratories across the globe, including at the ICTP, where several of our scientists now have an opportunity to see the experimental confirmation of their theories.

What's behind the headlines and excitement? A team of 120 scientists from the United States and Japan announced that the Super-Kamiokande experiment in Takayama, Japan, showed that neutrinos oscillate. This means they may periodically convert from one form into another. Such behavior is a tell-tale sign of mass.

The topic of neutrino oscillation has interested ICTP staff scientist, Alexei Smirnov, for more than a decade. In 1985, he proposed an elegant solution for the so-called neutrino solar deficit. Scientists know that the number of neutrinos arriving on Earth is too small to account for the number of neutrinos created by nuclear reactions that take place in the centre of the Sun. The unanswered question has been what happens to these neutrinos.

Following the concept first proposed by the Italian physicist Bruno Pontecorvo, Smirnov developed a model explaining why solar-created neutrinos may oscillate on their journey from the Sun to Earth. That is, why neutrinos are transformed from one type into another and thus become difficult to detect. Neutrinos, in fact, exist in three different forms or flavours: electron, muon, and tau (there may also be a fourth flavour, sterile).

Now Smirnov's theory appears on the verge of confirmation. Nevertheless, much work remains to be done because the parameters of the observed oscillations, uncovered in experiments, still must compare to the parameters proposed in theories. Moreover, if neutrinos oscillate, they must have mass. The experiment in Japan offered indirect proof of neutrino mass.

Goran Senjanovic, also a staff scientist at the ICTP, has studied the origin of neutrino mass since the late 1970s, when with Gustavo Branco he published the first paper on the issue of neutrino mass in the context of modern gauge theories. Soon after, with Rabindra Nath Mohapatra, he coauthored a paper on the so-called see-saw mechanism of neutrino mass.

And why is the experimental discovery so important? Because such a finding, if confirmed, would not only require us to add new chapters to textbooks on particle physics but could have a dramatic impact on how we view the expansion of the Universe.

More specifically, if neutrinos have mass, it will cause us to rethink--and modify--one of the fundamental pillars of modern physics, the Standard Model, which describes the interactions of elementary particles and assumes that neutrinos have no mass. And, although neutrinos may have a mass billion times less than electrons, they have emerged as the most likely candidates for so-called "dark matter" because they fill every corner of the Universe.

As a result, understanding the properties of the Universe's lightest particles could help explain the behavior of the Universe's deepest void. That's the kind of finding that brings headlines to newspapers around the world and raises the level of excitement among scientists. For physicists, the finding that neutrinos have mass could well be a dream come true, which allows the rest of us to dream about the Universe in ways that we could have never dreamed before.

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