Probing Question: What is a neutrino?

Oct 16, 2007 by Steve Miller
Probing Question: What is a neutrino?
Workers repair one of the 11,200 photomultiplier tubes that line the Super Kamiokande neutrino detector and track the light generated as neutrinos move at nearly the speed of light through the water. Courtesy Kamioka Observatory, ICRR, The University of Tokyo

Neutrinos are tiny -- really, really tiny -- particles of matter. They are so small, in fact, that they pass between, and even through, atoms without interacting at all. Neutrinos are everywhere: If you start counting now, more than 10 quintillion (that's 10 trillion billions) of them will have passed through your body by the time you finish this article. Yet only one of those 10 quintillion neutrinos will likely interact with an atom in your body. The rest will go merrily on their way.

Staggering numbers of neutrinos constantly pass right through the earth and off into distant space. Detecting them carries the idea of searching for a needle in a haystack to ridiculous extremes. Fortunately for scientists, there are a lot of them to look for. Irina Mocioiu, assistant professor of physics at Penn State, studies evidence of the rare interactions between neutrinos and other, more accessible, subatomic particles, such as protons and electrons.

How does she know when an interaction is taking place? "Neutrinos carry no charge, so they cannot be observed directly," Mocioiu acknowledges. But each collision can cause a characteristic change in a proton or an electron and transfer some of the energy of the moving neutrino into the motion of these charged particles. "There are many ways to see charged particles in a detector," she explains.

A neutrino detector -- one of the devices designed to confirm the activity of these infinitesimal particles -- typically contains a large body of liquid, which increases the chances for particle interaction. For example, noted Mocioiu, the Super Kamiokande detector in Japan holds 12.5 million gallons of water surrounded by more than 11,000 photomultiplier tubes, light sensors arrayed to pick up the radiation caused by interactions between neutrinos and water molecules.

To prevent interference from other types of radiation, this detector, like the dozen or so others operating around the world, was built about a kilometer below ground. Every day that it operates, Super K picks up information from a small handful of the almost unimaginable number of neutrinos that pass through it.

Because they are so tiny, neutrinos were long thought to have no mass at all. "In the last 10 years, however," Mocioiu noted, "neutrino oscillation experiments have definitively proved that neutrinos have masses, just extremely tiny ones."

So where did all these ghostly particles come from in the first place? Most of the neutrinos in the universe are believed to have been formed billions of years ago, during the Big Bang, said Mocioiu. "We believe that the neutrinos coming from the Big Bang are almost stationary and that there are 10,000,000 such neutrinos in every cubic foot of space throughout the universe," she added. These stationary neutrinos are almost impossible to detect.

The more active neutrinos that Mocioiu studies are products of the nuclear reactions that fuel stars and high energy cosmic events such as the explosions of dying stars. Even with so many neutrinos around, they still don't add up to much mass. According to our present understanding, Mocioiu saids, neutrinos account for at most a few percent of the total energy density of the universe, and maybe as little as a fraction of a percent.

Why study something that, on its face, seems so insignificant? "Neutrinos play an important role in particle physics, astrophysics and cosmology," said Mocioiu. "Because neutrinos have no electric charge and have only weak interactions, they can travel much longer distances without being absorbed by matter or deflected by magnetic fields. So neutrinos can provide new information about astrophysical objects and events."

The streams of neutrinos careening away from distant collapsing stars or galaxies, Mocioiu says, carry with them bits of data about the extremely high-energy events that produced them. It's a reminder that even the tiniest of particles have a tale to tell about the universe of which they are part.

Source: by Steve Miller, Research Penn State

Explore further: New method for non-invasive prostate cancer screening

add to favorites email to friend print save as pdf

Related Stories

Simulations for better transparent oxide layers

39 minutes ago

Touchscreens and solar cells rely on special oxide layers. However, errors in the layers' atomic structure impair not only their transparency, but also their conductivity. Using atomic models, Fraunhofer ...

Greater safety and security at Europe's train stations

49 minutes ago

When a suspicious individual fleas on a bus or by train, then things usually get tough for the police. This is because the security systems of the various transportation companies and security services are ...

DNA may have had humble beginnings as nutrient carrier

59 minutes ago

New research intriguingly suggests that DNA, the genetic information carrier for humans and other complex life, might have had a rather humbler origin. In some microbes, a study shows, DNA pulls double duty ...

Central biobank for drug research

59 minutes ago

For the development of new drugs it is crucial to work with stem cells, as these allow scientists to study the effects of new active pharmaceutical ingredients. But it has always been difficult to derive ...

Scientists get set for simulated nuclear inspection

7 hours ago

Some 40 scientists and technicians from around the world will descend on Jordan in November to take part in a simulated on-site inspection of a suspected nuclear test site on the banks of the Dead Sea.

Recommended for you

New method for non-invasive prostate cancer screening

14 hours ago

Cancer screening is a critical approach for preventing cancer deaths because cases caught early are often more treatable. But while there are already existing ways to screen for different types of cancer, ...

How bubble studies benefit science and engineering

15 hours ago

The image above shows a perfect bubble imploding in weightlessness. This bubble, and many like it, are produced by the researchers from the École Polytechnique Fédérale de Lausanne in Switzerland. What ...

Famous Feynman lectures put online with free access

16 hours ago

(Phys.org) —Back in the early sixties, physicist Richard Feynman gave a series of lectures on physics to first year students at Caltech—those lectures were subsequently put into print and made into text ...

Single laser stops molecular tumbling motion instantly

20 hours ago

In the quantum world, making the simple atom behave is one thing, but making the more complex molecule behave is another story. Now Northwestern University scientists have figured out an elegant way to stop a molecule from ...

User comments : 0