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Neutrino astronomy

I. What are neutrinos?

Section 1 text from “The Particle Adventure

Neutrinos are a type of lepton. Since they have no electrical or strong charge they almost never interact with any other particles. Most neutrinos pass right through the earth without ever interacting with a single atom of it.

Neutrinos are produced in a variety of interactions, especially in particle decays. In fact, it was through a careful study of radioactive decays that physicists hypothesized the neutrino’s existence. For example:

(1) In a radioactive nucleus, a neutron at rest (zero momentum) decays, releasing a proton and an electron. (2) Because of the law of conservation of momentum, the resulting products of the decay must have a total momentum of zero, which the observed proton and electron clearly do not. (Furthermore, if there are only two decay products, they must come out back-to-back.) (3) Therefore, we need to infer the presence of another particle with appropriate momentum to balance the event. (4) We hypothesize that an antineutrino was released; experiments have confirmed that this is indeed what happens.

neutrino production conservation momentum

Because neutrinos were produced in great abundance in the early universe and rarely interact with matter, there are a lot of them in the Universe. Their tiny mass but huge numbers may contribute to total mass of the universe and affect its expansion.

II. What is neutrino astronomy?


John Learned, High Energy Physics Group, Dept. of Physics and Astronomy, Univ. of Hawaii, writes

Almost all we know about the universe derives from the observation of photons… We are learning some further things about the cosmos beyond the solar system by observing cosmic rays, which are mostly made up of either atomic nuclei minus their orbiting electrons, or one of their basic components, protons. But these positively charged particles do not point to their place of origin due to the magnetic fields of our galaxy which affect their flight paths like a magnet affects iron filings.

What is needed for deep, sharply focused examination of the universe is a telescope that can see a particle that is not much affected by the gas, dust, and swirling magnetic fields it passes on its journey. Neutrinos are a candidate…. these neutral weakly interacting particles come to us almost without any disruption straight from their sources, traveling at very close to the speed of light. A (low energy) neutrino in flight would not notice a barrier of lead fifty light years thick. When we are able to see outwards in neutrino light we will doubtless get a wondrous new view of the universe.

Why neutrino astronomy?

Thierry Stolarczyk, at CEA, à l’Institut de recherche sur les lois fondamentales de l’Univers (Irfu)., writes

Most of our current knowledge of the Universe comes from the observation of photons.
Photons have many advantages as cosmic information carriers: they are copiously produced, they are stable and electrically neutral, they are easy to detect over a wide energy range, and their spectrum carries detailed information about the chemical and physical properties of the source.

Their disadvantage is that the hot, dense regions which form the central engines of stars, active galactic nuclei and other astrophysical energy sources are completely opaque to photons. Therefore we cannot investigate the properties of these regions by direct observation, but only by indirect inference.

For example, the photons we observe from the Sun come from its photosphere, far removed from the hydrogen-fusing core. Moreover, high energy photons interact with photons of the infrared radiation background and with the cosmic microwave background to create electron-positron pairs; this is the Greisen-Zatsepin-Kuz’min effect (GZK). This effect suppresses any possibility of surveying the sky over distances greater than 100Mpc with high energy (>10 TeV) gamma rays.

In order to observe the inner workings of the astrophysical objects and to obtain a description of the Universe over a larger range of energies, we need a probe which is electrically neutral, so that its trajectory will not be affected by magnetic fields, stable so that it will reach us from distant sources, and weakly interacting so that it will penetrate regions which are opaque to photons. The only candidate currently known to exist is the neutrino.


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The burgeoning field of neutrino astronomy. Symmetry magazine


Learning Standards

Massachusetts Science and Engineering Practices

• Ask questions:
o That arise from careful observation of phenomena, or unexpected results, to clarify and/or seek additional information.
o That arise from examining models or a theory, to clarify and/or seek additional information and relationships

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas

In fission, fusion, and beta decay processes, atoms change type, but the total number of protons plus neutrons is conserved. Beta processes involve an additional type of interaction (the weak interaction) that can change neutrons into protons or vice versa, along with the emission or absorption of electrons or positrons and of neutrinos. Isolated neutrons decay by this process.

…Beta processes involve an additional type of interaction (the weak interaction) that can change neutrons into protons or vice versa, along with the emission or absorption of electrons or positrons and of neutrinos….

…In forming a concept of the very small and the very large, whether in space or time, it is important to have a sense not only of relative scale sizes but also of what concepts are meaningful at what scale. For example, the concept of solid matter is meaningless at the subatomic scale, and the concept that light takes time to travel a given distance becomes more important as one considers large distances across the universe….

AAAS Benchmarks

Increasingly sophisticated technology is used to learn about the universe. Visual, radio, and X-ray telescopes collect information from across the entire spectrum of electromagnetic waves; computers handle data and complicated computations to interpret them; space probes send back data and materials from remote parts of the solar system; and accelerators give subatomic particles energies that simulate conditions in the stars and in the early history of the universe before stars formed. 4A/H3


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