By Miriam Aczel

What are neutrinos? They are almost massless— hardly feel the force of gravity They have no electric charge— don’t feel the electromagnetic force Have no color charge—don’t feel the strong nuclear force They feel the weak nuclear force

Their Flavors Electron neutrino Muon neutrino Tau neutrino And: they can change flavor from from one to another e  

Solar Neutrinos Are produced in weak-force interactions inside the Sun The “solar neutrino problem” (Paper in Rev. Mod. Phys. 60, 297 (1988) by John Bahcall of IAS, Princeton, and R. K. Ulrich of UCLA): Where are all the neutrinos? We detect less than half the expected number of s

The Sudbury Neutrino Observatory 1,000 tons of heavy water (D 2 O)

Super Kamiokande Neutrino Observatory 50,000 tons of ultrapure water

Two more observatories: at CERN- Gran Sasso, and Fermilab-Soudan CERN creates neutrinos in an accelerator and sends them through the Earth to Gran Sasso, in Italy. Fermilab does the same, sending neutrinos to the Soudan mine in Minnesota

How do these observatories work? CERN-Gran Sasso and Fermilab-Soudan study neutrino oscillations: flavor changes in flight (inside the Earth’s crust) from electron to muon to tau flavors and back Sudbury and Super K study naturally-occurring neutrinos that arrive on Earth from deep space.

The key element, in all four detectors, is a large tank of water (heavy water or regular super-pure water) that is isolated from the environment by being placed in a deep mine to minimize “noise”—extraneous signals. Scientists look for the rare weak-force interaction of an incoming neutrino with either an electron or one of the nuclei of the water molecule.

How is Such a Detection Achieved? In 1967, Steven Weinberg predicted the existence of the Z (neutral) boson, a carrier of the action of the weak force (two other weak- force bosons, W+ and W-, had been predicted earlier). In 1972, in the new Gargamelle detector at CERN, “neutral currents” were discovered: a neutrino produced in an accelerator interacted with an electron in the fluid inside Gargamelle, through the intermediation of the weak-force boson Z.

All four experiments use the same idea. Neutrinos don’t like to interact with other matter, but if enough of them impact water or another fluid, eventually a weak-force interaction will occur, intermediated by the Z boson. Particles that interact here will end up moving at faster than the speed of light in the fluid inside the detector (water, heavy water). This produces Cherenkov Radiation, which is then detected by photodetectors. Cherenkov radiation is a cone of bluish light that is like a “supersonic boom” for light rays.

Example: The latest result from the MINOS Experiment of Fermilab-Soudan (P. Adamson, et al. arXiV: [hep-ex])

The red histogram, for all energy levels of neutrinos arriving at Soudan, shows the expected frequency of detection for electron neutrinos. The black intervals below show the actual frequencies for all energy levels. The difference between the black ranges and the red curve is the deficit in electron neutrinos believed to have changed flavor to muon-neutrinos. This is an experimental verification (once enough muon neutrinos are discovered) of the electron-to- muon flavor change, or oscillation. Neutrino oscillations imply mass (neutrinos, if they oscillate, cannot be massless).

A Research Proposal In a similar way to how muons are now used as “natural x-ray machines” to inspect cargo containers and look into volcanoes, I suggest using neutrinos to peer into the Earth. Here is how I suggest it can be done.

Design a Satellite to orbit the Earth in such a way that it is always at “night”

The satellite carries a detector, a water tank, ultrapure ton of water, and Cherenkov detectors, shielded all around except on side facing Earth

SUN Neutrinos Earth eclipses the Sun Earth from vantage point of our satellite and itsneutrinos go through detectorEarth, are detected

How do you know it can be done? The AMS (Alpha Magnetic Spectrometer) is a very heavy detector with a superconducting magnet and a ton of liquid helium going up to the Space Station to look for antimatter from space, dark matter, supersymmetric particles, and other particles and interactions in space. If it can be lofted into space, so can my neutrino detector

How Does it Work and What Can it Reveal? Most neutrinos go through the earth, but some are stopped because of interactions. The rate of interaction is proportional to the density of levels they go through. Cherenkov counters running a long time will reveal a map of Earth’s interior

What Could the Results be Like? A better, more accurate map of the interior of our planet