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Can a subatomic particle change its identity like a shape-shifter in a science fiction movie? If the particle, the neutrino, indeed oscillates among the three possible “flavors,” it means that neutrinos have a small mass that may account for some of the missing mass in the universe.
To help answer this, scientists may use something like a vault full of safe-deposit boxes. The valuables they hold will be plates of lead and photographic film waiting for a neutrino to make a near-impossible collision with the nucleus of a lead atom and leave a debris trail in the film that betrays its presence. |
Looking for Cracks in the “Standard Model”
In the 1950s and ‘60s scientists faced a bewildering array of particles coming from particle accelerators as they pushed to ever higher energies. Order was offered in the 1960s when several scientists proposed what is now called the Standard Model.
 In it, six types of quark (and corresponding anti-quark) are the building blocks for heavy particles. Mesons (middleweight particles) are made of two quarks (or antiquarks). Baryons (heavyweights, including protons and neutrons in the nuclei of atoms) are made of three quarks (or antiquarks).
Electrons, buzzing in clouds around the nucleus, are in a separate category called leptons (lightweights). There are only six leptons: electrons, muons, and taus, plus three corresponding neutrinos. Leptons are their own fundamental particles. Like quarks, leptons are believed to be fundamental particles with no underlying structure.
Right: Everything in the universe comes from combinations of 12 types of particles and four forces in the Standard Model. The MINOS experiment will focus on the three types of neutrinos and whether each oscillates to become one type then another. The Standard Model is rounded out by four bosons: W and Z (weak force), photon (electromagnetic force), and gluon (strong force).
These panels were assembled at NASA Goddard Space Flight Center's Astrophysics
Data Center using results from satellites developed there and elsewhere.
| MINOS Team (with links) |
| The MINOS collaboration involves scientists from a wide range of institutions. The team leader is Dr. Stanley Wojcicki of Stanford University. Dr. Adam Para of Fermilab leads the study of emulsion detectors which would confirm the appearance of neutrinos in the Soudan mine detectors.
Other MINOS collaborators include the University of Athens (Greece), the California Institute of Technology, JINR Dubna (Russia), Harvard University, IHEP Beijing (Peoples Republic of China), ITEP(Russia), Lebedev Institute (Russia), University College London, the University of Pittsburgh, IHEP Protvino (Russia), Rutherford Laboratory (England), the University of Sussex (England), Stanford University, the University of Texas at Austin, Tufts University, and Western Washington University. Funding for the project comes from DOE, the National Science Foundation, the State of Minnesota, and science funding agencies abroad. |
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The neutrino was a “desperate remedy” proposed in 1930 by Wolfgang Pauli when physicists realized that some mass or energy was lost in certain types of radioactive decay. Pauli suggested that the missing energy was carried away by a particle that could not then be detected. Enrico Fermi named it the neutrino - “little neutral one.” They stayed invisible until 1956 when detector methods had advanced. By 1962, scientists had identified the electron and muon neutrinos.
As scientists learned to detect neutrinos, they discoverd a curious fact: some neutrinos seemed to be missing. Even though only one neutrino in several million is ever detected, scientists expected to snare a few dozen out of the countless numbers flooding to Earth from the fusion reactor inside our sun. In fact, immense underground detectors measured only half as many neutrinos as calculated.
A possible answer to this “solar neutrino deficit” had been offered in 1957 by Bruno Pontecorvo in the USSR. He suggested that neutrinos might also change “flavor” (one of several terms physicists use as convenient expressions in describing subatomic particles; the terms bear no real relationship to the particle’s physical condition). Such a flavor change would not only account for the low numbers of neutrinos detected from the sun, but also have implications for the mass of the universe. The Main Injector Neutrino Oscillation (MINOS) experiment is designed to investigate this possibility.
*Material on this web page courtesy of NASA's Marshall Space Flight Center and Science@NASA
URL: http://wwwssl.msfc.nasa.gov/newhome/headlines/ast30aug99_1.htm
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