Fermilab is an enduring source of strength for the US contribution to scientific research world wide.
Friday, May 17, 2013
“The world of particle physics and cosmology is full of invisible phenomena like dark matter, neutrinos and that latest spiffy object predicted by the theory of the week. When you think about it, it’s really quite hard to measure some of the properties of these invisible particles. So scientists had to come up with some clever ways to determine things like the mass of something that cannot be detected directly. One such way involves careful accounting of the energy observed in the experiment.
When scientists were first studying beta decay, they expected the electron to be emitted with a single unique energy, as depicted in red. However, they measured instead a range of energies for the emitted electron, shown in yellow, all lower than the expected energy, which the electron would carry if neutrinos didn’t exist. In the lower right hand corner, we see a closeup of the spectrum near the expected energy. The dashed line is what we see if the neutrino has no mass, while the magenta curve is what we’d see if the neutrino had a small but non-zero mass. CMS scientists employed this technique to study top quark production to validate the method.
This technique has been used in the past. A type of radioactivity called beta decay occurs when a neutron in the nucleus of an atom converts to a proton and emits an electron. Following the principle of energy conservation, scientists predicted the electron to be emitted with a single energy, but measurements showed that the energy of the electron can have many different values. In fact, it turned out that the predicted value of the electron’s energy was actually the maximum it could be. The measured values were always lower.
In 1930 Wolfgang Pauli proposed a solution to this curious situation: Not only were a proton and an electron emitted in beta decay, but a neutrino was emitted as well. Neutrinos are particles that interact only via the weak nuclear force and are therefore very, very hard to detect. Clyde Cowan and Frederick Reines showed the idea to be correct in 1955 when the neutrino was detected.
See the full article here.
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