INVASIONS IN PARTICLE PHYSICS. Compton Lectures Autumn 2001 Lecture 3 Oct. 20 2001. LECTURE 2 Cosmic Invasion. The muon and the pion. Previous Lecture. Rutherford Scattering. Most of the atom is empty space. The positive charge of the atom is concentrated in a tiny nucleus.
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Compton Lectures Autumn 2001
Oct. 20 2001
The muon and the pion
The idea in the 30s was that there existed a neutral object of ½ spin in the nucleus (Rutherford’s idea) and for example the Nitrogen nucleus is (7 protons+7 neutrons)
excited nucleus unexcited (same) nucleus + gamma
Electromagnetism wrong sign
The size of the nucleus is 10-15 m
to hold the nuclear pieces together
What makes a force “short range”??
The photon which has zero mass is the
messenger of the electromagnetic force.
The range of the electromagnetic (EM) force
is infinite -The EM force can be attractive or repulsive.
The nuclear force that keeps the nuclear pieces together must act at very small distances (the size of the nucleus).
A clue for the “short range” of this force: bigger heavier nuclei decay into smaller more stable nuclei.
Anderson had already discovered the positron in a cloud chamber in 1930.
Ionizing radiation that does not
originate from the earth :cosmic rays.
In 1905 Victor Hess performed a series of high-altitude balloon experiments and concluded that the
origin of this radiation is beyond the
Cosmic rays were the only source of
high energy particles to study until
accelerators were developed.
The alleged pi-meson that Anderson et al. discovered, seemed very reluctant to interact with the atomic nuclei: it penetrated Earth’s atmosphere and reached the cloud chamber at ground level!
The pion was discovered by Cecil Powel and Giuseppe Occhialini in 1947 using photographic emulsions at the Pic du Midi, high in the French Pyrrenees.
(200 times heavier that the electron)
(273 times heavier that the electron)
N(t)=N(0)exp(-Gt) at a later time t.
what is the force that causes the muon to decay?