Download Experimental foundations of subatomic physics The muon, the pion

Experimental foundations of subatomic physics
A. Andronic
The muon, the pion, the kaon
• The muon (µ)
• The pion (π)
...the only one predicted by theory
• The kaon (K)
Prelude: the status of things around 1940
A. Andronic
• Fermi proposes a theory of β decay, 1933
(it is still in place, in a modernized version)
• understanding of nuclear force, keeping p,n in the nucleus (Heisenberg and
others, 1932)
- pp and nn forces are equal (charge symmetry)
- pp, pn and nn forces are equal (charge independence)
- introduction of isospin and of new symmetries (SU(2), continuous; SU(4), approximate)
• better understanding of soft component of cosmic ray showers (started by photons), 1936: cascade of photons and electron-positron pairs and Bremsstrahlung
• Hideki Yukawa, Kyoto, 1934, proposes a theory of nuclear forces
“The nuclear force is effective at extremely small distances. My new insight was that this distance and the mass
of the new particle are inversely related to each other”
Recall Maxwell: Coulomb potential 1/r (infinite range), photon of mass zero
Yukawa: potential e−kr /r (range 1/k), mesons of mass µ, where 1/k = ~/µc
Nobel Prize, 1949
Discovery of the muon
A. Andronic
Neddermeyer and Anderson, 1936
analyzing the penetrating component of cosmic rays
was not part of showers like the electron
had a mass about 200 times that of electron
they called it “heavy electron”
in 1937 Oppenheimer and Serber propose that
the muon is the particle of Yukawa
Muon follow-up
A. Andronic
• it decays (lifetime 2.2 µs):
µ− → e− + ν̄e + νµ
µ+ → e+ + νe + ν̄µ
same decay mechanism as beta decay
• a new neutrino is needed (was later discovered) because it was noted later that
lepton types must be individually conserved
• µ in atoms and µ with e combinations were produced (for short time:)
• studies with the muon yield very precise physics results (ex.: proton radius)
• a still heavier brother, the tau τ (about twice heavier than the proton, 3500
heavier than e) was discovered in 1975 at Stanford, USA
Martin Perl, Nobel Prize 1995
...and it must have its own neutrino, ντ , of course (was discovered in 2000)
Anticipations of the pion
A. Andronic
• it became clear, from interactions with matter, that the muons are not the
mesons of Yukawa
• a new proposal: Yukawa’s mesons decays into muons (Tanikawa, 1942; Sakata
and Inoue, 1943; Marshak, 1947)
• in 1940 Sakata and Tanikawa had proposed a neutral meson, expected to decay
in 10−16 s into 2 photons
Discovery of the pion
A. Andronic
Powell, Lattes, Occhialini, 1947, Bristol, UK (Nobel Prize, 1950, Powell)
using photographic emulsions
“We must conclude that the particle entered the nucleus and produced a disintegration with the emission of heavy particles”
http://www.nature.com/physics/looking-back/lattes/
Pion follow-up
A. Andronic
mass: about 280 times e
decay (lifetime 2.6×10−8 s):
π + → µ+ + νµ
π − → µ− + ν̄µ
(much less probable, but decay into electron happens too)
by the same force as for the beta decay
picture at right:
π − → µ− + ν̄µ, µ− → e− + ν̄e + νµ
Cecil Powell, Nobel Lecture, 1950
neutral pion: π 0 → γ + γ
(an electromagnetic decay)
Discovery of the kaon
A. Andronic
Rochester and Butler, Manchester, UK, 1947 (cloud chamber):
the neutral kaon decaying into 2 pions (“V event”)
2 such photos out of 5000
decay similar to beta decay, pion decay: governed by the weak interaction
Leprince-Ringuet and L’Héritier had discovered charged kaons (1946)
More particles discovered in nice places
A. Andronic
with photographic emulsions
...also at accelerators, using the Bubble Chamber, Alvarez and others
(Nobel Prize 1968)
and since the things were not easy to understand, some of the new particles
were called “strange particles” (included kaons) ...not just a name (by A. Pais)