The muonx

beigecakeUrban and Civil

Nov 16, 2013 (3 years and 8 months ago)

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The
muon

(from the
Greek

letter
mu

(μ) used to represent it) is an
elementary particle

similar
to the
electron
, with negative
electric charge

and a
spin

of
1

2
. Together with the
electron
,
the
tauon
, and the three
neutrinos
, it is classified as a
lepton
. It is the unstable
subatomic particle

with the second longest
mean lifetime

(2.2

µs
), behind the
neutron

(~15

min
). Like all
elementary particles, the muon has a corresponding
antiparticle

of opposite charge but equal
mass

and spin:
the
antimuon

(also called a
positive muon
). Muons are denoted by μ


and
antimuons by μ
+
. Muons were sometimes referred to as
mu mesons

in the past, even though
they are not classified as
mesons

by modern particle physicists (
see
History
).Muons have a
mass

of 105.7

MeV
/
c
2
, which is about 200 times the mass of the electrons. Even so, muons
are the lightest particles of ordinary matter, after the electrons. Since the muon's in
teractions
are very similar to those of the electron, a muon can be thought of in most ways as simply a
much heavier version of the electron. Due to their greater mass, muons are not as sharply
accelerated when they encounter electromagnetic fields, and do

not emit as much
bremsstrahlung radiation
. For this reason, muons of a given energy are far more highly
penetrating of matter than electrons, since slowing of these particles
in matter to capture
velocities is primarily due to energy loss from this mechanism. Muons generated by cosmic
rays in the atmosphere are capable of penetrating to the ground and into deep mines.As with
the case of the other charged leptons, the muon has a
n associated
muon neutrino
.

Leptons

are a family of
elementary particles
, alongside
quarks

and
gauge bosons

(also
known as force carriers). Like quarks, leptons are
fermions

(
spin
-
1

2

particles) and are subject
to the
electromagnetic force
, the
gravitational force
, and
weak interaction
, but unlik
e quarks,
leptons do not participate in the interaction. There are six
flavours

of leptons, forming three
generations
. The first generation is the
electronic leptons
, comprising the
electrons

(e

) and
electron neutrinos


e
); the second is the
muonic leptons
, comprising
muons



) and
muon
neutrinos


μ
); and the third is the
tauonic leptons
, comprising
tauons



) and
tauon neutrinos


τ
). Each lepton has a corresponding antiparticle


these antiparticles are known as
antileptons
.Leptons are an important part of the
Standard Model
, especially the electrons
which are one of the components of
atoms
, alongside
proton
s

and
neutrons
.
Exotic atoms

with muons and tauons instead of electrons can also be synthesized.

Muons were discov
ered by
Carl D. Anderson

in 1936 while he studied
cosmic radiation
. He
had noticed parti
cles that curved in a manner distinct from that of electrons and other known
particles, when passed through a
magnetic field
. In particular, these new particles were
negatively

charged but curved to a smaller degree than electrons, but more sharply than
protons
, for particles of the same velocity. It was assumed that the magnitude of their
negative electric charge was equal to that of the electron, and so to account for the difference
in curvature, it was supposed that these particles were of intermediate mass (
lying somewhere
between that of an electron and that of a proton). The discovery of the muon seemed so
incongruous and surprising at the time that Nobel laureate
I. I. Rabi

famously qu
ipped, "Who
ordered that?"

For this reason, Anderson initially called the new particle a
mesotron
, adopting the prefix
meso
-

from the Greek word for "mid
-
". Shortly thereafter, additional particles of intermediate
mass were discovered, and the more general

term
meson

was adopted to refer to any such
particle. Faced with the need to differentiate between different types of mesons, the mesotron
was in 1947 renamed the
mu meson

(with the Greek letter
μ

(
mu
) used to approximate the
sound of the Latin letter
m
).

However, it was soon found that the mu meson significantly differed from other mesons; for
example, its decay products included a
neutrino

and an
antineutrino
, rather than just one or
the other, as was observed in other mesons. Other mesons were eventually understood to be
hadrons

that is, particles
made of
quarks

and thus subject to the
residual strong force
. In
the quark model, a
meson

is compo
sed of exactly two quarks (a quark and antiquark), unlike
baryons
, which are composed of three quarks. Mu mesons, however, were found to be
fundamental particles (leptons) like electrons, with

no quark structure. Thus, mu mesons were
not
mesons

at all (in the new sense and use of the term
meson
), and so the term
mu meson

was abandoned, and replaced with the modern term
muon
.