Particles can also be revealingly grouped according to what forces they feel between them. All particles (even those that are massless) are affected by gravity, since gravity affects the space and time in which particles exist. All charged particles are affected by the electromagnetic force, as are neutral particles that have an internal distribution of charge (such as the neutron with its magnetic moment). Special names are given to particles that feel the strong and weak nuclear forces. Hadrons are particles that feel the strong nuclear force, whereas leptons are particles that do not. The proton, neutron, and the pions are examples of hadrons. The electron, positron, muons, and neutrinos are examples of leptons, the name meaning low mass. Leptons feel the weak nuclear force. In fact, all particles feel the weak nuclear force. This means that hadrons are distinguished by being able to feel both the strong and weak nuclear forces.
Table 1 lists the characteristics of some of the most important subatomic particles, including the directly observed carrier particles for the electromagnetic and weak nuclear forces, all leptons, and some hadrons. Several hints related to an underlying substructure emerge from an examination of these particle characteristics. Note that the carrier particles are called gauge bosons. First mentioned in Patterns in Spectra Reveal More Quantization, a boson is a particle with zero or an integer value of intrinsic spin (such as s=0, 1, 2, ...s=0, 1, 2, ... size 12{s=0,`1,`2,` "." "." "." } {}), whereas a fermion is a particle with a halfinteger value of intrinsic spin (s=1/2,3/2,...s=1/2,3/2,... size 12{s=1/2,`3/2,` "." "." "." } {}). Fermions obey the Pauli exclusion principle whereas bosons do not. All the known and conjectured carrier particles are bosons.
Table 1: Selected Particle Characteristics
Category

Particle name

Symbol

Antiparticle

Rest mass
(
MeV
/
c
2
)
(
MeV
/
c
2
)

B
B

L
e
L
e

L
μ
L
μ

L
τ
L
τ
size 12{L rSub { size 8{τ} } } {}

S
S
size 12{S} {}

Lifetime (s)

Gauge 
Photon 
γ
γ
size 12{γ} {}

Self 
0 
0 
0 
0 
0 
0 
Stable 
Bosons 
W
W
size 12{W} {}

W
+
W
+
size 12{W rSup { size 8{+{}} } } {}

W
−
W
−
size 12{W rSup { size 8{  {}} } } {}

80
.
39
×
10
3
80
.
39
×
10
3
size 12{"80" "." "22" times "10" rSup { size 8{3} } } {}

0 
0 
0 
0 
0 
1.6
×
10
−
25
1.6
×
10
−
25
size 12{3 times "10" rSup { size 8{  "25"} } } {}

Z
Z
size 12{Z} {}

Z
0
Z
0
size 12{Z rSup { size 8{0} } } {}

Self 
91
.
19
×
10
3
91
.
19
×
10
3
size 12{"91" "." "19" times "10" rSup { size 8{3} } } {}

0 
0 
0 
0 
0 
1.32
×
10
−
25
1.32
×
10
−
25
size 12{3 times "10" rSup { size 8{  "25"} } } {}

Leptons 
Electron 
e
−
e
−
size 12{e rSup { size 8{  {}} } } {}

e
+
e
+
size 12{e rSup { size 8{  {}} } } {}

0.511 
0 
±
1
±
1
size 12{ + 1} {}

0 
0 
0 
Stable 
Neutrino (e) 
ν
e
ν
e
size 12{e rSup { size 8{  {}} } } {}

v
¯
e
v
¯
e
size 12{ { bar {v}} rSub { size 8{e} } } {}

0
7
.
0
eV
0
7
.
0
eV
size 12{0` left (<7 "." 0`"eV" right )} {}

0 
±
1
±
1
size 12{ + 1} {}

0 
0 
0 
Stable 
Muon 
μ
−
μ
−
size 12{μ rSup { size 8{  {}} } } {}

μ
+
μ
+
size 12{μ rSup { size 8{+{}} } } {}

105.7 
0 
0 
±
1
±
1
size 12{ + 1} {}

0 
0 
2
.
20
×
10
−
6
2
.
20
×
10
−
6
size 12{2 "." "20" times "10" rSup { size 8{  6} } } {}

Neutrino (μ)(μ size 12{μ} {}) 
v
μ
v
μ
size 12{v rSub { size 8{μ} } } {}

v

μ
v

μ
size 12{v rSub { size 8{μ} } } {}

0
(
<
0.27
)
0(<0.27)

0 
0 
±
1
±
1
size 12{ + 1} {}

0 
0 
Stable 
Tau 
τ
−
τ
−
size 12{τ rSup { size 8{  {}} } } {}

τ
+
τ
+
size 12{τ rSup { size 8{+{}} } } {}

1777 
0 
0 
0 
±
1
±
1
size 12{ + 1} {}

0 
2
.
91
×
10
−
13
2
.
91
×
10
−
13
size 12{2 "." "29" times "10" rSup { size 8{  "13"} } } {}

Neutrino (τ)(τ size 12{τ} {}) 
v
τ
v
τ
size 12{v rSub { size 8{τ} } } {}

v

τ
v

τ
size 12{ { bar {v}} rSub { size 8{τ} } } {}

0
(
<
31
)
0(<31) 
0 
0 
0 
±
1
±
1
size 12{ + 1} {}

0 
Stable 
Hadrons (selected) 
Mesons 
Pion 
π
+
π
+
size 12{π rSup { size 8{+{}} } } {}

π
−
π
−
size 12{π rSup { size 8{  {}} } } {}

139.6 
0 
0 
0 
0 
0 
2.60 × 10 ^{−8}

π
0
π
0
size 12{π rSup { size 8{0} } } {}

Self 
135.0 
0 
0 
0 
0 
0 
8.4 × 10 ^{−17}

Kaon 
K
+
K
+
size 12{K rSup { size 8{+{}} } } {}

K
−
K
−
size 12{K rSup { size 8{  {}} } } {}

493.7 
0 
0 
0 
0 
±
1
±
1
size 12{ + 1} {}

1.24 × 10 ^{−8}

K
0
K
0
size 12{K rSup { size 8{0} } } {}

K

0
K

0
size 12{ { bar {K}} rSup { size 8{0} } } {}

497.6 
0 
0 
0 
0 
±
1
±
1
size 12{ + 1} {}

0.90 × 10 ^{−10}

Eta 
η
0
η
0
size 12{η rSup { size 8{0} } } {}

Self 
547.9 
0 
0 
0 
0 
0 
2.53 × 10 ^{−19}

(many other mesons known) 
Baryons 
Proton 
p
p
size 12{p} {}

p

p

size 12{ { bar {p}}} {}

938.3 
± 1

0 
0 
0 
0 
Stable 
Neutron 
n
n
size 12{n} {}

n

n

size 12{ { bar {n}}} {}

939.6 
± 1

0 
0 
0 
0 
882 
Lambda 
Λ
0
Λ
0
size 12{Λ rSup { size 8{0} } } {}

Λ

0
Λ

0
size 12{ { bar {Λ}} rSup { size 8{0} } } {}

1115.7 
± 1

0 
0 
0 
∓
1
∓
1
size 12{ + 1} {}

2.63 × 10 ^{−10}

Sigma 
Σ
+
Σ
+
size 12{Σ rSup { size 8{+{}} } } {}

Σ

−
Σ

−
size 12{ { bar {Σ}} rSup { size 8{  {}} } } {}

1189.4 
± 1

0 
0 
0 
∓
1
∓
1
size 12{ + 1} {}

0.80 × 10 ^{−10}

Σ
0
Σ
0
size 12{Σ rSup { size 8{0} } } {}

Σ

0
Σ

0
size 12{ { bar {Σ}} rSup { size 8{0} } } {}

1192.6 
± 1

0 
0 
0 
∓
1
∓
1
size 12{ + 1} {}

7.4 × 10 ^{−20}

Σ
−
Σ
−
size 12{Σ rSup { size 8{  {}} } } {}

Σ

+
Σ

+
size 12{ { bar {Σ}} rSup { size 8{+{}} } } {}

1197.4 
± 1

0 
0 
0 
∓
1
∓
1
size 12{ + 1} {}

1.48 × 10 ^{−10}

Xi 
Ξ
0
Ξ
0
size 12{Ξ rSup { size 8{0} } } {}

Ξ

0
Ξ

0
size 12{ { bar {Ξ}} rSup { size 8{0} } } {}

1314.9 
± 1

0 
0 
0 
∓
2
∓
2
size 12{ + 2} {}

2.90 × 10 ^{−10}

Ξ
−
Ξ
−
size 12{Ξ rSup { size 8{  {}} } } {}

Ξ
+
Ξ
+
size 12{Ξ rSup { size 8{+{}} } } {}

1321.7 
± 1

0 
0 
0 
∓
2
∓
2
size 12{ + 2} {}

1.64 × 10 ^{−10}

Omega 
Ω
−
Ω
−
size 12{ %OMEGA rSup { size 8{  {}} } } {}

Ω
+
Ω
+
size 12{ %OMEGA rSup { size 8{+{}} } } {}

1672.5 
± 1

0 
0 
0 
∓
3
∓
3
size 12{ + 3} {}

0.82 × 10 ^{−10}

(many other baryons known) 
All known leptons are listed in the table given above. There are only six leptons (and their antiparticles), and they seem to be fundamental in that they have no apparent underlying structure. Leptons have no discernible size other than their wavelength, so that we know they are pointlike down to about 10−18m10−18m size 12{"10" rSup { size 8{  "18"} } m} {}. The leptons fall into three families, implying three conservation laws for three quantum numbers. One of these was known from ββ size 12{β} {} decay, where the existence of the electron’s neutrino implied that a new quantum number, called the electron family number LeLe size 12{L rSub { size 8{e} } } {} is conserved. Thus, in ββ size 12{β} {} decay, an antielectron’s neutrino veve size 12{ { bar {v}} rSub { size 8{e} } } {} must be created with Le=−1Le=−1 size 12{L rSub { size 8{e} } =  1} {} when an electron with Le=+1Le=+1 size 12{L rSub { size 8{e} } "=+"1} {} is created, so that the total remains 0 as it was before decay.
Once the muon was discovered in cosmic rays, its decay mode was found to be
μ−→e−+ve+vμ,μ−→e−+ve+vμ, size 12{μ rSup { size 8{  {}} } rightarrow e rSup { size 8{  {}} } + { bar {v}} rSub { size 8{e} } +v rSub { size 8{μ} } ","} {}
(1)which implied another “family” and associated conservation principle. The particle vμvμ size 12{L rSub { size 8{μ} } } {} is a muon’s neutrino, and it is created to conserve muon family numberLμLμ size 12{L rSub { size 8{μ} } } {}. So muons are leptons with a family of their own, and conservation of total LμLμ size 12{L rSub { size 8{μ} } } {} also seems to be obeyed in many experiments.
More recently, a third lepton family was discovered when ττ size 12{τ} {} particles were created and observed to decay in a manner similar to muons. One principal decay mode is
τ−→μ−+vμ+vτ.τ−→μ−+vμ+vτ. size 12{τ rSup { size 8{  {}} } rightarrow μ rSup { size 8{  {}} } + { bar {v}} rSub { size 8{u} } +v rSub { size 8{τ} } "."} {}
(2)Conservation of total LτLτ size 12{L rSub { size 8{μ} } } {} seems to be another law obeyed in many experiments. In fact, particle experiments have found that lepton family number is not universally conserved, due to neutrino “oscillations,” or transformations of neutrinos from one family type to another.
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