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Chapter: 4 Atomic and nuclear physics
    Section: 4.8 Nuclei and particles
        SubSection: 4.8.4 Subatomic particles

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4.8.4 Subatomic particles

Classification

Subatomic particles are classified according to whether they do or do not respond to the strong nuclear force. Those that do are named ‘hadrons’, of which the protons and neutron are particular examples, while those that do not respond to the strong force are called ‘leptons’, and the electron and neutrino are examples.

The leptons appear to be pointlike even when probed to the highest resolution currently available ( 10−18 m). The hadrons are known to be extended objects ( 10−15 m diameter) which are built from pointlike particles known as ‘quarks’. Five varieties of quark have been identified which are distinguished by their electrical charges and further intrinsic properties named ‘strangeness’, ‘charm’, ‘bottom’ (or ‘beauty’) which are additive quantum numbers. Thus a hadron containing two charmed quarks will have net 2 units of charm and is referred to as a charmed hadron; the more strange quarks there are contained in a hadron, the more will be its net strangeness. The total electrical charge of a hadron is the sum of the electrical charges of its constituent quarks.

The quarks respond to the strong nuclear force but otherwise behave very much like leptons. This has led to conjectures that leptons and quarks may be fundamental and related to one another, though the precise nature of this relationship is still unclear. Recent evidence for a sixth variety of quark (variously called the ‘top’ quark or sometimes ‘truth’) completes a parallelism with the six leptons.

For each variety of particle listed nature also has an antiparticle with the same mass and spin but opposite charge, strangeness and other additive quantum numbers. Their symbols are the same as those for their particle equivalents but for a bar above them, e.g. p is a proton and is the antiproton. The anti-electron is denoted , or most commonly e+, and named the ‘positron’.

The data on antiparticles are much poorer than for particles. General theorems require that antiparticles have the same mass and lifetimes as their particle equivalents: the poor data are all consistent with this and are not listed here.

In addition, there are particles classified by the family name ‘gauge bosons’. These are carriers of the fundamental forces and have spin. The photon is the carrier of the electromagnetic force; the W and Z bosons are the carriers of the weak forces.

Fundamental particles with spin 

Leptons

    Name and
      symbol

  Charge
 

Mass/MeV
 

Mean life/s†
 

Principal decay modes 
 

 

 

 

 

 

 

Electron

e

−e

    0.510 999 06 ± 0.000 000 15

    Stable

    Stable

 

 

 

 

    (> 1.9 × 1023 years)

 

Electron-

νe

0

    < 7.3 × 10−6

    Stable

    Stable

    neutrino

 

 

 

 

 

Muon

μ

−e

   105.658 389 ± 0.000 034

    2.197 03 × 10−6

    ev

 

 

 

 

    ± 0.000 04

 

Muon-

vμ

0

    < 0.27

    Stable

    Stable

    neutrino

 

 

 

 

 

  

 

 

 

 

 

Tau

τ

−e

    1777.1

+ 0.4
− 0.5

    2.956 × 10 −13 ±  0.031

    Hadron + neutrals
     π π0ν, μνν, eνν

 

 

 

              

 

  

Tau-

ντ

0

    < 31

    Stable

    Stable

    neutrino

 

 

 

 

 

 

 

 

 

 

 

† The observed mean life of a particle in flight is longer than its mean life at rest by a factor

(1−β2) − 1/2 = 1 + Kinetic enery
Rest energy


where β is the ratio of its velocity in the observer's frame to the velocity of light.



Quarks

    Name and symbol 

Charge 

Z-component (IZ) 
of isospin

Baryon number 

Other non-zero
quantum numbers

Mass/GeV 

 

 

 

 

 

 

 

Down

d

1

e

3

1

              

2

    

1

    

3

0

0.35

Up

u

 

2

e  

3

+

1

         

2

    

1

    

3

0

0.35

Strange

s

1

e

3

0     
    

1

    

3

Strangeness − 1

0.5  

Charm

c

 

2

e  

3

0     
    

1

    

3

Charm + 1

 1.3−1.7

Bottom

b

1

e

3

0     
    

1

    

3

Bottom + 1

 4.7−5.3

Top

t

 

2

e  

3

0     
    

1

    

3

Top + 1

    175

 

 

 

 

 

 

 

No individual quarks have been isolated so the concept of mass is not well defined. The listing is merely a qualitative guide extracted from the masses of the lightest hadrons built from the respective quarks. For more details see F. E. Close (1979) An Introduction to Quarks and Partons, Academic Press.




Stable and metastable hadrons

These data refer only to particles immune to decay via the strong interaction; they are derived by the Particle Data Group (Phys. Rev., D50, July 1994) which contains a considerable expansion of the data of this table together with data on unstable mesons and baryons. At present these data are updated biennially by the Particle Data Group.

The quark content is listed by symbols, e.g. the proton built of two up quarks and one down quark is denoted uud. Several metastable heavy particles have been found built from a charmed quark (c) and charmed antiquark ( ) or from a bottom quark (b) and a bottom antiquark ( ). The resulting bound states of these quarks appear to be effectively non-relativistic systems with spin 0 or 1 and orbital angular momentum similar to positronium. By analogy they are known as ‘charmonium’ and ‘bottomonium’ respectively. The spectrum is listed under Heavy quark spectroscopy, below. The charmonium and bottomonium states all have zero baryon number, strangeness, charm and bottom quantum numbers.

The charm (C), strangeness (S) and baryon number (B) of the hadrons which appear in the subsequent tables are as follows:

    Hadron type

Mesons (B = 0)

S

C

Baryons (B = 1)

S

C

 

 

 

 

 

 

 

Non-strange    .    .    .    .    .    .

 π, η

  0

 0

 p, n    

   0

    0 

Strange       .    .    .    .    .    .    .

K+

+1

 0

 Λ, Σ    

 −1

   0

 

K

−1

 0

 Ξ     

 −2

   0

 

     K

0

,

s

K

0

L

~ 50%
~ 50%

+1

 0

Ω−  

 −3

   0

 

−1

 0

 

 

 

 

 

 

 

Charmed   .    .    .    .    .    .    .

D+, D0   

  0

 1

Λ

+

c

   0

 +1

 

 D− , 0   

  0

 −1  

 

 

 

 

D

+

s

  +1  

 +1  

 

 

 

 

D

s

  −1 

  −1  

 

 

 

 

 

 

 

 

 

 


Mesons consist of a quark and an antiquark; baryons (B = 1) consist of three quarks (each with B = ). The superscripts denote charges in units of the proton charge.
 
 

Nonstrange hadrons

    Name and symbol 

Quark content

Spin

Mass/MeV

Mean life/s

Principal
modes of
decay

 

 

 

 

 

 

Pion π+, (π)

u()

 0

 139.5699 ± 0.00035

2.6030 × 10−8 ± 0.0024  

    μ±ν

π0     .     .     .     .     .     .

  and d  0   134.9764 ± 0.0006   

0.84 × 10−16 ± 0.06   

    γγ

Eta η0   .     .     .     .     .

 , d and s

 0

  547.45 ± 0.19   

7.93 × 10−19 ± 1.1     

    γγ,

 

   

 

 

    π0π0π0,

 

 

 

 

 

    π +ππ0

Proton      .    .    .

 uud
 

1

 

2

 938.272 31 ± 0.000 28

Stable (> 1.6 × 1025 years)

   Stable

Neutron n    .    .    .

 ddu
 

1

 

2

939.5653† ± 0.00028 

887.0 ± 2.0

    pe

 

 

 

 

 

 

† The difference (mpmn) between the proton and neutron masses is known very accurately to be − 1.293 318 ± 0.000 009 MeV.


Strange hadrons

   Name and symbol 

Quark content

Spin

Mass/MeV

Mean life/s

Principal modes of
decay

 

 

 

 

 

 

K-mesons

       

 

    K + (K )    .   .   .

 u, (sū)  0  493.677 ± 0.016

1.2371× 10 −8 ± 0.0029

  μ±ν, π±π0

K

0

S

K

0

 

L

~ 50% s
~ 50% d

 0

 497.671 ± 0.031

0.8922 × 10 −10 ± 0.0020
5.17 × 10 −8 ± 0.04

  π+π, π0π0
  π0π0π0, π+ππ0
  π±ev, π± μν

         

 

Hyperons

 

 

 

 

 

 Λ    .    .    .    .

uds  
 

1

 

2

   1115.68 ± 0.01

2.632 × 10 −10 ± 0.020

  pπ, nπ0

 Σ+    .    .    .    .    .

uus

 

1

 

2

   1189.36 ± 0.06

0.800 × 10 −10 ± 0.004

  pπ0, nπ+

 Σ−    .    .    .    .    .

dds

 

1

 

2

   1197.34 ± 0.05

1.482 × 10 −10 ± 0.011

  nπ

 Σ0    .    .    .    .    .

uds

 

1

 

2

   1192.46 ± 0.08

5.8 × 10 −20 ± 1.3

  Λγ

 Ξ−    .    .    .    .    .

dss

 

1

 

2

   1321.32 ± 0.13

1.641 × 10 −10 ± 0.016

  Λπ

 Ξ0    .    .    .    .    .

uss

 

1

 

2

   1314.9 ± 0.6

2.90 × 10 −10 ± 0.10

  Λπ0

 Ω−    .    .    .    .    .

sss

 

3

 

2

   1672.45 ± 0.32

0.819 × 10 −10 ± 0.027

  ΛK, Ξ0π, Ξ π0

 

 

 

 

 

 


The difference (MKLMKS) between the K 0L and K 0S masses is known very accurately to be 
                       MKLMKS = (3.510 ± 0.018) × 10 −12 MeV


Charmed hadrons

    Name and symbol 

Quark content

Spin

Mass/MeV

Mean life/s

Principal modes of
decay

 

 

 

 

 

 

D+ (D)     .    .    .

c (d)

 0

1869.4 ± 0.4

10.57 ± 0.15 × 10 −13  

  0π±π0
  
0π±π+π

Do (o)     .    .    .

c (u)

 0

1864.5 ± 0.5

4.15 ± 0.04 × 10 −13

  π+π0
  π+π
0

Ds+ (Ds)   .    .    .

c (s)

 0

1968.8 ± 0.7

4.67 ± 0.17 × 10 −13

  K, π, η (many

 

     

 

  combinations)

Λc+  .    .    .    .    .

cud
 

1

 

2

2284.9 ± 0.6

2.00 ± 0.01 × 10 −13

  pK π+

Ξc+   .    .    .    .    .

 cus 
 

1

 

2

2465.1 ± 1.6

   3.5  

+ 0.7

 × 10 −13

− 0.4

  ΛK π+ π+

Ξco   .    .    .    .    .

cds
 

1

 

2

2470.3 ± 1.8

   0.98

+ 0.23

 × 10−13

− 0.15

  Ξ π+

 

 

 

 

 

 

Bottom hadrons

    Name and symbol 

Quark content

Spin

Mass/MeV

Mean life/s

Principal modes of
decay

 

 

 

 

 

 

B+ (B)     .    .    .

u (bū)

 0

5278.6   ± 2.0

(12.9 ± 0.5) × 10 −13*

  Charmed and/or
  strange hadrons

Bo (o)     .    .    .

b (d)

 0

5279.0† ± 2.0

(12.9 ± 0.5) × 10 −13 

  Charmed and/or
  strange hadrons

 

 

 

 

 

 

* This is averaged over all B mesons.
† Mass difference mBo − m = 0.34 ± 0.29 MeV.



Heavy quark spectroscopy

The massive charmed and bottom quarks form non-relativistic bound states with their corresponding antiquarks. The resulting spectroscopy is similar to that of positronium and is known as ‘charmonium’ (charmed quark and charmed antiquark) or ‘bottomonium’. The low-lying energy levels are metastable. These energy levels yield important information on the nature of the interquark forces. The quark and antiquark couple their spins to a total of spin 0 or 1 and are in a state of relative orbital angular momentum L. The spectrum is listed in standard 2S + 1LJ notation.


Charmonium spectroscopy

State

c configuration

Spin

Mass/MeV

Width/MeV

Principal modes of decay

 

 

 

 

 

 

ηc (2980)

1S0

0

2978.8 ± 1.9

10.3 

+ 3.8
− 3.4

Hadrons

J/ψ(3100)

3S1

1

3096.9 ± 0.1

0.086 ± 0.006

e+ e, μ+ μ, γ+ hadrons

χc0(3415)

3P0

0

3415.1 ± 1.0

14 ± 5 

2(μ+ π )

χc1(3510)

3P1

1

3510.5 ± 0.1

0.88 ± 0.14

γ J/ψ(3100)

χc2(3555)


3P2

2

3556.2 ± 0.1

2.00 ± 0.18

γ J/ψ(3100)

ψ(3685)

3S1

1

3686.0 ± 0.1

0.278 ± 0.032

J/ψπ+ π, J/ψπoπo
  γχ(3415), γχ(3510)
  γχ(3555)

ψ(3770)

3D1

1

3769.9 ± 2.5

23.6 ± 2.7

D

 

 

 

 

 

 

Heavier charmonium states decay rapidly to charmed hadrons.


Bottomonium spectroscopy

State

b configuration

Spin

Mass/MeV

Width/MeV

Principal modes of decay

 

 

 

 

 

 

(9460)

13S1

1

9460.32 ± 0.22

0.052 ± 0.002

     e+ e, μ+ μ hadrons

χb0(9873)

13P0

0

9859.8 ± 1.3

 

     γ(9460)

χb1(9894)

13P1

1

9891.9 ± 0.7

Unknown

     γ(9460)

χb2(9914)


13P2

2

9913.2 ± 0.6

 

     γ(9460)

(10020)

23S1

1

10 023.3 ± 0.3   

0.043 ± 0.008

 

χb(10231)

23P0

0

10 232 ± 1   

 

 

χb(10249)

23P1

1

10 255 ± 1   

Unknown

(9460)γ, (10020)γ

χb(10264)

23P2

2

10 268 ± 1   

 

 

(10350)

33S1

1

10 355 ± 1   

0.024 ± 0.003

 

Unknown

(10570)

43S1

1

10 580 ± 1   

24 ± 2  

 

Unknown

 

 

 

 

 

 

Heavier bottomonium states decay rapidly to bottom hadrons.


Gauge bosons with spin 1

Name and symbol

Charge

Force transmitted

Mass/MeV

Width/GeV

Principal modes of
decay

 

 

 

 

 

 

Photon γ   .    .    .

0

  Electromagnetic

0

Stable

  Stable

Gluon g    .    .    .

0

  Interquark colour
  and strong forces

0

Stable

  Stable

Weak bosons W±

±e

  Charged weak
  (radioactivity)

80.22 ± 0.26

2.08 ± 0.07

  e ± v, μ ± v, τ ± v
  hadrons

Z0   .    .    .    .    .

0

  Neutral weak

91.187 ± 0.007

2.490 ± 0.007

  e+ e, μ+ μ, τ+ τ
  hadrons

 

 

 

 

 

 

F.E.Close

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