Transcript Slide 1

ParticleZoo
The Standard Model
The body of currently accepted views of structure and interactions of
subatomic particles.
Interactions
2
Interaction
Coupling
Charge
Field Boson
Mass/
GeVc-2
Jp
strong
color
gluons (8)
0
1-
elmgn
electric (e)
photon (g)
0
1-
weak
weak
W+, W-, Z0
100
1
Weak
interactions
violate certain
symmetries
(parity, helicity)
see later
Standard Model Q&L
Particles
Fermions
Family
Q/e
Color
Spin
Weak
Isospin
Quarks
u c t
d s b
+2/3
-1/3
r, b, g
½
½
Leptons
ne nm nt
e m t
0
-1
none
½
½
W. Udo Schröder
The Standard Model ct’d
Combine weak and elm interactions “electro-weak”
Type of isospin-symmetry: same particles carry weak and elm charge.
Vqq
Force range
3
Electromagnetic: ∞
0
1 fm
Standard Model Q&L
2mqc2
r
Weak: 10-3fm
Strong qq force increases with
distance
There are no free quarks. All free physical particles are colorless.
W. Udo Schröder
Confinement and Strings
4
Why are there no free quarks? Earlier: symmetry arguments.
Property of gluon interaction between color charges
(“string*-like character).
Q: Can one dissociate a qq pair?
Standard Model Q&L
field lines: color strings
energy in strings
proportional to length
0.9GeV/fm
successive q/q-bar creation, always in pairs!
W. Udo Schröder
Baryon Production with Strong Interactions
Typically: Energetic projectile hits nucleon/nucleus,
new particles are produced.
5
Rules for strong interactions:
•Energy, momentum, s, charge, baryon numbers, etc., conserved
•q existing in system are rearranged, no flavor is changed
Standard Model Q&L
•q-q-bar pairs can be produced
p
p+
u
u
d
_
d
u
W. Udo Schröder
time 
annihilation
d, d-bar
u
u
s
_
s
u
creation
s, s-bar
S+
Example
p + p +  S+ + K +
K+
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time 
Baryon Resonances
p
p+
_
d u
u u d
Typically: Energetic projectile hits
nucleon/nucleus, intermediate
particle is produced and decays into
other particles.
Example
p + p +  ++  p + p +
Standard Model Q&L
u u
u
++
++ produced as short-lived intermediate state,
t = 0.5·10-23s
corresp. width of state: G = ħ/t = 120 MeV
u u d
p
W. Udo Schröder
_
d u
p+
This happens with high probability when a
nucleon of 300 MeV/c, or a relative energy of
1232 MeV penetrates into the medium of a
nucleus.  Resonance
Conservation Laws
7
Quantum numbers are additive.
Anti-quarks have all signs of quark quantum numbers reversed,
except spin and isospin.
Derived quantities:
Charge
Q  e T3 + (1 2) B + S + C + B * +Top
Standard Model Q&L
Hypercharge Y  B + S
In a reaction/transmutation, decay, the following quantities
are conserved (before=after):
•The total energy, momentum, angular momentum (spin),
•The total charge, baryon number, lepton number
W. Udo Schröder
Conservation Laws in Decays
Decay
8
A  B + C
possible, if
mAc2 ≥ mBc2 + mCc2
Otherwise, balance must be
supplied as kinetic energy.
Relativistic energy of particle
with rest mass m, momentum p :
E
 pc 
2
+  mc

2 2
 Ekin + mc 2
Standard Model Q&L
Example: Conservation of charge, baryon number, lepton number in
neutron decay.
n  p + e +n e n  decay
p + m   n +n m m  capture
Q 0  e e +0  0
B  1 1 + 0 + 0  1
Le  0  0 + 1  1  0
e e  0+0
1 + 0 1 + 0
0 + 0  0 +0
Lm  0  0 + 0 + 0  0
0 +1 0 +1
W. Udo Schröder
Weak Interactions
10-5 weaker than strong interaction, small probabilities for
9
reaction/decays. Mediated by heavy (mass ~100GeV)
intermediate bosons W± ,Z0.
Weak bosons can change quark flavor
d
u
Standard Model Q&L
W+
u
up-down
conversion
carries +e
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u
Z0
W-
s
strange-non-strange
conversion
carries –e
u
no flavor change
carries no charge
Decays of W± and Z0 Bosons
Standard Model Q&L
10
Hadronic decays to quark pairs are dominant (>90%), leptonic
decays are weak. All possible couplings:




l
,
n

e
,
n
,
m
,
n
,
t
,n t  leptonic decays
  
e 
m 

W 
 q, q    d , u  ,  s, c  ,  b, t  hadronic decays
  l ,n    e + ,n e  ,  m + ,n m  , t + ,n t 

+
W 
 q , q '   d , u  ,  s , c  ,  b , t 
 l , l    e  , e +  , n e ,n e  ,  m  , m + ), (n m ,n m  ,

0

+
Z 
t
,
t
 ), (n t ,n t 

 q, q   (d , d ), (u , u ),  s, s ), (c, c  , (b, b ), (t , t )
W. Udo Schröder
Examples of Weak Decays
Can you predict, which (if any) weak boson effects the change?
_
p
ne

p
n
e
m
ne
?
?
time
11
?
n
Standard Model Q&L
n-decay?
W. Udo Schröder
p
n
neutrino scattering
off protons?
nm
e-
neutrino-induced
reaction off e-?
Examples of Weak Decays
Answer: Yes, all processes are possible. These are the bosons,
p
e-
_ p
ne
n
ne
m
W+
Z0
time
12
W-
n
Standard Model Q&L
n-decay
p
n
nm
e-
neutrino scattering
neutrino-induced
off protons
reaction off e-
Method:
•Balance conserved quantities at the vortex, where boson originates.
Remember W± carries away charge ±|e|.
•Balance conserved quantities at lepton vortex.
W. Udo Schröder
Particle Production
13
probability
In electron-positron collisions,
particle-anti-particle pairs can be
created out of collision energy,
either via electromagnetic or weak
interaction.
 collision energy (GeV)
m+
Standard Model Q&L
m-
e+
electromagnetic
W. Udo Schröder
e-
Z0
e+
weak
m+
m-
Z0
g
e-
antifermion
fermion
e-
e+
example
Standard Model Q&L
14
The End
W. Udo Schröder