Transcript Slide 1
ParticleZoo
The Standard Model
The body of currently accepted views of structure and interactions of
subatomic particles.
Interactions
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
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
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
Electromagnetic: ∞
0
1 fm
2mqc2
r
Weak: 10-3fm
Strong qq force increases with
distance
There are no free quarks. All free physical particles are colorless.
Confinement and Strings
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?
field lines: color strings
energy in strings
proportional to length
0.9GeV/fm
successive q/q-bar creation, always in pairs!
Baryon Production with Strong Interactions
Typically: Energetic projectile hits nucleon/nucleus,
new particles are produced.
Rules for strong interactions:
•Energy, momentum, s, charge, baryon numbers, etc., conserved
•q existing in system are rearranged, no flavor is changed
•q-q-bar pairs can be produced
p
p+
u
u
d
_
d
u
time
annihilation
d, d-bar
u
u
s
_
s
u
creation
s, s-bar
S+
Example
p + p + S+ + K +
K+
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 +
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
_
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
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
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
Conservation Laws in Decays
Decay
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
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
Weak Interactions
10-5 weaker than strong interaction, small probabilities for
reaction/decays. Mediated by heavy (mass ~100GeV)
intermediate bosons W± ,Z0.
Weak bosons can change quark flavor
d
u
W+
u
up-down
conversion
carries +e
u
Z0
W-
s
strange-non-strange
conversion
carries –e
u
no flavor change
carries no charge
Decays of W± and Z0 Bosons
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 )
Examples of Weak Decays
Can you predict, which (if any) weak boson effects the change?
_
p
ne
p
n
e
m
ne
?
?
time
?
n
n-decay?
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
W-
n
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.
Particle Production
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+
m-
e+
electromagnetic
e-
Z0
e+
weak
m+
m-
Z0
g
e-
antifermion
fermion
e-
e+
example
The End