particlezoo. the standard model the body of currently accepted views of structure and interactions...
TRANSCRIPT
ParticleZoo
The Standard ModelThe Standard ModelThe body of currently accepted views of structure and interactions of subatomic particles.
Interaction
Coupling Charge
Field Boson
Mass/GeVc-
2
J
strong color gluons (8) 0 1-
elmgn electric (e) photon () 0 1-
weak weak W+, W-, Z0 100 1
Interactions
Fermions
Family Q/e Color SpinWeak
Isospin
Quarksu c td s b
+2/3-1/3
r, b, g ½ ½
Leptons
e
e
0-1
none ½ ½
Particles
Weak interactions violate certain symmetries (parity, helicity) see later
The Standard Model ct’dThe Standard Model ct’d
Combine weak and elm interactions “electro-weak”Type of isospin-symmetry: same particles carry weak and elm charge.
Force range
Electromagnetic: ∞
Weak: 10-3fm
Strong qq force increases with distance
2mqc2
Vqq
r1 fm
0
There are no free quarks. All free physical particles are colorless.
Confinement and StringsConfinement and StringsWhy 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?
energy in strings proportional to length 0.9GeV/fm
field lines: color strings
successive q/q-bar creation, always in pairs!
Baryon Production with Strong InteractionsBaryon 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
uu
u
d_d
uu
u
s_s
p
Example
p K
annihilation creation d, d-bar s, s-bar
time
Baryon ResonancesBaryon Resonances
Typically: Energetic projectile hits nucleon/nucleus, intermediate particle is produced and decays into other particles.
Example
p p
u u u ++
u u d
_ d u
tim
e
u u d
_ d u
p +
p +
++ produced as short-lived intermediate state, = 0.5·10-23s
corresp. width of state: = ħ/ = 120 MeV
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 LawsConservation LawsQuantum numbers are additive.
Anti-quarks have all signs of quark quantum numbers reversed, except spin and isospin.Derived quantities:
3 (1 2) *Charge Q e T 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 DecaysConservation Laws in Decays
Decay A B + C possible, if mAc2 ≥ mBc2 + mCc2
Otherwise, balance must be supplied as kinetic energy.
22 2 2
, :
kin
Relativistic energy of particle
with rest massm momentum p
E pc mc E mc
Example: Conservation of charge, baryon number, lepton number in neutron decay.
0 0 0 0 0
1 1 0 0 1 1 0 1 0
0 0 1 1 0 0 0 0 0
0 0 0 0 0 0 1 0 1
e
e
capn decan p e p n
Q e e e e
B
L
L
y ture
Weak InteractionsWeak Interactions10-5 weaker than strong interaction, small probabilities for reaction/decays. Mediated by heavy (mass ~100GeV) intermediate bosons W± ,Z0. Weak bosons can change quark flavor
u
d
W+ W- Z0
u
s
u
u
up-down strange-non-strange no flavor change conversion conversion carries +e carries –e carries no charge
Decays of WDecays of W± ± and Zand Z0 0 BosonsBosons
0
, , , , , ,
, '
, , , , , , ), ( , ,
, ), ( ,
, ( , ), ( , ), , ), ( , , ( ,
, , ,
), ( ,
, , , ,
, , ,
, , , , ,
,
,
e e
e
el eW
q q d u s c b
l e leptonic decaysW
q q d u s c b t hadronic decay
l l e e
Z
q q d d u u s s c c b b t t
t
s
)
Hadronic decays to quark pairs are dominant (>90%), leptonic decays are weak. All possible couplings:
Examples of Weak DecaysExamples of Weak Decays
Can you predict, which (if any) weak boson effects the change?
n
? ??
p
pe-
_e
p
e-
e
tim
e
n-decay? neutrino scattering neutrino-induced
off protons? reaction off e-?
Examples of Weak DecaysExamples of Weak DecaysAnswer: Yes, all processes are possible. These are the bosons,
n
W- W+Z0
p
p e- _e
p
e-
e
tim
e
n-decay neutrino scattering neutrino-induced
off protons reaction off e-
Method:Method:
•Balance conserved quantities at the vortex, where boson originates. Remember W± carries away charge ±|e|.
•Balance conserved quantities at lepton vortex.
Particle ProductionParticle Production
e- e+
- +
e- e+
fermion
e- e+
- +
anti-fermion
electromagnetic weak example
In electron-positron collisions, particle-anti-particle pairs can be created out of collision energy, either via electromagnetic or weak interaction.
collision energy (GeV)
pro
bab
ility
The EndThe End