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