Download
1 / 29
290 likes | 305 Views
This article delves into the internal structure of subatomic particles, including quarks and leptons, and explains the predictions made by the quark model. It also explores the color charge and confinement, as well as the production of particles through strong interactions.
E N D
ParticleZoo The Zoo of Subatomic Particles The Standard Model of Quarks and Leptons
e- p hadron jet excited states of the proton scatter probability ground state of the proton Bartel etal. PL28B, 148 (1968) energy of scattered electron Nucleons Are Not Elementary Particles! e- Scatter high-energy electrons off protons. If there is no internal structure of e- or p, then well-defined “elastic” e- energy for each angle. See structure!! Each line in the energy spectrum of scattered electrons corresponds to a different energy state of the proton. elastic x1/8.5
The quark model represents a relatively simple picture of the internal structure of subatomic particles and makes predictions of their production and decay. It uses a minimum of adjusted quark parameters and has great predictive power, e.g., for the composite-particle masses, magnetic moments, and lifetimes. There are no contradictions to this model known so far, (but many questions remain). The Quark Model
e- e- p D S=3/2 N S=½ Internal Nucleonic Structure The proton has internal structure, so-called quarks (u,u,d). Quarks combine to nucleon states of different excitations. Proton is the (u,u,d) ground state 1200 MeV N: one doublet with a splitting of onlyDm = 1.3 MeV D: one quadruplet with a splitting of only Dm = 8 MeV 938 MeV p S=0 Mesons 135 MeV
Nucleons (q,q,q) Mesons (q, q-bar) q-bar:anti-quark The Quark-Lepton Model of Matter Explains the consistency of the known particles in all of their states. 3 families of quarks (3 “colors” each) and associated leptons. All are spin-1/2 particles, quarks have non-integer charges
Particle Spectrum Mass (GeV/c2) Leptons Hadrons Baryons Mesons Y'’ 4 n, e Y' J/Y 3 t 2 W X*Y* D 10 X S L N 8 K*w r 1 8 hK p m 0 Spin ½ ½ 3/2 0 1 Simplified scheme of stable or unstable subatomic particles. Families have different interactions, Leptons: weak+elm, Hadrons: weak+elm+strong Each particle also has an anti-particle, with inverse quantum numbers. “strange”
Quark Quantum Numbers All: spin=1/2, baryon number B=1/3 T,T3: isospin; S: strangeness; C: charm; B*: bottom qu.#, Top: top qu.#
_u _u _u _u _u _u u u u s u s s u s u u u u d d s d s d d s d d u d d d d d _s _s _s _s s s s s quarks antiquarks _d _d T3 0 K+ p n K0 p0 S- S0 S+ p- L0 h h’ _K0 K- X- X0 Structure of Composite Particles There are only 3-quark (q,q,q) Baryons and quark-antiquark configurations. No free quarks or higher quark multiplicities. s= 1/2 s= 0 Baryon Octet Meson Nonet p+ S
D- D0 D+ D++ T3 0 u u u d s s u s u s s s s s u u u u d d d s d d d s d d d u S*- S*0 S*+ X*- X*0 W- S Baryon Decuplet s = 3/2
Meson Wave Functions Examples to interpret the graphic shorthand in these figures: Meson spins are integer, vector sum of half-integer quark and anti-quark spins, and their integer orbital angular momentum l. In ground state, mostly l =0.
Baryon Wave Functions Examples to interpret the graphic shorthand: These Baryon and Meson wave functions are schematic, do not have proper (anti-)symmetry property required by Pauli Principle: The total particle wave function must be antisymmetric under quark exchange (quarks are fermions)
u d u d u d d d d _d _d D- _d D++ s3,T3 s3,T3 d quarks anti-d quarks Pauli Principle and Color Coordinate Quarks are Fermions no two same quarks can be in the same state have both 3 identical fermions (same quarks) with same spins (S=3/2) and isospin (T3=+3/2) states Violates Pauli Principle !? Conclusion: There must be an additional quantum number (degree of freedom), “color”. Need 3 colors and their anti-colors Color and complementary color (anti-color) add up to color-less (white)
d d d _d _d _d d quarks anti-d quarks Color Wave Function D++ : Flavor and spin configurations symmetric, spatial configuration symmetric (no orbital angular momentum, l=0) color configuration must be antisymmetric. All colors are present with equal weights. All physical particles are “white.” Necessity of color rules out combinations such as There are no free quarks Confinement
Gluons Gluons carry color and the corresponding anticolor. Color can be transferred but particle remains colorless. Bound quark systems (physical particles) by q-q interactions. Field quanta: 8 Gluons (not actually pions!) Spin and parity 1- like a photon. _q qc’ q qc gluon emission q-qbar creation self coupling changes color of the color charges Usual conservation laws apply to reactions between quarks.
_b _ b,g _ b,g b b g r _g _ r,g g _ b,g g _b _ b,g g b _ r,b b _r r g b r _d u u u d p p+ Gluon Exchange time Gluons are exchanged back and forth between q-q, changing q colors and momenta dynamically r, g, and b are visited with equal probability
u u u d s _d _s u u 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 time u S+ p K+ p+ annihilation creation d, d-bar s, s-bar
p p+ time _ d u _ d u u u d u u d p p+ Baryon Resonances Typically: Energetic projectile hits nucleon/nucleus, intermediate particle is produced and decays into other particles. D++ produced as short-lived intermediate state, t = 0.5·10-23s corresp. width of state: G = ħ/t = 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 u u u D++
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? Confinement and Strings energy in strings proportional to length 0.9GeV/fm field lines: color strings successive q/q-bar creation, always in pairs!
Leptons Leptons have their own quantum number, L, which is conserved. It seems likely, but is not yet known, whether electronic, muonic and tau lepton numbers are independently conserved in reactions and decays.
Conservation Laws Quantum numbers are additive. Anti-quarks have all signs of quark quantum numbers reversed, except spin and isospin. Derived quantities: • 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. Example: Conservation of charge, baryon number, lepton number in neutron decay.
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 u Z0 W+ W- u u s up-down strange-non-strange no flavor change conversion conversion carries +e carries –e carries no charge
Decays of W± and Z0 Bosons Hadronic decays to quark pair are dominant (>90%), leptonic decays are weak. All possible couplings:
Can you predict, which (if any) weak boson effects the change? Examples of Weak Decays _ne p ne p n m- e- ? ? ? time n n nm p e- n-decay? neutrino scattering neutrino-induced off protons? reaction off e-?
Answer: Yes, all processes are possible. These are the bosons, Examples of Weak Decays _ne p ne p e- n m- W+ Z0 W- time n n nm p e- n-decay 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 In electron-positron collisions, particle-anti-particle pairs can be created out of collision energy, either via electromagnetic or weak interaction. probability collision energy (GeV) anti-fermion fermion m+ m- m+ m- Z0 g Z0 e- e- e+ e+ e- e+ electromagnetic weak example
The Standard Model Interactions Weak interactions violate certain symmetries (parity, helicity) see later The body of currently accepted views of structure and interactions of subatomic particles. Particles
Combine weak and elm interactions “electro-weak” Type of isospin-symmetry: same particles carry weak and elm charge. Vqq 0 r 1 fm The Standard Model ct’d Force range Electromagnetic: ∞ Weak: 10-3fm Strong qq force increases with distance 2mqc2 There are no free quarks. All free physical particles are colorless.
