Discovery of Pions and Kaons in Cosmic Rays in 1947

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1 Discovery of Pions and Kaons in Cosmic Rays in 947 π + µ + e + (cosmic rays) Points to note: de/dx Bragg Peak Low de/dx for fast e + Constant range (~600µm) (i.e. -body decay) small angle scattering

2 Strange Particles

3 Protons Have Inner Structure -resonance First measurements studied total absorbtion cross section using a 4-fold coincidence

4 Quarks The name was coined in 964 by Murray Gell-Mann, who said that he was strongly influenced by a poem from James Joyce's book, Finnegans Wake: Three quarks for muster Mark! Sure he hasn't got much of a bark And sure any he has it's all beside the mark. In Joyce's poem, the word's meaning seems to be a kind of squawk or birdlike sound, but Gell-Mann simply liked the sound of it.

5 Hadrons: Bound States of the Strong Interaction Baryons: Quarks Mesons (Quark/Anti-Quark)

6 Quarks and Color Discovery of the ++ -baryon by Fermi et al. in 95 p uud n udd ++ uuu (J/) Not only forbidden by Pauli principle, but no qq or q states observed. Therefore, introduce color charge. Hadrons must be colorless or else many states must exist (e.g. u R u R d G, u R u B d G ). There must be color charges (RGB, and the corresponding anti-charges) for baryons and mesons to be colorless.

7 Mesons with u, d, and s Quarks (ds) (us) (du) (ud) (su) (sd) Spin

8 Mesons and Mixing Angles η 8 η 0 6 uu + d d ss uu + d d + ss qq qq + ss ss η η cos sin η ( Θ ) ( ) ( ) ( ) PS sin Θ PS 0 Θ Θ PS cos PS η 8 Θ V Θ PS 7 o o 0... o Small deviation from ideal mixing angle in the φ/ω system: decoupling of the light and strange quarks

9 Baryons with u, d, and s Quarks uuu,ddd,sss states missing for S/: Pauli principle

10 Determination of Quantum Numbers in Hadron-Hadron Interactions Cross sections and decay rates ρ Γ f dσ dω π M h dn de hc 8π if g f ρ f M s if r p r p f i a { + b c{ + d i f (g f spin/isospin degeneracy)

11 Determining the Pion Spin Using π + +d p+p (95) and p+p π + +d data and the principle of Detailed Balance M if M fi σ σ ( + pp π d ) ( + π d pp) ( s + )( s + ) π π ( ) s + p p d p p Factor because two identical p Data from show clearly that s π 0

12 Mass Formulas and Hyperfine Interaction M Meson m Quark + m Quark + M SS M SS σ ι ( qq) 8π α s 9 h σ j c h + c σι σ j Ψ(0) m m i ( Pseudoscalar) ( Vector) Equivalent formulas for baryons j

13 Magnetic Moments ( ) ( ) ( ) ( ) d u d d u p i i i m e Q µ µ µ µ µ µ ϕ χ ϕ χ µ,,0,,, + Ψ Dirac (pointlike) particle (For m u m d )

14 Do Quarks Really Exist? Enhancement of the scattering cross section at backward angles (or large momentum transfers q ) indicate substructure

15 Gluons Exist as Well e + e - q qbar gluon bremstrahlung -jet event (Petra-DESY) Hadronization

16 Development of the Standard Model Important distinction between current quarks and constituent quarks Origin of the masses: Higgs Mechanism different lecture

17 Quantum Chromodynamics (QCD): the Theory of the Strong Interaction Fundamental gauge theory, like QED Symmetry group SU() ( color charges) 8 exchange bosons (Gluons) (no color singlet) Gluons carry color charge Self interaction of the gluons Coupling constant α s is strongly energy dependent at low energies (hadron structure) perturbation theory does not work! > Lattice calculations > Models with effective degrees of freedom (Baryons, Mesons)

18 QCD at Short Distances α s << Perturbative treatment: example, jet production in pbar p collisions at s.8 TeV (Fermilab) Quantitative description of the data over 0 orders of magnitude by perturbative QCD!

19 Strong QCD: Nonperturbative Regime Running coupling constant a s ( Q ) π ( ) ( n log Q / Λ ) f Λ is the mass scale (i.e. renormalization) where α s get very large

20 Charge Screening and Quark Confinement Comparison: dielectric medium ferromagnet

21 Hadrons as effective degrees of freedom Only color singlet states at large distances

22 Production of J/Ψ c cbar states were discovered in 974 in both proton induced reactions at BNL and e + e - γ* J/Ψ at SLAC (Nobel prize 976). In e + e - reactions there is much less background, but one can only produce states with the same quantum numbers as the (virtual) photon J π - The states are very narrow, and the resolution is dominated by the detectors / beam.

23 Width of the J/Ψ Width determined from Breit-Wigner formula, branching ratios and total cross section: Breit-Wigner ( ) ( )( )( ) [ ] MeV ) ( 4 / 4 / ) ( 0 / Γ Γ Γ Γ + Γ + + Γ Ψ tot e e R e e e e J e e de E E E s s J E λ σ π λ σ The J/Ψ is so narrow (Γ ρ 50 MeV) because it is below the D + D - threshold, and the so called OZI rule (Okubu-Zweig-Iizuka) (a Breit-Wigner is the Fourier transform of an exponential decay)

24 OZI Rule and the Decay of Vector Mesons Processes with discontinuous quark lines are strongly suppressed. One needs at least a gluon exchange to annihilate the c-quarks and produce other quark flavors. OZI allowed, but below threshold for the J/Ψ OZI suppressed

25 Charmonium Spectroscopy Charmonium is the positronium of QCD: one can obtain information about the confining potential from the energy levels The Crystal Ball Detector The η c was confirmed 0 years later, 80 MeV higher!

26 Extracting the Potential from the Energy Levels The energy levels are similar to positronium for n, Coulomb type interaction at small distances Deviations at higher energies Field of el. Charges: Photons don t couple to themselves Field from q-qbar : Self coupling of the gluons creation of new q-qbar pairs at high energy

27 Determining α s at M(J/Ψ) The partial width for decays into e + e - Γ 6πα M ( + J e e ) ( ) / Ψ γ * ψ 0 Q 5keV The partial width for decays into hadrons Γ 60 π 9 8M Γ Γ ( ) ( ) J / Ψ g Hadrons α ψ ( 0) s 70keV ( J / Ψ g Hadrons) ( + J / Ψ γ * e e ) α α s α s 0.

28 The Electromagnetic Structure of Hadrons Elastic scattering of spinless electrons by (pointlike) nuclei (Rutherford scattering) ( ) ( ) Θ + Θ Ω cos sin 8 Mc E E E p p p q Z q M p p s M g c d d f i if i f if f α α π σ r r h A A Z α α /q ( ) 4 4 q E Z d d Rutherford α σ Ω