Physics 492 Lecture 28

Transcription

1 Physics 492 Lecture 28 Main points of last lecture: Feynman diagrams. Main points of today s lecture:. Nuclear forces: charge exchange pion exchange Yukawa force deuteron charge independence, isospin symmetry Introduction to H.E.P

2 Strong interaction diagrams The short range of the nuclear force suggests that it can result from the exchange of a massive particle. candidate is pion 3 such particles Range of force Born Approximation

3 NN forces Relevant diagrams for NN forces in one pion exchange limit:

4 Charge exchange in N-N scattering n-p differential cross section is backward peaked.

5 Charge symmetry Pions have nearly equal masses: m(π + )= m(π -- ) m(π 0 ): (m(u)=m(d)) charge symmetry for Z=N±1

6 Charge symmetry is general feature Charge independence Similar (analogue) states

7 Charge independence: Isospin Symmetry Consider p and n to be two different states of a nucleon. T 3 =-1 T 3 =1 6 He 0 + g.s. 6 Li 0 + ex.s. 6 Be 0 + g.s.

8 Deuteron Deuteron is also T=0 isospin is completely analogous to spin The reason for this formalism is that charged pion exchange switches neutrons into protons and vice versa; it is useful to consider neutrons and protons to be different states of a nucleon or to consider their quark substructure where u and d quarks have equal masses.

9 Physics 492 Lecture 29 Main points of last lecture: Nuclear forces: charge exchange pion exchange Yukawa force deuteron charge independence, isospin symmetry Main points of today s lecture: Short lived particles. measurements of unstable particles. Measurements of resonances. Quark model spin ½ nucleons pseudoscaler mesons vector mesons. spin 3/2 baryons

10 Deuteron Deuteron is also T=0 isospin is completely analogous to spin The reason for this formalism is that charged pion exchange switches neutrons into protons and vice versa; it is useful to consider neutrons and protons to be different states of a nucleon or to consider their quark substructure where u and d quarks have equal masses.

11 Detection of fast particles Tools for measurement: de/dx for charged particles silicon use magnets to measure p + h+ e-

12 TPC: 3-D coordinates, event reconstruction Z coordinate from drift time in Electric field X coordinate and y coordinates from charge induced on cathode pad below the wire plane 3 dimensional event reconstruction, with energies and particle types. B v z π track E v projected track y B v E v x wire plane and pads below

13 neutral particles? neutrals? Use Time of Flight (TOF)

14 Very Short lived particles Study resonances ( )( ( ) ( ) 2 p p p p 2 2 p k p k p k p k p g c m k p p π π ν π ν μ π μ μν Δ μ π μ μ Δ v v + = + + = + =

15 Quark model Define constituents: quarks start with lightest quarks u and d: Organize particles and resonances: Baryons (First nucleons)

16 Nucleon Wavefunction Skematic wavefunction in terms of quarks: Need another quantum number (color) to satisfy Pauli principle: Explains charge of nucleon. How about spin? What is probability of 2 u quark spin up and one d quark spin down?

17 Quark model Mesons first pions

18 Intrinsic Parity

19 Physics 492 Lecture 30 Main points of last lecture: Short lived particles. measurements of unstable particles. Measurements of resonances. Quark model spin ½ nucleons Main points of today s lecture: Quark model pseudoscaler mesons vector mesons. spin 3/2 baryons Quark-flow diagrams for reactions S- quarks mesons baryons Production and decay of strange particles C and B quarks

20 From last lecture: Pion quark wave function

21 η (eta) meson

22 Progress so far

23 Vector Mesons The quark-quark interaction is spin dependent. Baryons with spins aligned have higher energies.

24 S=3/2 Baryons Aligned spins have higher energies. Color is needed to satisfy Pauli exclusion principle.

25 Higher L?

26 Quark-flow diagrams Particle production can be described by following the quark lines.

27 Strangeness s quark Mesons:

28 Baryons Strangeness

29 Overall picture: strange baryons