Experimental particle physics introduction

1. Experimental particle physics introduction

2. What holds theworld together?

3. <0.01 m crystal 10-9 m molecule 10-10 m atom 10-14 m nucleus 10-15 m proton <10-18 m electron

4. The atomic world • 400 B.C. Democritus’ atomic hypothesis • atoms believed to be indivisible • 1897 electron discovered • Thomson’s plum pudding model • roughly 1/1000th of the hydrogen ion mass • 1890s three types of radioacitivity • alpha particles are helium ions • 1909 Milikan measures the electron charge • electric charge is quantized e = 1.6∙10-19 C

5. Rutherford’s experiment • Alpha particle beam on thin gold foil • Plum pudding model • expect small deflection of alpha particles • Marsden & Geiger as detector… • saw also particles in the backwards region

6. Scattering experiments 1 1 2 2 3 3 4 4 5 5 6 6 7 7

7. Scattering experiments 1 2 3 4 5 6 7

8. Scattering experiments 4 3 2 1 7 5 6

9. Scattering experiments 3 2 1 4 5 6 7

10. we measure scattered particles detector acceptance & efficiency target (overlap) beam Rutherford Scattering • The cross section describes the likelihood of an interaction between particles. • It has the dimension of an area.

11. Experimental data • The outcome of the experiment is of a statistical nature  quantum mechanics • To get a more accurate understanding of things: • Switch on the light • Increase luminosity • Increase the resolution • Smaller wavelength • Higher energy

12. Smashing the walnut • What is the shape of the walnut in the shell? • Use a hammer as a tool and look at the pieces flying away…

13. Particle tracks • How can particles be measured? • High energetic particles ionize matter • First evidence of x-rays and radioactivity on photo plates

14. Particle Tracks • Bubble chamber • Superheated liquid hydrogen • Ionization along the particle track • Reduce the liquid pressure (metastable) • Liquid vaporizes around ions • Or the other way around: cloud chamber • Ions act as condensation nuclei in supersaturated water or alcohol vapor

15. v FL B The Lorentz force • Charged particle moving through a magnetic field experiences a central force:

16. Tracking chambers • Spark chamber • Plates with high voltage • Wire chamber • Measure the charge

17. classic relativistic Particle identification • Particles lose energy in matter • Bremsstrahlung • Pair production • If we stop the particle completely, the measured energy is the particles energy. • We can identify the particle

18. Particle calorimetry • Measure how deep a particle penetrates heavy material • Light produced in scintillators • Energy resolution limited by number of layers • Heavy scintillating materials (PbWO4)

19. So…? An event from the JADE experiment at the PETRA accelerator (e+e- collider). • First measure the momentum • Then measure the energy • Identify the particle • (it’s not always that easy, but for now let’s assume it is…)

20. gravitation the nuclear force 1 ∞ range 1038 10-15 m the electromagnetic force the weak force 1025 10-18 m 1036 ∞ range The four forces

21. The standard model • Quarks carry color charge. • Baryons are three quark states. • Mesons are quark-antiquark states. • Color is confined – can not be isolated.

22. x t Exchange character of forces Forces are described by exchange of particles Feynman diagram

23. The gluon in the strong force The gluon carries color charge itself. (The photon does not carry electric charge.)

24. The beauty of particle physics This is an e+e- collision. We see three distinct jets (of hadrons) with similar total energies. First direct evidence of gluons in the proton! The spin of the gluon can be determined from the angular distribution of the jets: Sz = 1

25. How is a proton built up?