1 Experimental particle physics introduction

2 What holds the world together?

3 <0.01 m crystal m molecule m atom m nucleus m proton < 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 1/1000 th 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∙ 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

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

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 The Lorentz force Charged particle moving through a magnetic field experiences a central force: B v FLFL

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

17 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 classicrelativistic

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 (PbWO 4 )

19 So…? 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…) 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…) An event from the JADE experiment at the PETRA accelerator (e + e - collider).

20 The four forces the nuclear forcegravitation the electromagnetic forcethe weak force m 1 ∞ range ∞ range m

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

23 The gluon in the strong force The gluon carries color charge itself. (The photon does not carry electric charge.) 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: S z = 1

25 How is a proton built up?

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