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Experimental particle physics

introduction

Experimental particle physics

introduction

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What holds theworld together?What holds theworld together?

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<0.01 mcrystal

10-9 mmolecule

10-10 matom

10-14 mnucleus

10-15 mproton

<10-18 melectron

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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

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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

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Scattering experiments

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Scattering experiments

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Scattering experiments

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Scattering experiments

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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

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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

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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…

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Particle tracks

• How can particles be measured?

• High energetic particles ionize matter

• First evidence of x-rays and radioactivity on photo plates

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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

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The Lorentz force

• Charged particle moving through a magnetic field experiences a central force:

B

vFL

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Tracking chambers

• Spark chamber– Plates with high voltage

• Wire chamber– Measure the charge

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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

classic relativistic

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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)

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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).

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The four forces

the nuclear forcegravitation

the electromagnetic force the weak force

1038

10-15 m1

∞ range

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∞ range

1025

10-18 m

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The standard model

• Quarks carry color charge.

• Baryons are three quark states.

• Mesons are quark-antiquark states.

• Color is confined – can not be isolated.

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Exchange character of forces

Feynman diagram

t

x

Forces are described by exchange of particles

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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.)

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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

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How is a proton built up?

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