EXPERIMENTAL EVIDENCE FOR THE QUARK AND STANDARD MODELS

J 5.1, 5.2, 5.3, 5.4, 5.5A particle-arly interesting presentation by

LIAM HEGARTY 2012Edited by JCA 2014

Deep Inelastic Scattering

Experimental Conclusions

Asymptotic Freedom

Neutral Current

As evidence for the Standard Model

J 5.1

Assessment Statement:

State what is meant by deep inelastic scattering

Teacher’s Notes:

None, but it’s quite self explanatory

What is it?

Deep Inelastic ScatteringDeep:

Large transfer of energy and momentum from a lepton to a hadron

The leptons are able to penetrate deeply into the hadron and rebound, or scatter, at different angles with a reduction in energy

Inelastic:

Energy is lost when the electron hits the proton

After the collision, new hadrons are formed if the energy is high enough.

Diagram of the collision. The first shows an elastic collision, the second shows a high energy collision

Feynman diagram of the deep inelastic scattering experiment

J 5.2

Assessment Statement:

Analyse the results of deep inelastic scattering experiments.

Teacher’s Notes:

Students should appreciate that these experiments provide evidence for the existence of quarks, gluons and colour.

Experimental Conclusions, Episode 1: Quarks!

There are three constituent particles within a baryon, and two within a meson

Lots of electrons are fired at the particle

The scattering angles and momenta of the particles are measured afterward

The results can be used to graph the structure function of the particle

‘What’s a structure function!?’ I hear you cry! If only you were that interested. The description is on the next slide

Structure FUNctionThe structure function graphs the probability of a single part of a particle having a given amount of momentum

Basically, we want to know how the momentum is shared out. By knowing this, we are able to guess how many particles make up a baryon/meson.

(What I said, but more physics-y)

As this graph shows that about 1/3 of the momentum is given to each particle, we can guess that there are 3 constituent particles.

Experimental Conclusions, Episode 2: Fractionally Charged Quarks!?

The constituent particles are charged, with an electrical charge of either +-⅓ or +-⅔

The energy and momentum is transferred via the virtual photon, meaning it’s an electromagnetic interaction

The strength of the interaction is therefore the electromagnetic interaction strength, or the ‘coupling constant’ (you don’t need to know what they are)

However, the interaction is not proportional to the result expected from a particle with a charge equal to that of an electron, but is instead smaller.

What does this mean??That the charge is smaller than +-1. Obviously. More detailed experiments give the above figures

Experimental Conclusions, Episode 3: Asymptotic Freedom!?

The particles within the hadron are essentially free – they are only loosely bound (more details will follow about asymptotic freedom; try to contain yourselves)

Incoming particles hit the quarks and scatter at smaller angles than expected

Particles colliding with a stationary object reflect at large angles, but hitting an object with a degree of freedom reduces this angle

Experimental Conclusions, Episode 4: Colour!

Each of the constituent particles seem to come in three different varieties

These varieties are the three colours of quarks. Red, Green, Blue.

Experimental Conclusions, Episode 5: A Glu(on) Hope

There appear to be electrically neutral particles within a hadron

In electromagnetic interactions, the electron can only ‘see’ charged particles

The momentum of all of the particles with which the electron couples can be measured, but the total momentum was found to be less than that of the proton

This gives evidence for new-tral particles within the proton, given the inventive name, gluons.

J 5.3

Assessment Statement:

Describe what is meant by asymptotic freedom.

Teacher’s Notes:

It is sufficient for students to know that the strength of the strong interaction decreases as the energy available for the interaction increases.

J 5.4

Assessment Statement:

Describe what is meant by neutral current.

Teacher’s Notes:

A simple description in terms of processes involving Z0 exchange is sufficient

BackgroundThe electromagnetic and weak interactions can be coupled in the electroweak theory, a theory based on symmetry which we don’t have to understand

However, the initial calculations showed there to be an infinite answer to the interactions in Feynman diagrams, rendering them useless.

A couple of Dutch physicists (Hooft and Veltman) managed to sort this out and gave us nice, possible calculations which involved the existence of a neutral boson (Zo).

The experimental breakthrough came in ‘83, when you thought CERN couldn’t get any cooler

Proton-Antiproton CollisionsProtons were accelerated through 270GeV, and collided with antiprotons accelerated to the same energy

Technically they were supposed to be searching for the , and Z0 bosons and evidence for the standard model. Another possible reason could be that they were motivated by a desire to smash things together really hard

When the beams were allowed to collide, Z0 bosons were created but decayed instantly into an electron-positron pair, which allowed them to be detected

Z0 e- + e+

CalculationsThe momentum and energy of the produced electron-positron pair within the curved path within the magnetic field was measured

This allowed calculations to be undertaken to discover the rest mass of the Z0 boson, around 90 GeV c-2

J 5.5

Assessment Statement:

Describe how the existence of a neutral current is evidence for the standard model.

Teacher’s Notes:

Students should know that only the standard model predicts weak interaction processes involving the exchange of a massive, neutral particle (the Z0 boson).

Neutral Currents and the Standard Model

The standard model is the only one to predict neutral currents

Neutral currents processes are the technical term for processes mediated by a massive, neutral particle (Z0)

The W was also discovered in the experiments, and the rest mass found to be 80GeV c-2

The W are charged bosons which decay into either a positron or an electron, and a neutrino or an antineutrino

W- e- + νe

W+ e+ + νe

Examples

Two examples of processes mediated by the Z0 boson:

Fin