4.1a How to construct a Feynman diagram (optional)

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来自 University of Geneva 的课程

Particle Physics: an Introduction

198 个评分

University of Geneva

Particle Physics: an Introduction

198 个评分

This course introduces you to subatomic physics, i.e. the physics of nuclei and particles. More specifically, the following questions are addressed: - What are the concepts of particle physics and how are they implemented? - What are the properties of atomic nuclei and how can one use them? - How does one accelerate and detect particles and measure their properties? - What does one learn from particle reactions at high energies and particle decays? - How do electromagnetic interactions work and how can one use them? - How do strong interactions work and why are they difficult to understand? - How do weak interactions work and why are they so special? - What is the mass of objects at the subatomic level and how does the Higgs boson intervene? - How does one search for new phenomena beyond the known ones? - What can one learn from particle physics concerning astrophysics and the Universe as a whole? The course is structured in eight modules. Following the first one which introduces our subject, the modules 2 (nuclear physics) and 3 (accelerators and detectors) are rather self contained and can be studied separately. The modules 4 to 6 go into more depth about matter and forces as described by the standard model of particle physics. Module 7 deals with our ways to search for new phenomena. And the last module introduces you to two mysterious components of the Universe, namely Dark Matter and Dark Energy.

从本节课中

Electromagnetic interactions

We now start a series of three modules discussing the three fundamental forces described by the Standard Model of particle physics. In this forth module, we go into more details about the properties of electromagnetic interactions. We discuss spin and how it intervenes in measurements. And we give a few examples of basic electromagnetic processes to point out common features. You will notice that the intellectual challenge and also the level of mathematical description rises somewhat as we go along. This is why we first remind you how to describe the intensity of a reaction using the cross section and the decay rate and how to construct a Feynman diagram.

4.1 Reminder: Describing particle interactions6:22

4.1a How to construct a Feynman diagram (optional)4:45

与讲师见面

Martin Pohl
Professeur ordinaire
Département de physique nucléaire et corpusculaire
Mercedes Paniccia
Collaboratrice scientifique
Département de Physique Nucléaire et Corpusculaire
Anna Sfyrla
Assistant Professor
Nuclear and Particle Physics

[MUSIC]

In this video we will give you a few hints on how to construct a Feynman diagram

for the two-body process a+b -> c+d.

We first have to decide wether this can be

an annihilation process in which the two initial state particles annihilate into a gauge boson.

In that case the momentum transfer will be equal

to the total energy-momentum.

If they cannot annihilate, they have to

exchange a gauge boson in a scattering process.

In this case the q^2 is equal to the difference between

the final particle momentum and the initial particle momentum. Now we must decide, which gauge boson can be exchanged.

First of all we have to remember that at each vertex quantum numbers are strictly conserved.

First of all baryon and lepton numbers must be conserved,

and, if we are not talking about weak interactions,

also the particle flavor.

Then we must consider the charge at each vertex.

If the electric charge is not 0 and it does not change at the vertex, we can exchange a photon.

If the particle’s weak isospin is not 0 and the charge does not change,

we can exchange a Z. If the charge does change

by ±1 unit, we can exchange a W±.

Finally, if the color is not 0 and it changes at the vertex, we can exchange a gluon.

So here is a first example,

the process e+e- -> u ubar.

We clearly cannot couple the electron directly to a u quark

so this is an annihilation process in which the

e+ e- annihilate into a u bar pair.

This can proceed via the exchange of a photon

or a Z, since both the electron

and the u quark carry both electric

and weak isospin charge.

It cannot go via the exchange of a gluon or a W

because the e+ e- does not carry color

and the charge does not change

at the vertex.

A second example is the process e+ e- -> nu unbar.

Unless the neutrinos are electron-type neutrinos,

this is again an annihilation process.

It proceeds via the exchange of a Z boson.

It cannot proceed via photon exchange, because

the neutrino does not carry electric charge.

Neither the electron nor the neutrino are colored,

so gluons cannot be exchanged.

The electric charge is not changing at the vertex, so

W± exchange is also excluded. The third

example is the process e- u -> nu_e d.

In this case it must be a scattering process, since the e- cannot directly couple to the u quark.

They must in fact exchange a W boson

since the charge changes by 1 unit at each vertex.

This is possible because both the e- and the u quark

carry weak isospin.

They cannot exchange a photon since the charge changes

neither can they exchange a Z. And since the e- and the nu do not

carry color, they cannot exchange a gluon either.

The next example is the decay of a d quark into a u

e- and an electron-antineutrino. To conserve baryon and lepton

number we must connect the d quark to the u quark and the electron to the antineutrino.

This means that at each vertex, the charge changes by 1 unit

and we can exchange a W boson. We cannot exchange a photon

because the charge changes, likewise we cannot exchange a Z.

And since the e- and the nu don’t carry color

we cannot exchange a gluon.

The last example is a red d quark scattering off a blue d quark,

giving a blue u quark and a red d quark. This is clearly a strong process since it involves color exchange.

Since strong interactions conserve flavor,

we must connect the two u quarks and also the two d quarks.

That means that the two

vertices are connected by a gluon

of red-antiblue color

such that the color is conserved at each vertex.

We cannot exchange a photon since

the color changes, neither a Z or a W bosons

since all of these cannot change the color of a particle.

[MUSIC]

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