Quarks and Antiquarks

Particle Physics Lesson 9

Particle Physics

Particle physics is concerned with fundamental particles, which means that the particles can't be broken down any further.

It used to be thought that protons,

neutrons and electrons were the fundamental particles of matter. 

VideoVideo

In Search of Giants (5 of 15) The In Search of Giants (5 of 15) The Standard Model of Particle PhysicsStandard Model of Particle Physics

Hadrons are not Hadrons are not fundamental!fundamental!

It has been found that nucleons (proton and neutron) are made up of smaller particles, so nucleons are now not fundamental.

Particle accelerators fired electrons at protons and showed there were three scattering centres.

Quarks There are three main

quarks, up, down, and strange (for the exam). 

They have corresponding antiquarks. 

There are three others with even odder names, top, bottom and charm (not in exam).

Quark Properties Quarks are the fundamental particles

from which hadrons are made. 

They cannot exist on their own.

Notice how they go in matching pairs.

Three Generations of Matter

Quark NumbersQuark Numbers

There are three quantum numbers associated with quarks:

Charge, expressed as the fraction of the electronic charge.  1/3e = 5.33 × 10-20 C

Baryon number Strangeness number, when there are

strange quarks. Each antiquark has equal and opposite

values of charge, baryon number and strangeness.

Quark Numbers

Quark Charge (Q)

Baryon number  (B)

Strangeness (S)

Down -1/3 1/3 0

Anti-down +1/3 -1/3 0

Up +2/3 1/3 0

Anti-up -2/3 -1/3 0

Strange -1/3 1/3 -1

Anti-strange

+1/3 -1/3 1

d

d

u

u

s

s

Quark Properties Baryons are made of three quarks; antibaryons

of three antiquarks. Mesons are made up of one quark and one

antiquark. Mesons are not made up of two quarks.

Gluons bind quarks together; they are subject to the strong interaction.

Important Notes: Strangeness is conserved in strong interactions only (next lesson).

Hadron Quark Combinations

Notice how you Notice how you have these three have these three options only:-options only:-

Baryon:Baryon:

Anti-BaryonAnti-Baryon

Meson:Meson:

qqq

qqq

qq

Meson Quark Meson Quark CombinationsCombinations

Complete the table for the following hadrons:

Particle Composition

Baryon no.

Charge

Proton (p) uud 1 +1e

Antiproton

Neutral Pion (π0)

Neutral Kaon (K0)

Lambda (Λ0)

Complete the table for the following hadrons:

Particle Composition

Baryon no.

Charge

Proton (p) uud 1 +1e

Antiproton ūūđ -1 -1e

Neutral Pion (π0)

uū or dđ 0 0

Neutral Kaon (K0)

dŝ or sđ 0 0

Lambda (Λ0) uds 1 0

Beta decay

Question Some mesons such as the neutral Kaon

and the neutral Pion (Pi-meson) can be made up in different ways.

Why does a meson always have a baryon number of 0?

It is made up of a quark and an antiquark (P)

The baryon numbers add up to zero (P)

discovery of the Σ++c baryon