2.4 Quarks and Antiquarks

Question 1

What were kaons called when they were first discovered

Answer 1

V particles

As the cloud chamber photographs often showed V-shaped tracks

Question 2

Although the ‘strange particles’ all decay through the weak interaction:

Answer 2

• Those that decay into pions only were referred to as kaons
• The others, such as sigma (Σ) particle were found to have different rest masses which were always greater than the protons rest mass, and decay either in sequence or directly into protons and pions
• Strange particles are created in twos

Question 3

Why was the ‘strangeness number’ (S) introduced

Answer 3

To explain why certain reactions were not observed

Strangeness always conserved in strong interactions

Question 4

Strangeness numbers of particles

Answer 4

+1 for the K+ meson

0 strangeness for non-strange particles (proton, neutron, pions, leptons)

Question 5

Strangeness is always conserved in a strong interaction, whereas strangeness can change by…

Answer 5

… 0, +1, or -1 in weak interactions

Question 6

How can the properties of the hadrons such as charge, strangeness and rest mass be explained

Answer 6

By assuming they are composed of smaller particles known as quarks and antiquarks

Question 7

What are three types of quarks

Answer 7

up, down and strange

u, d and s

Question 8

Mesons are ______, each consisting of a ______ and an ______.

Answer 8

Hadrons
Quark
Antiquark

Question 9

A π 0 meson can be…

Answer 9

…any quark-corresponding antiquark combination

Question 10

Each pair of charged mesons is a…

Answer 10

… particle-antiparticle pair

Question 11

There are two uncharged kaons

Answer 11

_

K0 and the K0 meson

Question 12

The antiparticle of any meson is a…

Answer 12

… quark-antiquark pair and therefore another meson

Question 13

Baryons and antibaryons are hadrons that consist of three quarks for a baryon or three antiquarks for an antibaryon

Answer 13

A proton is the uud combination
A neutron is the udd combination
                                  \_\_\_
An antiproton is the uud combination
The Σ particle is a baryon containing a strange quark

Question 14

What is the only stable baryon

Answer 14

The proton

A free neutron decays into a proton, releasing an electron and an electron antineutrino as in β- decay

Question 15

β- decay, a ______ in a ______-rich nucleus change into a ______, releasing an electron and an electron ______. In quark terms, a down quark changes to an ______ quark.

Answer 15

Neutron
Neutron
Proton
Antineutrino
Up

Question 16

In β+ decay, a ______ in a ______-rich nucleus changes into a ______, releasing a positron and an electron ______. In quark terms, an up quark changes to a ______ quark.

Answer 16

Proton
Proton
Neutron
Neutrino
Down