Physics A Level
Chapter 15: Atomic structure 15.11 Families of particles
Physics A Level
Chapter 15: Atomic structure 15.11 Families of particles
Today, sub-atomic particles are divided into two families:
- Leptons such as electrons and neutrinos. These are particles that are unaffected by the strong nuclear force.
- Hadrons such as protons and neutrons. These are all particles that are affected by the strong nuclear force.
The word ‘lepton’ comes from a Greek word that means ‘light’ (in mass) while ‘hadron’ means ‘bulky’. It is certainly true that protons and neutrons are bulky compared to electrons.
Leptons
Leptons are (currently) considered to be fundamental particles, although, in principle, we can never know for certain whether a particle, such as the electron, is truly fundamental; the possibility will always remain that a physicist will discover some deeper underlying structure.
Hadrons
Figure 15.16 shows the Large Hadron Collider at the CERN laboratory in Geneva. Physicists are experimenting with hadrons in the hope of finding answers to some fundamental questions about this family of particles. In 2013, they announced the discovery of the Higgs boson, a particle that was predicted 50 years earlier and which is required to explain why matter has mass.
higher energies as scientists have sought to look further and further into the fundamental nature of
matter. This is one of the particle detectors of the Large Hadron Collider (LHC), as it was about to be
installed. The entire collider is $27 km$ in circumference
| Type of quark | up | down | charm | strange | top | bottom |
| Symbol | u | d | c | s | t | b |
| Charge | $ + \frac{2}{3}e$ | $ - \frac{1}{3}e$ | $ + \frac{2}{3}e$ | $ - \frac{1}{3}e$ | $ + \frac{2}{3}e$ | $ - \frac{1}{3}e$ |
| Type of antiquark | antiup | antidown | anticharm | antistrange | antitop | antibottom |
| Symbol | $\overline u $ | $\overline d $ | $\overline c $ | $\overline s $ | $\overline t $ | $\overline u $ |
| Charge | $ - \frac{2}{3}e$ | $ + \frac{1}{3}e$ | $ - \frac{2}{3}e$ | $ + \frac{1}{3}e$ | $ - \frac{2}{3}e$ | $ + \frac{1}{3}e$ |
Quarks
To sort out the complicated picture of the hadron family of particles, Murray Gell-Mann in 1964 proposed a new model. He suggested that they were made up of just a few different particles, which he called quarks.
There are many surprising things about quarks. First, quarks have charges of less than the fundamental charge, e. However, quarks are never found outside a hadron. The quarks combine so that the resulting hadron will have a charge of e or a multiple of e.
There are six types (or ‘flavours’) of quark, each with an associated antiquark. Table 15.5 lists these quarks together with their charges.
In addition to the property of charge, quarks have other properties such as strangeness, charm, upness and downness. We do not need to concern ourselves about these properties; however, recognising that they exist should help you to understand how a large number of different hadrons can be made up from these half-dozen flavours of quark.
There are two ways in which quarks can combine to produce hadrons:
- three quarks make up a class of hadrons called baryons
- a quark and an antiquark make up a class of hadron called mesons.
Baryons
Examples of baryon are the proton and the neutron.
- A proton is made up of two up quarks and a down quark; proton = (uud).
- A neutron is made up of one up quark and two down quarks; neutron = (udd).
Mesons
There are many examples of mesons; here are two described:
- $A{\pi ^ + }$ meson is made up of an up quark and a down antiquark; ${\pi ^ + }$ meson = $(u\overline d )$.
- $A\phi $ meson is made up of a strange quark and an antistrange quark; $\phi $ meson = $(s\overline s )$.
Questions
14) a: Show that the charges on the quarks making up a proton give it a charge of $+1e$.
b: Show that the charges on the quarks making up a neutron give it a charge of 0.
15) A $\rho - $meson is made up of an up quark and an antidown quark. Calculate its charge.
16) Suggest which quarks or antiquarks make up $a{\pi ^ - }$ meson.
17) Show that the $\phi - $meson is neutral.