The document discusses the historical development of particle physics from the discovery of subatomic particles like protons and neutrons to the modern quark model. Key developments include the proposal of nuclear forces to bind protons and neutrons, the discovery of many new hadron particles in accelerators, and the classification of hadrons into baryons and mesons. The quark model was proposed to explain hadrons as composed of more fundamental quarks, with six flavors eventually discovered. Leptons like electrons were also discovered as fundamental particles not affected by strong forces. The quark model successfully explained phenomena like beta decay in terms of quark interactions and changes.

1. Topic for senior secondary or A-level physics students
~Lalit Kishore

2. Historical perspective
 Till 19th centaury atom was considered to the fundamental
particle (ultimate: not formed by any other particles or sub-
atomic prticles)
 In 20th centaury, Chadwick discovered independent neutrons
and protons...protons were found in solar flares...some stars
behaved as if they were neutron stars. Studies of cosmic rays led
to discovery of new fundamental particles
 Nuclear force was – the strongest force that can could bind like
charges – studied
 With high power / energy particle accelerators, very many new
particles and their anti-particles were discovered and are still
being discovered
 Families / classifications/ series have created to find the missing
particles including the God particle

3. Hadrons particles
Hadrons as new fundamental particles
Quark model of hadrons with six
flavours and their charge value and
strangeness
 With discovery of new particles using
particle accelerators (collision experiments)
to be classified in categories of those
affected by strong forces and those not
affected
 2 conclusions from collision experiments /
nuclear reactions
 Total charge always remains constant
 Total number of nucleons at times but
rarely changed
 The particles with properties different from
neutrons and protons discovered which
were called hadrons
 From the collision studies lead to quark
model of hadrons
 It was proposed that hadrons was further
made of fundamental particles called
quarks which led to discovery more
particles
 Many types or ‘flavours’ (six)of quarks were
discovered and proposed

4. Hadrons’ classification as baryons
and mesons
 With discovery of anti-quarks and various
combination with quarks led to new
classification of hadrons called baryons
and mesons
 Baryon family : protons, Neutrons and
their anti-particles, they consist of 3
quarks
 Meson family (consisting of quarks / anti-
quarks): Pions and kappas

5. Leptons
Definition and neutron
decay
Quark model reaction
 The fundamental particles
not affected by strong
forces – no more composed
of smaller particles
 Examples: Electron;
positron, neutrino, anti-
neutrino
 In the decay of a neutron;
proton, electron and
antineutrino are emitted

6. Beta decay explanation in terms of
quarks and leptons
Weak force presence Proton decay quark model reaction
 Beta decay is not due strong
force and, therefore, must be
due to another force called
‘weak force’ or ‘weak
interaction’
 In beta decay, the lepton
number before and after
decay remains the same
 During the decay of a proton
in the nucleus, a neutron is
formed and a neutrino is
emitted

7. Quark model facts
Quark model attributes Some quark compositions
 Quark model has 6 flavours of
quark
 Up
 Down
 Strange
 Charm
 Bottom
 Top
 Quarks have fractional charges
 A meson contains a quark and
an antiquark
 Baryons contain three either 3
quarks or 3 antiquarks
 Protons are composed of
quarks up, up, and down
 Neutrons are composed of
quarks up, up and down
 During minus beta decay, a
down quark quark changes to
an up quark
 During plus beta decay, an
up quark changes to a down
quark