1. ELEMENTARY PARTICLES and their
classification
Mrs.P.Kanmani, M.Sc.,M.Phil,
Assistant Professor of Physics

2. What are Elementary Particles?
• An elementary particle or fundamental particle is a particle
that does not have a substructure.
• It is not known to be made up of smaller particles.
• Particles that have no substructure could be considered the
basic building blocks of the universe from which all other
particles are made.

3. History of Elementary of particles
• The first subatomic particle to be discovered was the electron,
identified in 1897 by J. J. Thomson.
• The nucleus of the atom was discovered in 1911 by Ernest
Rutherford, the nucleus of ordinary hydrogen was recognized
to be a single proton. In 1932 the neutron was discovered.
• An atom was seen to consist of a central nucleus—containing
protons and, except for ordinary hydrogen, neutrons—
surrounded by orbiting electrons.
• Historically they were once regarded as fundamental
constituents of every matter.

4. History of Elementary of particles
• In 1928 the relativistic quantum theory of P. A. M. Dirac
hypothesized the existence of a positively charged electron, or
positron, which is the antiparticle of the electron; it was first
detected in 1932.
• Difficulties in explaining beta decay led to the prediction of
the neutrino in 1930, and by 1934 the existence of the
neutrino was firmly established in theory (although it was not
actually detected until 1956).
• Photon was suggested by Einstein in 1905 as part of his
quantum theory of the photoelectric effect .

5. History of Elementary of particles
• In 1935 Hideki Yukawa suggested that a meson (a charged
particle with a mass intermediate between those of the
electron and the proton) might be exchanged between
nucleons.
• The meson emitted by one nucleon would be absorbed by
another nucleon; this would produce a strong force between
the nucleons, analogous to the force produced by the
exchange of photons between charged particles interacting
through the electromagnetic force.

6. History of Elementary of particles
• In 1947 the particle predicted by Yukawa was discovered and
named the pi meson, or pion which was 200 times heavier
than electron .
• Both the muon and the pion were first observed in cosmic
rays .
• Further studies of cosmic rays turned up more particles.
• Each elementary particle is associated with
an antiparticle with the same mass and opposite charge.

7. History of Elementary of particles
• Some particles, such as the photon, are identical to their
antiparticle.
• Such particles must be neutral, but not all neutral particles are
identical to their antiparticle.
• By the 1950s these elementary particles were observed in the
laboratory as a result of particle collisions produced by
a particle accelerator .

8. Classification of Elementary Particles
• The fundamental particles may be classified into groups in
several ways.
• First, all particles are classified into fermions, which obey
Fermi-Dirac statistics and bosons, which obey Bose-Einstein
statistics.
• Fermions have half-integer spin, while bosons have integer
spin.

9. Classification of Elementary Particles
• All the fundamental fermions have spin 1/2. Electrons and
nucleons are fermions with spin 1/2.
• The fundamental bosons have mostly spin 1. This includes the
photon.
• The pion has spin 0, while the graviton has spin 2. There are
also three particles, the W+, W− and Z0 bosons, which are
spin 1. They are the carriers of the weak interactions.

10. Classification of Elementary Particles
• They can also be classified based on their interactions

11. Classification of Elementary Particles
• The electron and the neutrino are members of a family
of leptons.
• The leptons are distinguished from other particles called
hadrons in that leptons do not participate in strong
interactions.

12. Classification of Elementary Particles
• Hadrons are strongly interacting particles. They are divided
into baryons and mesons.
• The baryons are a class of fermions, including the proton and
neutron, and other particles which in a decay always produce
another baryon, and ultimately a proton.
• The mesons are bosons.

13. Classification of Elementary Particles
• Protons and neutrons are made of still smaller particles
called quarks.
• It appears that the two basic constituents of matter are the
leptons and the quarks.
• There are believed to be six types of each. Each quark type is
called a flavor, there are six quark flavors.
• Each type of lepton and quark also has a
corresponding antiparticle, a particle that has the same mass
but opposite electrical charge and magnetic moment.

14. Classification of Elementary Particles
• In the current theory, known as the Standard Model there are
12 fundamental matter particle types and their corresponding
antiparticles.
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15. Classification of Elementary Particles
• In addition, there are gluons, photons, and W and Z bosons,
the force carrier particles that are responsible for strong,
electromagnetic, and weak interactions respectively. These
force carriers are also fundamental particles.
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