Transcript Particle Classification - Department of Physics, HKU

Classification of Particles
[Secs 2.1, 14.1, 14.2 Dunlap]
The basic structure of matter
Fortunate that the electron is not effected by the strong force – if it did it would be
“sucked in” to the nucleus and we would have not atoms – no chemistry and no life!
Leptons, Mesons, Baryons,
Hadrons, Bosons, Fermions – what
are they?
• It is important when coming at particle physics to
realize that much of the classification of particles
(i.e. Leptons, Mesons, Baryons, Hadrons, Bosons,
and Fermions) have their roots in history. If we
had to classify these particles today with what we
now know about them we would probably choose
different names. Looking at the history is however
highly instructive
Going back in history
1913
Life in 1913 was simple – only
three known particles – the
proton, electron and photon.
1933
By 1933 life had become more complicated. The
neutron, and positron had been discovered. The
neutrino has a strong evidence
The Yukawa Particle
In the 1930s Yukawa tackled the problem of what keeps
the nucleus together despite the repulsive force
between protons. He knew the interaction was
extremely strong and short range but what caused it?
In 1936 Yukawa published his theory that there was a
particle (the Yukawa particle) that had to have a
mass of around 100MeV that interacted with the
nucleons to produce this ultra strong force.
Photon (EM) field obeys:
Particle of mass m obeys:
E 2  p 2c 2  0
Hideki Yukawa
(1907 – 1981) Nobel
laureate (1949)
Put Quantum operators
E 2  p2c2  m2c4
p  i  and E  i

t
1  2
  2
0
2
c t
1  2
  2
 m2 c 4
2
c t
Thus short range explanation
for the strong force was that it Photon has no
was mediated by a particle gap or no mass
with mass, estimated ~100
MeV, or 200 times of
electron mass
The particle has a mass
and short ranged,
exponentially decay like
screened Coulomb force
2
2
How can a particle mediate a force?
Attractive and repulsive forces
arise from particle exchange
In the ball players analogy – the
players throw the ball and get a
momentum “kick” backwards. If on
the other had they exchange the
balls as shown below then there is
an attractive force.
The analogy breaks though –
classically the ball exists – in the
quantum world where does the
mediating particle come from?
Answer: it comes through the
borrowing of energy (to make the
particle E=mc2 for a short time) as
allowed by the uncertainty
principle
E.t 
2
What is the Feynman diagram for the
nuclear strong force?
Well it looks like this
n
p


p
n
Latter we will see what this looks like at the quark level. Note that the n and p do
0
not have to change – there is also a 
The line for \pi is usually drawn as dashed line

Discovery of the muon
Those were exciting days the 1930s – for Carl Anderson
in particular – who discovered the positron in 1932 and
in 1936 the muon. How did he do this – using the cloud
chamber to study Cosmic Rays.
Originally Anderson thought this must be the Yukawa
particle since it had 207 electron masses. Yukawa had
predicted his particle should have ~200 electron masses.
Soon it became clear though that this could not be
Yukawa’s particle since it did not interact with the
nucleus of atoms. It behaved like a heavy electron!
Carl Anderson
(1905 – 1991)
1939 Nobel Prize
(140MeV)


Physicist Rabbi philosophically said “Who ordered it”?
3x 10-8 sec
(106 MeV)
Usually, we use bar to represent
anti-neutrino, and electron is a
particle, positron anti-particle, and
same to \muons depending on
their charges.


+ 
anti-neutrino
2x 10-6 sec
(0.5 MeV)

e + e +  
The discovery of the pion

There were 3 major players in the discovery of the pion.
The discovery took place in 1947 in two laboratories
Marietta Blau
(1894 – 1970)
Donald Perkins
(1925)
Discovered the
technique of
nuclear emulsions
for looking at short
lived particle tracks
Was the first see a
pion event - i.e. a
particle of the right
mass interacting
with a nucleus
Cecil Powell
(1903 – 1969)
Nobel laureate 1950
And his research
group in Bristol saw
the 2nd and 3rd events
– and saw that the
pion decayed to a
muon
Going back in history to 1947
To begin with particles were classified
according to their weight (mass). The
diagram shows what physicists knew about
particles in 1947. There do seem to be
three groupings by weight (mass).
(i) Light particles (which they called
LEPTONS) =
(e-, e+, e, )
[from Greek; leptos = small]
(ii) Middle weight particles (which they

+
+
0
called MESONS) (  ,  ,  ,  ,  )
[from Greek, mesos=middle]
(iii) Heavy weight particles (which they called
BARYONS) = p, n,
[from Greek, barus=heavy).
1947
BARYONS
MESONS
Note that muons are
actually leptons, and
gamma is not lepton!
LEPTONS
Because the MESONS and BARYON
groups seemed so much heavier than the
leptons, they were collectively known as
“bulky” particles or HADRONS (Gk:
hadros = bulky)_
i.e.
HADRONS = MESONS + BARYONS
Modern classification - leptons
• Leptons are FERMIONS (spin half particles) that do not
participate in the strong interaction.
• Leptons interact only through the electro-weak force (i.e.
electric force plus weak force) and thus we may think of the
leptons as now meaning “light – as in delicate - interaction”
• Leptons appear to be point like particles having no internal
structure (size <10-19m). Electrons must have size greater
than 10-50m otherwise they would be black-holes. String
theories put their size around 10-30m
• Leptons are seen in the left hand column of mass in the Figure
in previous slide.
• The muon is not referred to now as a mu-meson, because it
not a meson – ( a meson is strongly interacting and it is a
boson)
Modern classification - mesons
• Mesons are BOSONS of either spin 0 (usual) or spin 1 (rare)
that DO interact via the strong interaction (also electric and
weak)
• Mesons tend to have masses between the electron mass and
the p, n masses. The reason is as we shall see latter – that
the mesons comprise of 2 quarks bound together.
• Mesons have internal structure and measurable size (~1F)
• Mesons cause the nucleon-nucleon force – they are “go
between” or “in the middle” particles – a new meaning of
“mesos”
• Not all “go between” – mediating particles are mesons. The
photon “sticks” the electron to the atomic nucleus in the same
kind of way – as a pion. The photon is not a meson however
because it does not interact via the strong interaction, Moreover
the photon is massless.
Modern classification – Baryons
and Hadrons
• BARYONS are FERMIONS (spin ½ or 3/2) that
interact via the strong interaction (also electric and
weak).
• Baryons have masses equal to or greater than the
proton (because as we shall see they comprise of
three bound quarks)
• Baryons have internal structure and measurable
size (~1fm)
•
HADRONS: is a collective term for strongly
interacting particles. i.e the hadron family contains
MESONS + BARYONS
Classification of Fermions and Bosons
LEPTONS
HADRONS
The fundamental force carriers
(gauge bosons)
GLUONS - mediate the STRONG interaction between quarks (not pions)
PHOTONS – mediate the ELECTRIC interaction between quarks and between
charged leptons and between quarks and leptons
W and Zs – mediate the WEAK interaction between quarks and leptons
GRAVITONS – mediate the GRAVITATIONAL interaction (questionably)
The particle
situation 1983
Notable additions are the
TAON – super heavy electron
discovered 1974
K- MESONS
D- MESONS
HYPERONS such as



