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

1913

Going back in history

1933 Life in 1913 was simple – only three known particles – the proton, electron and photon.

By 1933 life had become more complicated. The neutron, and positron had been discovered. The neutrino has a strong evidence

The Yukawa Particle

Hideki Yukawa (1907 – 1981) Nobel laureate (1949) 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:

E

2  2

p c

2  0 Put Quantum operators  Thus short range explanation for the strong force was that it was

mediated by a particle with mass, estimated ~100 MeV, or 200 times of electron mass

2   1

c

2  2  

t

2  0 Photon has no gap or no mass

p

Particle of mass m obeys:

E

2   and

E



i

 

t

 2   1

c

2  2  

t

2  4  The particle has a mass and short ranged, exponentially decay like screened Coulomb force

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=mc 2 for a short time) as allowed by the uncertainty principle

E t

2

What is the Feynman diagram for the

Well it looks like this

nuclear strong force?

p n   p n Latter we will see what this looks like at the quark level. Note that the n and p do not have to change – there is also a  0 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.

Carl Anderson (1905 – 1991) 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!

1939 Nobel Prize (140MeV)   Physicist Rabbi philosophically said “Who ordered it”?

3x 10 -8 sec

(106 MeV) anti-neutrino Usually, we use bar to represent anti-neutrino, and electron is a   +  

2x 10 -6 sec

particle, positron anti-particle, and same to \muons depending on their charges.

(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) Discovered the technique of nuclear emulsions for looking at short lived particle tracks Donald Perkins (1925) 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 2 nd and 3 rd 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).

1947 BARYONS

(i) Light particles

(which they called

LEPTONS

) = (e-, e+,  e , ) [from Greek;

leptos = small

]

(ii) Middle weight particles

called

MESONS

  ,  + (which they ,  + ,  0 ,  [from Greek,

mesos=middle

]

(iii) Heavy weight particles

(which they called

BARYONS

) =

p, n

, [from Greek,

barus=heavy

). Note that muons are actually leptons, and gamma is not lepton!

MESONS 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

participate in the strong interaction

.

do not

• Leptons interact only through the electro-weak force electric

leptons

force plus weak (i.e. force) and thus we may think of the as now meaning “light – as in delicate - interaction” • Leptons appear to be point like particles having no internal structure (size <10 -19 m). Electrons must have size greater than 10 -50 m otherwise they would be black-holes. String theories put their size around 10 -30 m • 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    