There are two types
of hadron that we have to know about:- the baryon and meson.
Baryon
- 3 quarks - baryon number of 1- proton and neutron
Anti-baryon
- 3 anti-quarks - bayon number of -1 - ant-proton and anti neutron.
Meson -
a quark and an anti-quark - baryon number 0 ('cos its not a baryon!)
- kaon, pion
You only need to learn
about three quarks - the up 'u', down 'd' and strange
's' but there are others. you don't need to worry about them for this
course, but it would be a good idea to look them up and give yourself
a more complete picture.
The examiner could give you baryons that contain other quarks and expect
you to classify them - if they did that they would have to give you the
quark properties in a table.... but you are expected to recall the general
types above!
Each quark has characteristics
that are listed in your data book - you do NOT have to learn these,
you should be able to use them to work things out!
Quark
Charge
Baryon
Number
Strangeness
up 'u'
down 'd'
strange 's'
u
d
s
2/3
-1/3
-1/3
-2/3
1/3
1/3
1/3
1/3
1/3
-1/3
-1/3
-1/3
0
0
-1
0
0
1
The only two
types of meson you need to know about are the pion p
and the kaon k.
The kaon involves
'strangeness'. It is the only strange particle you have to
deal with in this course.
The pion has no
strange quark.
After the symbol
comes a charge sign - which you can work out from the data book.
Quark
Antiquark
Meson
u
u
u
d
d
d
s
s
d
s
u
d
s
u
d
u
p+
k+
po
po
k+
p-
ko
k-
As we are limited
to u, d and s (and I have already told you that the only hadron we
have to deal with that has a strange quark is the kaon) we only have to
concern ourselves with two baryons: the proton and the neutron.
You
know that a neutron is neutral and a proton has a charge of +1.
Using the
information in the table above (charge!) deduce what arrangement of
'u' and 'd' (three in total) makes each of the baryons we have to know
about.
proton
quark composition: ?
neutron
quark composition: ?
The only two
types of baryon you need to know about are: the proton 'p'
and the neutron 'n'.
Neither is 'strange'.
(The kaon (a meson) is the only strange particle you have to deal
with in this course). They are both made up of only 'u' and 'd'.
They are each
composed of three quarks.
You know their
charge so you can work out their composition from the data book.
What do hadrons
do?
Mesons are
involved in the strong force - they help to hold the nucleus together.
Baryons are
particles that participate in strong interactions They are composed
of three quarks, and are therefore generally more massive than mesons..
The
Pion
The pion is a semistable
meson produced either in a neutral form with a mass 264 times that of
an electron and a mean lifetime of 8.4 × 10-17 seconds or in a
positively or negatively charged form with a mass 273 times that of
an electron and a mean lifetime of 2.6 × 10-8 seconds. It is also
called a pi meson.
It is a meson involved
in holding the nucleus together - it is the exchange
particle for the strong nuclear force.
It is produced as
the result of high-energy particle collision - cosmic rays interacting
in the upper atmosphere result in pion production.
These then decay:
- negatively charged
ones into a muon and a muon antineutrino by weak interaction
- positively charged
ones into an anti-muon and a muon neutrino by weak interaction
- the uncharged
pion decays to an electron, positron, and gamma ray by the electromagnetic
interaction
Lifetimes:
The neutral pions
have a lifetime of about 10-16 seconds.
The positive and
negative pions have longer lifetimes of about 2.6 x 10-8
seconds.
The
kaon
The kaon is an unstable
meson. It consists of an electrically charged form with a mass 966 times
that of an electron or a neutral form with a mass 974 times that of
an electron. It is produced as a result of a high-energy particle collision.
It is also called K-meson
It is produced as
the result of high-energy particle collision
There are two types
of neutral Kaons K0S and K0L.
The 'L' one has a long life (5.2 x10-8 seconds) and the other 'S' one
a short life (0.89 x10-10 seconds). They are formed by the mixtures
of the quark combinations down-antistrange and antidown-strange.
The charged kaons
are mesons which have a quark composition of up-antistrange for the
positive kaon and antiup-strange for the negative kaon. They decay in
about 1.24 x 10-8 seconds:
They decay as follows:
Decay
times in this range indicate decay by the weak interaction. Note that
NONE of the decay products has a strange quark, so this decay violates
conservation of strangeness and cannot proceed by the strong interaction.
Describing
the neutral kaons is much more complex. There are two versions of this
particle with the same mass but different decay lifetimes - very strange!!
Remember
that weak decays proceed via w-bosons!
The
neutral kaons are important historically for their part in advancing
our understanding of quark processes. When first discovered in the late
1940s they were labelled the t and q mesons, and their decay was a great
mystery, labelled the t-q puzzle.
The
particles werefound to be identical in mass, and the only thing which
apparently differentiated them was their decay processes and the fact
that the two different sets of decay products had different parity.
Particle
decays by the strong or electromagnetic interactions had been observed
to conserve parity in their decays. Theoretical physicists T. D. Lee
and C. N. Yang proposed in 1956 that parity need not be conserved in
weak interaction decays. In 1957 Chien-Shiung Wu showed this violation
of parity conservation in the beta decay of cobalt. Even with the violation
of parity, it was thought that the combination of charge conjugation
and parity would leave the system invariant (CP invariance). An experiment
by Cronin and Fitch in 1964 showed that there was a small CP violation
in the kaon decay, so the kaon has played a central role in the discussions
of these symmetries since that time.
When
Rochester and Butler discovered the short-lived version of the neutral
kaon in 1946, they did so by observing a characteristic "V"
pattern in a cloud chamber. The decay they saw was
The
neutral kaon did not leave a track in the cloud chamber, but the "V"
track of pions revealed its presence upon decay.
You
know that a neutron is neutral and a proton has a charge of +1.
