Particle Reactions and Decays - I
[Secs 16.1, 16.2 Dunlap]
REACTION
?
No
enough energy
DECAY
Q<0
?
Yes
Q>0
CAN IT HAPPEN ?
No
Check B, Li, Qc
Yes
IS IT WEAK?
T, S, C violation ?
Yes
WEAK
 W+, or W- involved.
 Flavor change occurs with
one unit of charge change.
No
QUARKS
LINK UP?
Yes
STRONG
No
γ in
products
Yes
E.M.
No
TO HADRONS
TO LEPTONS
Classification of Decays
No
Type of Decay
Particles
Guage Bosons
Example
1
Leptonic decay
leptonleptons
weak
   e     e
2
Hadronic lepton
decay
leptonlepton+hadron
weak
      
3
Hadronic decay
hadronhadrons
strong
     p
4
Nonleptonic
hadron decay
hadronhadrons
strong+
weak
5
Semileptonic
hadron decay
hadronhadron+leptons weak
n  p  e   e
6
Leptonic hadron
decay
hadronleptons
      
7
Electromagnetic
hadron decay
hadronhadron+photon EM
weak
K      
  p  
LEPTON DECAYS
(i.e. decay into leptons)
LEPTONIC
DECAY
HADRONIC –
LEPTON DECAY
HADRONIC DECAYS
HADRONIC DECAY
SEMI-LEPTONIC HADRON DECAY
LEPTONIC HADRON DECAY
NON-LEPTONIC HADRON DECAY
ELECTROMAGNETIC HADRON DECAY
In 1963 The UFI is back
The quarks can exist either as
eigenstates of the WEAK interaction or
the STRONG interaction.
Indeed it is best to think of the QUARKS
as being fundamentally having FLAVOR
STATES dw and sw determined via
the WEAK interaction. Then comes
along the STRONG interaction which
MIXES these flavors into.
In 1963 theoretical
physicist Nicolo
Cabibbo gave an
explanation in terms of
quark state mixing and
introduced an angle The Cabibbo angle
d  d w cos   sw sin 
s   d w sin   sw cos 
How the Quark Mixing works
dw
After switching on the STRONG
interaction – these are the new
quark states.
d
d  d w cos c  sw sin c
θc
sw
θc
s
s   d w sin c  sw cos c
The Cabibbo Angle turns out to be
~15°
Cabbibo allowed – Cabibbo Surpressed
d
d
d
u
s
s
Cabibbo Allowed
d
d
d
d
s
u
Cabibbo Surpressed
MEASURING THE CABIBBO ANGLE

u


W+

d

u
W+
K+
s

K sin 2  c
2


tan
c
2
 cos  c
from which one finds
tan  c  0.075
Example 1
BETA MINUS DECAY
n
 p  e  e

All the primary conservation laws (above the line) are ok, so the reaction
should go. But is it S, W, or EM? There are two things that indicate the
primary classification is WEAK. These are (i) the fact that this T of the
final state is N.D (not defined) and (ii) the fact that leptons are present in
the final state. Having established that it is a W (weak) decay we then
make the inference:
WEAK DECAY  INVOLVEMENT OF “W” PARTICLE
Example 1
BETA MINUS DECAY
n
 p  e  e

FEYNMAN DIAGRAM
CLASSIFICATION = Semileptonic Hadron Decay
Example 2
ASSOCIATED PRODUCTION
 n
   K

0

All the primary conservation laws (above the line) are ok.
Also the secondary conservation laws (obeyed by the
strong interaction, ones below the line) are ok. This
means the reaction must be mediated by the STRONG
force.  There will be NO Ws
 All quarks will “link up”.
Example 2
ASSOCIATED PRODUCTION
 n
   K

0

FEYNMAN DIAGRAM
CLASSIFICATION = PURE HADRONIC
Example 3
K- MESON DECAY
K 
   


All the primary conservation laws (above the line) are ok. But
here the secondary conservation laws (obeyed by the strong
interaction, ones below the line) are NOT OK. This is a clear
indication that this cannot be a strong process. The presence of
leptons also confirms that this must be a WEAK interaction
process.  the presence of W particles.
Example 3
K- MESON DECAY
K 
   


FEYNMAN DIAGRAM
DECAY CLASSIFICATION = LEPTONIC HADRON DECAY
Example 4
LAMDA ZERO DECAY
As with the K+ decay we see failure on isospin and
strangeness. Again this is clear indication that the WEAK
interaction is responsible. [We saw in the lecture on
strangeness that a strong interaction could only occur if the
strange quark that had been produced could find another
strange quark for pairing up with – and annihilating with]
 W PARTICLE involved in FLAVOR changing
Example 4
LAMDA ZERO DECAY
FEYNMAN DIAGRAM
DECAY CLASSIFICATION = Nonleptonic Hadron decay
Example 5
DELTA ++ PRODUCTION
So all is OK for an allowed reaction and one going by
the STRONG interaction. This a fully hadronic
process.
Example 5
DELTA ++ PRODUCTION
FEYNMAN DIAGRAM
CLASSIFICATION = Hadronic Reaction
Example 6
D ZERO DECAY
All is ok above the line – so the process is possible.
Below the line we see a violation of strangeness and
charm. Thus it looks as if we have a charmed quark
changing into a strange quark.

Involvement of a W boson. This is a WEAK
process.
Example 6
D ZERO DECAY
FEYNMAN DIAGRAM
DECAY CLASSIFICATION = Nonleptonic Hadron decay
Example 7
D ZERO DECAY
We see again an allowed decay but one which
involves flavor changing on a quark. Here, however
the presence of leptons in the final state makes
finding the Feynman diagram easy.
Example 7
D ZERO DECAY
FEYNMAN DIAGRAM
DECAY CLASSIFICATION = Semileptonic Hadron decay
Example 8
e e 
  


 
  
0