Transcript Physics 7802.01 Introduction
P780.02 Spring 2002 L4 Richard Kass
Conservation Laws
When something doesn’t happen there is usually a reason!
n
\
pe
-
or p
ne
+
v or
e
Read: M&S Chapters 2, 4, and 5.1, That something is a conservation law !
A conserved quantity is related to a symmetry in the Lagrangian that describes the interaction. ( “Noether’s Theorem” ) A symmetry is associated with a transformation that leaves the Lagrangian invariant.
time invariance leads to energy conservation translation invariance leads to linear momentum conservation rotational invariance leads to angular momentum conservation Familiar Conserved Quantities Quantity Strong EM Weak Comments energy sacred Y Y Y linear momentum ang. momentum electric charge Y Y Y Y Y Y Y Y sacred sacred Y
P780.02 Spring 2002 L4
Other Conserved Quantities
Richard Kass Quantity Baryon number Lepton number(s) top strangeness charm bottom Isospin Charge conjugation (C) Parity (P) CP or Time (T) CPT G Parity Strong EM Weak Comments Y Y Y no p + 0 Y Y Y no e Nobel 88, 94, 0?
Y Y Y Y
N N
discovered 1995 discovered 1947 Y Y Y Y Y Y
N
Y
N N N N
discovered 1974, Nobel 1976 discovered 1977 proton = neutron (m u m d ) particle anti-particle Nobel prize 1957 Y Y Y Y Y Y Y
N N
y/n Y
N
small sacred
No,
Nobel prize 1980 works for pions only Neutrino oscillations give first evidence of lepton # violation!
These experiments were designed to look for baryon # violation!!
Classic example of strangeness violation in a
decay
: p (S=-1 S=0) Very subtle example of CP violation: expect: K o long + 0 BUT K o long + ( 1 part in 10 3 )
P780.02 Spring 2002 L4
Some Reaction Examples
Richard Kass Problem 2.1 (M&S page 43): a) Consider the reaction v u +p + +n What force is involved here? Since neutrinos are involved, it must be WEAK interaction.
Is this reaction allowed or forbidden?
Consider quantities conserved by weak interaction: lepton #, baryon #, q, E,
p
,
L
, etc.
muon lepton number of v u =1, + =-1 (particle Vs. anti-particle) Reaction not allowed!
b) Consider the reaction v e +p e + + +p Must be weak interaction since neutrino is involved. conserves all weak interaction quantities Reaction is allowed c) Consider the reaction e + + + (anti-v e ) Must be weak interaction since neutrino is involved. conserves electron lepton #, but not baryon # (1 0) Reaction is not allowed d) Consider the reaction K + + 0 + (anti-v ) Must be weak interaction since neutrino is involved. conserves all weak interaction (e.g. muon lepton #) quantities Reaction is allowed
P780.02 Spring 2002 L4
More Reaction Examples
Let’s consider the following reactions to see if they are allowed: a) K + p + + b) K p n c) K 0 + Richard Kass
K
s u K
s u
First we should figure out which forces are involved in the reaction.
All three reactions involve only strongly interacting particles (no leptons)
K
0
s d
so it is natural to consider the strong interaction first. a) b) c) Not possible via strong interaction since strangeness is violated (1 -1) Ok via strong interaction (e.g. strangeness –1 -1) Not possible via strong interaction since strangeness is violated (1 0)
If a reaction is possible through the strong force then it will happen that way!
Next, consider if reactions a) and c) could occur through the electromagnetic interaction.
Since there are no photons involved in this reaction (initial or final state) we can neglect EM.
Also, EM conserves strangeness.
Next, consider if reactions a) and c) could occur through the weak interaction.
Here we must distinguish between interactions (collisions) as in a) and decays as in c).
The probability of an interaction (e.g. a) involving only baryons and mesons occurring through the weak interactions is so small that we neglect it.
Reaction c) is a decay. Many particles decay via the weak interaction through strangeness changing decays, so this can (and does) occur via the weak interaction.
To summarize: a) Not possible via weak interaction c) OK via weak interaction Don’t even bother to consider Gravity!
P780.02 Spring 2002 L4 Richard Kass
Conserved Quantities and Symmetries
Every conservation law corresponds to an invariance of the Hamiltonian (or Lagrangian) of the system under some transformation.
We call these invariances symmetries.
There are 2 types of transformations: continuous and discontinuous Continuous give additive conservation laws x x+dx or +d examples of conserved quantities: electric charge momentum baryon # Discontinuous give multiplicative conservation laws parity transformation: x, y, z (-x), (-y), (-z) charge conjugation: e e + examples of conserved quantities: parity (in strong and EM) charge conjugation (in strong and EM) parity
and
charge conjugation (strong, EM, almost always in weak)
P780.02 Spring 2002 L4 Richard Kass
Conserved Quantities and Symmetries
Example of classical mechanics and momentum conservation.
In general a system can be described by the following Hamiltonian : H=H(p i ,q i ,t) with p i =momentum coordinate, q i =space coordinate, t=time Consider the variation of H due to a translation q
dH
i
3 1
H q i dq i
i
3 1
H p i dp i
i
H t
only.
dt
For our example dp i =dt=0 so we have:
i
dH
We can rewrite dH as:
H
p i
dH i
3 1
H q i dq i
Using Hamilton’s canonical equations:
i
3 1
i H
H dq i
q i
i
3 1
with
i dq i
i
dp i dt q i
If H is invariant under a translation (dq) then by definition we must have: This can only be true if:
dH i
3 1
i i
3 1
H
q i
0
dq i
or
d dt i
3 1
i
3 1
i p i dq i
0 0 Thus each
p
component is constant in time and momentum is conserved.
P780.02 Spring 2002 L4 Richard Kass
Conserved Quantities and Quantum Mechanics
In quantum mechanics quantities whose operators commute with the Hamiltonian are conserved.
Recall: the expectation value of an operator Q is:
Q
*
Q
d x
with (
x
,
t
) and
Q
Q
(
x
,
x
,
t
) How does change with time?
d dt Q
d dt
*
Q
d x
*
t Q
d x
*
Q
t
d x
*
Q
t d x
Recall Schrodinger’s equation:
i
t
H
and
i
*
t
*
H
H + = H *T = hermitian conjugate of H Substituting the Schrodinger equation into the time derivative of Q gives:
d dt Q
d dt
*
Q
d x
1
i
*
H
Q
d x
*
Q
t
d x
1
i
*
QH
d x
Since H is hermitian ( H + = H ) we can rewrite the above as:
d dt Q
* (
Q
t
1
i
[
Q
,
H
] )
d x
So if Q/ t=0 and [Q,H]=0 then is conserved.
P780.02 Spring 2002 L4 Richard Kass
Conservation of electric charge and gauge invariance
Conservation of electric charge: S Q i = S Q f Evidence for conservation of electric charge: Consider reaction e v e which violates charge conservation but not lepton number or any other quantum number.
If the above transition occurs in nature then we should see x-rays from atomic transitions. The absence of such x-rays leads to the limit: t e > 2x10 22 years There is a connection between charge conservation, gauge invariance, and quantum field theory.
Recall Maxwell’s Equations are invariant under a gauge transformation: vector potential :
A
A
scalar potential : 1
c
t
A Lagrangian that is invariant under a transformation U=e i is said to be gauge invariant.
There are two types of gauge transformations: local: global: = (x,t) =constant, independent of (x,t) Maxwell’s EQs are locally gauge invariant Conservation of electric charge is the result of global gauge invariance Photon is massless due to local gauge invariance
P780.02 Spring 2002 L4 Richard Kass
Gauge invariance, Group Theory, and Stuff
Consider a transformation (U) that acts on a wavefunction ( y ): y U y Let U be a continuous transformation then U is of the form: If U=e i In the language of group theory is an operator.
is a hermitian operator ( = *T ) then U is a unitary transformation: U=e i U + =(e i ) *T = e -i *T = e -i UU + = e i e -i =1 Note: U is not a hermitian operator since U U + is said to be the generator of U There are 4 properties that define a group: 1) closure: if A and B are members of the group then so is AB 2) identity: for all members of the set I exists such that IA=A 3) Inverse: the set must contain an inverse for every element in the set AA -1 =I 4) Associativity: if A,B,C are members of the group then A(BC)=(AB)C If = ( 1 , 2 , 3 ,..) then the transformation is “Abelian” if: U( 1 )U( 2 ) = U( 2 )U( 1 ) i.e. the operators commute If the operators do not commute then the group is non-Abelian.
The transformation with only one forms the unitary abelian group U(1) The Pauli (spin) matrices generate the non-Abelian group SU(2)
x
0 1 1 0
y
0
i
0
i
z
1 0 0 1 S= “special”= unit determinant U=unitary n=dimension (e.g.2)
P780.02 Spring 2002 L4 Richard Kass
Global Gauge Invariance and Charge Conservation
The relativistic Lagrangian for a free electron is:
L
i
y
u
u
y
m
y y y is the electron field (a 4 component spinor)
c
1 This Lagrangian gives the Dirac equation:
i
u
y
mc
m is the electrons mass u = “gamma” matrices, four (u=0,1,2,3) 4x4 matrices that satisfy u = ( 0 , 1 , 2 , 3 )= ( / t, / x, / y, / z) u
u
v + v u y =2g uv 0 Let’s apply a global gauge transformation to L y
e i
y y y
e
i
L
i
y
u
u
y
m
y y
L
i
y
e
i
u
u e i
y
m
y
e
i
e i
y
L
i
y
e
i
e i
u
u
y
L
i
y
u
u
y
m
y y
m
y
e
i
e i
y
L
since λ is a constant By Noether’s Theorem there must be a conserved quantity associated with this symmetry!
P780.02 Spring 2002 L4 Richard Kass
Global Gauge Invariance and Charge Conservation
We need to find the quantity that is conserved by our symmetry.
In general if a Lagrangian density, L=L( , x u ) with a field, is invariant under a transformation we have:
L
0
L
L
x u
x u
Result from field theory For our global gauge transformation we have: ( 1
i
)
i
and
x u
x u
i
x u
Plugging this result into the equation above we get (after some algebra…)
L
0
L
L
x u
x u
L
x u
(
L
x u
)
x u
L
x u
The first term is zero by the Euler-Lagrange equation.
The second term gives us a continuity equation.
E-L equation in 1D
L x
d dt
(
L
x
) 0
P780.02 Spring 2002 L4 Richard Kass
Global Gauge Invariance and Charge Conservation
The continuity equation is:
x u
L
x u
x u
L
x u i
J u
x u
0 with
J u
i
L
x u
Result from quantum field theory Recall that in classical E&M the (charge/current) continuity equation has the form:
t
J
0 (J 0 , J 1 , J 2 , J 3 ) =( , J x , J y , J z ) Also, recall that the Schrodinger equation give a conserved (probability) current:
i
y
t
2 2
m
2 y
V
y
c
y * y and
J
i
c
[ y * y ( y * ) y ] If we use the Dirac Lagrangian in the above equation for L we find:
J u
y
u
y Conserved quantity This is just the relativistic electromagnetic current density for an electron.
The electric charge is just the zeroth component of the 4-vector:
Q
J
0
d x
Therefore, if there are no current sources or sinks (
J
=0) charge is conserved as:
J
x u u
0
J
t
0 0
P780.02 Spring 2002 L4 Richard Kass
Local Gauge Invariance and Physics
Some consequences of local gauge invariance: a) For QED local gauge invariance implies that the photon is massless.
b) In theories with local gauge invariance a conserved quantum number implies a long range field.
e.g. electric and magnetic field However, there are other quantum numbers that are similar to electric charge (e.g. lepton number, baryon number) that don’t seem to have a long range force associated with them!
Perhaps these are not exact symmetries! evidence for neutrino oscillation implies lepton number violation.
c) Theories with local gauge invariance can be renormalizable, i.e. can use perturbation theory to calculate decay rates, cross sections, etc.
Strong, Weak and EM theories are described by local gauge theories.
U(1) local gauge invariance first discussed by Weyl in 1919 SU(2) local gauge invariance discussed by Yang&Mills in 1954 (electro-weak) y y e e i t i ( t
x
( ,t)
x
,t) y t y t is represented by the 2x2 Pauli matrices (non-Abelian) SU(3) local gauge invariance used to describe strong interaction (QCD) in 1970’s is represented by the 3x3 matrices of SU(3) (non-Abelian)
P780.02 Spring 2002 L4
Local Gauge Invariance and QED
Richard Kass Consider the case of local gauge invariance, = (
x
,t) with transformation: y
e i
(
x
,
t
) y y y
e
i
(
x
,
t
The relativistic Lagrangian for a free electron is ) NOT invariant under this transformation.
L
i
y
u
u
y
m
y y
c
1 The derivative in the Lagrangian introduces an extra term: y
e i
(
x
,
t
)
u
y
e i
(
x
,
t
) y
u
[
i
(
x
,
t
)] We can MAKE a Lagrangian that is locally gauge invariant by adding an extra piece to the free electron Lagrangian that will cancel the derivative term.
We need to add a vector field A u which transforms under a gauge transformation as: A u A u + u (
x
,t) with (
x
,t)=-q (
x
,t) (for electron q=-|e|) The new, locally gauge invariant Lagrangian is:
u
1
uv L
i
y y
m
y y
F
q
y
u
16
F uv u
y
A u
P780.02 Spring 2002 L4 Richard Kass
The Locally Gauge Invariance QED Lagrangian
L
i
y
u
u
y
m
y y 1 16
F uv F uv
q
y
u
y
A u
Several important things to note about the above Lagrangian: 1) A u is the field associated with the photon.
2) The mass of the photon must be zero or else there would be a term in the Lagrangian of the form: m A u A u However, A u A u is not gauge invariant!
3) F uv = u A v v A u and represents the kinetic energy term of the photon.
4) The photon and electron interact via the last term in the Lagrangian. A u This is sometimes called a current interaction since:
q
y
u
y
A u
J u A u
In order to do QED calculations we apply perturbation theory e J u (via Feynman diagrams) to J u A u term.
5) The symmetry group involved here is unitary and has one parameter U(1) e -