lepton physics & weak interactions
TRANSCRIPT
Lepton physics &weak interactions
The neutrino hypothesis
†
ZA P Æ Z +1
A D + b- + n
Given that you can only measure
†
ZA P Æ Z +1
A D + b-
What physics observations/measurementsprompted the addition of the neutrino?
How was the neutrino hypothesis confirmed?
The neutrino hypothesis
ElectronicsMany PMTs
Liquid scintillatorFissionreactor
Shielding
The neutrino hypothesisNeutrino source(Fission reactor)
neutron rich nuclides
Hydrocarbon scintillator“target”†
ZA P Æ Z +1
A D + b- + n
†
n + p Æ n + e+
†
e+ + e- Æ g + g
†
n + 113Cd Æ 114Cd + g Eg = 9.1 MeV
Light - PMTsCoincidence expt
(a)Æ [10ms] Æ (b)Reines & Cowan (Nobel Prize)
(a)(b)
The neutrino hypothesis
†
ZA P Æ Z +1
A D + b- + n
†
n +n Æ p + e-
†
n + p Æ n + e+
†
ZA P Æ Z -1
A D + b+ + n
†
n + p Æ n + e+
†
n +n Æ p + e-
The two neutrino hypothesis
†
ZA P Æ Z +1
A D + b- + n
†
n +n Æ p + e-
†
n + p Æ n + e+
†
ZA P Æ Z -1
A D + b+ + n
†
n + p Æ n + e+
†
n +n Æ p + e-
Neutrino helicity
†
h ≡r s • r p r s • r p
; h ≡r s r p ˆ s • ˆ p ( )
r s • r p
†
r s
†
r s
†
r p
†
r p Right-handed particles
Left-handed particles
†
h = -1 for all nh = +1 for all n
(Not arbitrary assignments!
Neutrino helicityEu-152
Sm-152*
Sm-152
e
E1 g
†
152 Eu + e- Æ 152Sm* + n
†
152 Sm + g
0
1
0
†
152 Eu
†
152 Sm*
†
n
Neutrino helicity
†
152 Eu
†
152 Sm*
†
n Z
ISm
ISm
se
se
sn
sn
Neutrino helicity
†
152 Eu
†
152 Sm*
†
n Z
†
152 Eu
†
152 Sm*
†
n Zg
†
152 Sm
†
DI =1
Method:Measure the direction of thecircular polarization of theoutgoing photons by anasymmetry experiment using NaI
Result:The neutrino emitted
here hasnegative helicity!
Lepton helicity
†
r s
†
r p
†
r s
†
r p
†
h = -vc
h = +vc
For electrons (e-)
For positrons (e+)
Not fundamental properties of e- (e+); property ofweak interaction - directly related to helicity of n
Lepton Lepton familynumber
Lepton Lepton familynumber
e- +1 ne +1e+ -1 n e -1m- +1 nm +1m+ -1 n m -1t- +1 nt +1t+ -1 n t -1
Lepton families
Arbitrary number assignmentto conserve leptons - by family!
Each lepton has its own anti-lepton
Lepton Lepton familynumber
Lepton Lepton familynumber
e- +1 ne +1e+ -1 n e -1m- +1 nm +1m+ -1 n m -1t- +1 nt +1t+ -1 n t -1
Lepton families
†
n Æ p + p - Æ m- +
ne + p Æ m+ Æ e+ +
nm + p Æ
†
m+ Æ e+ + g ??
MEGA
Example: m+ decay
†
m+
†
ne
†
e+
†
n m
m+ at rest
Note helicitiesof neutrinos
Note: helicity of e+ dueto neutrino helicities
Example: m- decay
†
m-
†
n e
†
e-
†
nm
m- at rest
Note helicitiesof neutrinos
Note: helicity of e- dueto neutrino helicities
Example: m+ decaym+ at rest
Maximummomentum for e+
†
m+
†
ne
†
e+
†
n m
†
m+
†
ne
†
e+
†
n mMinimum
momentum for e+
†
ve+ ª 0