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 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
†
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