P461 - decays II 1

Parity Violation in Beta Decays

The Parity operator is the mirror image and is NOTconserved in Weak decays (is conserved in EM andstrong)

non-conservation is on the lepton side, not thenuclear wave function side

spin 1/2 electrons and neutrinos are (nominally)either right-handed (spin and momentum in samedirection) or left-handed (opposite)

Parity changes LH to RH

),,(),,(),,(),,(

pfqpfq +----

rrPzyxzyxP

RH

LHLprLP

ppPrrrr

rr

==

-=

)(

)(

P461 - decays II 2

Handedness of Neutrinos

handedness is call chirality. If the mass of aneutrino = 0 then:

all neutrinos are left-handedall antineutrinos are right-handed

Parity is maximally violated As the mass of an electron is > 0 can have both LH

and RH. But RH is suppressed for large energy (aselectron speed approaches c)

fraction RH vs LH can be determined by solvingthe Dirac equation which naturally incorporatesspin

P461 - decays II 3

Polarized Beta Decays

Some nuclei have non-zero spin and can bepolarized by placing in a magnetic field

magnetic moments of nuclei are small (1/M factor)and so need low temperature to have a highpolarization (see Eq 144 and 14-5)

Gamow-Teller transition with S(e-nu) = 1 if Co polarized, look at angular distribution of

electrons. Find preferential hemisphere (down)

21

21

6060

,45 ===++ -

siieNiCo n

Co

Pnu

pe

Spin antinu-RH

Spin e - LH

P461 - decays II 4

Discovery of Parity Violation inBeta Decay by C.S. Wu et al.

Test parity conservation by observing adependence of a decay rate (or cross section) ona term that changes sign under the parityoperation. If decay rate or cross section changesunder parity operation, then the parity is notconserved.

Parity reverses momenta and positions but notangular momenta (or spins). Spin is an axialvector and does not change sign under parityoperation.

neutron

Pe

Pe

q

180o-q

mirror

Beta decay of a neutron in areal andmirror worlds:If parity is conserved, then theprobability of electronemission at q is equal to that at180o-q.Selected orientation of neutronspins - polarisation.

P461 - decays II 5

Wus experiment Beta-decay of 60Co to 60Ni*. The

excited 60Ni* decays to theground state through twosuccessive g emissions.

Nuclei polarised through spinalignment in a large magneticfield at 0.01oK. At lowtemperature thermal motion doesnot destroy the alignment.Polarisation was transferred from60Co to 60Ni nuclei. Degree ofpolarisation was measuredthrough the anisotropy ofgamma-rays.

Beta particles from 60Co decaywere detected by a thinanthracene crystal (scintillator)placed above the 60Co source.Scintillations were transmitted tothe photomultiplier tube (PMT)on top of the cryostat.

P461 - decays II 6

Wus results Graphs: top and middle - gamma

anisotropy (difference in countingrate between two NaI crystals) -control of polarisation; bottom - basymmetry - counting rate in theanthracene crystal relative to therate without polarisation (after theset up was warmed up) for twoorientations of magnetic field.

Similar behaviour of gammaanisotropy and beta asymmetry.

Rate was different for the twomagnetic field orientations.

Asymmetry disappeared when thecrystal was warmed up (themagnetic field was still present):connection of beta asymmetry withspin orientation (not with magneticfield).

Beta asymmetry - Parity notconserved