parityKudryavtsev

8/12/2019 parityKudryavtsev

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The Development of Particle Physics

Dr. Vitaly KudryavtsevD36a, Tel.: 0114 2224531

[email protected]


2/15

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

Parity violation

t-q - puzzle; is the parity conserved? Experiment by Wu et al. with 60Co beta decay (1956).

Experimental set-up Measurements Results and outcomes

Parity violation in p + + e+ - decay (1956). Detector and pion/muon beam Results

Conclusions.


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t -q puzzle

Parity - a quantum number describing thesymmetry of the mirror reflection. The parityoperation reverses the sign of the spatialcoordinates of the wavefunction:

P (r, t )= (- r, t ). Parity is even if P =+ , parity is odd if P = - . For a state withorbital angular momentum l the parity is (-1) l .

q + decays into p + p 0 and has a parity (-1) J ifits spin is J : i.e. J P =0 + or 1- or 2+ ...

t +

has J P

=0-

or 2-

or ... according to Dalitzanalysis of t p + p + p - - decay. However their masses and lifetimes were known to be similar. Are they the same particle?

If yes, the parity is not conserved in weak interactions and decays. This means that the weakforce behaves differently in left-handed and right-handed coordinate systems: it candistinguish left from right, image from mirror image.


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Test of parity conservation

Lee and Yang analysed all available data and demonstrated that there is noevidence for or against parity conservation in weak interactions (unlike strongand electromagnetic interactions).

Test: to observe a dependence of a decay rate (or cross section) on a term thatchanges sign under the parity operation. If decay rate or cross section changesunder parity operation, then the parity is not conserved.

Parity reverses momenta and positions but not angular momenta (or spins). Spinis an axial vector and does not change sign under parity operation.

neutron

Pe

Pe

q

180 o-q Beta decay of neutron in a real and

mirror worlds:If parity is conserved, then the probabilityof electron emission at q is equal to thatat 180 o-q.Selected orientation of neutron spins -

polarisation.


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Wus experiment

Beta-decay of 60Co to 60 Ni *. The excited 60 Ni * decays to the ground state through two successive gemissions with g energies 1.173 and 1.332 MeV.

National Bureau of Standards (Ambler et al.) -nuclear polarisation through spin alignment in alarge magnetic field at 0.01 oK. At low temperaturethermal motion does not destroy the alignment.Polarisation was transferred from 60Co to 60 Ninuclei. Degree of polarisation was measuredthrough the anisotropy of gamma-rays.

Beta particles from 60Co decay were detected by athin anthracene crystal (scintillator) placed abovethe 60Co source. Scintillations were transmitted tothe photomultiplier tube (PMT) on top of thecryostat.


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Wus experiment

Photons were detected by two NaI crystals(scintillators). Difference in the countingrate ( g anisotropy) showed the degree of

polarisation. The time of experiment - several minutes

(before the set up warmed up and the polarisation disappeared).

Polarising magnetic field was applied in

both directions (up and down).


7/15Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

Wus results

Graphs: top and middle - gamma anisotropy(difference in counting rate between two NaIcrystals) - control of polarisation; bottom - b asymmetry - counting rate in the anthracene

crystal relative to the rate without polarisation(after the set up was warmed up) for twoorientations of magnetic field.

Similar behaviour of gamma anisotropy and betaasymmetry.

Rate was different for the two magnetic fieldorientations.

Asymmetry disappeared when the crystal waswarmed up (the magnetic field was still present):connection of beta asymmetry with spinorientation (not with magnetic field).



Parity violation in beta decay

Conclusion: clear indication of parity violation. Angular distribution of electron intensity:

where a =-1 for electrons and +1 for positrons. P - polarisation.Two terms: the first term (unity) is scalar (even parity, does not changesign under reflection), J is an axial vector and does not change the sign

either,P

e is the polar vector and change sign. So, the productJ P

e changes sign and is pseudoscalar (odd parity). The presence of bothterms implies a parity mixture.

Solution to the t-q - puzzle: they are the same particle K + , but parity isnot conserved in weak decays and K + decays in several different modes.

I (q ) 1 a

J Pe

E e 1 a

v

c cosq P

a

v

c



Outcomes

Previous result applied to neutrino (assuming m=0),implies that it should be fully polarised, P =-1 forneutrino and P =+ 1 for antineutrino, so it is in a purehelicity state P H 1.

Consider p + + e+ decay. Since neutrinos are left-handed P H -1 , muons should be also polarised(negative helicity on average, see figure for the piondecay in the pion rest frame) with polarisation P=-v/c(muons are non-relativistic, so both helicity states areallowed) .

If muons conserve polarisation when they come to rest,the electrons from muon decay should also be

polarised (see figure for muon decay at rest in themuon rest frame) and have an angular dependence:

n p

J n J

p n

e+ n

J e J nn

J n

J

e n e + n

I (q ) 1 -a

3cosq



Parity violation in p + + e+ decay

Experiment by Garwin, Lederman, Weinrichaimed to confirm parity violation through themeasurements of I( q ) for positrons.

85 MeV pion beam ( p + ) from cyclotron. 10% of muons in the beam: need to be

separated from pions. Pions were stopped in the carbon absorber

(20 cm thick) Counters 1-2 were used to separate muons Muons were stopped in the carbon target

below counter 2. The arangement is optimised to have

maximum number of muons stopped in thecarbon target.



Parity violation in p + + e+ decay

Positrons from muon decay were detected by a telescope 3-4, which required particlesof range >8 g/cm 2 (25 MeV positrons).

Events: concidence between counters 1-2

(muon) plus coincidence between counters3-4 (positron) delayed by 0.75-2.0 s. Goal: to measure I( q ) for positrons. Conventional way: move detecting system

(telescope 3-4) around carbon targetmeasuring intensities at various q . But verycomplicated.

More sophisticated method: precession ofmuon spin in magnetic field. Verticalmagnetic field in a shielded box around thetarget.

The intensity distribution in angle wascarried around with the muon spin.



Results of the experiment by Garwin et al.

Changing the field (the magnetisingcurrent), they could change the rate(frequency) of the spin precession,which will be reflected in the

angular distribution of the emitted positrons. Garwin et al. plotted the positron

rate as a function of magnetisingcurrent (magnetic field) andcompared it to the expecteddistribution:

The agreement proved the initialassumption about parity violation.

I (q ) 1 -a 3

cosq



Other results and systematic tests

If CP (charge-parity) is conserved, then the violation of parity results in the violationof the invariance under charge conjugation.

The rate of precession is a function of the ratio of magnetic moment to spin. This

ratio was measured as 2.0 0.1. Reduction of the thickness of carbon shield - pions were stopped in the target,

muons were emitted isotropically by pions at rest, no variation in counting rate withmagnetising current.

Shifting telescope 3-4 to 65 o with respect to the incident muon direction (initialangle was 100 o) - similar curve but shifted to the right by a value corresponding to a

precession angle of 37 o, in agreement with the spatial rotation of the counter system. The results were confirmed by Friedman and Telegdi, who measured positron

asymmetry from p + + e+ decay in nuclear emulsions.



Conclusions

Parity is not conserved in weak interactions. Invariance under charge conjugation is violated. CP was still considered to be a good symmetry. Neutrinos were found to be left-handed (negative helicity H =-1),

while antineutrinos were right-handed ( H =+1). This was confirmedin an experiment by Goldhaber et al. with electron capture reaction:e- + 152 Eu 152 Sm * + n.

Substantial progress in the theory of weak interactions: V-A theory(more in a few weeks).


15/15Dr Vitaly Kudryavtsev The Development of Particle Physics L t 6

References

C. S. Wu et al. "Experimental test of parity conservation in betadecay", Phys. Rev., 105 (1957) 1413.

R. L. Garwin et al. "Observation of the failure of conservation of

parity and charge conjugation in meson decays: the magnetic momentof the free muon", Phys. Rev., 105 (1957) 1415.

J. I. Friedman and V. L. Telegdi. "Nuclear emulsion evidence for parity non-conservation in the decay chain p - - e+ ", Phys. Rev., 106 (1957) 1290.

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