The Development of Particle Physics

Dr. Vitaly Kudryavtsev

D36a, Tel.: 0114 2224531

v.kudryavtsev@sheffield.ac.uk

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 + + e+ - decay (1956).– Detector and pion/muon beam– Results

• Conclusions.

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

- puzzle

• Parity - a quantum number describing the symmetry of the mirror reflection. The parity operation reverses the sign of the spatial coordinates of the wavefunction:

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

• + decays into + 0 and has a parity (-1)J if its spin is J: i.e. JP=0+ or 1- or 2+ ...

• + has JP=0- or 2- or ... according to Dalitz analysis of + + -

- 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 weak force behaves differently in left-handed and right-handed coordinate systems: it can distinguish left from right, image from mirror image.

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

Test of parity conservation

• Lee and Yang analysed all available data and demonstrated that there is no evidence for or against parity conservation in weak interactions (unlike strong and electromagnetic interactions).

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

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

neutron

Pe

Pe

q

180o-q

mirror

Beta decay of neutron in a real andmirror worlds:If parity is conserved, then the probability of electron emission at q is equal to that at 180o-q.Selected orientation of neutron spins - polarisation.

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

Wu’s experiment

• Beta-decay of 60Co to 60Ni*. The excited 60Ni* 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 a large magnetic field at 0.01oK. At low temperature thermal motion does not destroy the alignment. Polarisation was transferred from 60Co to 60Ni nuclei. Degree of polarisation was measured through the anisotropy of gamma-rays.

• Beta particles from 60Co decay were detected by a thin anthracene crystal (scintillator) placed above the 60Co source. Scintillations were transmitted to the photomultiplier tube (PMT) on top of the cryostat.

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

Wu’s experiment

• Photons were detected by two NaI crystals (scintillators). Difference in the counting rate ( 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).

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

Wu’s results

• Graphs: top and middle - gamma anisotropy (difference in counting rate between two NaI crystals) - 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 two orientations of magnetic field.

• Similar behaviour of gamma anisotropy and beta asymmetry.

• Rate was different for the two magnetic field orientations.

• Asymmetry disappeared when the crystal was warmed up (the magnetic field was still present): connection of beta asymmetry with spin orientation (not with magnetic field).

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

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 change sign under reflection), J is an axial vector and does not change the sign either, Pe is the polar vector and change sign. So, the product J Ј Pe changes sign and is pseudoscalar (odd parity). The presence of both terms implies a parity mixture.

• Solution to the - t q - puzzle: they are the same particle K+, but parity is not conserved in weak decays and K+ decays in several different modes.

I()1 J

P e

Ee

1v

ccos

P v

c

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

Outcomes

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

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

• 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 the muon rest frame) and have an angular dependence:

n m+

p+

Jn Jm

p+ Ѓ m+ + nm

e+nm+

Je Jnn

Jn

Jm

m+ Ѓ e+ + ne + nm

I()13

cos

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

Parity violation in + + e+ decay

• Experiment by Garwin, Lederman, Weinrich aimed to confirm parity violation through the measurements of I(q) for positrons.

• 85 MeV pion beam (+ ) 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 the carbon target.

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

Parity violation in + + e+ decay

• Positrons from muon decay were detected by a telescope 3-4, which required particles of range >8 g/cm2 (25 MeV positrons).

• Events: concidence between counters 1-2 (muon) plus coincidence between counters 3-4 (positron) delayed by 0.75-2.0 ms.

• Goal: to measure I(q) for positrons.• Conventional way: move detecting system

(telescope 3-4) around carbon target measuring intensities at various q. But very complicated.

• More sophisticated method: precession of muon spin in magnetic field. Vertical magnetic field in a shielded box around the target.

• The intensity distribution in angle was carried around with the muon spin.

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

Results of the experiment by Garwin et al.

• Changing the field (the magnetising current), 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 magnetising current (magnetic field) and compared it to the expected distribution:

The agreement proved the initial assumption about parity violation.

I()13

cos

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

Other results and systematic tests

• If CP (charge-parity) is conserved, then the violation of parity results in the violation of 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 with magnetising current.

• Shifting telescope 3-4 to 65o with respect to the incident muon direction (initial angle was 100o) - similar curve but shifted to the right by a value corresponding to a precession angle of 37o, in agreement with the spatial rotation of the counter system.

• The results were confirmed by Friedman and Telegdi, who measured positron asymmetry from + + e+ decay in nuclear emulsions.

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

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 confirmed in an experiment by Goldhaber et al. with electron capture reaction:

e- + 152Eu Ѓ 152Sm* + n.

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

Dr. Vitaly Kudryavtsev The Development of Particle Physics Lecture 6

References

• C. S. Wu et al. "Experimental test of parity conservation in beta decay", 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 moment of 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+ - m+ - e+ ", Phys. Rev.,106 (1957) 1290.