4 Introduction Parity Parity invariance The mirror image of any physical process also representsa perfectly possible physical process.Physicist held the mirror symmetry of the laws of nature to be self-evident.But in 1956 , Lee and Yang discovered that although therewas ample evidence for parity invariance in strong andelectromagnetic processes , there was no confirmation in thecase of weak interactions.
5 IntroductionParityLee and Yang proposed a test , which was carried out laterthat year by Wu .In this famous experiment radioactive cobalt 60 nuclei werecarefully aligned , so that their spins pointed in same direction.Cobalt 60 undergoes beta decay andWu recorded the direction of the emitted electrons.What she found was that most of them came out in the “southerly” direction , opposite to the nuclear spin.
6 IntroductionParityParity is not an invariance of weak interactions.If it were, the electrons in Wu’s experiment would have tocome out in equal numbers , ‛north’ and ‛south’.P
7 Introduction Parity Helicity : h = +1 right-handed h = -1 left-handed For massive particle , helicity is not Lorentz-invariant.For massless particle , helicity is Lorentz-invariant.By experiment ,photons have right-handed and left-handed ;neutrinos are left-handed ;antineutrinos are right-handed.
9 Parity is a multiplicative quantum number IntroductionParityAccording to QFT , the parity of fermion must opposite tothat of the corresponding antiparticle , while the parity of aboson is the same as its antiparticle.We take the quarks to have positive intrinsic parity , so theantiquarks are negative.Parity is a multiplicative quantum number
10 Introduction Parity Tau-theta puzzle Two strange mesons , called at the time θ and τ , appearedto be identical in every respect - same mass, same spin (0) ,same charge , and so on – expect that one of them decayedinto two pions and the other into three pions .Lee and Yang suggested that θ and τ are really same particle( now known as the ) , and parity is simply not conservedin one of the decays.
11 Introduction Charge Conjugation Classical electrodynamics is invariant under change in thesign of all electric charges , we introduce an operation thatgeneralizes this notion of changing the sign of the charge.It is called charge conjugation (C) , and it converts eachparticle into its antiparticle.Charge conjugation changes the sign of all the “ internal "quantum numbers (charge , baryon number , lepton number,strangeness , charm , ······ ) while leaving mass , momentumand spin untouched.
14 Introduction Charge Conjugation Charge conjugation is a " multiplicative " quantum number ,it is conserved in the strong and electromagnetic interactions.For example , the pion decays into two photons but never decays into three photons.
26 Introduction CP violation In 1964 , Cronin and Fitch was reported an experiment. Atthe end of a beam 57 feet long , they counted 45 two pionevents in a total of decays . That’s a tiny fraction , butunmistakable evidence of CP violation.
28 Introduction CP violation Parity is maximally violated in the weak interaction . Bycontrast , CP violation is a small effect by any measure .Within Standard Model , it can be accommodated byincluding an empirical phase factor (δ) in CKM matrix ,provided that there are three generations of quarks.
29 Introduction CP violation 32% of all KH’s decay by 3π mode we have discussed ,41% go toCP takes (a) into (b) , so if CP were conserved , and KLwere a pure eigenstate , (a) and (b) would be equallyprobable. But experiment show that KH decays more ofteninto positron than into an electron.CP violation is a necessary condition for matter-antimatter asymmetry!
30 Introduction CP violation In 1981, Carter and Sanda pointed out that the violationshould also occur with the neutral B mesons.A B-Factory is a machine created by particle physicists to produce large numbers of B mesons. Currently there are two B-Factories running: BaBar at SLAC in California, and Belle at KEK in Japan respectively.
31 Introduction CP violation By 2001 , their detector had recorded evidence of CP violation in neutral B decays.CP violation of B meson decays is larger effct than K meson decays.
32 Introduction ? Is T a good symmetry ? CPT theorem T-reversal : Strong and Electromagnetic interaction→ T invariance .Weak interaction → expect not T invariance ,but experiments are tough to do.Is T a good symmetry ?
33 Introduction CPT theorem CPT theorem states that the combined operations of timereversal (T) , charge conjugation (C) , and parity (P) is anexactly symmetry of any interaction .Here are two reason to support this theorem :All observations indicate that CPT is indeed a symmetry of nature.It is impossible to construct a Lorentz-invariant QFT with a HermitianHamitonian that violates CPT .
34 ? Introduction CPT theorem MINOS ( Main Injector Neutrino Oscillation Search)νμ Disappearance –Oscillation Parameters Contour New at Neutrino 2010?