FYST17 Lecture 2 Symmetries and CP violation Thanks to A Hocker and M. Bona 1 Today’s topics • Symmetries – Broken symmetries • Neutral kaon mixing • CP violation – Matter / anti-matter asymmetry • The CKM matrix 2 3 Continuous Symmetries and Conservation Laws In classical mechanics we have learned that to each continuous symmetry transformation, which leaves the scalar Lagrange density invariant, can be attributed a conservation law and a constant of movement (E. Noether, 1915) Continuous symmetry transformations lead to additive conservation laws No evidence for violation of these symmetries seen so far 4 Continuous Symmetries and Conservation Laws In general, if U is a symmetry of the Hamiltonian H, one has: H, U 0 H U† HU fHi UfHUi fUHUi† fHi Accordingly, the Standard Model Lagrangian satisfies local gauge symmetries (the physics must not depend on local (and global) phases that cannot be observed): Conserved additive quantum numbers: Electric charge (processes can move charge between quantum fields, but the sum of all charges is constant) Similar: color charge of quarks and gluons, and the weak charge Quark (baryon) and lepton numbers (however, no theory for these, therefore believed to be only approximate asymmetries) → evidence for lepton flavor violation in “neutrino oscillation” 5 Discrete Symmetries Discrete symmetry transformations lead to multiplicative conservation laws The following discrete transformations are fundamental in particle physics: Parity P (“handedness”): In particle physics: Reflection of space around an arbitrary center; P e e P invariance → cannot know whether we live in this world, or in its mirror world LR P 00 Particle-antiparticle transformation C : P n n Change of all additive quantum numbers (for example the C e e electrical charge) in its opposite (“charge conjugation”) LL C u u Time reversal T : C d d The time arrow is reversed in the equations; C 00 T invariance → if a movement is allowed by a the physics law, the movement in the opposite direction is also allowed Time reversal symmetry (invariance under change of time direction) does certainly not correspond to our daily experience. The macroscopic violation of T symmetry follows from maximising thermodynamic entropy (leaving a parking spot has a larger solution space than entering it). In the microscopic world of single particle reactions thermodynamic effects can be neglected, and T invariance is realised. 6 Discrete Symmetries Discrete symmetry transformations lead to multiplicative conservation laws The following discrete transformations are fundamental in particle physics: Parity P (“handedness”): In particle physics: Reflection of space around an arbitrary center; P e e TheseP invariance arecannot interestingknow whether we live inbecause this world, or in itsit mirroris notworld obvious whetherLR 00 the laws of nature should look the same for anyP ofthese Particletransformations,-antiparticle transformationand the Canswer: was surprisingP nwhen n Change of all additive quantum numbers (for example the C e e electricalthese symmetriescharge) in its oppositewere (“chargefirst conjugation”)tested ! LL C u u Time reversal T : C d d The time arrow is reversed in the equations; C 00 T invariance → if a movement is allowed by a the physics law, the movement in the opposite direction is also allowed Time reversal symmetry (invariance under change of time direction) does certainly not correspond to our daily experience. The macroscopic violation of T symmetry follows from maximising thermodynamic entropy (leaving a parking spot has a larger solution space than entering it). In the microscopic world of single particle reactions thermodynamic effects can be neglected, and T invariance is realised. 7 C, P, T Transformations and the CPT Theorem The CPT theorem (1954): “Any Lorentz-invariant local quantum field theory is invariant under the successive application of C, P and T ” proofs: G. Lüders, W. Pauli; J. Schwinger Fundamental consequences: Relation between spin and statistics: fields with integer spin (“bosons”) commute and fields with half-numbered spin (“fermions”) anticommute → Pauli exclusion principle Particles and antiparticles have equal mass and lifetime, equal magnetic moments with opposite sign, and opposite quantum numbers m m/ m 1018 Best experimental test: KKK0 0 0 8 EM and strong interactions are (so far) C, P, and T invariant Example: 0 but not 0 1 휋0 = 푢푢ത − 푑푑ҧ 퐶|휋0ۧ = +|휋0ۧ 2 퐿=0,푆=0 퐶 ∙ 퐵ത, 퐸ത = −퐵ത, 퐸ത ⇒ 퐶|훾ۧ = −|훾ۧ Thus initial and final states are C even, C is conserved In general: 푃|푞푞തۧ = (−1)퐿+1|푞푞തۧ, 퐶|푞푞തۧ = (−1)퐿+푆|푞푞തۧ Experimental tests of P and C invariance of the EM interaction: C invariance: BR(0 3) < 3.1 x 10-8 P invariance: BR(40) < 6.9 x 10-7 Experimental tests of C invariance of strong interaction: Compare rates of positive and negative particles, like 푝푝ҧ → 휋+휋−X, 퐾+퐾−푋, … And … the Surprise in Weak Interaction ! T.D. Lee and C.N. Yang pointed out in 1956 (to explain the observation of the decays K → 2π and 3π - the cosmic-ray θ/τ puzzle) that P invariance had not been tested in weak interaction → C.S. Wu performed in 1957 the experiment they suggested and observed parity violation Angular distribution of electron intensity: P v I( ) 1 e 1 cos Ece helicity where: - spin vector of electron Pe - electron momentum Ee - electron energy 1 for electron 1 for positron TCO ~ 0.01 K polarized in It was found that parity is even maximally magnetic field violated in weak interactions ! 10 Full details in chapter 10 11 Neutral Kaon Mixing An initially pure K0 state, will evolve into a superposition of states: K()()() t g t K00 h t K The time dependence is obtained by solving the time-dependent Schrödinger equation: K00()() t K t with 2x2 matrices M, Γ, of which the off-diagonals d proportional to Δm, ΔΓ govern the mixing i M i 00 2 dt K()() t K t ITI( )/ (0) The respective time-dependent intensities are found to be (neglecting CP violation): K 0 I t eLLtt 2 e /2 cos m t K 0 0 tt /2 I t eLL 2 e cos m t KL K 0 K 0 After several KS lifetimes, only KL are left Tt /S 14 Neutral Kaon Mixing and CP Violation Since KS and KL are not CP eigenstates, the time dependence has to be slightly modified by the size of , giving rise to an additional term. KK00 Neglecting other sources of CP Asymmetry:Let’s get backA to the + – decay rates: cos m t violation & assuming arg(ε) = π/4. KK00 CPLEAR 1999 + – dominated by KS K 0 N(K + –) ~ N(K + –) amplitude |ε| LargeS interferenceL with opposite sign + – dominated by KL K 0 15 There are in Fact Four Meson Systems with Mixing Pairs of self-conjugate mesons that can be transformed to each other via flavor changing weak interaction transitions are: 0 0 0 0 K sd D cu Bd bd Bs bs They have very different oscillation properties that can be understood from the “CKM couplings” (see later in this lecture) occurring in the box diagrams for the plot x = 15 NTN()/ 0 00 xsD = 0.02 BBss ysD== 0.100 00 Ds == 00 0000 BBKKDDss00 BBdd mixing probability: ~~= 250%18%x10–6 0 KL 00 BB 00 ddKK T 16 CP violation CP From Schrödinger eqn: KS KL 3 types of CP violation: violation in mixing Parameter Prob(KKKK0 0 ) Prob( 0 0 ) “indirect” CP violation violation in interference Prob(K00 ( t ) ) Prob( K ( t ) ) violation in decays “direct” Prob(K f ) Prob( K f ) CP violation Parameter ’ 17 The Discovery of CP Violation in the Decay Due to the smallness of the effect, it took several experiments and over 30 years of effort to establish the existence of direct CPV Feynman graphs: u Experimental average Indeed, a very small CPV effect ! W d s u 0 K d d “Tree” (born-level) amplitudes s u 0 0 u K d d 0 d (16.7 ± 2.3)x10–4 Interference W t,, c u d “Penguin” s (loop-level) K 0 g amplitude d 20 Anti-particles Combining quantum mechanics with special relativity, and the wish to linearize /t, leads Dirac to the equation (1928) i x, t m x , t 0 for which solutions with negative energy appear Dirac, imagining holes and seas in 1928 Vacuum represents a “sea” of such negative-energy Energy particles (fully filled according to Pauli’s principle) E Dirac identified holes in this sea as “antiparticles” with m e opposite charge to particles … (however, he conjectured 0 that these holes were protons, despite their large difference in mass, because he thought “positrons” would have been discovered already) m e E An electron with energy E can fill this hole, emitting an s 1/ 2 s 1/ 2 energy 2E and leaving the vacuum (hence, the hole This picture fails for bosons ! has effectively the charge +e and positive energy). 21 Particles and Antiparticles Annihilate What happens if we bring particles and antiparticles together ? A particle can annihilate with its antiparticle to form gamma rays An example whereby matter is converted into pure energy by Einstein’s formula E = mc2 Conversely, gamma rays with sufficiently high energy can turn into a particle-antiparticle pair Particle-antiparticle tracks in a ALEPH bubble chamber Higgs candidate Particles and Antiparticles Annihilate What happens if we bring particles and antiparticles together ? A particle can annihilate with its antiparticle to form gamma rays An example whereby matter is converted into pure energy by Einstein’s formula E = mc2 Conversely, gamma rays with sufficiently high energy can turn ALEPHinto a particle -antiparticle pair Higgs candidate Particle-antiparticle tracks in a e e ZH() Z qqbb bubble chamber So the Standard Model can handle both particles