Part I Phenomenology of Elementary 36 12/15/2008 transformation modulo a simple rotation. invariance requires all situations to be equally likely to occur, e.g. right-handed and left-handed electrons should be equally likely, and so should right-handed anti- and left-handed anti-neutrinos. Experimental evidence is quite different: situation A is found very much less frequently than situation B! All is consistent with the statement that “Parity is maximally broken” in weak interactions, i.e. situation A is completely suppressed in the limit θ→0. Another way of viewing this is the statement that weak interactions involve only left-handed (and right-handed anti-fermions) in the limit that the (rest- )masses are small compared to their energies. The form of the angular distribution that is predicted for the weak decay by the is parity non-conserving and has the form: pJ⋅ ^ e 60Co I()θ ∝ 1−=−⋅ 1 βe J (I.66) E J e 60Co

even parity odd parity term term where both terms have equal maximum magnitudes (|βe|≈1 for high-energy electrons); I(θ) has no definite parity. Many more, and more so- phisticated experiments in atomic, nuclear, and physics have since confirmed the violation of parity invariance by the weak interac- tion. The θ–τ puzzle can now be explained by the finding that the weak decay of the (~495 MeV) does not conserve parity and can result in final states of either two or three .

3.5 The Charge Conjugation Operator The transformation of “Charge Conjugation” is somewhat of a misnomer: the transformation C is defined as a full switch in sign of all dis- crete additive quantum numbers of particles: number, number, , , , etc., as well as Charge (but NOT the space-time properties like mass, momentum, ; and NOT parity which is a multiplicative q.n.). In this way, C transforms a par- ticle into its anti-particle: e.g. C|p〉 =⎯|p〉, C|π−〉 = |π+〉, C|e−〉 = |e+〉, etc. Particles and have exactly the same masses. Note that only the neutral objects (in the general sense of having zero charge, isospin, baryon-number, etc.), i.e. (⎯qq states) like π0, ρ0, ϕ, ω, J/ψ, or particle- systems like e+e− or ⎯nn are actually eigenstates of C. For example, for charged objects C and Q do not commute: CQqqqqq==− C , and QC q =−=−−⇒= Q q qq,[,]2CQ CQ (I.67) The eigenvalue of C for the neutral is easily derived using the fact that it decays into two . A is represented by the vec- tor field A, which is generated by a circulating current of electrons. The C operation transforms the charge carriers into their antiparticles, and therefore, under C the current changes direction and so must A: C|γ〉 = −|γ〉. Therefore C|π0〉 = C|γγ〉 = (−1)2|γγ〉 = +|π0〉. In analogy with the neutral pion, one defines C|π+〉 = +|π−〉: the pion has JPC=0−+. One can turn this fact around, and search for π0→γγγ decays whose existence would signal the violation of Charge Conjugation symmetry in the electromagnetic interaction; experimentally one finds: Γ(π0→γγγ)/Γ(π0→γγ)<3.8×10-7 (90%CL). Part I Phenomenology of Elementary Particles 37 12/15/2008

For charged particles, an extended operator, the so-called G-parity operator, has been defined, which combines an I3 flip (e.g. a θ=180° rota- iπI ½iπσ π π ⎛⎞01 tion around the 2-axis in isospin space: e 2 = e 2 = 1cos /2 + iσ2sin /2 = ⎜⎟) of the with C (see also (I.47)): ⎝⎠−10

− ⎛⎞01 − GG||π 〉= −du 〉= C⎜⎟ |()()|()()|||du−〉=C() ud 〉= ud 〉=−−〉=− ud π 〉, (I.68) ⎝⎠−10 so that also charged (but not strange, nor charmed, etc.) mesons are eigenstates of this new operator. In general: G=C(−1)I. The G-operator is useful in predicting whether an even or an odd number of pions is produced in a : e.g. it codifies the fact that the ± ± 0 ± ± 0 0 1 ρ →π π but not: ρ →π π π ; the G-parity of the ρ-meson (Cρ=−1) is Gρ=(−)(−1) = +1; thus the ρ will only decay in even numbers of pi- 0 ons. In contrast the I=0 omega-meson ω (783 MeV, Cω=−1) has Gω=(−)(−1) = −1, and the ω must therefore decay in odd numbers of pions.

3.5.1 The CP Operation J60 J60 The Charge-Conjugation, as well as Parity quantum numbers are con- Co pe⇑ Co pe+⇑ served in strong and electromagnetic interactions (with the caveat about only the neutral states being eigenstates of C; which can be amended θ θ by using G, see below, instead of C for charged objects). It is clear that in addition to P, also C is violated in the weak interac- π−θ C π−θ tion: applying the C operation to the 60Co decay: because situation A is NOT observed (i.e. the electrons are emitted preferentially in the direc- A B tion opposite to the 60Co spin), the does NOT couple with left-handed antineutrinos, but with right-handed antineutrinos. + pν⇑ Similarly, in β decays it is found that only left-handed neutrinos par- p⎯ν ⇑ ticipate in the weak interaction, as are present in situation B: situations A and B have very different probabilities, contrary to the expectation of C invariance. After the bad shock of parity violation in the weak interaction, it was hypothesized that at least the combined operation CP might be a sym- metry of all interactions, including the weak interaction. That means that I(θ) and⎯I(π – θ) would be equal, where⎯I is the distribution of in the decay of anti-60Co. However, it was soon found that, on a small scale, weak interactions also broke this symmetry. Evidence for that violation was first found in the neutral Kaon system.

3.5.2 Neutral Kaons are produced in large quantities in strong interactions. The prime example is the production of the K0 in π− + p → K0(S=+1,B=0) + Λ0(S=−1,B=1). As is easy to see, this strong interaction conserves baryon number and strangeness. In fact, the⎯K0 is not so easy to produce: as the K0, because the additive quantum number strangeness changes sign under C, the⎯K0 has S=+1, and it cannot be produced together with a Λ0. (In fact the easiest way to produce a⎯K0 in a πp interaction is in: π+ + p →⎯K0(S=−1,B=0) + K+(S=+1,B=0) + p which requires more energy, and has an inherently smaller production rate.) Both the K0 and the⎯K0 decay frequently into two pions (π+π− and π0π0) with a lifetime typical for the weak interaction. Note that the states π+π− and π0π0 are actually eigenstates of CP, but not the K0 and⎯K0: CP|K0〉 = Part I Phenomenology of Elementary Particles 38 12/15/2008

−⎯|K0〉, and CP⎯|K0〉 = −|K0〉. The minus sign comes from the parity operation: there is arbitrariness in the parity assignment of the Kaon; nor- mally one defines the parity of the Kaon to be odd, identical to the parity of the pion (they are members of the same spin-0 multiplet), but apart from some sign changes, the discussion below stays valid if the parity assignment is chosen to be even. Weak interactions do not conserve strangeness S and strangeness-violating transitions between K0 and⎯K0 will, and do, occur. The transition can be seen to occur via the common 2π intermediate states, or via the underlying diagrams that explicitly involve the weak interme- diate vector , e.g.: W+ π+ ⎯s ⎯d 0 0 K0 c c ⎯K K0 ⎯K d s W− π− Because the connection (via the W’s or the pions) is awfully weak, we may treat the process using perturbation theory: we start with the in- 0 0 teraction Hamiltonian H0 (=Hstrong+Helectromagnetic) which conserves strangeness, and therefore cannot couple the K and⎯K states. The eigen- 0 0 states of H0 are |K 〉 and ⎯|K 〉. Once we “switch on” the weak interaction, transitions between the two Kaon states become possible, and the total Hamiltonian H contains the term Hweak: H=Hstrong+Helectromagnetic+ Hweak=H0+Hw. If CP is a good symmetry, it follows that [H,CP]=0, and we should be able to construct common eigenstates of H and CP; indeed: 0 001 ⎫ 0001 |K〉≡ |K 〉− |K 〉 ⎧ 00 1 2 ( ) |K〉=() |K12 〉+ |K 〉 ⎫ ⎪⎪ 2 ⎪ ⎧CP|K11〉=+ |K 〉 ⎬⎨ or 0 ⎬⎨ then: 00, (I.69) 001 0 1 00 CP|K〉=− |K 〉 |K〉≡ |K 〉+ |K 〉 ⎪⎪|K〉=() − |K12 〉+ |K 〉 ⎪ ⎩ 22 2 2 ( )⎭ ⎩ 2 ⎭ proving that K1 and K2 are proper CP eigenstates. They are also eigenstates of H; because they are constructed from eigenstates of H0, and, further because for Hw: if [HCPw , ]= 0 00 000000 0 0† 〈〉=−〈K|Hww |K K|HCP |K〉====−〈K|CPH w |K〉=−〈 K|(CP ) H ww |K〉=〈 K|H |K 〉. (I.70) Hw does not connect nor mix the K1 and K2 states: if [HCPw , ]= 0 000 0 0 00†000 〈〉=〈K|212Hww |K K|HCP |K 1〉====〈 K| 2CPH w |K 12〉=〈 K|(CP ) H w |K 1〉=−〈 K| 21H w |K 〉= 0. (I.71) and similar for H0, which commutes with CP. Thus, the K1 and K2 states must be eigenstates of the total H. 0 0 We may define the expectation values of Hw in K and⎯K :

if [HCPw , ]= 0 0000 0 0 0 0 EE'≡〈 K |HHww |K 〉=〈 K | |K 〉 , and Δ ' ≡〈 K | Hww |K 〉 ==== 〈 K | H |K 〉, (I.72) from which: Part I Phenomenology of Elementary Particles 39 12/15/2008

00 0 0 0 0 1 0000 00 0 0 〈〉=〈〉+〈〉=+〈〉−〈〉−〈〉+〈〉=+−ΔK|11HHH |K K| 1011 |K K|ww |K 10EEK| HHHH |K K|w |K K|w |K K|w |K) 0 E 'E ' 2 ( . (I.73) 00 and similarly: 〈〉=++Δ K220 |H |KEE ' E ' where E0 is the eigenvalue of H0. In the Kaon rest system we have E0=mK–iΓK/2, where mK, ΓK are the mass, respectively the width of the neutral Kaon, which differ for the K1 and K2: the K1 and K2 have different energies and (in their rest frames) different masses; they are dif- ferent states: each has definite CP (+1 and −1 respectively); they decay very differently (e.g. K1→2π + Q≈220 MeV, K2→3π + Q≈80 MeV) –11 –8 and with quite different rates because of the difference in phase space: τ(K1)=8.958×10 s, and τ(K2)=5.11×10 s. The true ‘particles’ are 0 0 not the K and⎯K , but the K1 and K2. To quote Gell-Mann and Pais (Phys. Rev. 97 (1955) 1387.): “To sum up, our picture of the K0 implies that it is a particle mixture exhibiting two distinct lifetimes, that each lifetime is associ- ated with a different set of decay modes, and that not more than half of all K0’s can undergo the familiar decay into two pions.” ... “Since we should properly reserve the word ‘particle’ for an object with a unique lifetime, it is the K1 and the K2 quanta that are the true ‘particles.’ The K0 and the⎯K0 must, strictly speaking, be considered ‘particle mixtures’.” Gell-Mann and Pais predicted the observation that some meters after a target the short-lived K1 would have died out, and purely K2→3π de- cays should remain. Several other beautiful and purely quantum mechanical phenomena occur in the Kaon system: ‘Strangeness oscilla- tions’ and ‘Regeneration.’ However, it is also clear that the K1 nor the K2 has a well defined strangeness nor a well defined quark content, so 0 0 it remains very much a of taste whether one calls the set (K1, K2) more elementary than the set (K ,⎯K )!

K0 ↔⎯K0 Oscillations 0 0 Starting with a pure K state at the (strong interaction) production site, the K , a superposition of the weak eigenstates K1 and K2, will oscil- late back-and-forth between a⎯K0 and a K0 as time passes. Physically this is analog to the system of two weakly coupled pendula (e.g. two pendula hanging off a crossbar that is somewhat flexible): setting one pendulum in motion, the other pendulum starts slowly to pick up power, and while the first pendulum deceases in amplitude, the swing of the next one is picking up. Energy oscillates back-and-forth between the two, with a frequency dependent on the coupling strength. The added complication in the Kaon system is, of course, that the Kaon decays, so the oscillation is superimposed on the decay rate of the particle. 0 1 Mathematically the oscillation depends on the coupling strength between the two states, i.e. on ΔE’. Starting with ψ(t=0) = |K 〉 = ( /√2)(K1 + K2), we find at time t >0: ∂ψ ()t it=+()HHψ () (I.74) ∂t 0 w 0 0 Developing ψ(t) in terms of the two t=0 eigenfunctions of H0 we write: ψ(t) = a(t)|K 〉 + b(t) ⎯|K 〉, with the normalization and boundary con- ditions: a2+b2=1; a(t=0)=1. Filling this general solution into (I.74), we obtain a set of coupled differential equations for a(t) and b(t): Part I Phenomenology of Elementary Particles 40 12/15/2008

∂at() ⎫ iEEatEbt(')()'() =+0 +Δ ⎪ ∂t ⎪ at()=−+ exp{} iE (0 E ') t cos( Δ Et ') ⎬ ⇒ ∂bt() bt()=− i exp{} − iE ( + E ') t sin( Δ Et ') iEEbtEat=+(')()'() +Δ ⎪ 0 (I.75) ∂t 0 ⎭⎪

−+iE(') E t 0 0 and: ψ (tEtiEt )=Δ〉−Δ〉 e0 ( cos( ' ) |K sin( ' ) |K ) Clearly, the probability of finding a⎯K0 sometime after production oscillates: Prob(|⎯K0〉,t) = 〈⎯K0|ψ(t)〉 = sin2(ΔE’t); it oscillates between 0 and 1, with average value ½ (in the absence of decays)! The appearance of⎯K0 after production can be verified experimentally by detecting strong interactions in a secondary target from Kaons produced in a primary target up- stream: after being produced as K0, the beam is inspected at various distances by inter- posing a secondary target and collecting the interactions. One observes final states of the type π++Λ0, which can only be produced in strong, strangeness conserving, K0p in- teractions, see Figure 3.12 If it were possible to create a mono-energetic K0 beam, the oscillations would be sim- ply observable as function of distance – note that the time t is in the rest system of the Kaon. However, by measuring the interaction products of a particular final state fully Figure 3. Number of⎯K0 interactions as function of time as function of distance, one can derive the initial Kaon energy, and then correct to the (units of proper lifetime τ1). The curves are for two values proper time t. Indeed, a beautiful confirmation of the oscillation is found. of Δm (natural units). Note that the number of events goes to zero for small times. Regeneration 0 0 Instead of considering the K and⎯K states, one can also consider the produced Kaon as a superposition of K1 and K2. Because only the K1 state (CP-even) can decay into 2π, it has a much shorter lifetime than the K2 state (CP-odd), which must go to at least 3π. Therefore, after some tens of centimeters away from the production target, the K1 component of the neutral Kaon beam has died out completely, and a pure K2 beam remains (in vacuum), as evidenced by the observation of only 3π decays in the vacuum for many meters after the production tar- get. 0 0 When one interposes a secondary target (“matter”) into this pure K2 beam, the K and⎯K components interact with different strengths (see the section above); the⎯K0 (S=−1) component is much more likely to undergo strong interactions than the K0 component:

12 Camerini et al., Phys. Rev. Lett. (197x) ~365. see also: Gewenniger et al.; Part I Phenomenology of Elementary Particles 41 12/15/2008

0 + K(SS=− 1) +p →π 0 + Λ0 (1) =− ()n

+ K(00SS=+ 1) +p →π 0 + Λ(1), =− (I.76) ()n

⎛ 0 pp0 ⎞ ⎜e.g. only: K (SBSBn=+ 1) + ( =+ 1) → Λ ( =+ 1, =− 1) + (BB =+ 1) + ( =+ 1)⎟ ⎝ ()nn()⎠ Formally, after “regeneration” (of the K0 component) in the interposed matter, the total amplitude can be written as: 0 ψ = 111aa|K00〉+ |K 〉 = ( aaaa − ) |K 〉+ ( + ) |K 0 〉, (I.77) reg 2 ( ) 2212 where the coefficients a and⎯a are now different from one another after passing through matter, and a K1 component reappears, as evidenced by a multitude of 2π decays in the first few centimeters after the secondary target!

K1 – K2 Mass Difference Coherent regeneration is used to determine the K1 – K2 mass difference. Coherence exists when only the regenerated K1 component in the forward direction (θ=0) is considered. Again, for simplicity, assume that the incoming K2 beam is monochromatic (=mono-energetic), and is represented by a traveling wave exp(ip2x). The regenerated K1 component in the forward direction is then also mono-energetic, but, because the K1 has a slightly different mass, will have a slightly different momentum p1, and is represented by a traveling wave exp(ip1x). The K1 component is generated all along the path inside the material, and waves generated at x=0 and at x=L/2 may interfere destructively when the L phase difference Δϕ = 2π ( /2)(1/λ2 – 1/λ1) = 2π L/(2) (p2 – p1) = (2n−1)π: waves from the first half of the material will pair-wise extinguish waves generated in the second half. In general, interference will take place depending on exp[iL(p2 – p1)] ≈ exp[iL(m1 – m2)m/p], where λ2=/p2 m≡(m1+m2)/2 and p≡(p1+p2)/2, and where we used dp/dm = −m/p. K2 These strength variations in the number of regenerated K1 can be measured as function of average Kaon momentum p (controlled by K2 the energy of the production π− beam) and material thickness L, and K1 −6 lead to a very sensitive determination of m2 – m1 = 3.48×10 eV. λ1=/p1 This is to be compared to the measurement of a possible difference in “matter” K0 and⎯K0 masses (a test of the validity of invariance under the com- 0 0 –10 x bined operation of TCP): mK – m⎯K < 5×10 eV. L

CP Violation in Weak Interactions In 1973 a Brookhaven experiment found that even the combination of C and P was not a perfect symmetry of the weak interaction: far from a production target, where – if CP would be a perfect symmetry – only the CP-odd state K2 should be present (in vacuum), a few 2π (CP- even) were found! Part I Phenomenology of Elementary Particles 42 12/15/2008

0 Γ→(KL ππ ) 45 −3 @57 m: 0 =≈× 2 10 , Γ→(KL all) 22,700 where we used the notation of KL to indicate the long-lived Kaon, and to distinguish it from the true CP-eigenstate K2; KL would be identical to K2 if CP were conserved in the weak interaction. Similarly, one uses KS for the short-lived variant (≈K1). The “wrong” decay rate is small, about 1 in 500 of all decays, but it means that CP is not a symmetry of the weak interaction either, and that one can actually distinguish particles from antiparticles by the weak interaction. The KL and KS are not pure CP-eigenstates, and are mixtures of the CP- eigenstates K1 and K2: 0 +− 1 Γ→++()Keπνe KKK,2.310,0003=+≈×()εε − and: L ≠1 (I.78) L212 0 −+ 1+ ε Γ→++()KeLeπν The finding of CP violation implies that T – Time Reversal symmetry – must also be violated in the weak interaction, because TCP invariance is a feature of (almost) all possible quantum theories and very hard to get around.

3.6 Time Reversal Symmetry Time reversal symmetry is a somewhat funny symmetry, because no conservation law is related to it. Instead of a unitary transformation it is a anti-unitary transformation. This can be seen already in the simple Schrodinger equation: a replacement of t by –t, changes not only the wavefunction but also the sign of the time derivative (which is to first power, unlike the second-order spatial derivative!), and thus changes the form of the equation! The proper T transformation is therefore: T ψ (,)rrtt⎯⎯→−ψ * (, );i.e. Not an eigenvalue equation!

* * (I.79) ∂∂−∂−ψψ(,)rrrtttT (, )* ⎛⎞ ψ (, ) * ititit=⎯⎯Hrψψ(,)→=−==− H (, r )⎜⎟() Hr ψ (, ) ∂−∂∂ttt⎝⎠ We will not further discuss T.