1chris parkes part ii cp violation in the sm chris parkes

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1Chris Parkes Part II CP Violation in the SM Chris Parkes Slide 2 2 Outline THEORETICAL CONCEPTS I.Introductory concepts Matter and antimatter Symmetries and conservation laws Discrete symmetries P, C and T II.CP Violation in the Standard Model Kaons and discovery of CP violation Mixing in neutral mesons Cabibbo theory and GIM mechanism The CKM matrix and the Unitarity Triangle Types of CP violation Slide 3 Kaons and discovery of CP violation Slide 4 4Chris Parkes What about the product CP? ++ ++ ++ ++ Intrinsic spin P C CP Initially CP appears to be preserved in weak interactions ! Weak interactions experimentally proven to: Violate P : Wu et al. experiment, 1956 Violate C : Lederman et al., 1956 (just think about the pion decay below and non-existence of right-handed neutrinos) But is C+P CP symmetry conserved or violated? Slide 5 5Chris Parkes Kaon mesons: in two isospin doublets Part of pseudo-scalar J P =0 - mesons octet with , Introducing kaons K + = us K o = ds K - = us S=+1S=-1 I 3 =+1/2 I 3 =-1/2 Kaon production: (pion beam hitting a target) K o : - + p o + K o But from baryon number conservation: K o : + + p K + + K o + p Or K o : - + p o + K o + n +n Requires higher energy Much higher S 0 0 -1 +1 S 0 0 +1 -1 0 S 0 0 +1 -1 0 0 Slide 6 6Chris Parkes What precisely is a K 0 meson? Now we know the quark contents: K 0 =sd, K 0 =sd First: what is the effect of C and P on the K 0 and K 0 particles? (because l=0 q qbar pair) effect of CP : Bottom line: the flavour eigenstates K 0 and K 0 are not CP eigenstates Neutral kaons (1/2) Slide 7 7Chris Parkes Nevertheless it is possible to construct CP eigenstates as linear combinations Can always be done in quantum mechanics, to construct CP eigenstates |K 1 > = 1/ 2(|K 0 > + |K 0 >) |K 2 > = 1/ 2(|K 0 > - |K 0 >) Then: CP |K 1 > = +1 |K 1 > CP |K 2 > = -1 |K 2 > Does it make sense to look at these linear combinations? i.e. do these represent real particles? Predictions were: The K 1 must decay to 2 pions given CP conservation of the weak interactions This 2 pion neutral kaon decay was the decay observed and therefore known The same arguments predict that K 2 must decay to 3 pions History tells us it made sense! The K 2 = K L (K-long) was discovered in 1956 after being predicted (difference between K 2 and K L to be discussed later) Neutral kaons (2/2) Slide 8 8Chris Parkes How do you obtain a pure beam of K 2 particles? It turns out that you can do that through clever use of kinematics Exploit that decay of neutral K (K 1 ) into two pions is much faster than decay of neutral K (K 2 ) into three pions Mass K 0 =498 MeV, Mass 0, +/- =135 / 140 MeV Therefore K 2 must have a longer lifetime thank K 1 since small decay phase space 1 = ~0.9 x 10 -10 sec 2 = ~5.2 x 10 -8 sec (~600 times larger!) Beam of neutral kaons automatically becomes beam of |K 2 > as all |K 1 > decay very early on Looking closer at K L decays Initial K 0 beam K 1 decay early (into ) Pure K 2 beam after a while! (all decaying into ) ! Slide 9 9Chris Parkes Incoming K 2 beam Decay of K 2 into 3 pions If you detect two of the three pions of a K 2 decay they will generally not point along the beam line Essential idea: Look for (CP violating) K 2 decays 20 meters away from K 0 production point The Cronin & Fitch experiment (1/3) J.H. Christenson, J.W. Cronin, V.L. Fitch, R. Turley PRL 13,138 (1964) 00 ++ -- Vector sum of p( - ),p( + ) Slide 10 10Chris Parkes Incoming K 2 beam Decaying pions If K 2 decays into two pions instead of three both the reconstructed direction should be exactly along the beamline (conservation of momentum in K 2 decay) The Cronin & Fitch experiment (2/3) J.H. Christenson et al., PRL 13,138 (1964) Essential idea: Look for (CP violating) K 2 decays 20 meters away from K 0 production point Slide 11 11Chris Parkes Result: an excess of events at =0 degrees! K 2 decays (CP Violation!) K 2 decays Note scale: 99.99% of K decays are left of plot boundary The Cronin & Fitch experiment (3/3) K 2 X p = p + p = angle between p K2 and p If X = 0, p = p K2 : cos = 1 If X 0, p p K2 : cos 1 Weak interactions violate CP Effect is tiny, ~0.05% ! Weak interactions violate CP Effect is tiny, ~0.05% ! Slide 12 12Chris Parkes Slide 13 13Chris Parkes Almost but not quite! Slide 14 14Chris Parkes with ||