Presentation on theme: "Particle Physics II Chris Parkes Heavy Flavour Physics Weak decays – flavour changing Mass states & flavour states GIM mechanism & discovery of charm CKM."— Presentation transcript:
Particle Physics II Chris Parkes Heavy Flavour Physics Weak decays – flavour changing Mass states & flavour states GIM mechanism & discovery of charm CKM matrix 3rd Handout
2 Weak decays Weak decays are mediated by: – W bosons charged current interactions –Z bosons neutral current interactions Weak interaction does not respect conservation of flavour flavour changing interactions are possible Will discuss how this happens and difference between flavour changing charged (W ± ) currents, and neutral (Z 0 ) currents
3 Weak Decays: Charged currents e e W W c s W u d W gWgW gWgW gWgW gWgW ud vertex: allowed us vertex: not-allowed but observed! Have vertices: Assume quarks have similar vertices: Consider observed reactions: d u W -- -- s u W -- K-K-
4 Mass States & Flavour states ‘flavour’ state is a superposition of the ‘mass’ states –Flavour states = states that couple to W –Mass states = states of definite mass, ‘free’ quark states Flavourmass d d’ s s’ θCθC θ C is known as Cabibbo angle
5 Quark mixing: udsc quarks ud’ W gWgW us W g us u d W g ud =+ Flavour states as mixture of mass states: udW preferred to usW
6 Cabibbo-allowed/suppressed decays (2 generations) g us and g cd are Cabibbo suppressed with respect to g ud and g cs e.g. consider Cabibbo-allowed decays of charm quarks, D + : c s+l+ and c s+u+dbar Charmed meson decays most commonly include strange mesons Also explains c decays to Kbars ( c s+u+dbar) preferred to c decays to K (c d+u+sbar) Example of: (p270 Bettini)
7 Weak decays: neutral currents Z u u Z d’ Z l l Z gZgZ gzgz gZgZ gZgZ Why no flavour changing neutral currents (FCNC)? p261 Bettini Charged current W neutral current Z
8 GIM Mechanism: Add in charm u u Z d’ Z gZgZ gZgZ c c Z s’ Z gZgZ gZgZ d’ s’ No flavour changing neutral Currents (FCNC) Glasgow, Iliopoulos, Maiani 1970, used this to suggest another quark was needed (at tree level in SM)
9 Discovery of charm Introduction of charm solved FCNC problem Cancellation of FCNC predicted mass of charm to be ~1.5-2GeV Charm observed as J/ψ=cc in 1974 Ψ: R-measurement in e+e- J: Hadron production p+Be J+X
10 Charmonium – charm width p+N: Experimental resolution hides small width in mass reconstruction e+e-: Extract width from line shape of resonance –91 keV, small width, large lifetime –Strong decay - Why so small width ? 1 gluon – 2* mass D > m ψ 2 gluon – ψ C=-1, g C=-1 3 gluon allowed but s 3 g c c u u c c D0D0 D0D0 Not possible energetically for ψ Ψ’’ allowed 24MeV width g c c d d u u Allowed d d π-π- π+π+ π0π0
11 Generalise mixing to 3 Families: CKM Matrix Generalise cabibbo matrix To three generations Kobayashi Maskawa 2008 Nobel
12 Measuring Elements V ud -decay (u d) V us K-decays (s u) V ub B-decays (b u) rare difficult to measure, B- factories have improved this V cd production of charm of valence quarks in -DIS V cs Semi-leptonic D-decays (c s) V cb B-decays (b c) V td top-decay limits V ts top-decay limits V tb top-decays t Wb See Bettini p265 et seq
13 Example: W decays revisited u d W V ud u s W u b W c d W c s W c b W In branching fraction calculation we assumed Vud=Vcs=1, and neglected others Q) Why did we get answer right ?
14 CKM Unitary And six equations of off-diagonal elements=0, e.g. 1 st row * 3 rd column: For probability elements need only be real, but for CP violation (see next) need to be complex Q) If the measurements of these were to add to < 1, how would you interpret this?