1. CP violation and D Physics:
Part V
CP violation and D Physics
Chris Parkes

2. PHENOMENOLOGY AND EXPERIMENTS
Outline
PHENOMENOLOGY AND EXPERIMENTS
CP violation and Kaon physics
Mixing in the neutral kaon system
Neutral kaon decays
Rare kaon decays
CP violation and B physics
B factories, old and future experiments
Mixing in neutral B mesons
Benchmark B decays
Rare B decays
CP Violation and D physics
Mixing in neutral D mesons
Direct and Indirect CP Violation Searches
Future prospects
Concluding remarks

3. D mesons c 
Nucl.Phys. B871 (2013) 1-20

4. Why study mixing and CP violation in the D sector?
D0 mixing slightly different from K0 and B0(s) mixing
only ‘up’ type quark system that can mix
D0 mixing only recently observed experimentally (2007 onwards)
Beyond Standard Model (BSM) effects may enhance mixing
(Accurate) SM and BSM theoretical calculations/predictions are difficult and show a broad range of estimates
why?: D-mesons are too light to be treated as heavy … and too heavy to be treated as light!
CP violation:
Predicted to be negligible in the SM
 large effects would be evidence for New Physics …
Experimental evidence for CP violation with D mesons not yet clear
(CPV in Kaons got a Nobel prize, CPV in B got a Nobel prize…)

5. Mixing in neutral D mesons
BaBar and Belle find first evidence for D0 mixing
Well established by combining multiple channels
Announcement at the Rencontres de Moriond Conference in March 2007
First single channel 5σ measurment D0 mixing
LHCb, November 2012

6. - - D0 D0 D0 D0 Mixing in the neutral D0 – D0 system c u _ _ _ W+
Feynman (box) diagrams for D0 mixing: down type quarks in loops c u _ _ _ W+ - D0
d, s, b
d, s, b D0 _ _ W- u c c u
d, s, b - _ _ D0 W- _ W+ D0 _ _
d, s, b u c
b loop CKM suppressed from Wolfenstein parametrisation is order (λ3λ2)2
and s,d loops GIM suppressed
Long distance effects important
– but even more difficult to calculate

7. Only small fraction of an oscillation before decay
reminder x<<1
Only small fraction of an oscillation before decay
suppressed
suppressed
suppressed
suppressed
DCS
interference
mixing

8. 1. Flavour at production 2. Decay Diagrams Key points on D mixing
Hence, don’t need to use the flavour tagging procedures we discussed for B,
mistag rate will be low
2. Decay Diagrams
Right sign – Cabibbo Favoured
Wrong sign – Double Cabibbo Suppressed

9. 1. Wrong sign / right sign Key points on D mixing
Consider events that are either wrong sign (rare and time independent)
or mixed (rare and number will increase with time).
to cancel systematics take ratio to right sign events
Flat with time? no mixing, increases with time? mixing

11. 1. Wrong sign / right sign Key points on D mixing
Flat with time? no mixing, increases with time ? mixing
See just first part of oscillation curve

12. yCP Another way to measure D mixing CP Violation Am Weak Phase ϕ
Measure lifetime of decays into CP eigenstates → here CP even, e.g. K+K-
Assume D2 ≡ p|D0⟩ - q|D̅0⟩ is CP even → then this measures τCP+ = 1 / Г2
Compare to decay into flavour-specific final state, e.g. K-π+ → this measures τ = 2 / (Г1+Г2) ≡ 1 / Г̅
Therefore τ / τCP+ - 1 = Г2 – Г1 / (Г1 + Г2) = ΔГ / (2Г) ≡ y
For non-zero CP violation D2 is not the physical state DH → measure yCP ≡ τ(Kπ) / τ(KK) – = (1 – AM2/8) y cos ϕ – AM/2 x sin ϕ
Our group
Update imminent with ~100x more data
CP Violation Am
Weak Phase ϕ

13. Indirect CPV

14. * AΓ : indirect CP violation
Same concept as B or K measurements we saw before
But experimentally different since don’t see oscillations as x is so small
Measure from lifetimes of decay to CP eigenstate
* Actually it has a direct CP violation contribution if direct CPV is large → see M. Gersabeck, C. Parkes et al., J.Phys. G39 (2012)

15. D0 D0 D0 D0 D0 D0 t=0 t Oscillation & Decay CP Violation Am Ad
Weak Phase ϕ D0

16. Prompt/Secondary Separation
Experimental Complication
D come from both production in proton-proton (prompt)
And from decay of B to D (secondary)
Separate out to get prompt D for time dependent measurement
Use impact parameter that is the distance of flight vector of reconstructed D from the primary vertex (proton-proton interaction point)
World’s best measurement
No sign of CP Violation yet….
Phys.Rev.Lett. 112 (2014) (our group)

17. Direct CPV

18. ΔACP: Direct CP violation in two-body charm
Search for CP asymmetry
Use decays
where slow pion charge determines the D flavour at production
Note time-integrated not time dependent
What we
measure
What we
want
What we
don’t want (1)
What we
don’t want (2)

19. ΔACP: Direct CP violation in two-body charm
Measure difference of raw asymmetries for KK and ππ final states
Leads to difference of CP asymmetries (at first order)
Araw(KK) - Araw(ππ) ≈ ACP(KK) - ACP(ππ)
Expect similar magnitude and opposite sign for ACP(KK) and ACP(ππ)
→ ACP(KK) - ACP(ππ) ≈ 2 |ACP(KK)| ≈ 2 |ACP(ππ)|
Enhanced sensitivity in difference measurement!
Chris Parkes

20. Production Asymmetry of D*+
Technical Scale Drawing of LHC Collision
Proton (Matter)
Proton (Matter)
+ve charge
-ve charge
Cancel by measuring difference of KK and ππ final states
Detection Asymmetry of D and of π+s
Symmetric final states
no D detection asymmetry
Difference of final states
cancel slow π+ detection asym.
Periodic reversal of magnetic field
Chris Parkes
Phys. Rev. Lett. 108, (2012)

21. ΔACP : direct CP violation
World Average Results
(some tension more results needed )
ΔACP : direct CP violation
AΓ : indirect CP violation

22. Status / Future prospects
Mixing
Fully established - 5σ single channel measurement
CP violation:
So far there is no clear evidence for CP violation with D mesons – slight discrepancy in direct, none in indirect
Large CP Violation would be evidence for New Physics, as SM prediction is small
Prospects:
Enormous samples at LHCb under analysis > 10 billion charm events recorded during LHC run 1!
Picture changing very quickly
In addition to two-body decays discussed here, there is active work on 3-body (and 4-body) final state analyses using Dalitz plots
(and a number of good LHCb PhD thesis topics…)

23. Key Points – D section Neutral meson mixing only ‘up’ type system
x<<1, very slow oscillations
Long distance effects important, not only box diagrams
D0 -> K+π-, Wrong sign / right sign event ratio with time
5σ single channel discovery LHCb
No CP Violation observed (yet)
Small in SM, could be larger due to new physics
Search for indirect CP Violation
Search for direct CP Violation