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

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

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

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…)

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

- - 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

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

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

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

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

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 = (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

Indirect CPV

* 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) 045005

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

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) 041801 (our group)

Direct CPV

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)

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

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, 111602 (2012)

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

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…)

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