Part V CP violation and D Physics Chris

: Part V CP violation and D Physics Chris Parkes

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

D mesons Nucl. Phys. B 871 (2013) 1 -20 c

Why study mixing and CP violation in the D sector? Mixing: q D 0 mixing slightly different from K 0 and B 0(s) mixing q only ‘up’ type quark system that can mix q D 0 mixing only recently observed experimentally (2007 onwards) q Beyond Standard Model (BSM) effects may enhance mixing q (Accurate) SM and BSM theoretical calculations/predictions are difficult and show a broad range of estimates Ø why? : D-mesons are too light to be treated as heavy … and too heavy to be treated as light! CP violation: q Predicted to be negligible in the SM large effects would be evidence for New Physics … q Experimental evidence for CP violation with D mesons not yet clear q (CPV in Kaons got a Nobel prize, CPV in B got a Nobel prize…) Chris Parkes 4

Mixing in neutral D mesons Ba. Bar and Belle find first evidence for D 0 mixing Well established by combining multiple channels Announcement at the Rencontres de Moriond Conference in March 2007 First single channel 5σ measurment D 0 mixing LHCb, November 2012

Mixing in the neutral D 0 – D 0 system q Feynman (box) diagrams for D 0 mixing: down type quarks in loops D D 0 0 c _ u + d, s, b W W- W- d, s, b _ __ d, s, b _ _ _ d, s, b W+ u _ c 0 D -0 D q b loop CKM suppressed from Wolfenstein parametrisation is order (λ 3λ 2)2 q and s, d loops GIM suppressed q Long distance effects important – but even more difficult to calculate Chris Parkes 6

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

Key points on D mixing 1. Flavour at production 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 Chris Parkes Wrong sign – Double Cabibbo Suppressed 8

Key points on D mixing 1. Wrong sign / right sign 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 Chris Parkes 9

Chris Parkes 10

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

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 Chris Parkes * Actually it has a direct CP violation contribution if direct CPV is large → see M. Gersabeck, C. Parkes et al. , J. Phys. G 39 (2012) 045005 13

Oscillation & Decay t=0 t D 0 D 0 D 0 CP Violation Am Ad Weak Phase ϕ D 0 14

Prompt/Secondary Separation l Experimental Complication D come from both production in proton -proton (prompt) l And from decay of B to D (secondary) l l Separate out to get prompt D for time dependent measurement l Use impact parameter that is the distance of flight vector of reconstructed D from the primary vertex (proton-proton interaction point) l World’s best measurement – No sign of CP Violation yet…. Phys. Rev. Lett. 118 (2017) 261803 (our group) 15

Direct CPV

ΔACP: Direct CP violation in two-body charm l Search for CP asymmetry Note time-integrated not time dependent l Use decays where slow pion charge determines the D flavour at production What we measure want don’t want (1) don’t want (2) 17

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 18

Production Asymmetry of D*+ l Technical Scale Drawing of LHC Collision Proton (Matter) • Cancel by measuring difference of KK and ππ final states Detection Asymmetry of D and of π+s +ve charge -ve charge Chris Parkes • Symmetric final states no D detection asymmetry • Difference of final states cancel slow π+ detection asym. • Periodic reversal of magnetic field Phys. Rev. Lett. 108, 111602 (2012) 19

World Average Results ΔACP : direct CP violation (slight tension more results needed) AΓ : indirect CP violation 20

Status / Future prospects Mixing Fully established - 5σ single channel measurement CP violation: q So far there is no clear evidence for CP violation with D mesons – slight discrepancy in direct, none in indirect q Large CP Violation would be evidence for New Physics, as SM prediction is small Prospects: q Enormous samples at LHCb under analysis > 10 billion charm events recorded during LHC run 1 alone! q q Chris Parkes 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 Ph. D thesis topics…) 21

Key Points – D section • Neutral meson mixing • only ‘up’ type system • x<<1, very slow oscillations • Long distance effects important, not only box diagrams • D 0 -> 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 Chris Parkes 22