Measurements of time-integrated CP and other asymmetries ) r k - PowerPoint PPT Presentation

Measurements of time-integrated CP 
 and other asymmetries ) r 
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Outline Raw asymmetries Production and detection asymmetries Multi-body final states Two-body final states CP violation 2

Measured asymmetries N(D → f) − N( D ̅→ f ̅ ) 
 • Measure A raw (D → f) = N(D → f) + N( D ̅→ f ̅ ) particle tagging 
 • Get to first order D and D ̅ A raw (D → f) = A CP (D → f) + A prod (D) + A det (f) + A det (tag) • Need to constrain ➡ Production asymmetry ➡ Detection asymmetry (final state and flavour tag) • General idea ➡ Use similar Cabibbo-allowed processes 
 and assume A CP (D → f) = 0 3

Production asymmetries • Particular to pp collider + e − and ➡ “Replaces” forward-backward asymmetry at e p p ̅ • Valence quarks favour the production of matter baryons ➡ Favours antimatter mesons • Production asymmetry can 
 depend on kinematics Fit ➡ Accounted through 
 binning / re-weighting PLB 718 (2013) 902 D + 4

Detection asymmetries • Material interaction 
 Cross-section in mb - 100 K d can be asymmetric + K d 80 60 ➡ Strange quark can 
 40 produce hyperons 20 Data from K.A. Olive et al. (PDG), CPC 38 (2014) 090001 • Detector can be 
 0 2 10 1 10 p in GeV/c lab asymmetric ➡ Causes asymmetry 
 through different 
 average bending of positive 
 JHEP 07 (2014) 041 and negative tracks ➡ Regularly revert dipole polarity 5

Results Two-body decays

First example • Measurement • Extract CP asymmetries using control modes JHEP 10 (2014) 025 7

Results for K S h • Charged D two-body modes are 
 challenging due to neutral 
 particles involved • Measurement based on 3 fb -1 • Uses weighted control mode kinematics and average of dipole magnet polarities • All approximately zero total signal low-mass cross-feed combinatorial JHEP 10 (2014) 025 8

The Δ a CP saga * • What is Δ a CP ? • Interplay of direct and indirect CP violation • Individual asymmetries are expected to have opposite sign due to CKM structure * after A. Lenz @ CHARM 2013, arXiv:1311.6447 EPJC 73 (2013) 2373 9

Latest results • D*-tagged (1 fb -1 , preliminary) D 0 π s+ LHCb-CONF-2013-003 • muon-tagged (3 fb -1 ) D 0 μ - B JHEP 07 (2014) 041 10

Individual asymmetries a D ( μ + ) a P (B) a raw (K - K + ) a CP (K - K + ) measure want average 11

Individual asymmetries a D ( μ + ) a P (B) a raw (K - K + ) a CP (K - K + ) measure want D from B average 11

Individual asymmetries a D ( μ + ) a P (B) a raw (K - K + ) a CP (K - K + ) measure want D 0 → K - π + D from B average 11

Individual asymmetries a D ( μ + ) a P (B) a raw (K - K + ) a CP (K - K + ) measure want D 0 → K - π + D from B a D (K - π + ) average 11

Individual asymmetries a D ( μ + ) a P (B) a raw (K - K + ) a CP (K - K + ) measure want D 0 → K - π + D from B a D (K - π + ) average D + → K - π + π + Prompt D 11

Individual asymmetries a D ( μ + ) a P (B) a raw (K - K + ) a CP (K - K + ) measure want D 0 → K - π + D from B a D (K - π + ) average D + → K - π + π + Prompt D a P (D + ), a D ( π + ) 11

Individual asymmetries a D ( μ + ) a P (B) a raw (K - K + ) a CP (K - K + ) measure want D 0 → K - π + D from B a D (K - π + ) average D + → K - π + π + Prompt D a P (D + ), a D ( π + ) D + → K S π + 11

Individual asymmetries a D ( μ + ) a P (B) a raw (K - K + ) a CP (K - K + ) measure want D 0 → K - π + D from B a D (K - π + ) average D + → K - π + π + Prompt D a P (D + ), a D ( π + ) a CP/I (K S ) D + → K S π + 11

Individual asymmetries a D ( μ + ) a P (B) a raw (K - K + ) a CP (K - K + ) measure want D 0 → K - π + assume no CPV in a D (K - π + ) Cabibbo-favoured average final states D + → K - π + π + a P (D + ), a D ( π + ) a CP/I (K S ) D + → K S π + 11

Individual asymmetries a D ( μ + ) a P (B) a raw (K - K + ) a CP (K - K + ) measure want D 0 → K - π + a D (K - π + ) average D + → K - π + π + JHEP 07 (2014) 041 a P (D + ), a D ( π + ) a CP/I (K S ) D + → K S π + 11

( Δ )a CP results • Ignoring contribution from indirect CPV JHEP 07 (2014) 041 12

Results Multi-body decays

On Dalitz plots • Many ways to reach multi-body final states through intermediate resonances • Resonances interfere and can carry different strong phases ➡ Superb playground for CP violation • Look for local asymmetries ➡ Model-dependent: 
 Fit all contributions to phase-space and 
 ρ (770) 0 look for differences in fit parameters K * (892) + ➡ Model-independent: 
 Look for asymmetries in regions of 
 K * (892) - phase space by “counting” Courtesy of S. Reichert 14

D + → 3 π PLB 728 (2014) 585-595 • Model-independent 
 searches for CP violation ➡ Over 3M D + & D - decays in 1 fb -1 ➡ Search for asymmetry significances in bins of phase space ➡ Search for local asymmetries through un- binned comparison with nearest neighbours 15

Binned PLB 728 (2014) 585-595 LHCb method removes sensitivity to global asymmetries p-values for no-CPV hypothesis 
 > 50% for different binnings 16

Binned PLB 728 (2014) 585-595 LHCb method removes sensitivity to global asymmetries Similar results also obtained with un-binned kNN method * p-values for no-CPV hypothesis 
 > 50% for different binnings 16 * reduced sensitivity due to inclusion of few neighbours

Why not un-binned? • Need to compare each event with every other ➡ Computationally challenging for O(1M) events ➡ Use GPUs to exploit massive parallelisation 0 →π + π − π 0 decays ➡ Applied to D • Energy test (M. Williams, PRD 84 (2011) 054015) ➡ Test statistic (T) comparing pairwise 
 weighted distances in phase space ➡ Compare 
 0 ↔ D 0 
 D 0 ↔ D ̅ 0 
 D ̅ 0 ↔ D ̅ 0 D ➡ Expect T~0 (no CPV) or T>0 (CPV) PLB 740 (2015) 158 17

All π 0 s 0 s • Reconstructing merged and resolved π • Merged photon clusters γ π 0 γ + π − ) ➡ High energy, small opening angle, small m( π • Resolved photon clusters (includes conversions) γ + π − ) ➡ Small energy, large opening angle, large m( π π 0 γ • Complementary phase-space coverage resolved π 0 merged π 0 PLB 740 (2015) 158 18

Results • 8 × larger sample than BaBar ] Significance 4 PRD 78 (2008) 051102 3 3 c LHCb simulation / 2 ) [GeV 2 4 σ = 0.3 GeV / c 2 ➡ 420k resolved π 0 , 250k merged π 0 1 2 0 π − π 0 ➡ Similar or better sensitivity ( 2 m -1 1 • Using permutations with randomly -2 -3 assigned flavour tags to obtain no- 0 0 1 2 3 2 2 4 m ( + 0 ) [GeV / c ] π π CPV sample ➡ Reference T distribution • Result based on 1000 permutations ➡ P-value as fraction above nominal T value ➡ (2.6±0.5)% PLB 740 (2015) 158 19

CP violation in decay • Range of new measurements with increasing precision in several decay modes ➡ 2-body (K S h, hh) 0 ) ➡ Multi-body (model-independent, including π • Route forward: ➡ Measurements in related modes (two-body, resonances) to identify potential sources of CP violation ➡ Model-independent measurements are discovery strategies ➡ Need model-dependent measurements for quantitative interpretation • Future expectations ➡ See Chris’s talk on Friday 20