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Measurement of ϕ_s at LHCb: Results and Implications for Flavour Physics

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30 diapositives · 468 Views · Jul 30, 2014

This presentation by Yasmine Amhis, on behalf of the LHCb Collaboration, explores the measurement of the CP-violating phase ϕ_s in B_s0 decays, focusing on both the theoretical and experimental frameworks used. The results, derived from a dataset of 1 fb⁻¹, reveal ϕ_s values compatible with the Standard Model while showing sensitivity to potential new physics. The analysis discusses techniques for flavour tagging, the challenges of time-dependent CP asymmetry, and the significance of systematic uncertainties. Insights into the implications for the Bs decay dynamics and future experimental prospects are provided.

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  1. Measurement(s) of ϕs at LHCb Yasmine Amhis On behalf of the LHCb Collaboration

  2. From Y. Nir Lecture @ CERN April 2012 Yasmine Amhis - HQL 2012

  3. The B0s neutral system Time Evolution Diagonalizing this Hamiltonian leads to two mass eigenstates with masses MH(L)and decay width ΓH(L) Yasmine Amhis - HQL 2012

  4. Bs decay to a flavour eigenstate ϕs in a box • Measure relative phase difference : • ϕs=ϕM − 2ϕD between two “legs”. • In SM + Ignoring penguins : • ϕD~ 0 • ϕsSM~ϕM • is predominantly determined by arg(Vts) • is predicted to be small ≈ − 0.04 • [Charles et al. Phys. Rev. D84 (2011) 033005] • New Physics (NP) can add large phases: ϕs = ϕsSM+ϕsNP Yasmine Amhis - HQL 2012

  5. Dramatis personae du jour ϕs Ambiguity Measurement Yasmine Amhis - HQL 2012

  6. Dramatis personae du jour Ambiguity Measurement ϕs

  7. Flavour Tagging Time dependent CP asymmetry needs to identify the initial flavour of reconstructed Bs0 mesons (initial state a b or b quark). Currently use Opposite Side Tagger, fully optimized and calibrated on data Effective tagging efficiency (2.29 ±0.07 ±0.26)% arXiv:1204.5675 Yasmine Amhis - HQL 2012

  8. Decay time resolution • We measure from data using prompt J/ψ which decay at t = 0 ps. • Width is found to be about 40 - 45 fs. • Use per-event estimate of the decaytime resolution. Yasmine Amhis - HQL 2012

  9. The Golden Channel Yasmine Amhis - HQL 2012

  10. Theoretically : • b→ccs tree dominance leads to precise prediction of ϕs in SM. • P → VV, admixture of CP-odd and CP-even states, measure also ΔΓs. • Experimentally : • Relatively large branching ratio. • Easy to trigger on muons from J/ψ → μ+μ-. • The Observables • 3 “P-wave” amplitudes of KK system. • 1 “S”-wave amplitude. • 10 terms with all the interferences. • ϕs, ΔΓs, , Γs … Yasmine Amhis - HQL 2012

  11. There is a two-fold ambiguity in the solutions : • ϕs and π -ϕsand ΔΓschanges sign Yasmine Amhis - HQL 2012

  12. LHCb-CONF 2012-004 • Full 2011 data set of 1 fb−1 is used. • Simple cut selections based on kinematics. • Most background removed by decay time cut t > 0.3 ps. • Very clean signal∼ 21200 signal candidates. Yasmine Amhis - HQL 2012

  13. Fit projections CP-odd CP Content clearly seperated CP-even Yasmine Amhis - HQL 2012

  14. Systematic uncertainties Yasmine Amhis - HQL 2012

  15. Results ΔΓs is the first observed non-zero value. All the measurements are compatible with SM. Yasmine Amhis - HQL 2012

  16. Us and the rest of the world Yasmine Amhis - HQL 2012

  17. Resolving the ambituity Yasmine Amhis - HQL 2012

  18. Y. Xie et al., JHEP 09 (2009) 074] Resolving the ambiguous solution The sign of ΔΓs K+K− P-wave : Phase of Breit-Wigner amplitude increases rapidly across φ(1020) mass region K+K− S-wave: Phase of Flatté amplitude for f0(980) relatively flat (similar for non-resonance) Phase difference between S- and P-wave amplitudes Decreases rapidly across φ(1020) mass region Resolution method: choose the solution with decreasing trend of δs −δP vs mKK in the φ(1020) mass region Yasmine Amhis - HQL 2012

  19. Submitted PRL arXiv:1202.4717v2 Resolving the ambiguous solution The sign of ΔΓs • Use a sample of 0.37 fb -1 • Split data into 4 bins of mKK • In each bin measure : • Fractionof S-wave • Measure relative strong phase difference : • δS – δperp ΔΓs< 0 δS –δperp ΔΓs > 0 Solution I is selected by the data

  20. Yasmine Amhis - HQL 2012

  21. Previous analysis was Bs J/ψf0(980) PL B707 (2012) 497 All m(ππ) range found to be CP-odd : 97.7% @ 95% C.L [arXiv:1204.5643] Now use wider m(ππ) range hence : Bs J/ψππ Yasmine Amhis - HQL 2012

  22. Selection using a Boosted Decision Tree. • Maximum likelihood fit to • time and mass. • Approx. 7400 signal events in 1fb-1 Simpler decay rate expression since there is no angular structure Yasmine Amhis - HQL 2012

  23. Result • Uses Γsand ΔΓsfrom the • Bs J/ψΦanalysis (+ correlation) • PRL 108 (2012) 101803 • Uses Δms from Bs→ Ds (3)π • PL B709 (2012) 177 ϕs = -0.02 ± 0.17(stat.) ± 0.02(syst.) rad. arXiv1204.5675 Submitted to PLB Yasmine Amhis - HQL 2012

  24. Conclusion • LHCb has presented new preliminary results using the full 2011 data (1 fb-1) • From an analysis of the J/ψϕ channel we find: Γs= 0.6580 ± 0.0054(stat.) ± 0.0066(syst.) ps-1; ΔΓs= 0.116 ± 0.018(stat.) ± 0.006(syst.) ps-1; ϕs = -0.001 ± 0.101(stat.) ± 0.027(syst.) rad. • From an analysis of the J/ψππchannel we find: ϕs = -0.02 ± 0.17(stat.) ± 0.02(syst.) rad. • Combining both results we find: ϕs = -0.002 ± 0.083(stat.) ± 0.027(syst.) rad. • We resolve the 2-fold ambiguity and find: ΔΓs > 0 . Yasmine Amhis - HQL 2012 24

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  26. Systematics Yasmine Amhis - HQL 2012

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  29. Conclusion Yasmine Amhis - HQL 2012

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