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ANGELO CARBONE INFN Bologna (for the LHCb Collaboration) PHYSICS AT LHC Prague, 6-12 July, 2003

ANGELO CARBONE INFN Bologna (for the LHCb Collaboration) PHYSICS AT LHC Prague, 6-12 July, 2003. Overview. Physics Motivation Event selection Sensitivity on CP-violating observables Determination of the  angle of the unitarity triangle Conclusions. . *. V td V tb. *. V ud V ub.

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ANGELO CARBONE INFN Bologna (for the LHCb Collaboration) PHYSICS AT LHC Prague, 6-12 July, 2003

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  1. ANGELO CARBONE INFN Bologna (for the LHCb Collaboration) PHYSICS AT LHC Prague, 6-12 July, 2003

  2. Overview • Physics Motivation • Event selection • Sensitivity on CP-violating observables • Determination of the angle of the unitarity triangle • Conclusions

  3. * VtdVtb * VudVub   * VcdVcb Physics Motivation • LHCb can collect large samples of B(d, s) hh decays • Bd + - Bd K+ - Bs + K- Bs K+ K- • Due to large penguin contributions the Bd + - channel does no longer provide a clean strategy to extract sin(2) … • ... but the combined determination of the Bd + - andBs  K+ K- CP asymmetries can be used to extract R. Fleischer, Phys. Lett. B459 (1999)

  4. LHCb re-optimized detector Material reduction: • Cut the tracking stations by 1/2 • Improved RICH-1 design • Improved VELO design Material up to RICH-2 now: 40% X0 12% λI

  5. Event simulation • Full GEANT simulation • Full Pattern Recognition • Selection cuts optimized separately for each decay under study to maximize efficiency and reject backgrounds • Two sources of background considered • Two-charged-body decays of b-hadrons (particle mis-identification) • Inclusive bb decays (combinatorial background) • MC samples used: size (number of events) • Bd + - 300k • Bs K+ K- 300k • Bd K+ - 50k • Bs + K- 50k • b p K- 50k • b p - 50k • inclusive bb 10M

  6. , K B0(s) , K IP IPB L IP Selection cuts • Charged tracks • Each leg identified as a K, or a particle lighter than K Cuts on • p • max. pT • min. pT • max. IP/IP • min. IP/IP • 2 of common vertex • Assume combinatorial background to be dominated by generic bb events • With these cuts we can reject all generated bb background also when relaxing mass cut • B candidate: Cuts on • pT • IP/IP • L/L • mass

  7. Rejection of two-body background Relying on – Particle ID performance (RICH) – mass resolution (~18 MeV/c2) Bd + - mass with RICH PID Bd + - mass without RICH PID GeV/c2 GeV/c2

  8. Annual yields and B/S ratios • Annual yields after L0xL1 triggers and offline selection • B/S include physics and combinatorial background

  9. Bd + - Bs K+ K-Proper time acceptance and resolution Bd + - Bs K+ K- Proper time acceptance Acceptance loss due to cuts to reject prompt combinatorics(distance of flight, IP) ps ps Bd + - Bs K+ K- Proper time resolution Excellent proper time resolution~40 fs fs fs

  10. Bd + - Bs K+ K-CP sensitivities • Tagged event samples generated by means of a fast MC fast MC inputs Values used as illustrative example • Assume identical lifetime for signal and background • Assume negligible CP violation in the background Preliminary assumptions: need more study and MC statistics

  11. Max likelihood method to extract CP asymmetries • One year of data taking Standard error ellipse 68% 95% 99% B0  + - Bs K+ K-

  12. Example of binned asymmetries • Results of log likelihood fit superimposed on binned asymmetries Bd + -(1 year) Bs K+ K-(1 year)

  13. Bs K+ K-Dependence of CP sensitivityon Ms • Ms > 14.1 ps-1(PDG 2003, CL 95%) • Error increases by a factor ~1.7 from Ms=15 ps-1 to Ms=30 ps-1

  14. Strategy to extract  Details in: R. Fleischer, PLB 459 (1999) 306 • d exp(i) = function of Tree and Penguinamplitudes in Bd  • d’ exp(i’) = function of Tree and Penguin amplitudes in Bs KK • d= Bdmixing phase • s= Bsmixing phase •  • Measuring the four CP-violating observables: 7 unknowns and 4 equations

  15. Using U-spin symmetry of strong interactions • Bd  + -and Bs  K+ K- decays are related to each other by interchanging all d and s quarks. U-spin flavour symmetry Unique solution for  • The mixing phases d and sare measured by the decays BdJ/ Ks and BsJ/  • Four equations and 3 unknowns • Within the validity of U-spin symmetry the system can be solved … • … but the size of U-spin symmetry breaking is still an open question • This illustrative example:

  16. f(d, ) [arbitrary units] d  (degrees) Bayesian approach to determine  • Infer a joint p.d.f. f (d,, ) by using the four CP asymmetry coefficients as constraints 2D projection On (d, ) plane 3D magnified Joint p.d.f. for d and  (1 year) f (d,) =  f (d,, ) d

  17. Resolution on  95% confidence region for d and  p.d.f. for  f () =  f (d,, ) ddd 1 year 2 years 3 years 4 years • R.M.S. of f () after 4 years: 2.2º (for  = ~60º)

  18. Conclusions • The current simulations show large event yields for the decaysBd, s hh • CP sensitivities forBd + -andBs K+ K-(one year of data taking) • Combining theBd + -andBs K+ K-CP measurements it is possible to extract the gamma angle (within U-spin symmetry validity) • Resolution on :2.2° after 4 years of data taking (for  = ~60°) Bd + - Bs K+ K-

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