Vincenzo Vagnoni INFN Bologna CKM Workshop Durham, April 8th 2003

Physics motivation LHCb can collect large samples of B0(s)  hh decays The combined determination of the B0  + - and Bs  K+ K- CP asymmetries can be used to extract the  angle of the unitarity triangle R. Fleischer, Phys. Lett. B459 (1999) The decays are related by U-spin flavour symmetry interchanging d s quarks Within the validity of U-spin symmetry the system can be solved and  is determined d =2 s 0 Bs  J/ 

VELO: 21stations (Rmin= 8mm) Si 220 mm, strips R e φ ~65 m2 3 Tracking stations IT : Si strips OT: straw tubes TT~1.41.2 m2 Si microstrips

B0(s)  hh Monte Carlo simulation MC samples used: size (number of events) B0  + - 60k Bs  K+ K- 65k B0  K+ - 62k Bs  K- + 24k b  p K- 20k b  p - 21k inclusive bb background 1.144M Full Geant simulation Full Pattern Recognition MC mass production going on now to increase statistics in order to improve background studies only ~20 times the signal statistics, but BR(signal) << 1/20 only ~1/2 minute of data-taking at nominal LHCb luminosity !

B0(s)  hh Selection cuts , K , K IP IPB L B candidate: Cuts on pT IP/IP L/L mass Charged tracks Each leg identified as a K, or a particle lighter than K (using RICH) 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

Rejection of physics background Relying on – RICH PID – mass resolution (~18 MeV/c2 single Gaussian) B0  + - mass with RICH PID B0  + - mass without RICH PID GeV/c2 GeV/c2

Annual yields and B/S ratios Event type B/S preliminary estimate (90% CL) Untagged annual yield B0  + - < 0.8 27000 B0  K+ - < 0.25 115000 Bs  K+ K- < 0.55 35000 B/S include physics and combinatorial background MC mass production for a better B/S estimate going on now

B0  + - Proper time acceptance and resolution Proper time resolution ps Single Gaussian fit: =41±1 fs

B0  + - BS  K+ K- CP sensitivities Tagged event samples generated by means of a toy MC B0  + - BS  K+ K- Annual event yield (1 year) 27000 35000 Background over signal ratio 0.8 0.55 Mixing parameter 0.755 20 Effective Tagging Power 6.4% -0.30 0.16 0.58 -0.17 TOY MC inputs Values used as illustrative example Assume background lifetime and signal lifetime to be the same Assume negligible CP violation in the background Preliminary assumptions: need more study and MC statistics

Max likelihood estimation to extract CP asymmetries After one year of data taking Standard error ellipse B0  + - 68.27% 95.45% 99.73% Bs  K+ K-

Example of binned asymmetries Results of log likelihood fit superimposed on binned asymmetries B0  + - Bs  K+ K-

Dependence of CP sensitivity on B/S and on the effective tagging power Current estimate of B/S ratio Bs  K+ K- Current estimate of B/S ratio On B/S B0  + - TP tagging power Bs  K+ K- TP tagging power On tagging

Dependence of CP sensitivity on xs Error increases by a factor 1.6 from xs=20 to xs=40 Bs  K+ K-

Dependence of CP sensitivity on s Bs  K+ K- A can be determined from untagged Bs rates In this example A is fixed with negligible error in the fit

Summary The current simulations show large event yields for the decays B0(s)  hh CP sensitivity after one year of LHCb data taking has been estimated LHCb can measure CP violation in the Bs  KK @ xs=40 with an error increase on the asymmetry amplitudes by a factor 1.6 w.r.t. xs=20 Event type B/S preliminary estimate (90% CL) Untagged annual yield B0  + - < 0.8 27000 B0  K+ - < 0.25 115000 Bs  K+ K- < 0.55 35000 B0  + - BS  K+ K-

Towards … R. Fleischer Phys. Lett. B459 (1999)