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

2. 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  Bs  J/ 

3. 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

4. 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 !

5. 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

6. 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

7. 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

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

9. 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

10. 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-

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

12. 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

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

14. 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

15. 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-

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