1. CP violation in Bs→ff and Bs → K*0K*0
5th International Workshop on the CKM Unitary Triangle
Rome, Italy, 9-13 September 2008
CP violation in Bs→ff and Bs → K*0K*0
Yuehong Xie
University of Edinburgh
(on behalf of the LHCb collaboration)

2. Weak phases in Bs penguin decays
u,c,t
W, ? b s + b s
Φ,K*0
u, c, t
u, c, t
s,d
B0s
B0s
B0s
s,d
Φ,K*0 s s b s
W , ?
+ tiny correction
New particles can enter both mixing and decay diagrams
To be measured in Bs→J/Yf
CKM 2008
Yuehong Xie

3. Observables Polarization fractions: FL, F// and F┴
CP asymmetries in each polarization f = L, //, ┴
Simplified for null test of SM: assume no NP in b→s decay, then just like in Bs→J/Yf
Only one phase F to measure
F ≠ 0 indicates NP
CKM 2008
Yuehong Xie

4. Analysis method Angles in helicity basis Maximum likelihood fit
Likelihood function models mass, proper time and angular distributions of signal and background candidates
Bs invariant mass: separate signal and background
Angular variables: separate different polarization components
Time evolution: extract CP parameters. An example
Signal
background
B mass
Angles in helicity basis
CKM 2008
Yuehong Xie

5. SM predictions and measurements
SM expectations
Sf ≈ 0 and Cf ≈ 0 for f = L, // and ┴, or F ≈ 0
BR(Bs→ff) ~ ( ) x10-6
BR(Bs→ K*0K*0) ~ 10 x10-6
FL > F// and F// ≈ F┴, values vary due to large hadronic uncertainties
CDF measured BR in data
sample of 180 pb-1
BR(Bs→ff) =(14+6-5±6)x10-6
(PRL 95:031801, 2005)
4.7 s significance
CKM 2008
Yuehong Xie

6. LHCb selection of Bs→ ff
Simulated 100k signal events and 22M b-inclusive events
Pass standard L0 trigger, mainly hadron trigger
Pass HLT tracking and event selection
Suppress b-inclusive background using
K particle ID, Pt
f impact parameter, vertex quality, Pt and invariant mass
Bs vertex quality, pointing angle and invariant mass
inclusive-b B vertex c2
inclusive-b K+ Pt
inclusive-b f mass
Signal K+ Pt
signal f mass
signal B vertex c2
CKM 2008
Yuehong Xie

7. Bs→ ff event yields
Final signal yield in 2 fb-1(1 nominal year): 3380 ± 450 in ±50 MeV Bs mass window based on central value of CDF BR measurement
Inclusive-b B/S < 95% CL
Specific background study ongoing
Core gaussian:
fraction 91%
s = 14 MeV
Bs mass
CKM 2008
Yuehong Xie

8. Detector effects in simulation
core st ~ 40 fs
Time acceptance
Angular resolution
effect negligible
cosq acceptance:
~ flat
f Acceptance:
~ flat
CKM 2008
Yuehong Xie

9. Flavour tagging Total tagging efficiency: 0.706 ± 0.008
Opposite side
High Pt leptons
K± from b → c → s
Vertex charge K+
Qvertex ,QJet
Same side
Opposite side PV
e-, m-
Bs0signal D
K + K-
B0opposite f f
Same side
Fragmentation K± accompanying Bs K+ K-
Total tagging efficiency:
± 0.008
Total tagging power: ±
Equivalent average mistag rate:
0.294
combining taggers
NN trained for each tagger
Final decision is made using NN outputs of all taggers
Categorized according to per event mistag rate
HCP2005, 4-9 July 2005
Yuehong Xie

10. Toy Monte Carlo generation
Physics inputs
Widths: ΓL = 0.738, ΓH = 0.654
Mass difference: Δms = ps-1
F// = 0.25 and F┴ = 0.25
d1 = arg(A┴/A//) = 0, d2 = arg(A┴/A0) = p
Weak phase F = 0
Experimental inputs
3380 signal events with B/S=1
Tagging efficiency 71% and mistag rate 30%
Single gaussian resolutions for mass and time
Time acceptance taken from simulation
Angular resolution and acceptance ignored
All background distributions taken from simulation
CKM 2008
Yuehong Xie

11. Fit results Fit parameters: F//, F┴ , Nsig, Nbg, d1, d2 and F
1000 toy experiments
Result: s(F) ≈ with 2 fb-1
This resolution only weakly depends on input values of F//, F┴, d1, d2 and F
Calibration methods are being developed to obtain knowledge of tagging performance, background distributions, resolution and acceptance effects in order to control systematics.
CKM 2008
Yuehong Xie

12. Bs→ K*0K*0 Can be studied in direct analogy to Bs→ff
A method to combine Bs→ K*0K*0 and Bd→ K*0K*0 to extract hadronic parameters for SM prediction of CP asymmetries in Bs→ K*0K*0 ( PRL 100:031802, 2008 ) is also being studied
Events are selected in a similar way to Bs→ff with relatively tighter cuts
Signal yields with 2 fb-1 w/o considering trigger
7.6k Bs→ K*0K*0 with B/S = 1
1.9k Bd→ K*0K*0 with B/S = 4
Angular fit studied. Time dependent analysis to measure CP asymmetries is under way.
CKM 2008
Yuehong Xie

13. Angular fit of Bq→ K*0K*0 Do combined fit to separate Bs and Bd
B mass
Do combined fit to separate Bs and Bd
Polynomial model of angular acceptance works well
Fit curve
Bs projection
Bd projection
Background projection
cosq1
In 2 fb-1 (1 nominal year): s(FL(Bs)) =0.005,
s(FL (Bd)) = and s(BR(Bs)/BR(Bd) ) =0.59
CKM 2008
Yuehong Xie

14. Conclusions
Bs→ff and Bs→ K*0K*0 provide a new window into CP violation in hadronic b → s penguins
Updated study shows LHCb can achieve a resolution of s(F) ≈ in Bs→ff with 2 fb-1 (1 nominal year), making it an ideal place to search for the anticipated unexpected physics
Study of Bs→ K*0K*0 is progressing well in LHCb
LHCb is about to take over the penguin world!
CKM 2008
Yuehong Xie