Measuring CP violation in B s →  with LHCb

1. Measuring CP violation in Bs→ with LHCb Jim Libby (University of Oxford) for the LHCb collaboration CKM 2006 Nagoya

2. Outline • Motivation for the study Bs→ • Brief introduction to LHCb • Expected signal event yields and backgrounds • CP sensitivity studies • LHCb sensitivity to sin 2βeff from B0→KS • Conclusions CKM 2006 Nagoya

3. Bs→  • FCNC gluonic penguin decay • Bs analogue of BdK0,ηK0 etc • Dependence on Vtsin both the decay and Bs mixing amplitudes leads to the SM CP violation being < 1% • for example M. Raidal, PRL 89, 231803 (2002) • Large CP asymmetry unambiguous signature of New Physics • PVV decay requires full angular analysis to extract CP asymmetries CKM 2006 Nagoya

4. LHCb in a slide Muon Detector Tracking stations interaction region • Forward geometry: • 1012 bb/2 fb-1 produced • both B hadrons in acceptance for tagging • excellent proper time resolution (40 fs) • RICHs for hadron ID from 1 to 100 GeV/c • Level-0 trigger: high ptl±, hadron and  hardware trigger 40→1 MHz • Software Higher Level Trigger (HLT): • ensure high pt object associated with large impact parameter tracks • inclusive and exclusive selections to reduce storage rate to 2 kHz proton beam Trigger Tracking CKM 2006 Nagoya

5. Selection studies • Selection studies performed on PYTHIA/EVTGEN/GEANT4 simulated samples of signal and background events • Limited statistics available for background estimates • 34 million b-inclusive events corresponds to ~5 minutes of data taking at nominal luminosity • Trigger simulation is applied for Level-0 and large impact parameter with pt HLT • Reconstruct →K+K− only • Selection criteria based on: • RICH K± ID • ptand impact parameter of K± and  candidates • Bs and invariant mass • Bs and vertex quality b inclusive Signal (Arbitrary norm.) Entries/MeV/c2  mass (GeV/c2) CKM 2006 Nagoya

6. Signal yield and background • Assuming branching fraction measured by CDF (PRL 95 031801 (2005)) • Signal yield per 2 fb-1 is 5100 events with a 60% uncertainty from BF alone • Background estimated from b-inclusive sample is 0.4 < B/S < 2.1 at 90% c.l. • Dominated by combinatorics • The efficiency of the  inclusive HLT for this signal mode is ~80% • Expected number of events selected per 2 fb-1 is 4000 b inclusive Signal (Arbitrary norm. & no trigger) σmB=12 MeV/c2 mB(GeV/c2) mB(GeV/c2) CKM 2006 Nagoya

7. Formalism for sensitivity studies I • A PVV decay is a mixture of CP odd and even amplitudes • Use helicity amplitudes to calculate differential decay distributions • The transversity basis is used which is related to the helicity basis by: • A0 and A|| are CP even and A┴ is CP odd • Time-dependent differential distribution in full is: • fi(θ1, θ2,) are 6 symmetric angular functions under -meson interchange c=1+ 2 CKM 2006 Nagoya

8. Formalism of sensitivity studies II • Assume a single new physics CP violating phase NPsuch that: • λicould all be different but this assumption simplifies the time dependent analysis • only fine tuning of the individual phases would lead to a null result in the presence of new physics • An example of the time-dependence: Strong phase differences with A┴ CKM 2006 Nagoya

9. Experimental inputs to sensitivity studies • Sensitivity studies are performed with maximum likelihood fit to toy MC • Fit variables are mB, proper time, transversity angles and B tag • 500 toy experiments are performed with each set of input parameters • The signal yield is assumed to be 4000 with a B/S=0.9 • Toy MC studies include: • Proper time resolution = 42 fs • Proper time acceptance function • Mistag dilution: ε(1−2ω)2=9.6% (estimated from BsDsK simulation) • Flat background in mBand transversity angles • Exponential lifetime distribution for background • Angular acceptance assumed flat and resolution ignored (good assumptions from BsJ/ψstudies) Proper time acceptance function Impact parameter selections in trigger and offline Proper time (ps) CKM 2006 Nagoya

10. Sensitivity to CP asymmetry • 2 fb-1 sensitivity to NPis 0.10 • Pull normally distributed NP pull NP (rad) CKM 2006 Nagoya

11. Variation with input assumptions Benchmark number in red • No significant variation seen as a function of input NP, Rp,t strong phases and proper time resolution • Expect systematics from mistag rate, proper time acceptance and background distributions. Will be estimated from control channels CKM 2006 Nagoya

12. B0 →K0S sensitivity to sin 2βeff • Selection studies predict an event yield of 800 events/2 fb−1 with a B/S<2.4 at 90% c.l. • Mistag dilution ε(1−2ω)2=5.0% and proper time resolution 60 fs • Values obtained from the full simulation used in a Toy MC sensitivity study to sin 2βeff • 10% CP-even K+K- S-wave also included • Sensitivity to sin 2βeff with 2 fb-1 is 0.32 • Close to current individual e+e− B-factory uncertainties CKM 2006 Nagoya

13. Conclusions • Bs→ the best channel for gluonic penguin measurements at LHCb • With 2 fb-1 of data a full time-dependent angular analysis of Bs→ yields a: 0.10 radian statistical sensitivity to any New Physics induced CP violation via gluonic penguins in this decay • With an LHCb data set of 10 fb-1 (~5 years) 0.04 radian statistical sensitivity assuming systematic uncertainties are controlled • LHCb will make an important contribution to searches for New Physics in b→sss transitions CKM 2006 Nagoya

14. Back-up CKM 2006 Nagoya

15. Variation with input assumptions II Benchmark number in red CKM 2006 Nagoya