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LHCb performance for B s  D s  (B s mixing) and B s  D s K  decays (  )

LHCb performance for B s  D s  (B s mixing) and B s  D s K  decays (  ). London. ADVANCED STUDIES INSTITUTE "PHYSICS AT LHC" Prague, Czech Republic. July 6 - 12, 2003. Richard White Imperial College. Outline. The Physics CKM matrix and Unitarity Triangles

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LHCb performance for B s  D s  (B s mixing) and B s  D s K  decays (  )

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  1. LHCb performance for BsDs(Bs mixing) and BsDsK  decays () London ADVANCED STUDIES INSTITUTE "PHYSICS AT LHC" Prague, Czech Republic. July 6 - 12, 2003 Richard White Imperial College

  2. Outline • The Physics • CKM matrix and Unitarity Triangles • Mixing in Bs0system • Event selection at LHCb • Annual event yields and backgrounds • Sensitivity to ms and  • Extraction method • Results • Current B-physics experiments can’t explore the Bs sector • Need LHCb for this

  3. Unitarity Triangles   +    -   By exploiting the Unitarity of the CKM matrix, one can form triangles in the complex plane. For the B-system: VcdV*cb + VtdV*tb + VudV*ub = 0 VtsV*us + VtdV*ud + VtbV*ub = 0 BsDs BsDsK ( - 2) BsJ/

  4. Formalism The two mass eigenstates |Bl and |Bh are given by: Time-dependent rates for initial flavour eigenstates Bs and Bs decaying into final states f or f are: Where Af is the instantaneous decay amplitude for Bs f In the Standard Model:

  5. BsDs u + W+ d b c Bs Ds- s s This channel has a flavour specific final state hence: Can be used to measure s, s and ms Branching ratio assumed same as Bd D-+:3x10-3

  6. BsDsK u c T1 T2 Ds- K+ s s W- W+ b u c b K+ Bs Ds- Bs s s s s Bs and Bs can go to same final state. • From the two time-dependent Rates • measure  - 2 and strong-phase difference • Branching ratio assumed same as Bd D-K+ and Bd Ds+-: 2x10-4 and 4.8x10-5. • Rapid oscillations make time-dependent analysis essential

  7. BsDsh Kinematics at LHCb • The Bs is heavily boosted in the beam direction (Z) • Vertex resolution in Z dominated K or + Bs0 K+ Ds∓ K- 144 m  440 m 47m • Choose Ds KK since fully reconstructible and high branching ratio • Ds acts as a background to DsK (12 – 15 times x-section) • Very small kinematic difference between Ds and DsK – the K/  separation provided by the two RICH systems is critical.

  8. Proper Time Resolution Define proper time as: • Double Gaussian fits to reconstructed – MC truth values • Ds core resolution of 57 fs with 78% in core • Bs core resolution of 42 fs with 77% in core ms = 30 ps-1 gives period of 209 fs – sufficient resolution

  9. Event selection K/separation of ‘bachelor’ particle BsDs BsDsK LL Common selection for BsDsand BsDsK • Track impact parameter, quality and momentum • Vertex fits • Mass windows • Lifetimes • Exploit Ds resonance decays • Mass and angular cuts RICH PID critical for separating the two channels

  10. Separation of BsDsK from BsDs Without RICH you are dead in the water With it, can handle main background

  11. Yields and BackgroundsAnnual number for untagged events Signals current selections yield: 87K BsDs6.1K BsDsK Specific backgrounds DsK pollution of Ds signal negligible Ds pollution of DsK signal B/S = [0.06,0.21] 90% CL bb background DsK B/S = [0.0,4.2] 90% CL zero selected in 10.2M sample Ds B/S = [0.01,0.53] 90% CL

  12. Acceptance Functions BsDs BsDsK • Short lifetimes suppressed because visible secondary vertex needed • Large lifetimes can give large Bs impact parameter

  13. Evaluation of Sensitivities • R+(t) is rate for specific initial state to required final state e.g. Bs Ds-+ • R-(t) is rate for CC of specific initial state to required final state e.g. Bs Ds-+ • So: Rsig(t) = (1-tag) R+(t) + tagR-(t) • Background rate Rbkg(t) = e-t • Decay at some proper time t reconstructed at time trec • Error in proper time, trec, from momentum and vertex resolution, determined event by event Then Where Can extract s, s, ms, , , arg() and arg() Likelihood

  14. Examples Initial pure Bs decaying to Ds-K+ at time t. No smear and perfect acceptance Realistic picture with tag = 0.3

  15. Examples BsDs with two different ms

  16. Sensitivity to msFrom BsDs • Standard Model predicts : ms =14.3 – 26 ps-1 and s /ms = (4  2)10-3 • Error onms irrelevant: Sensitive up to ms = 60 ps-1

  17. Sensitivity to tagging and Background Level • Nominal  = 30% and f = 20% • Linear dependence, but (ms)largelyirrelevant anyway

  18. Sensitivity to  - 2From BsDsK Different values of strong phase difference: xs = ms/ = 30 xs = ms/ = 15 Sensitivity only weakly dependent on

  19. Summary • (ms)/ms < 1% for ms < 60 ps-1: Good sensitivity up to this point •  - 2 only weakly dependent on strong phase difference • LHCb capable of measuring Bs mixing and  to high precision • BsDsK gives very clean extraction of 

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