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LHCb Strategies for g from B → DK

Beauty 2006, Oxford UK, 25-29 September 2006. LHCb Strategies for g from B → DK. Yuehong Xie University of Edinburgh for the LHCb Collaboration. ADS with B + →DK + and B 0 → DK* 0. Dalitz with B + →DK + and B 0 → DK *0. Physics Aim.

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LHCb Strategies for g from B → DK

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  1. Beauty 2006, Oxford UK, 25-29 September 2006 LHCb Strategies for g from B→DK Yuehong Xie University of Edinburgh for the LHCb Collaboration ADS withB+→DK+ and B0 →DK*0 Dalitz withB+→DK+ and B0 →DK*0

  2. Physics Aim • It is generally assumed tree processes are dominated by SM contributions • Disagreement between the tree-determined triangle (|Vub|+ g from B→DK) and the loop-determined triangle (eK, Dmd, Dms, sin2b, … ) indicates new physics. • They are consistent within current measurement precision. • Have big uncertainty in g used for tree fit. Improvement needed! • Require sensitivity of g from tree ~ 5º to match the precision of indirect determination • LHCb: g from Bs→DsK is clean and unaffected by new physics, but has sensitivity of 14º with 2 fb-1 of data. We will see B→DK is more promising for LHCb. g (tree) Loop fit Tree fit Yuehong Xie

  3. LHCb detector RICH PID performance 2<p<100 GeV <e(KK,P)> = 93% <e(K,P)> = 4.7% PID: RICH1/RICH2 SPD/PS ECAL HCAL M1-M5 Detector status: Talk by Lluis Garrido  Trigger: Talk by Eduardo Rodrigues Tracking: Vertex Locator TT T1-T3 Magnet Tracking efficiency 94% for physics tracks with p>10GeV Yuehong Xie

  4. Data simulation • LHCb will start data-taking in 2007. • Full Pythia+Geant4 simulations are used for trigger, reconstruction and event selection studies. • Used event samples include • 260m minimum bias events for trigger study, • 140m Inclusive bb events for background study, • Dedicated signal events. • Sensitivities are obtained using fast simulations according to efficiencies, resolutions and background level from full simulations. Yuehong Xie

  5. Event selection criteria • Hadron particle identification using RICH • Composite particle invariant masses (B, D, K*, Ks) • Impact parameters and transverse momentum of B and decay products • Vertex c2 of B, D, K*, Ks • Angle between B momentum and flight direction • Event topology Cuts are optimized to reject most background events from a large inclusive bb sample and retain high signal efficiencies. Final B/S ratios are assessed based on a separate bb sample. Yuehong Xie

  6. B→DK decays s ubar c b u b cbar s ubar ubar c b ubar u b D0 D0 D0 cbar s s dbar dbar dbar dbar • Three parameters for CKM favoured B→DK and disfavoured B→DK • weak phase difference g = Arg(-(VubV*cs)/(VcbV*us)) • strong phase difference dB and • amplitude ratio rB= |A(B→DK )|/|A(B→DK )| • Interference allows to extract g, if same D0 and D0 final states B decays (rB  0.1) D0 ubar ubar B0 decays (both diagrams colour suppressed → rB  0.4) Yuehong Xie

  7. ADS: use Interference (1) (2) (3) (4) • Consider a final state common to D0 and D0,e.g. Kp DCS favoured • Define • There are four possible charged B decays Interference ~ O(1) for suppressed (2) and (4) Yuehong Xie

  8. ADS tailored for LHCb • D0→Kp: 3 observables from the relative rates of the 4 processes depends on 4 unknowns g, rB, dB, dDKp • rDKpis already well measured • may benefit from cos(dD) measurements in CLEO-c and/or BES III • Need another D0 decay channel to solve for all unknowns • 4-body decay D0→Kppp provides 3 observables which depends on 4 unknowns g, rB, dB, dDK3p– only dDK3p new (4-body Dalitz, p.17) • Each CP mode D0→KK/pp provides one more observables with no new unknowns The ADS method constrains g only if we can see suppressed decays! Good opportunity for LHCb. Yuehong Xie

  9. Example: B+→(Kp)DK+,rB=0.077 • Favoured modes • Background from D0p decays dominates (BR ~13 × D0K) • Use RICH information to separate D0K and D0p • Use dedicated sample of D0p decays → Expect ~17k bkgrd events/year from D0p • Use bb sample to assess combinatorial background → Expect ~0.7k bkgrd events/year • Suppressed modes • bb sample indicates that the combinatorial contribution dominates : → Expect ~0.7k background events/year • D0p: 17k*RDpBELLE ~ 60 events/year Efficiency / % e(KK ,) = 93% e(pK ,) = 4.7% ~28k/year B+→ (K+p-)DK+ B/S ~ 0.6 ~28k/year B- → (K-p+)DK- B/S ~ 0.6 Momentum / GeV MC B±→D0(Kp)p± events 687 / 580k pass all cuts except B mass 387 / 580k inside 3s B mass cut • ~530/year B+→ (K-p+)DK+ B/S ~ 1.5 • ~180/year B- → (K+p-)DK- B/S ~ 4.3 • (depending on rB ) B± mass /MeV Yuehong Xie

  10. Performance with B+→DK+ w/o bkgrd • Take g = 60º, rB = 0.077 • dB = 130º • rDKp = rDK3p = 0.06 -25º< δDK < 25ºand -180º< δDK3p <180º • Combine Kp with • K3p similar yields and background level as Kp • KK and pp modes δDKp, δDK3p estimated bkgrd δDKp, δDK3p ~4.3k B+→ D0(hh)K+ B/S ~ 1 ~3.3k B- → D0(hh)K- B/S ~ 1 (HighlightedareRMS quoted for non-Gaussian distribution of fit results due to close ambiguous solutions, will disappear as statistics increase. Global analysis using all modes will also help.) s(g) ~ 5º -15º in a year, depending on rB, δDK and δDK3 Yuehong Xie

  11. ADS with D*K D0K mass • Attractive feature • D*→D0p0 (BR~2/3) – has strong phase δB • D*→D0g (BR~1/3) – strong phase δB+p → if can distinguish the two decays → powerful additional constraint ! • Potential • Including D*K without background improves precision to s(g) = 2º - 5º • However • Reconstruction efficiency is small for soft g while background is enormous • Non-trivial to separate D0p0 and D0g • Fit DK mass shape to get p0 and g components ignoring neutrals • all favoured modes can reach 10k/year or more with B/S ~ O(1) • Suppressed modes still problematic: under study • Idea to use the event topology to reconstruct momentum of p0/g is being investigated B±→D*(D0g)K± bb sample Background dominating: 1 bb event here represents 5k/year! B±→D*(D0p0)K± B±→D*(D0g)K± Charged reconstruction Yuehong Xie

  12. ADS with B0→DK*0 • The same method can be applied to B0→D0K* with D0→Kp and KK, (historically treated with “GLW” method) • rB ~ 0.4, big interference in CP modes (rDCP=1) () ~ 7º-10 in a year (2 fb-1)(55 <  < 105, rB~0.4, -20 < dB < 20) Yuehong Xie

  13. Dalitz: B+→(Ksp+p-)DK+ • Three body decay D0→Ksp+p-fully parameterized with two parameters m+2=m2(Ksp+) and m-2=m2(Ksp-) • Use pre-determined model to describe D0 decay amplitudes as a function of (m+2, m-2) Total B decay amplitude as sum of contributions via D0 and D0 N: number of resonances aj, aj: amplitude and phase parameters from B factories Aj: model-dependent parameterization of matrix element Interference has sensitivity to g G(m-2,m+2) = |f(m-2,m+2)|2 + rB2|f(m+2,m-2)|2 + 2 rB Re [ f(m-2,m+2)f*(m+2,m-2)ei (-g+dB) ] Yuehong Xie

  14. Comparison to BELLE data Naïve model amplitude Isobar amp. + non-res. Belle data with model fit Yuehong Xie

  15. Performance of B+→(Ksp+p-)DK+ for rB = 0.077 LHCb generator studies m- • 5k events/year assuming “good” Ks efficiency at HLT, combinatorial B/S < 0.7, D(Kspp)p B/S in (0.11, 12) at 95% C.L. • Need large D(Kspp)p sample to clarify • Model parameters from B and charm factories • s(g) ~ 8º + model uncertainty, w/o b.g. and detector effect in fit • Two-fold ambiguity (g, dB), (g-p, dB-p) r(770) K* and DCS K* m+ Statistical precision depends on rB, e.g., s(g)~4º for rB=0.15 Current Ks problem in HLT: Only 25% of Ks decay inside active region of VELO, giving 1.5k events/year. Online reconstruction of the pion tracks from other Ks requires special treatment and is challenging due to HLT time limit. Great efforts are being made to speed up the online reconstruction of no-VELO tracks. Depending on how this will be accomplished, annual yield can vary between 1.5k - 5k, leading to d(g)~8º-16º for rB = 0.077. Yuehong Xie

  16. Acceptance studies • Selection tuned to maximise signal-to-background ratio • Flat phase space MC generated • Overall acceptance not flat as a result of trigger and offline selection % m- % m- m+ m+ Acceptance evaluation with isobar model in progress – as expected similar acceptance function is indicated. Can be checked with B→Dp data. Yuehong Xie

  17. Dalitz: B+ (KsK+K-)D K+ • Same method works for D0 KsK+K-decay • BR reduced by factor of 6 BR(D0 K0K+K-) =(1.03±0.10)% • But less background because of two more particle identification constraints from RICH • Dalitz model has fewer resonances (, a0) but complex threshold effects (Babar hep-ex/0507026) • Work on event selection and sensitivity ongoing Yuehong Xie

  18. “Dalitz”: four-body D decay The idea for 3-body D0 decays can be extended to 4-body D0 decays. Need 5 parameters to describe D0 decays: not really a Dalitz plot. • ~ 1.5k events in a year • Preliminary number d(g) ~ 15º w/o b.g. and detector effect B+→(ppKK)DK+ for g=60º, dB=130º, rB=0.08 B+→(pppK) DK+ • The same channel as used in ADS, but to take into account strong phase dependence on Dalitz space • rB~rD big interference • Sensitivity under study Yuehong Xie

  19. Dalitz: B0 (KSp+p-)D K*0 • Branching ratio reduced by factor 10 compared with B+ mode • Same method as in B+ • Annual yield < 600 events • Big interference (rB~0.4) • Will complement the “GLW” modes D0→Kp, KK and pp • Also suffersKS HLT problem PDG ‘04 Yuehong Xie

  20. Summary of performances: 2 fb-1 no b.g., detector effect More modes are being investigated. Combined precision should not be far away from 5º Yuehong Xie

  21. Sources of systematics As we approach statistical error of 10º, understanding and controlling systematical uncertainty becomes more and more important for LHCb • Dalitz model dependence • Improve D decay model by work of B and Charm factories (LHCb for 4-body) • Other contributions to K* region in B→DK* • To be studied • Dalitz acceptance not flat • Currently obtained using Mont Carlo simulation, should be calibrated with data • Production asymmetry and detection charge asymmetry to be studied • D0-D0 mixing and CP violation • CP-conserving D0 mixing is a very small effect and can be neglected • New physics with big CP violation in D0 mixing may change the value of g extracted from B→DK. Not really a systematic error but a new physics effect. • D0 mixing and CP violation can be probed in LHCb. See Raluca Muresan’s talk Friday. g = 92º ± 41º ± 11º ± 12º (Babar) g = 58º ± 18º ± 3º ± 9º (Belle) Yuehong Xie

  22. Conclusions • LHCb will be able to extract g from B→DK to the required 5º precision to match the indirect determination with ~2 fb-1 • Comparison of g from B→DK and indirect determination will become a stringent test of the SM • Together with improvement on |Vub| the measured g from B→DK will enable to construct a reference Unitarity Triangle needed for new physics search • A lot of work to do before data-taking Yuehong Xie

  23. Offline track finding strategy T track Upstream track VELO seeds Long track (forward) Long track (matched) VELO track T seeds Downstream track Multiple pass track finding: Velo seeds T seeds Long tracks  highest quality for physics Downstream tracks  needed for efficient KS finding Upstream tracks  lower p, worse p resolution, useful for RICH1 pattern recognition Yuehong Xie

  24. CKM global fit Yuehong Xie

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