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B 0  rp , B 0  rr and the measurement of a at LHCb

B 0  rp , B 0  rr and the measurement of a at LHCb. Patrick Robbe, LAL Orsay, 26 September 2006, for the LHCb Collaboration. Introduction. How to measure a a Measurements in LHCb: Performances and sensitivity with B 0  p + p - p 0 Performances and sensitivity with B 0  rr

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B 0  rp , B 0  rr and the measurement of a at LHCb

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  1. B0rp, B0rr and the measurement of a at LHCb Patrick Robbe, LAL Orsay, 26 September 2006, for the LHCb Collaboration

  2. Introduction • How to measure a • a Measurements in LHCb: • Performances and sensitivity with B0  p+p-p0 • Performances and sensitivity with B0  rr • Combination and LHCb sensitivity to a

  3. The a angle of the Unitarity Triangle CKM Matrix g b Vud* Vtb Vtd* a Vub B0 decays to charmless CP final states are sensitive to b+g = p-a, the relative weak phase between tree and penguin contributions.

  4. Current status of a measurements • Direct measurements at B factories: • B pp • B rr • B rp • Combined (with rp from BABAR only): • Indirect (without direct a measurements): rp + a = ° 60 . 9 BABAR : ( 91 . 5 ) - 18 . 2 rp + + a = ° È ° 13 . 5 12 . 0 Belle : ( 83 . 5 ) ( 176 . 5 ) - - 22 . 8 46 . 1

  5. a measurements in LHCb • In LHCb, a measurement is performed with: • Time dependant Dalitz analysis of B0  p+p-p0 • SU(2) analysis of B0  r+r-, B0  r0r0 and B+  r+r0. • Typical event in LHCb: • Challenge to reconstruct multi-track final states and final states with p0.

  6. LHCb Calorimeter • 4 devices: Scintillator Pad Detector (SPD), Preshower (PRS), Electromagnetic Calorimeter (ECAL) and Hadronic Calorimeter (HCAL). • Provides with acceptance 30 mrad to 300 (250) mrad: • Level-0 trigger information (high transverse momentum hadrons, electrons, photons and p0, and multiplicity) • Kinematic measurements for g and p0 with sE/E = • Particle ID information for e, g, and p0. 10%  1% E

  7. e Merged Resolved Transverse energy (GeV) p0 reconstruction at LHCb • Resolved p0: reconstructed from 2 isolated photons • sm = 10 MeV/c2 • Merged p0: pair of photons from high energy pion which forms a single ECAL cluster, where the 2 showers are merged. • The pair is reconstructed with a specific algorithm based on the expected shower shape. • sm = 15 MeV/c2 • Reconstruction efficiency: ep0 = 53 % for B0p+p-p0 Resolved π0 Merged π0 π0 mass (Mev/c²)

  8. a with Brp (1) • Assuming that the decay B0p+p-p0 proceeds through the rpp resonance, 6 interfering decay modes contribute to the p+p-p0 Dalitz plot: • B0r+p-, B0r-p+, and B0r0p0 • B0r-p+, B0r+p-, and B0r0p0 • Tree or Penguin transitions contribute to each decay mode. The time dependant analysis of the tagged Dalitz plot gives enough information to determine at the same time a and the relative amplitudes and strong phases between all processes. [Snyder, Quinn, 1993] B0 s- s- s- r0p0 r+p- s+ s+ s+ B0 s- s- r-p+ s- s+ s+ s+ t (ps) 0 2 6 10

  9. a with Brp (2) Maximize a Likelihood with 9 parameters a ( + background fractions r ) Theoretical ingredients Phenomenological ingredients The ρ line-shape a = ( a , T-+ , f-+ , T00 , f00 , P-+ , d-+ , P+- , d+- ) é ù N N bkg evt ( ) Õ å å 2 p + - p + - a = - w a + Ä s s s 3 tag 3 bkg bkg , r ( 1 r ) ξ (s , s , t ) M (s , s , t , ) r G ( , , ) ê ú L L + - k k k b b k k k k t s s ê ú ë û = k b B , B bkg Experimental (mis)tagging tag = +1/0/-1 Experimental acceptances Background contamination Experimental resolutions Event Yield Experimental ingredients

  10. Brp Selection • Multivariate Selection based on: • Particle Identification: Charged pion ID, neutral p0 clusters, … • Kinematical criteria: transverse momenta, … • Vertexing criteria: impact parameters, vertex isolation • Combined PDF: Momenta sum π± identification signal e,μ,K,p inclusive bb π± ΣPt(π±o)/B (GeV) ΔLL(π) Inclusive bb Signal π+π- vertex isolation vertices-B-momentum. alignment signal inclusive bb inclusive bb signal dmin (mm) XPDF Θ(PV-SV,PB)

  11. Brp Results and Backgrounds • 1 million of fully simulated Brp events ➛ 10 days of LHCb at 2.1032 cm-2s-1 ➛ 1300 events selected = BABARρπstatistics up to 2004, efficiency of 7x10-4 ➛50% with merged π0s • 33 millions of inclusive BB events ➛15 min of LHCb at 2.1032 cm-2s-1 • 3 signal events selected and passing the trigger • 5 background events in side-bands (D(s)π,D(s)r) and rejected by the trigger N3π = 14x103 events / 2 fb-1 Consistent with : B/S = 20% (B/S < 80% @ 90% CL) • Few millions of specific charmless B decays Assume B/S = 1 in the following

  12. Brp Fit: Signal acceptance • Acceptance in Dalitz plane: e (%) produced selected s- s- s+ s+ The lower corner of the Dalitz plot is highly depopulated due to the cut on the p0 energy. However, the upper region of the Dalitz figure contains enough information to allow the a extraction. • Proper time acceptance: ε (%) Region of low lifetime depopulated due to the large impact parameters required in the selection t (ps)

  13. B mass (GeV/c²) Δt (ps) Brp Fit: Resolution and Tagging σ = 50 fs σ = 60 MeV/c² • Expected resolutions: Resolutions are dominated by calorimeter energy resolution • Flavour tagging: • Performance estimated from full MC simulation: → Tagging efficiency ε= 40±2 % → Wrong tag fraction ω= 31±2 % εeff = ε(1-2ω)²= 6±2 % • The tagging performance actually depends on the position in Dalitz plane. • In the real experiment, the wrong tag fraction will be extracted from data (for example, using the self-tagged K+π-π0 decay) • NB : the untagged sample also enters in the global fit.

  14. B(Kρ,K*π)Kππ ρ resonant Flat + + 5% 50% 45% LHCb Sensitivity onawith Brp: Method • Assume a set of theoretical parameters agen • Simulate a set of toy experiments accordingly Yield = 104 signal events = 2 fb-1 • Simulate backgrounds according to rgen ratios Background structure poorly known. Assume B/S = 1 and use a mixture made of: The same proper time distribution, resolutions and tagging dilution as signal are assumed. On real data, information on background will be extracted from the side-bands. • Simulate the experimental effects (resolution, acceptance, wrong tag, ...) • Maximize the likelihood with respect to: afitand the background ratios rfit(12-D fit)

  15. <D²>=2 <D²> = 6.5 gen αgen LHCb Sensitivity onawith Brp: Results 70 toy experiments super-imposed ( L = 2 fb-1) 15% converge to a pseudo-mirror solution. Fraction decreases with increasing luminosity (<1% @ 10 fb-1) Average αgen Distribution of fit error 85% converge to the correct solution The correct solution generally corresponds to a deep (if not deepest) minimum. 90% of experiments with σ < 10°

  16. ρ η LHCb Sensitivity ona A typical LHCb toy experiment (2fb-1): Current BABAR measurement (Stat + Syst):

  17. a Measurement Systematics Estimates What is the impact of imperfect knowledge of quantities included in the likelihood: Extracting  via the 3π Dalitz analysis requires an accurate control of the inputs The final analysis will be much more difficult than this prospective study → Not likely to be a ‘first year’ analysis for LHCb but very promising results

  18. a with Brr • Method: Recent measurements showed that the B0r+r- decay is predominantly longitudinally polarized, and then a pure CP eigenstate. • The time dependant asymmetry gives access to aeff = a + Da, shift due to penguin contributions: • Constraints on Da can be obtained measuring B+r+r0 and B0r0r0 branching fractions. • Analysis similar to B0p+p- but with many advantages: • B(B+r+r0) and B(B0r+r-) 5 times larger, • B(B0r0r0) is small: (1.20.40.3)x10-6 • Time dependant analysis of B0r0r0 could also provide additional information. with Aoo A+-/√2 Δα Aoo A+-/√2 A+o= A-o

  19. LHCb performances for B0r+r-and B+r+r0 • Selection for B0 ρ+ρ- and B+ρ+ρ0 • Multivariate selection as for B ρπ • 2 and 1 neutral pion(s) in the final state, respectively • Overall efficiency : 0.01 % and 0.045 % • B mass resolution dominated by ECAL resolution : 80 MeV/c² and 52 MeV/c² • Proper time resolution : 85 fs and 47 fs • Expected annual yields (2 fb-1): • B ρρ0 : 9000 B/S ~ 1 • B0ρ+ρ- : 2000 B/S < 5 @ 90%CL One year of LHCb probably not competitive with current B factory performance. Will need several years to provide a sizeable contribution to C+-, S+- measurement. The main contribution of LHCb to the Bρρ analysis could be the improvement of the measurement of the B0ρ0ρ0 mode

  20. LHCb performances for B0r0r0 σ(mB) = 16 MeV/c² Selection: • multivariate selection • overall efficiency : 0.16% Expected annual yield ( 2fb-1/year): B mass Background contamination: σ(t) = 32 fs Proper time

  21. a with Brr LHCb sensitivity(1) B00 = 1.2x10-6 σB00/B00 = 20% • Measuring B00 • + Measuring C00 with time dependant analysis C00 = 0.51 ; σC00= 0.4 • + Measuring S00 S00 = 0.30 ; σS00= 0.4 With 2 fb-1: 2006 WA :

  22. a with Brr LHCb sensitivity(2) • a resolution depends on the actual C00/S00 central value The current indirect measurement is weakly constrained e.g. large S00: S00 = -0.61 ; σS00= 0.4

  23. rr and rp : 2 fb-1 at LHCb WA 2006 • LHCb a measurement with rp • LHCb r0r0 (C00/S00) + B factories r+r- and r0r+ • LHCb p+p- (C+-/S+-) + B factories p+p0 and p0p0 LHCb 2 fb-1

  24. Conclusions At LHCb, 2 complementary analyses developed to measure a: • The time-dependent B0(ρπ)0 Dalitz plot analysis: • No ambiguity on  in [0,π] but pseudo-mirror solutions. • With 2 fb-1 LHCb may achieve σstat< 10° on  • Require an accurate control of the ρ-lineshapes and the experimental efficiencies. • Ambitious but promising. • Probably several years to setup the analysis. • The time-dependent B0r+r- asymmetry and SU(2) analysis: • 8-fold ambiguity on  in [0,π]. • Several years of LHCb needed to improve the current B0ρ+ρ- measurement. • With 2 fb-1the main LHCb contribution could be the improved measurement of B0 ρ0ρ0. • Accessing the ρ0ρ0 time-dependent asymmetry will reduce the degeneracy of mirror-solutions and improve the current  determination. • Performance strongly depends of the actual values of C00 and S00. • During LHCb era the stat. error on  could reach the few degrees level SU(2) breaking effects, electroweak penguin contributions could be an issue

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