V ub measurements with the BaBar detector

Transcription

1 V ub measurements with the BaBar detector Daniele del Re University of California San Diego (on behalf of the BaBar collaboration) Outline: Inclusive b ulν measurements: Endpoint ν reconstruction m X on recoil of Breco tags m X vs q 2 on recoil of Breco tags Exclusive b ulν measurements: B 0 π + lν on recoil of Partial tags B (π,ρ,ω,η,η,a 0 )lν on recoil of Breco tags

2 08/17/04 Daniele del Re (UCSD) - ICHEP 04 2 b ulν decays and V ub Charmless semileptonic B decays, B X u lν, allow for the measurement of V ub Experimentally challenging, because B X c lν background (50 times higher) Tight cuts are needed and signal is analyzed in limited region of phase space extrapolation introduces uncertainties I will present two approaches: 1. Inclusive: look at kinematic quantities inclusively, use duality assumption and study b ulν. Once total B(b ulν) is obtained V ub can be extracted with small uncertainties. 2. Exclusive: study exclusive decays, use form factor to extract branching ratio and V ub b q V ub u q ν B 0,± π,ρ l π,ρ...

3 Three Experimental Techniques 08/17/04 Daniele del Re (UCSD) - ICHEP 04 3 Three ways of studying B X u lν decays in BaBar: Untagged Partial Tags Method signal recoil No recoil and ν reco from miss. momentum Recoil of partially reconstructed D* Recoil of fully recon. B D(*)X X u l ν X u l ν X u l ν (nh ±,mγ) π soft l(d 0 νx) D (*) X efficiency purity ν D 0 ν h l + π + h - γ γ π Fully Partial Untagged Had. Reco Y(4S) Reco ed Missed (or not all used) π + l - ν

4 Inclusive: Endpoint and ν Reco 08/17/04 Daniele del Re (UCSD) - ICHEP 04 4 Endpoint: one high energy electron (2.0GeV<E e< 2.6GeV) Untagged cut on the missing momentum ( pmiss = p + p ) and event shape cuts ee vis Continuum bkg using off-peak data and on-peak for E l >2.8GeV. BB bar bkg fitting E l spectrum (Deν, D * eν, D ** eν, D (*) πeν, X u eν and non semileptonic components) ν Reconstruction: Same approach as in the endpoint method (same p * >2GeV/c) but it selects a refined subset of events (much better S/B) b clν background separated by: 2 max β q 1 β sh = mb + q mbee 2mB 1+ β 4Ee 1+ β where β = 0.06 is the boost of the B in they(4s) rest frame. Less sensitive to non perturbative parameters than m X efficiency and shape modeling checked using pure B D 0 lν(x) control sample

5 Inclusive: Endpoint and ν Reco Endpoint Data (continuum sub) MC for BB background 80fb -1 ν reco Untagged Data (eff. corrected) MC BB ( Xeν ) = (1.73± 0.22 ± 0.33 ) 10 u exp (m,a) syst b 3 BB ( Xeν ) = (2.37 ± 0.22 ± 0.26 ± 0.34 ) u stat det syst bkg syst 0.30(m,a) syst b V ub exp (m,a) syst HQET = (3.94 ± 0.25 ± 0.37 ± 0.19 ) 10 b V ub = (4.63± 0.21 stat ± 0.25 det syst ± bkg syst 0.29 (m,a) syst 0.21 HQET ) 10 ± 08/17/04 Daniele del Re (UCSD) - ICHEP 04 5 b

6 Inclusive: m X vs. q 2 08/17/04 Daniele del Re (UCSD) - ICHEP 04 6 Extension of the already published result (PRL92,071802). Recoil selection and reconstruction of X system: One and only one lepton with p*> 1 GeV/c Correlation between lepton charge and B reco flavor Cut on the missing mass: M miss2 < 0.5GeV 2, charge conservation: Q tot =0 Partially reconstructed neutrino to reject B 0 D* l ν events kinematic fit (2-C): improve hadronic mass resolution Kaon veto Systematics due to lepton ID and tag normalization reduced by measuring a ratio of BRs R u/ sl = B( B Xulν ) B( B Xlν ) m X <1.7GeV and q 2 >8GeV 2 using the approach by Bauer et al. Dependence on shape function parameters is much reduced This approach and sample have been also used to unfold the m X spectrum.

7 Inclusive: m X vs. q 2 08/17/04 Daniele del Re (UCSD) - ICHEP fb -1 Stability of V ub as a function of the q 2 cut Unfolded m X spectrum and cumulative distribution for b ulν decays SF 2 SF GeV / c ; λ SF 2 SF GeV / c ; λ SF 2 SF GeV / c ; λ Λ = = Λ = = Λ = = BB ( Xlν ) = (0.88 ± 0.14 ± 0.13 ± 0.02 ) 10 u stat exp syst (m,a) syst V ub stat exp syst theo syst = (4.92 ± 0.39 ± 0.36 ± 0.46 ) 10 b 3 3

8 Exclusive Decays on Recoil 08/17/04 Daniele del Re (UCSD) - ICHEP 04 8 Semil Tags (B 0 D * lν): measurement in bins of q 2 (0-8-16GeV 2 /c 4 ) Signal yields are extracted by fitting M ν2 (event missing mass). (B D (*) X): 9 B X u lν modes: X u =π +,π 0,ρ +,ρ 0,ω,η,η,a 00, a 0 + Partial Tags Approach similar to the inclusive analysis but resonances are exclusively and fully reconstructed on recoil. 80fb -1 BB ( π lν) = (1.46 ± 0.27 ± 0.28 ) BB ( π lν) = (1.08 ± 0.28 ± 0.16 ) BB ( ρ lν) = (2.57 ± 0.52 ± 0.59 ) (upper limits for η,η,a 00, a 0+, results in backup slides) q 2 < 8GeV 2 /c 4 0 B + π l ν B π l ν B + ρ l ν

9 Conclusions 08/17/04 Daniele del Re (UCSD) - ICHEP 04 9 We measured V ub with different experimental inclusive techniques: SF param. from CLEO b sγ SF param. from BELLE b sγ Dominant error from modellization of non-perturbative effects (Shape Function, SF). Fitted on b sγ events; two fits available not yet combined: CLEO(2001), Belle(2004) We measured the unfolded m X distribution. In future b ulν decays can be used to put constraints on SF parameters.

10 08/17/04 Daniele del Re (UCSD) - ICHEP V ub measurements with BaBar method S/B Pros&Cons V ub (x10-3 ) Untagged inclusive Electron spectrum endpoint E e >2.0GeV Total rate using DeFazio-Neubert High statistics Duality valid for tight E e cuts? Bkg subtraction 3.94 ± 0.23 ± 0.42 exp theo Untagged inclusive E e vs q 2 and neutrino reconstruction E e >2.0GeV and s h <3.5 GeV 2 /c 4 Total rate using DeFazio-Neubert ~0.5 High statistics Lower syst. on shape functions Bkg subtraction ± 0.47 exp 0.36 theo inclusive m X analysis (1-D) m X <1.55 GeV/c 2 Total rate using DeFazio-Neubert ~1.7 Low background High resolution Low statistics Shape func. syst ± 0.40 exp theo inclusive m X vs q 2 analysis m X <1.7 GeV/c 2 and q 2 >8.0 GeV 2 /c 4 V ub using Bauer et al. ~2 Low background Very small syst. on SF param. Small statistics 4.92 ± 0.53 ± 0.46 exp theo Partial Tags exclusive Total rate using Form Factors calc Very small bkg ~no cut on kinem Small statistics ( BB ( l ) = (1.22 ± 0.26) 10 ) π ν

11 BACKUP SLIDES 08/17/04 Daniele del Re (UCSD) - ICHEP 04 11

12 Incl. Decays: Theory and Uncertainties Relevant issues hadronization effects and Fermi motion (b quark mass) non-perturbative parametrizations (Shape Function, SF) affected by large uncertainties. Two approaches to extract V ub and estimate theo. systematics: 1. DeFazio-Neubert paper (DFN ), tri-differential parametrization (E e,m X,q 2 ) to extrapolate. V ub extracted by V BB ( Xl u ν )1.61ps = (1 ± ± ) b τ ub OPE m B 2. q 2 vs m X approach by Bauer et al. (BLL ). partial BR with q 2 vs m X cut. V ub is extracted using V ub = BB ( Xlν; m < 1.7 GeV / c, q > 8 GeV / c ) 3 192π u X 2 5 BGm F b τ Dependence on SF (here in G) is much reduced. Theo. uncertainties on non-perturbative effects are evaluated using λ SF and Λ SF ellipse from b sγ from CLEO. Belle ellipse as an alternative. G Our default Alternative 08/17/04 Daniele del Re (UCSD) - ICHEP OPE CLEO BELLE

13 Excl. Decays: Theory and Uncertainties 08/17/04 Daniele del Re (UCSD) - ICHEP primary challenge is calculation of the form factors (containing hadronization effects and non-pert. contributions) Large uncertainties both extrapolation to full phase space and determination of V ub different theoretical models predict different q 2 distributions. discriminate among models by a precise measurement of differential BRs. The differential branching ratio can be related to V ub by the following relation db( B π l ν) dq d Gk F π 2 2 = V 2 ub τ 0 θ B 3 Wl f q (cos θwl ) 32π sin ( ) The bigger the integrated region the smaller the uncertainty on V ub 2 B ρlν

14 Inclusive: Endpoint Method 08/17/04 Daniele del Re (UCSD) - ICHEP Untagged one high energy electron (p * >1.1GeV) cut on the missing momentum ( pmiss = p + pvis ) and event shape cuts ee Continuum bkg using off-peak data and on-peak for E l >2.8GeV. BB bar bkg fitting E l spectrum (Deν, D * eν, D ** eν, D (*) πeν, X u eν and non semileptonic components) 80fb -1 For 2.0GeV<E l <2.6GeV we obtain (using DFN): BB ( Xeν ) = (1.73± 0.22 ± 0.33 ) 10 u exp (m,a) syst V ub exp (m,a) syst HQET = (3.94 ± 0.25 ± 0.37 ± 0.19 ) 10 Results are stable by applying tighter cuts on E l. b b 3 3 Main exp. systematics from signal modeling (~2-8%) and event selection efficiency (~6%), and D(*,**)eν description (~3%)

15 Inclusive: ν Reconstruction Method 08/17/04 Daniele del Re (UCSD) - ICHEP Same approach as in the endpoint method but it select a refined subset of events (much better S/B) : one high energy electron (p * >2GeV/c) is identified Untagged visible 4-momentum by using charged tracks (with particle ID) and energy deposits in the calorimeter missing momentum defined as pmiss = p + p ee vis and used to estimate neutrino cuts on the missing momentum and on the event shape. b clν background separated by: 2 max β q 1 β sh = mb + q mbee 2mB 1+ β 4Ee 1+ β where β = 0.06 is the boost of the B in the Y(4S) Data-MC agreement for p miss rest frame. Less sensitive SF parameters than m X efficiency and shape modeling checked using a pure B 0 D *+ lν(x) control sample

16 Inclusive: ν Reconstruction Method 08/17/04 Daniele del Re (UCSD) - ICHEP BR extracted by applying the following cuts E l >2GeV and s h <3.5 GeV 2 /c 4 The region s h >4.25 GeV 2 /c 4 is used to normalize the MC bkg to the data De Fazio-Neubert parametrization to extrapolate to full phase space Results: Untagged BB ( Xeν ) = (2.37 ± 0.22 ± 0.26 ± 0.34 ) u stat det syst bkg syst 0.30(m,a) syst V ub = (4.63± 0.21 stat ± 0.25 det syst ± bkg syst 0.29 (m,a) syst 0.21 HQET ) 10 ± b b 80fb -1 Main experimental systematics are from: Neutrals (~6%), K L (~7%), B X c lν modeling: (7%), stability in scans (~12%)

17 Inclusive: m X vs. q 2 08/17/04 Daniele del Re (UCSD) - ICHEP For Breco tags the reconstructed modes are (~1000 modes): B D(*)π, D(*)π π 0, D(*)3π, etc... The eff. is ~0.4% corresponding to ~4000 B/fb -1 (~2500 B/fb -1 B + and B ~1500 B/fb -1 B 0 ) The sample has low purity but it improves a lot once cuts on recoil are applied. p * > 1GeV Recoil selection and reconstruction of X system: One and only one lepton with p*> 1 GeV/c Correlation between lepton charge and B reco flavor Cut on the missing mass: M miss2 < 0.5GeV 2, charge conservation: Q tot =0 Partially reconstructed neutrino to reject B 0 D* l ν events kinematic fit (2-C): improve hadronic mass resolution Separate B X u lν in signal enriched and depleted based on veto on K ± and K S Systematics reduced by measuring R u/ sl BB ( Xl u ν ) = BB ( Xlν )

18 Inclusive: m X and m X vs. q 2 08/17/04 Daniele del Re (UCSD) - ICHEP 04 18

19 Inclusive: Unfold Had. Mass Spectrum 08/17/04 Daniele del Re (UCSD) - ICHEP m X spectrum can be converted in a universal variable by unfolding detector and selection effects. matrix, in general is non-invertible. relationship between measured and true spectra is: 80fb -1 where is the detector response xmeas = A x A true Unfolding method is based on procedure specified in hep-ph/ Systematics effects are properly taken into account. First and second moment of the b ulν m X distribution are extacted:

20 Comparison Using Belle s (m b,a) Ellipse 08/17/04 Daniele del Re (UCSD) - ICHEP ν reco CLEO ellipse V ub = (4.63± exp 0.36 theo ) 10 BELLE ellipse V ub = (4.99 ± exp 0.32 theo ) 10 Shift as expected. Theo. error reduced Endpoint V ub exp theo = (3.94 ± 0.23 ± 0.42 ) 10 3 V ub exp theo = (4.37 ± 0.24 ± 0.29 ) 10 3 Shift as expected. Theo. error reduced. With Belle ellipse cut variation on E e shows results that are less consistent among them m X V ub = (4.77 ± exp 0.45 theo ) V ub = (5.22 ± exp 0.32 theo ) 10 Shift as expected. Theo. error reduced m X vs q 2 V ub exp theo = (4.92 ± 0.53 ± 0.46 ) 10 3 V ub exp theo = (4.98 ± 0.56 ± 0.47 ) 10 3 Small change demonstrates that this method is less affected by SF params. With Belle ellipse theo. error is larger with respect to the others methods unfolding Unfolded spectrum seems to prefer Belle ellipse. Comparison with expected m X shape is better.

21 Exclusive: B 0 π + l - ν 08/17/04 Daniele del Re (UCSD) - ICHEP Partially reco ed sample is selected by following variables combined in a likelihood ratio: Some preselection cuts on shape and multiplicity lepton mom. (p*>1.3gev) soft pion mom. (50MeV<p π <200MeV) lepton-π Vtx probability Signal yield is extracted via M ν2 (mis. mass squared). D * is assumed to be collinear with the soft π Then: Eπ M * s D ED = Mν = ( p 0 pl p *) B D M * 2 2 π Partial Tags Additional cuts to: remove b-b bar background (like the charge correlation of the two reconstructed leptons and quantities related to the remainder of the event) remove continuum background (like the invariant mass and the opening angle of the two electrons) Right sign wrong sign

22 Exclusive: B 0 π + l - ν 08/17/04 Daniele del Re (UCSD) - ICHEP Measurement is performed in bins of q 2 Three bins: (q 2 < 8GeV 2 ; 8GeV 2 < q 2 <16GeV 2 ; q 2 >16GeV 2 ) Partial Tags Signal yields are extracted by fitting M ν2 (missing mass of the full event). We obtain: BB ( π lν) = (1.46 ± 0.27 ± 0.28 ) Main experimental systematics are due to detector (~8%), background composition (~10%) and shape of BB bar background (~15%) 80fb -1

23 Exclusive: B (π,ρ,ω,η,a 0 )lν 08/17/04 Daniele del Re (UCSD) - ICHEP B X u lν modes are studied: X u =π,π 0,ρ +,ρ 0,ω,η,η,a 00, a 0 + Resonances are exclusively reconstructed on the recoil. Similar cuts as for the inclusive m X analysis, such as p*>1gev and miss. mass squared of the event. Further per mode cuts to reject b c and Mass(MeV) Width(MeV) Decay modes π ± lν π 0 lν γγ crossfeed background are used 80fb -1 ρ ± lν π 0 π ± ρ 0 lν π π + ωlν π π + π 0 (89%) ηlν η lν γγ (39.4%) ππ + π 0 (22.6%) π 0 π 0 π 0 (32.5%) ρ 0 γ(29.5%) ηπ π + (44.3%) a 0 ± lν ηπ ± (~all) a 0 0 lν ηπ 0 (~all)

24 Exclusive: B (π,ρ,ω,η,a 0 ) lν 08/17/04 Daniele del Re (UCSD) - ICHEP Similar approach to the inclusive analysis but resonances are exclusively and fully reconstructed on recoil. We measure: BB ( π lν) = (1.08 ± 0.28 ± 0.16 ) BB ( ρ lν) = (2.57 ± 0.52 ± 0.59 ) BB ( + ην l ) < (90% CL) 4 BB ( + η' lν) < (90% CL) BB ( + al 0 ν) Ba ( 0 ηπ ) < (90% CL) 4 BB ( + al + ν) Ba ( + ηπ + ) < (90% CL) 0 0 These two results make use of: B B l B B l ( π ν) = 2 ( π ν) B B ρ l ν B B ρ l ν ( ) = 2 ( ) BB ρ lν BB ωlν ( ) = ( ) Systematics are dominated by MC statistics. Large systematics due to non-resonant contribution in B ρlν. Theoretical systematics are small (~ 4-7%) 0 B + ρ l ν 0 B + π l ν B π l ν fb -1

25 Results (80fb -1 ) 08/17/04 Daniele del Re (UCSD) - ICHEP B ± π 0 lν B 0 ρ ± lν BB ( π lν) = (0.89 ± 0.34 ± 0.12 ) BB ( π lν) = (0.91± 0.28 ± 0.14 ) B ± ρ 0 lν B 0 ωlν BB ( ρ lν) = (3.5± 1.1 ± 0.7 ) BB ( " ρ " lν) = (1.04 ± 0.39 ± 0.16 ) BB ( ων l ) = (1.26 ± 0.55 ± 0.24 )

26 Results (80fb -1 ) 08/17/04 Daniele del Re (UCSD) - ICHEP B ± ηlν B ± η lν + 4 BB ( ην l ) = (0.39 ± 0.41 ± 0.22 ) 10 stat syst BB l < CL + 4 ( ην) (90% ) BB ( η' lν) = (2.7 ± 1.2 ± 0.5 ) BB l < CL + 4 ( η' ν) (90% ) B ± a 0 0 l ν B 0 a ± 0 l ν + BB ( alν) Ba ( ηπ ) = (2.7 ± 1.4 ± 0.9 ) 10 stat syst BB ( alν ) < (90% CL) BB ( alν) Ba ( ηπ ) = (0.7 ± 1.6 ± 0.3 ) 10 stat syst 4 BB al < CL ( 0 ν ) (90% )

27 08/17/04 Daniele del Re (UCSD) - ICHEP V ub measurements with BaBar method S/B Results Untagged inclusive Electron spectrum endpoint E e >2.0GeV Total rate using DeFazio-Neubert Untagged inclusive E e vs q 2 and neutrino reconstruction E e >2.0GeV and s h <3.5 GeV 2 /c 4 Total rate using DeFazio-Neubert inclusive m X analysis (1-D) m X <1.55 GeV/c 2 Total rate using DeFazio-Neubert inclusive m X vs q 2 analysis m X <1.7 GeV/c 2 and q 2 >8.0 GeV 2 /c 4 Total rate and V ub using Bauer et al. Partial Tags exclusive Total rate by using Form Factors calc ~0.3 ~1.7 ~ V ub exp theo = (3.94 ± 0.23 ± 0.42 ) V ub = (4.63± exp 0.36 theo ) V ub = (4.77 ± exp 0.45 theo ) 10 V ub exp theo = (4.92 ± 0.53 ± 0.46 ) 10 BB ( π lν) = (1.22 ± 0.19 ± 0.18 ) BB ( ρ lν) = (2.57 ± 0.52 ± 0.59 )

28 V ub measurements 08/17/04 Daniele del Re (UCSD) - ICHEP 04 28

29 Conclusions for Exclusive Decays 08/17/04 Daniele del Re (UCSD) - ICHEP We also developed novel methods to measure EXCL. CHARMLESS decays: Breco tags: BB ( π lν) = (1.08 ± 0.28 ± 0.16 ) ρ l + ν = ± stat ± syst BB ( ) ( ) 10 Semil tags: AVERAGE BB ( π lν) = (1.46 ± 0.27 ± 0.28 ) BB ( π lν) = (1.22 ± 0.19 ± 0.18 )