Godang, Romulus

U., /South Alabama

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Proceedings of the DPF-2011 Conference, Providence, RI, August 8-13, 2011BABAR Results on CP Violation in B DecaysRomulus Godang (On Behalf of the BABAR Collaboration)Department of Physics, University of South Alabama, Mobile, Alabama 36688We report on the study of the decay B+ → D0(D̄0)K+ where D0 and D̄0 decaying to Kππ0, with the AtwoodDunietz and Soni (ADS) method. We measure the ratios Rads, R+, R− since the processes B+ → D0K̄+ andB+ → D0K+ are proportional to Vcb and Vub, respectively, are sensitive to rB and to the weak phase γ.1. IntroductionDuring recent years, several methods have been proposed to obtain the information on the Cabibbo-Kobayashi-Maskawa (CKM) [1, 2], phase angle γ. In the Standard Model (SM), the angle γ is the relativephase between b → cūs and b → uc̄s transitions as indicated in Fig. 1 and Fig. 2.buBusc+KD0+buBus+KD+0uFigure 1: Feynman diagrams for B+ → D̄0K+ (b → uc̄s))buBucs+KD+0uB+bu DKsuu0+Figure 2: Feynman diagrams for B+ → D0K+ (b → cūs)This phase angle γ can be measured using a variety of methods involving B-meson decays that mediated byeither only tree-level or both tree-level and loop-level amplitudes. A theoretical source of information on theangle γ is provided by B− → D(∗)K− decays. The D(∗) represents an admixture of D(∗)0 and D̄(∗)0 states.These decays exploit the interference between B− → D(∗)0K− and B− → D̄(∗)0K− that occurs when the D(∗)0and D̄(∗)0 decay to common final states.In the Gronau-London-Wyler (GLW) method [3, 4], the D0 meson is reconstructed based on Cabibbo-suppressed to CP -eigenstates, such as K+K−. In order to determine the angle γ from B± decays, we definethe two experimental observables direct-CP -violating partial decay rate asymmetries asACP± ≡Γ(B− → DCP±K−) − Γ(B+ → DCP±K+)Γ(B− → DCP±K−) + Γ(B+ → DCP±K+)=±2rBsinδBsinγ1 + r2B± 2rBcosδBcosγ(1)We also define the two ratios of charged averaged partial rates usingD meson decays to CP and flavor eigenstatesasRCP± ≡ 2Γ(B− → DCP±K−) + Γ(B+ → DCP±K+)Γ(B− → D0K−) + Γ(B+ → D̄0K+)= 1 + r2B ± 2rBcosδBcosγ (2)SLAC-PUB-15501Work supported in part by US Department of Energy under contract DE-AC02-76SF00515.SLAC National Accelerator Laboratory, Menlo Park, CA 940252 Proceedings of the DPF-2011 Conference, Providence, RI, August 8-13, 2011where DCP± refer to the CP eigenstates of D meson system and δB is the difference of their strong phases, andrB is the magnitude of the ratio of the amplitudes for each decayrB ≡|A(B− → D̄0K−)||A(B− → D0K−)| (3)In the Adwood-Dunietz-Soni (ADS) method [5, 6], the D0 meson is reconstructed in the doubly Cabibbo-suppressed decay D0 → K+π− from the favored b → c, while the D̄0 meson from the favored b → u suppressedamplitude is reconstructed in the favored decay D̄0 → K+π−. By ignoring the possible effect due to the Dmeson mixing, we define the the charge ratio of B+ and B− decay rate to the ADS final states R+ and R−,respectively.R+ =Γ(B+ → [K−π+]K+)Γ(B+ → [K+π−]K+) = r2B + r2D + 2rBrDkD cos(γ + δB + δD) (4)andR− =Γ(B− → [K+π−]K−)Γ(B− → [K−π+]K−) = r2B + r2D + 2rBrDkD cos(γ − δB + δD) (5)where rB and rD are the suppressed to favored B and D amplitude ratios asrB ≡|A(B+ → D0K+)||A(B+ → D̄0K+))|(6)andr2D =Γ(D0 → K+π−)Γ(D0 → K−π+) (7)and δB and δD are the strong phase differences between the two B and the two D, respectively.2. BABAR DetectorThe results presented in this paper are based on the entire BB̄ data sample collected with the BABAR detectorat the PEP-II asymmetric-energy B factory at the SLAC National Accelerator Laboratory. The BB̄ pairs areproduced from the decays of Υ (4S) resonance (on-resonance) that originate in collisions of 9.0 GeV electronsand 3.1 GeV positrons. The on-resonance data sample has a mean energy of 10.58 GeV and an energy rmsspread of 4.6 MeV . The off-resonance (continuum) data sample has a center-of-mass (CM) energy 40 MeVbelow the resonance.A detailed description of the BABAR detector and the algorithms used for track reconstruction and particleidentification is provided elsewhere [7]. A brief summary is given here. High-momentum particles are recon-structed by matching hits in the silicon vertex tracker (SVT) with track elements in the drift chamber (DCH).Lower momentum tracks, which do not leave signals on many wires in the DCH due to the bending induced by amagnetic field, are reconstructed in the SVT alone. Electrons are identified by the ratio of the track momentumto the associated energy deposited in the calorimeter (EMC), the transverse profile of the shower, the energyloss in the drift chamber, and information from a Cherenkov detector (DIRC). The BABAR detector Monte Carlosimulation is based on GEANT4 [8]. We use EVTGEN [9] to model the kinematics of B meson decays and useJETSET [10] to model off-resonance process e+e− → qq (q = u, d, s, or c quark).3. Data SampleIn the GLW method [11], we use (467±5)×106 BB pairs, approximately equally divided into B0B0 and B+B−.The data have been collected in the years from 1999 until early 2008. We have reconstructedB± → DK± decays,with D mesons decaying to non-CP (Kπ), and CP -even (K+K+, π+π−), and CP -odd (K0sπ0,K0sφ,K0sω)eigenstates.In the ADS method, we use two results where D0 mesons are reconstructed into two modes, D0 → K+π− [12]and D0 → K+π−π0 [13], respectively. In D0 → K+π− mode we use a data of (467 ± 5) × 106 BB pairs. WeProceedings of the DPF-2011 Conference, Providence, RI, August 8-13, 2011 3present a search of the decays B− → D(∗)K−, where the neutral D mesons decay into K+π− final state (WS).In this paper, we first applied to B− → D(∗)π−, where the neutral D mesons decay into the Cabibbo-favored(K−π−) and doubly suppressed mode. In D0 → K+π−π0, we use (474 ± 5) × 106 BB pairs. An additionaloff-resonance data sample of 45 fb−1 , collected at a center-of-mass energy 40 MeV below the Υ (4S) resonance,is used to study the e+e− → qq background. In this paper, we studied the decays D0 and D̄0 in which decaysto K∓π±π0 final states.4. Measurement of the CKM angle γ4.1. CP Observables in B± → DCPK± (GLW)We identify signal B → DK and B → Dπ candidates using two defined kinematic variables. The firstvariable is the difference between the CM energy of the B meson (E∗B) and the beam energy (∆E) and thebeam-energy-substituted mass (mES), respectively.∆E = E∗B −√s/2 (8)We estimate the irreducible background yields in our sample by exploiting the fact that the D invariant massdistribution fir this background is approximately uniform, while the signal event peaks around the nominal Dmass.Figure 3 shows the ∆E projections of the final fits to the CP subsamples. The curves are the full PDF (solid,blue) and B → Dπ (dash-dotted, green) stacked on the remaining backgrounds (dotted, purple). The regionbetween the solid and the dash-dotted lines represents the contribution of B → DK. Figure 4 shows mESprojections as well as projections to the fit to the D0 → K−π+ flavor mode. The line definitions are the sameas described in Fig. 3. We obtain the most precise measurements of the GLW parameters ACP± and RCP±:ACP+ = 0.25± 0.06(stat)± 0.02(syst) ACP− = −0.09± 0.07(stat)± 0.02(syst) (9)RCP+ = 1.18± 0.09(stat)± 0.05(syst) RCP− = 1.07± 0.08(stat)± 0.04(syst) (10)We measure a value of ACP+ which is 3.6 standard deviations from zero, which constitutes the first evidencefor direct CP violation in B → DK decays.E (GeV)∆-0.05 0 0.05 0.1Events / ( 0.01 GeV )0102030405060Events / ( 0.01 GeV )- DK→ -B,-K+ K→D -π+π →D a)E (GeV)∆-0.05 0 0.05 0.1Events / ( 0.01 GeV )0102030405060Events / ( 0.01 GeV )+ DK→ +B,-K+ K→D -π+π →D b)Figure 3: ∆E projections of the fits to the data, split into subset of define CP of the D candidate and charge of the Bcandidate: a) B− → DCP+K− and b) B+ → DCP+K+4 Proceedings of the DPF-2011 Conference, Providence, RI, August 8-13, 2011)2 (GeV/cESm5.25 5.26 5.27 5.28 )2Events / ( 0.0008 GeV/c0510152025303540 )2Events / ( 0.0008 GeV/c- DK→ -B,-K+ K→D -π+π →D a))2 (GeV/cESm5.25 5.26 5.27 5.28 )2Events / ( 0.0008 GeV/c0510152025303540 )2Events / ( 0.0008 GeV/c+ DK→ +B,-K+ K→D -π+π →D b)Figure 4: mES projections of the fits to the data, split into subset of define CP of the D candidate and charge of the Bcandidate: a) B− → DCP+K− and b) B+ → DCP+K+4.2. CP Observables in B− → DK− and B− → D∗K− (ADS)We reconstruct B− → D(∗)K− and B− → D(∗)π− with the D meson decaying to K−π+ (RS = right-sign)and K+π− (WS = wrong-sign). Charged conjugate reactions are assumed throughout this paper. For decaysinvolving a D∗, both D∗ → Dπ0 and D∗ → Dγ are reconstructed. To study the BB background for each signalcategory and charge combination (RS and WS) we use a sample of e+e− → Υ (4S) → BB Monte Carlo eventscorresponding to about 3 times the data luminosity.In this paper the off-resonance background events are reduced by using a multilayer perceptron artificialneural network with 2 hidden layers, available in the framework of TMVA package [14]. We use the neuralnetwork to select the discriminating variables.Figure 5 shows the projections on mES and neural network (NN) of the fit results for DK+ mode for samplesenriched in signal with the requirements NN > 0.94 for mES projections or 5.2725 < mES < 5.2875 GeV/c2.The point with error bars are data. The curves represent the fit projections for signal plus background (solid),the sum of all background components (dashed), and qq background only (dotted). The results of fits to theB+ and B− sampleR+ = (2.2± 0.9± 0.3)× 10−2 R− = (0.2± 0.6± 0.2)× 10−2 (11)and we extracted the variables r(∗)BrB = (9.5+5.1−4.1)% r∗B = (9.6+3.5−5.1)% (12)4.3. CP Observables in B± → [K∓π±π0]K± (ADS)In this paper the off-resonance background events, in contrast to BB events, are characterized by a jet-liketopology. We use a Fisher discriminant F to discriminate between the two categories of events. The Fisherdiscriminant is a linear combination of six variables. We choose the coefficients of the linear combinationto maximize the separation between the signal and the off-resonance background. For the signal the Fisherdiscriminant F peaks at 1 and -1 for the off-resonance events. Since the correlations among the variables arenegligible, we write the PDFs as products of one dimensional distributions of the mES and F . We use MCsamples to check the absence of the correlation between these distributions.The PDF parameters are derived from data sample when possible in order to reduce the systematic uncer-tainties. The parameters for the continuum events are determined from the off-resonance data sample. Weuse the data sample of B+ → Dπ+ with D → K+π−π0 to extract the parameters for the mES distribution.The parameters for the non-peaking BB distributions and the signal Fisher PDF are determined from the MCsample.Proceedings of the DPF-2011 Conference, Providence, RI, August 8-13, 2011 5)2 (GeV/cESm5.2 5.22 5.24 5.26 5.28 5.3)2Events/(2.5 MeV/c02468101214)2Events/(2.5 MeV/c (a)NN-1 -0.5 0 0.5 1Events / ( 0.02 )051015202530Events / ( 0.02 )(d)Figure 5: Projections on mES and neural network (NN) of the fit results for DK+ mode for samplesB(*)r0 0.05 0.1 0.15 0.2 0.25 0.31- confidence level00.10.20.30.40.50.60.70.80.91σ1 σ2 -K0D-K0D*1- confidence level (deg)γ-150 -100 -50 0 50 100 1501- confidence level00.10.20.30.40.50.60.70.80.91σ1 σ2 1- confidence levelFigure 6: Constraints on r(∗)Bfrom combined B− → [Kπ]K− ADS measurements (left) and constraints on angle γ fromcombined B− → D(∗)[K+π−]K− ADS measurements (right).The fits to the mES for F > 0.5 and the Fisher discriminant F distribution with mES > 5.27 GeV/c2 forthe combined B+ and B− samples are shown in Fig. 7 and Fig. 8, respectively. The data are well describedby the overall fit results (solid blue line) which is the sum of the signal, continuum background, non-peakingBB background, and peaking BB background. We have presented a study of the decays B± → D0K± and (a)5.2 5.22 5.24)2Events/(1.8 MeV/c1020)2 (GeV/cESm5.26)2Events/(1.8 MeV/c0 5.2 5.22 5.24 5.26)2Events/(18 MeV/c100200300 Fullsignal peakBB non-peakBBcontinuum)2 (GeV/cESm)2Events/(18 MeV/c0Figure 7: Distribution of mES (a,b) with F > 0.5.B± → D̄0K±, in which the D0 and D̄0 mesons decay to K±π±π0 final state using the ADS method. The finalresults areR+ = (5+12+1−10−4)× 10−3 R− = (12+12+2−10−4)× 10−3 (13)6 Proceedings of the DPF-2011 Conference, Providence, RI, August 8-13, 2011(c)-2 0 2Events / ( 0.12 )050100150FisherEvents / ( 0.12 )(d)-2 0 2Events / ( 0.12 )0100200300FisherEvents / ( 0.12 )Figure 8: Distribution of F (c,d) with mES > 5.27 GeV/c2.from which we obtain 90% probability limitsR+ < 23× 10−3 R− < 29× 10−3 (14)Following a Bayesian approach [15, 16], the probability distributions for the R+ and R− ratio obtained in the fitare translated into a probability distribution for rB . Figure 9 shows the posterior probability distribution. SinceBr0 0.1 0.2 0.3 0.4 0.5Probability density00.20.40.60.8-310×Probability densityProbability densityFigure 9: Bayesian posterior probability density function for rB from our measurement of R+ and R− and the hadronicD decay parameters rD, δD, and kD.the measurements are not statistically significant, we integrate over the positive portion of that distributionand obtain the upper limit rB < 0.13 at 90% probability, and the rangerB ∈ [0.01, 0.11] (15)at 68% probability and 0.078 as the most probable value.AcknowledgmentsThe author would like to thank the organizers of DPF 2011 for their excellent program and kind hospitality.The supports from the BABAR Collaboration, the University of South Alabama, and the University of Mississippiare gratefully acknowledged. This work was supported by the U.S. Department of Energy under grant No. DE-FG02-96ER-40970.Proceedings of the DPF-2011 Conference, Providence, RI, August 8-13, 2011 7References1 N. Cabibbo, Phys. Rev. Lett. 10, 531 (1963).2 M. 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BABAR Results on CP Violation in B Decays

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