We present improved measurements of the branching fractions and CP
asymmetries in the two-body decays B0 --> pi0 pi0, B0 --> K0 pi0 and B0 --> K+
pi- as well as the quasi two-body B0 --> K_1(1270)+ pi- and B0 --> K_1(1400)+
pi- decays. These updated measurements are made using the complete set of BABAR
data taken at the Y(4S) resonance, collected between 1999 and 2007 at the
PEP-II collider at SLAC.Comment: Parallel talk at ICHEP08, Philadelphia, USA, July 2008. 4 pages,
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Ofte, Ingrid

English

arXiv

The authors present improved measurements of the branching fractions and CP asymmetries in the two-body decays B{sup 0} {yields} {pi}{sup 0}{pi}{sup 0}, B{sup 0} {yields} K{sup 0}{pi}{sup 0} and B{sup 0} {yields} K{sup +}{pi}{sup -} as well as the quasi two-body B{sup 0} {yields} K{sub 1}(1270){sup +} {pi}{sup -} and B{sup 0} {yields} K{sub 1}(1400){sup +} {pi}{sup -} decays. These updated measurements are made using the complete set of BABAR data taken at the Y(4S) resonance, collected between 1999 and 2007 at the PEP-II collider at SLAC

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Work supported in part by US Department of Energy contract DE-AC02-76SF0051534th International Conference on High Energy Physics, Philadelphia, 2008SLAC-PUB-13429BABAR-PROC-08/097hep-ex/0810.1067Charmless Two-Body and Quasi-Two-Body B-decays at BABARIngrid OfteSLAC, 2575 Sand Hill Road, Menlo Park, CA 94025, USARepresenting the BABAR CollaborationWe present improved measurements of the branching fractions and CP asymmetries in the two-body decays B0 →pi0pi0, B0 → K0pi0 and B0 → K+pi− as well as the quasi two-body B0 → K1(1270)+pi− and B0 → K1(1400)+pi−decays. These updated measurements are made using the complete set of BABAR data taken at the Y(4S) resonance,collected between 1999 and 2007 at the PEP-II collider at SLAC.1. INTRODUCTIONB-mesons decaying into charmless hadronic final states are rare events. The leading tree diagrams are CKMsuppressed and loop (penguin) diagrams typically contribute at comparable magnitude. Decay rates and CP asym-metries may deviate from Standard Model expectations if yet-unknown heavy particles contribute in the loop. Theabundance of related decays allows for multiple independent measurements of CKM matrix parameters and CPviolation.Direct CP asymmetry occurs when the magnitude squared of a decay amplitude differs from that of its CPconjugate process, |A(B¯ → f¯)|2 6= |A(B → f)|2. Experimentally, we measure ACP in terms of different yield ofone process compared to that of the CP conjugate process. We expect non-zero ACP if two or more amplitudes ofcomparable size contribute with different weak (φ) and strong (δ) phases, e.g. in the B → Kpi decay, where wehave similar-sized contributions from tree and penguin amplitudes. The direct CP asymmetry is given by ACP =2 sinφ sin δ/(|T/P | + |P/T | + 2 cosφ cos δ). Theoretically, the amplitudes ratio |T/P | and phase δ are extremelydifficult to compute, as these quantities involve long-distance effects.The hadronic uncertainties cancel to some extent in appropriately constructed ratios. Sum rules [1] based on isospinand flavor SU(3) symmetries relate decay rates and asymmetries in different final states and can make precision testspossible in spite of the hadronic uncertainties.In these proceedings we present preliminary results from the analysis of four charmless B0 decay modes to two-body or quasi two-body final states using data from the BABAR [2] detector. The analyses are described in detailin [3, 4].2. ANALYSIS TECHNIQUEWe use 467 million Υ (4S) decays collected between 1999 and 2007, which amounts to about 22% more BB pairsthan used previously. Improvements in track reconstruction have further increased the efficiency of our analyses andthereby improved the statistical significance. For some of the decay modes, additional sensitivity is obtained fromimproved analysis techniques.Charmless two-body and quasi two-body decays of B mesons typically have high reconstruction efficiency andlarge qq¯ backgrounds. To reduce this background we make use of event-shape variables. At the PEP-II collider, theB-mesons are produced almost at rest in the center-of-mass (CM) frame, and their decay products are isotropicallydistributed. Backgrounds from e+e− → qq¯ events, on the other hand, are produced with larger momenta and havea more jet-like event structure. To distinguish these from signal B decays, we use one or more of the following1October 2008Presented at the 34th International Conference on High Energy Physics, 7/29/2008 to 8/5/2008, Philadelphia, PA, USA34th International Conference on High Energy Physics, Philadelphia, 2008observables, all evaluated in the CM frame: the sums L0 ≡∑i|p∗i| and L2 ≡∑i|p∗i| cos2 θ∗i, where p∗iare themomenta and θ∗i are the angles with respect to the thrust axis [5] of the B candidate, of all tracks and clusters notused to reconstruct the signal B-meson candidate; | cos θ∗S|, where θ∗Sis the angle between the sphericity axes [6] ofthe B candidate’s decay products and that of the remaining tracks and neutral clusters in the event; | cos θ∗B|, whereθ∗Bis the angle between momentum vector of the signal B and the beam axis; and | cos θ∗T|, where θ∗Tis the anglebetween the thrust axis of the signal B-meson’s daughters and the beam axis.Correctly reconstructed B decays are selected based on their kinematic signatures exploiting the fact that each ofthe B mesons have half of the precisely-known beam energy. Most commonly we use the energy substituted mass,mES =√(ECMbeam)2 − (pCMB)2 and the energy difference, ∆E = EB −√s/2, where√s is the total CM energy. Inthe case of B0 → K0Spi0, we instead use a kinematic constraint. We define mmiss = |√s − qˆB |, where qˆB is thefour-momentum of the reconstructed B0 after a B0 mass constraint has been applied. Also used is mB , the invariantmass of the signal B.In the modes where the B0 decays into a CP eigenstate, B0CP, we rely on a multivariate technique [7] to determinethe flavor of the other B, Btag. Finally, we extract the signal yield and CP asymmetries via an extended unbinnedmaximum likelihood (ML) fit to the kinematic and event-shape variables.3. RESULTS3.1. Branching fraction of B0 → K0pi0We reconstruct K0S→ pi+pi− and pi0 → γγ and measure the branching fraction via a ML fit to mmiss, mB , L2/L0and cos θ∗B, as well as ∆t, the proper time difference between the decay of the signal B and that of the other B inthe event, to enable extraction of CP asymmetry [8, 9]. The fit extracts a yield of 556 ± 32 signal events. Witha selection efficiency ε = (34.2 ± 1.2)%, this translates into a branching fraction measurement B(B0 → K0pi0) =(10.1± 0.6± 0.4)× 10−6, in agreement with, and superseding, BABAR’s previous result [10].Sum rules assuming flavor SU(3) symmetry [1] can be used to make a prediction based on the other, more preciselymeasured, Kpi final states: 2Γ(B0 → K0pi0) + 2Γ(B+ → K+pi0) = Γ(B+ → K0pi+) + Γ(B0 → K+pi−). Using thisformula, we obtain the prediction B(B0 → K0pi0)Sum rule = (8.4 ± 0.8) × 10−6. Our measurement is in reasonableagreement with this prediction within experimental uncertainties.3.2. Direct CP asymmetry in B0 → K+pi−This decay is a self-tagging mode and the CP asymmetry is apparent from the different decay rates of B0 → K+pi−and B0 → K−pi+. We reconstruct the B-meson from two oppositely charged tracks that are both assumed to bepions for the purpose of calculating ∆E. The signal pipi and Kpi yields are extracted from a ML fit to the kinematicvariables mES and ∆E, a F isher discriminant based on L2 and L0, and ∆t and B flavor tagging in order to extract thetime-dependent CP parameters for pi+pi− (see [11]). In addition, particle-identification observables (the Cherenkovangle ΘC in the DIRC [12] and ionization-energy loss dE/dx in the main tracking detector) are used to separate Ktracks from pi tracks. The ∆E distribution is offset from zero for Kpi events, seen in figure 1, which further aids indistinguishing the pipi and Kpi events. The different rates for B0 and B0 is apparent from the figure and we measureAK+pi− = −0.107± 0.016+0.006−0.004 with 6.1σ significance, in agreement with and superseding our previous result [13].3.3. Branching fraction and time-integrated CP asymmetry in B0 → pi0pi0The decay B0 → pi0pi0 is useful for the extraction of the CKM angle α from an isospin analysis of the B → pipisystem [15]. Combined with the other two B → pipi modes, the rates and CP asymmetries can determine |∆α| =|α− αeff | with a four-fold ambiguity. This evaluation has been presented in [11].2Presented at the 34th International Conference on High Energy Physics, 7/29/2008 to 8/5/2008, Philadelphia, PA, USA34th International Conference on High Energy Physics, Philadelphia, 2008E (GeV)∆-0.1 0 0.1Events / (10 MeV)0100200300400Events / (10 MeV)BABARPreliminaryB0→ K+pi−B0 → K−pi+Figure 1: sPlot [14] of ∆E for K±pi∓ events. The different decay rates for B0 → K+pi− and B0 → K−pi+ are apparent.We reconstruct the B0 from a pair of pi0 candidates. pi0 → γγ are formed from pairs of clusters in the EMC thatare isolated from any charged tracks. For this mode, we also use pi0 candidates from a single EMC cluster containingtwo adjacent photons (a merged pi0), or one cluster and two tracks from a photon conversion to an e+e− pair insidethe detectors. The yield and time-integrated CP asymmetry is obtained from a ML fit to the kinematic variables mESand ∆E, as well as the output of a neural network based on event-shape variables. The use of a neural network hasimproved the statistical sensitivity compared to the previous result. The time-integrated CP asymmetry is measuredby the B-flavor tagging algorithm.We use an improved background model for the ML fit, where we allow the background shape parameter to belinearly dependent on the neural network output observable. With this, we obtain a better fit to the data than in ourprevious measurement [16]. We observe Npi0pi0 = 247± 29, with an efficiency ε = (28.8± 1.8)%. This results in thebranching fraction measurement B(B0 → pi0pi0) = (1.83± 0.21± 0.13)× 10−6. We also obtain the time-integratedCP asymmetry measurement ACP = −Cpi0pi0 = 0.43± 0.26± 0.05.3.4. Branching fraction of B0 → K1(1270)+pi− and K1(1400)+pi−There has been interest recently in B-meson decays to an axial vector and a pseudoscalar meson. Experimentshave found relatively large branching fractions and more measurements are needed to improve our understanding ofthese decays. These B0 decays are also of interest to the measurement of the CKM angle α. A measurement of αeffcan be obtained from B0(B0) → a1(1260)±pi∓ [17]. Using SU(3) flavor-symmetry [18], theoretical bounds can be seton the difference ∆α = α−αeff by relating B0(B0) → a±1 pi∓ with ∆S = 1 decays: B → a1K and B → K1Api, whereK1A is a mixture of K1(1270) and K1(1400).K1(1270)+ and K1(1400)+ are wide, overlapping axial mesons. Both are reconstructed through their decays toK+pi+pi− final state. B mesons are reconstructed from B0 → K1(1270)+pi− and K1(1400)+pi− in 454 million BBpairs. This year’s updated analysis uses an improved signal model [19] to describe the production of K1(1270) andK1(1400), including interference effects. The decay is described in terms of two real production parameters (θ, φ),related to the relative amplitude and phase between K1(1270)+ and K1(1400)+. We do a likelihood scan with respectto these two variables. For each likelihood point, we perform a ML fit. At the minimum −lnL, we obtain a combinedbranching fraction B(B0 → K1(1270)+pi− + K1(1400)+pi−) = (31.0± 2.7± 6.9)× 10−6. We evaluate a significanceof 5.1σ. We also set limits on the ratio of the production constants for the K1(1270)+ and K1(1400)+ mesons inB0 decays: 0.25 < θ < 1.32 and −0.51 < φ < 4.51 at 95% probability. This is the first attempt in B-decay data to3Presented at the 34th International Conference on High Energy Physics, 7/29/2008 to 8/5/2008, Philadelphia, PA, USA34th International Conference on High Energy Physics, Philadelphia, 2008measure the relative phase between K1(1270) and K1(1400).4. CONCLUSIONWe present preliminary results on several charmless two-body and quasi two-body decays of neutral B-mesonsbased on the complete BABAR data sample. We have three improved branching-fraction measurements: B(B0 →K0pi0) = (10.1 ± 0.6 ± 0.4) × 10−6, B(B0 → pi0pi0) = (1.83 ± 0.21 ± 0.13) × 10−6 and B(B0 → K1(1270)+pi− +K1(1400)+pi−) = (31.0 ± 2.7 ± 6.9) × 10−6. We have also two updated measurements of direct CP asymmetry:Api0pi0 = −Cpi0pi0 = 0.43± 0.26± 0.05 and AK+pi− = −0.107± 0.016+0.006−0.004. We have also made the first attempt tomeasure the relative production phase between K1(1270) and K1(1400) in the decays of B-mesons.AcknowledgmentsI would like to thank the organizers of ICHEP 2008 for an interesting conference, and my BABAR and PEP-IIcollaborators for their contributions. This work was partially supported by U.S. Department of Energy and NationalScience Foundation.References[1] H. J. Lipkin, Phys. Lett. B 445, 403 (1999)M. Gronau and J. L. Rosner, Phys. Rev. D 59, 113002 (1999)[2] B. Aubert et al. [BABAR Collaboration], Nucl. Instrum. Methods Phys. Res. A 479, 1 (2002).[3] B. Aubert et al. [BABAR Collaboration], arXiv:0807.4226 [hep-ex].[4] B. Aubert et al. [BABAR Collaboration], arXiv:0807.4760 [hep-ex].[5] S. Brandt et al., Phys. Lett. 12, 57 (1964); E. Farhi, Phys. Rev. Lett. 39, 1587 (1977).[6] J. D. Bjorken and S. J. Brodsky, Phys. Rev. D 1, 1416 (1970).[7] B. Aubert et al. [BABAR Collaboration], Phys. Rev. Lett. 94, 161803 (2005).[8] B. Aubert et al. 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Charmless Two-Body and Quasi-Two-Body B-decays at BABAR

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