We present recent results on time integrated and time dependent CP violation
for charmless hadronic B decays using BABAR detector at the PEP-II B-factory.Comment: 7 pages, 3 postscript figues, contributed to the Proceedings of
  ICHEP200

Di Marco, Emanuele

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We present recent results on time integrated and time dependent CP violation for charmless hadronic B decays using BABAR detector at the PEP-II B-factory

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SLAC-PUB-12148BABAR-PROC-06/094Measurement of Direct CP Asymmetriesin Charmless Hadronic B DecaysEmanuele Di MarcoUniversità di Roma “La Sapienza”Physics Department, P.le Aldo Moro 2, 00185 Roma, Italy(representing the BABAR Collaboration)October 6, 2006AbstractWe present recent results on time integrated and time dependent CP violation for charmlesshadronic B decays using BABAR detector at the PEP-II B-factory.Submitted to the the Proceedings of 33rd International Conference on High-Energy Physics,ICHEP 06,26 July—2 August 2006, Moscow, Russia.Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309Work supported in part by Department of Energy contract DE-AC02-76SF00515.October 20061 IntroductionCP violation has been established in processes involving B0-B0 oscillations through measurementsof the time dependence of neutral B-meson decays to final states which include charmonium [1].Direct CP violation occurs when the amplitude for a process i → f is different from the CPconjugate one: ī → f̄ . In the Standard Model (SM) this can occur as a result of interference amongat least two contributing amplitudes which carry different weak and strong phases. For a decayprocess B → f and its CP conjugate B̄ → f̄ the direct CP asymmetry is defined as:ACP = Γ(B̄ → f̄) − Γ(B → f)Γ(B̄ → f̄) + Γ(B → f) (1)where B can be either a charged or neutral B meson. If A1 and A2 are two of the contributingamplitudes to the decay, which have Δφw weak phase difference and Δφs strong phase difference,the direct CP asymmetry can be expressed by:ACP = 2 sin Δφw sinΔφs|A1/A2| + |A2/A1| + 2cos ΔφwΔφs . (2)This implies that large direct CP violation is expected when the contributing amplitudes have largeΔφs, Δφw and similar magnitudes. This happens in charmless B decays where there are oftencontributions from Cabibbo suppressed tree amplitudes as well as from penguin diagrams whichhave different phases but similar magnitudes. The best candidate in this sense is B0 → K+π−decay.The simplest approach to the direct CP violation measurement is to measure time integratedasymmetries, for example different rates for the process B+ → f+ versus B− → f−, or in selftagging neutral B decays, for which the final state particles indicates the flavour of the B.Direct CP violation can also be observed as a time dependent asymmetry. In neutral B decaysto a CP eigenstate the time dependent asymmetry can be expressed by:ACP (Δt) = S sin(ΔmdΔt) + C cos(ΔmdΔt) (3)where Δmd is the B0-B0 mixing frequency and Δt is the difference between the signal and tag Bdecay times. If the parameter C is different from zero this is an indication of direct CP violation.In this case we define the direct CP violation ACP = −C.CP violation in charmless B decays can be also interpreted in terms of parameters of theUnitarity Triangle which describes the unitarity of the Cabibbo-Kobayashi-Maskawa quark mixingmatrix [2].In this note we briefly review some recent measurements of direct CP violation at the BABARexperiment situated at the PEP-II e+e− storage ring at the Stanford Linear Accelerator Center [3].The analyses presented mostly uses about 316 fb−1 of data recorded at the Υ (4S) resonance.2 Basic Measurement StrategyEvents are fully reconstructed combining tracks and neutral clusters in the detector. In order toselect B candidates we use a set of two kinematic variables: the beam-energy-substituted massmES =√(s/2 + pi · pB)2/E2i + p2B , and the energy difference ΔE = E∗B −√s/2. Here, (Ei,pi) isthe four-vector of the initial e+e− system,√s is the center-of-mass energy, pB is the reconstructed2momentum of the B0 candidate, and E∗B is its energy calculated in the e+e− rest frame. For signaldecays, the mES distribution peaks near the B0 mass with a resolution of about 2.5 MeV/c2, andthe ΔE distribution peaks near zero with a resolution of 10−50 MeV, depending on the final state.ΔE depends on the mass hypothesis assigned to the tracks and is so used in final states with kaonsor pions. Particle identification (PID) also uses the the Cherenkov detector (DIRC), which givesexcellent K/π separation (> 2.5σ for momenta < 4.0 GeV/c).The main background comes from light quark continuum production, which is different by signalin the more jet-like distribution of the decay products. Different variables, as the Legendre polyno-mials, combined using algorithms which maximize the separation between signal and background,are used to reject most of this background. Another source of background comes from decays of Bmesons which mimic the signal. This background is typically more difficult to suppress.The analysis strategy, common to the most of the analyses, is to perform an unbinned maximumlikelihood (ML) fit to several selection variables to extract CP asymmetries simultaneously to thesignal yields. Large sidebands, i.e. regions where the signal contribution is negligible, are kept inthe selection in order to characterize the background directly on data. Background from other Bdecays is usually estimated from Monte Carlo samples.3 Results on direct CP Violation3.1 B0 → K+π−, B0 → π+π−In B0 → K+π− decay the charge of the kaon can be used to infer the tag of the B meson, so atime integrated CP violation measurement is feasible. The analysis reconstructs simultaneouslyB0 → h+h−, with h = K,π, making use of mES, ΔE, a Fisher discriminant F with event shapevariables [4] and the Cherenkov angle θc. ΔE and θc lead to the separation of the h+h− modes. Theθc PDFs are determined separately for positive and negative tracks from dedicated D∗+ → D0π+,D0 → K−π+ data control samples. The fit to 347 million BB̄ pairs returns 2542 ± 67 B0 → Kπevents, and an asymmetry AKπ = −0.108 ± 0.024 ± 0.008. As a cross-check, the measured CPasymmetry for the background is consistent with zero. Fig. 1 shows the background-subtractedΔE distribution for K+π− events and K−π+ events. The significance of the CP violation, includingsystematics, is 4.3σ [5].The CP asymmetry for B0 → π+π− is measured with a time-dependent fit to the B0 → h+h−sample. The fit yields 675±42 signal events, with a direct CP asymmetry parameter ACP = −C =0.16 ± 0.11 ± 0.03, consistent with zero.3.2 B+ → h+π0In the SM the charge asymmetry in B+ → K+π0 is expected to be of the same order of theB0 → K+π− one, while no asymmetry is expected in B+ → π+π0, since the decay is mediatedby only one amplitude. The main B backgrounds for these modes are B → ρπ, B → ρK andB → K∗π decays. The fit strategy is similar to the one of B0 → K+π−. The measured yields,branching fractions and CP asymmetries are shown in Table 1. The asymmetries are consistentwith zero [5].3E (GeV)Δ-0.1 0 0.1Events / (30 MeV)0200400Events / (30 MeV)BABARPreliminaryFigure 1: The background-subtracted distribution of ΔE for signal K±π∓ events, comparing B0(solid) and B0 decays (dashed).B+ → π+π0 B+ → K+π0NS 572 ± 53 1239 ± 52B(10−6) 5.12 ± 0.47 ± 0.29 13.3 ± 0.56 ± 0.64ACP −0.02 ± 0.09 ± 0.01 0.02 ± 0.04 ± 0.01Table 1: Signal yields (NS), branching fractions (B) and CP asymmetries for B+ → h+π0.3.3 B+ → φK+, B → φπThe B+ → φK+ and B±,0 → φπ±,0 in the SM proceed only through b → s and b → d gluonicpenguins, respectively, then the direct CP asymmetry is expected to be zero. However, these decaysare sensitive to new physics entering the loop amplitude and providing additional phases, whichcould provide large CP asymmetries and a B(B → φπ) > 10−8. The analysis uses an extendedML fit to kinematic and event shape variables, the helicity of the φ meson and θc. The φ mesonis defined as the K+K− pair with an invariant mass in 1.0045 < mK+K− < 1.0345 GeV/c2. Theextracted yield for B+ → φK+ in 347 million BB̄ pairs is 624 ± 30 events, with a direct CPasymmetry ACP = 0.046 ± 0.046 ± 0.017, which is consistent with zero [6].A fit to 232 million BB̄ pair returns no signal events for B → φπ decays, allowing to set upperlimits on B’s: B(B+ → φπ+) < 2.4 × 10−6 and B(B0 → φπ0) < 2.8 × 10−6 [7].3.4 B+ → K0h+, B0 → K̄0K0In SM, decays B+ → K̄0h+ and B0 → K0K̄0 proceed only through penguin transition b → d,then the direct CP asymmetry is expected to vanish. In a way analogous to B+ → φK+, newphysics effects could enhance it. The direct CP asymmetry in the neutral mode is measured with atime-dependent fit. From an extended ML fit to mES, ΔE and F (including Δt for B0 → K̄0K0),in a sample of 347 million BB̄ pairs, the signal yields and the CP asymmetries are extracted.The results are reported in Table 2. We observe for the first time the decays B+ → K̄0K+ andB0 → K̄0K0 with significances of 5.3σ and 7.3σ, respectively. The measured direct CP asymmetriesare consistent with zero [8], in agreement with theoretical expectations. In Fig. 2 the asymmetry4Mode B(10−6) ACPB+ → K0π+ 23.9 ± 1.1 ± 1.0 −0.03 ± 0.04 ± 0.01B+ → K̄0K+ 1.6 ± 0.4 ± 0.1 0.10 ± 0.26 ± 0.03B0 → K0K̄0 1.1 ± 0.3 ± 0.1 0.40 ± 0.41 ± 0.06Table 2: Signal yields (NS), branching fractions B(×10−6) and CP asymmetries for B+ → K0h+,B+ → K0K̄0.for K0SK0S events and the confidence level contours in the C vs. S plane are shown. T [ps]Δ-6 -3 0 3 6/ 2 ps            0510/ 2 ps             tags 0B T [ps]Δ-6 -3 0 3 6Events0510Events tags 0B t (ps)Δ-6 -3 0 3 6Asymmetry-101AsymmetryC    -1 -0.5 0 0.5 1S     -2-1.5-1-0.500.51σ1σ2σ3Figure 2: Left: Δt distributions for B0 → K0SK0S decays in data tagged as B0 (top) or B0 (middle),and the asymmetry (bottom). The solid (dotted) curve represents the total (background only)likelihood projection. Right: Confidence level contours in the C vs. S plane. The circle indicatesthe physically allowed region, while the point with errors is the result on data.3.5 B0 → (ρπ)0A time-dependent analysis of the Dalitz plot of the B0 → π+π−π0 decays is performed, in orderto measure the CP violation taking into account the interference between the intermediate states:B0 → ρ±π∓ and the color suppressed B0 → ρ0π0. QCD factorization predicts null direct CPviolation, due to the lack of penguin contributions, but non-factorizable effects could be present,modifying this conclusion. Through the interference between these amplitudes this measurementcan lead to an unambiguous determination of the angle α of the Unitarity Triangle [9]. An extendedML fit to mES, ΔE, F , Dalitz plot variables and Δt is performed to 347 million BB̄ pairs. Since thefinal states are not CP eigenstates the CP violation can be expressed in terms of the time-dependent5C parameter and time- and flavour-integrated CP asymmetry Aρπ. These can be rearranged interms of A+−ρπ which describes the direct CP violation of the amplitude where the ρ is emitted fromthe W , and A−+ρπ which describes amplitudes where the π is emitted from the W . The fit returnsa signal yield of 1847 ± 69 events, dominated by B0 → ρ±π∓ decays, and direct CP asymmetriesA+−ρπ = 0.03 ± 0.07 ± 0.03, A−+ρπ = −0.38+0.15−0.16 ± 0.07. These results show an evidence of direct CPviolation, including systematics, of 3.0σ [10]. Fig. 3 shows confidence level contours for A+−ρπ andA−+ρπ .-1-0.75-0.5-0.2500.250.50.751-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1A– +ρπA+ – ρπBABARP R E L I M I N A R YFigure 3: Confidence level contours for the direct CP asymmetries A+−ρπ and A−+ρπ . The shadedareas represent 1σ, 2σ and 3σ contours, from darker to lighter.4 ConclusionsThe direct CP violation has been measured in B0 → K+π− decays to be of 0.108 ± 0.025, resultingin a 4.3σ effect. We observed for the first time the decays B+ → K̄0K+ and B0 → K0K̄0, withno CP violation. The direct CP asymmetry is consistent with zero in other rare decay modes,consistently with the SM predictions for those decays which are mostly sensitive to new physicseffects, as B+ → φK+. In the time-dependent analysis of B0 → (ρπ)0 we see an evidence of directCP violation with 3.0σ significance. The increasing of the recorded luminosity in the next yearswill provide a powerful tool to investigate the SM picture of the nature of CP violation.5 AcknowledgementsI am grateful to my BABAR collegues Gianluca Cavoto, Maurizio Pierini, Denis Dujmic, and Maha-laxmi Krishnamurthy for their support in this work. I would like to thank also Daniele del Re andprof. Aaron Roodman for their support during the conference.6References[1] B. Aubert et al. [BABAR Collaboration], arXiv:hep-ex/0607107. K. Abe et al. [Belle Col-laboration], arXiv:hep-ex/0608039.[2] UTfit Collaboration, M. Bona et al., [hep-ph/0501199]. CKMfitter Group, J. Charles et al.,Eur. Phys. J. C 41, 1 (2005)[3] B. Aubert et al. [BABAR Collaboration], Nucl. Instrum. Meth. A 479, 1 (2002) [arXiv:hep-ex/0105044].[4] R. A. Fisher, Annals of Eugenics 7, 179 (1936).[5] B. Aubert et al. [BABAR Collaboration], arXiv:hep-ex/0607106.[6] B. Aubert et al. [BABAR Collaboration], arXiv:hep-ex/0607112.[7] B. Aubert et al. [BABAR Collaboration], Phys. Rev. D 74, 011102 (2006) [arXiv:hep-ex/0605037].[8] B. Aubert et al. [BABAR Collaboration], arXiv:hep-ex/0608036.[9] A. E. Snyder and H. R. Quinn, Phys. Rev. D 48, 2139 (1993).[10] B. Aubert [BABAR Collaboration], arXiv:hep-ex/0608002.7

Measurement of Direct CP Asymmetries in Charmless Hadronic B Decays

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Measurement of Direct CP Asymmetries in Charmless Hadronic B Decays

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