Using data collected during 2011 at a center-of-mass energy of

7 TeV LHCb selected samples of B± → Dh± and Hb → h+h− decays. Using these

samples the measurement of the full set of partial widths and CP asymmetries of

B± → Dh± decays has been performed. From the analysis of the selected charmless

charged two-body hadronic B decays LHCb measured the direct CP asymmetries

ACP (B0 → Kπ) and ACP (B0

s → πK), the branching ratios BR(B0 → K+K−) and

BR(B0

s → π+π−) and the time-dependent CP asymmetries of B0 → π+π− and

B0

s → K+K− decays

Perazzini, S.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2013-11398-4Colloquia: IFAE 2012IL NUOVO CIMENTO Vol. 36 C, N. 1 Gennaio-Febbraio 2013Recent LHCb results related to the measurementof the CKM phase γS. PerazziniINFN, Sezione di Bologna - Bologna, Italyricevuto il 31 Agosto 2012Summary. — Using data collected during 2011 at a center-of-mass energy of7TeV LHCb selected samples of B± → Dh± and Hb → h+h′− decays. Using thesesamples the measurement of the full set of partial widths and CP asymmetries ofB± → Dh± decays has been performed. From the analysis of the selected charmlesscharged two-body hadronic B decays LHCb measured the direct CP asymmetriesACP(B0 → Kπ) and ACP(B0s → πK), the branching ratios BR(B0 → K+K−) andBR(B0s → π+π−) and the time-dependent CP asymmetries of B0 → π+π− andB0s → K+K− decaysPACS 12.15.Hh – Determination of Cabibbo-Kobayashi & Maskawa (CKM) matrixelements.PACS 13.25.Hw – Decays of bottom mesons.1. – IntroductionIn this paper we present the results obtained by the LHCb experiment based on thereconstruction of B± → Dh± decays (where the D signifies a D0 or D¯0 meson and hcan be either a kaon or a pion) and charmless charged two-body hadronic B decays,Hb → h+h′− (where Hb = B0, B0s ,Λb and h, h′ = π,K, p). All the results are obtainedanalysing data collected during 2011 at a center-of-mass energy of 7TeV.The measurement of partial widths and CP asymmetries of B± → Dh± decays pre-sented here (in the cases of D mesons decaying into K+K−, π+π−, K−π+ and K+π−) isone of the most powerful ways [1,2] for determining the complex phase γ of the Cabibbo-Kobayashi-Maskawa matrix [3] using tree-level processes. For this analysis the full datasample has been used, corrisponding to an integrated luminosity of∫ Ldt = 1 fb−1.Also Hb → h+h′− decays provide sensitivity to γ and even have potential to revealNew Physics thanks to the presence of loop topologies inside the decay diagrams [4].Here we present the measurement of the direct CP asymmetries ACP(B0 → Kπ) andACP(B0s → πK) (using∫ Ldt = 0.32 fb−1), the measurement of the branching fractionsBR(B0 → K+K−) and BR(B0s → π+π−) (using∫ Ldt = 0.37 fb−1) and the measure-ment of the direct and mixing-induced components of time-dependent CP asymmetriesc© Societa` Italiana di Fisica 18RECENT LHCB RESULTS RELATED TO THE MEASUREMENT OF THE CKM PHASE γ 19of B0 → π+π− (Adirππ and Amixππ ) and B0s → K+K− (AdirKK and AmixKK) decays (using∫ Ldt = 0.69 fb−1).2. – Analysis strategyThe decays B± → Dh± and Hb → h+h′− are mainly selected by the two-levelhadronic trigger of LHCb [5] that exploits the high transverse momentum and impactparameter with respect to primary vertices typical of B hadron decay products. Twodifferent kinds of offline selections have been used to extract the final samples in theanalyses shown in this paper: B± → Dh± have been discriminated from combinatorialbackground using a boosted decision tree algorithm, while for Hb → h+h′− a cut basedselection has been applied. In both cases different sets of selection criteria have beenoptimized depending on the measurement of interest.A crucial point of both analyses is the calibration of the particle identification (PID),needed to discriminate between the various final states and to estimate the relativefractions of correctly identified or mis-identified decays. The two Ring Imaging Cherenkovdetectors of LHCb (RICH1 and RICH2) allow to distinguish charged hadrons in a widemomentum range (between few GeV/c up to and beyond 100GeV/c). The response ofthese detectors has been calibrated using large and pure samples of pions, kaons andprotons selected from D∗+ → D0(K−π+)π+ and Λ → pπ− decays (plus their chargeconjugates).In both B± → Dh± and Hb → h+h′− analyses, direct raw asymmetries and signalyields are obtained from a simultaneous maximum-likelihood fit of all the possible finalstates. Further details on the fit models can be found in [6-8].The measurements of CP asymmetries are affected by two main sources of biases:detection asymmetry and B hadron production asymmetry. The former is mainly dueto the different magnitude of the strong interaction of charge conjugate final states (e.g.K+π− and K−π+) with the detector material, and has been studied using D∗-taggedand untagged two-body hadronic D meson decays. The latter is due to the initial quarkflavour imbalance in the proton-proton interaction. This has been studied by measuringthe charge asymmetry of B± → J/ψK± and B0 → J/ψK∗0(K+π−) respectively forcharged and neutral B mesons.In order to measure the time-dependent CP asymmetries of B0 → π+π− and B0s →K+K− a central roˆle is played by the tagging of the initial flavour of the B meson. Bymeans of a multivariate algorithm the initial flavour of the B meson is determined byanalyzing the decay products of the other B hadron in the event. The response of thetagging algorithm is calibrated measuring the oscillation amplitude of the flavour specificdecay B0 → K±π∓ that results proportional to the effective tagging power. Using themeasured effective tagging power as an external input to a two dimensional (invariantmass and decay time) maximum likelihood fit of the K+K− and π+π− spectra, thedirect and mixing-induced CP asymmetry terms Adirππ , Amixππ , AdirKK and AmixKK have beenmeasured [9].3. – ResultsUsing the full data sample collected during 2011 LHCb measured of 3 partialwidths [6]:RfK/π =Γ(B− → [f ]DK−) + Γ(B+ → [f ]DK+)Γ(B− → [f ]Dπ−) + Γ(B+ → [f ]Dπ+) ,(1)20 S. PERAZZINIwhere f represents KK, ππ and the favoured Kπ modes; 6 CP asymmetries:Afh =Γ(B− → [f ]Dh−)− Γ(B+ → [f ]Dh+)Γ(B− → [f ]Dh−) + Γ(B+ → [f ]Dh+) ,(2)and 4 charge-separated partial widths:R±h =Γ(B± → [π±K∓]Dh±)Γ(B± → [K±π∓]Dh±) .(3)Using such measurements the following quantities sensitive to γ have been computed:RCP+ ≈〈RKKK/π, RππK/π〉/RKπK/π = 1.007± 0.038± 0.012,(4)ACP+ =〈AKKK , AππK〉= 0.145± 0.032± 0.010,(5)RADS(K) =(R−K + R+K) /2 = 0.0152± 0.0020± 0.0004,(6)AADS(K) =(R−K −R+K) / (R−K + R+K)= −0.52± 0.15± 0.02,(7)RADS(π) =(R−π + R+π) /2 = 0.00410± 0.00025± 0.00005,(8)AADS(π) =(R−π −R+π) / (R−π + R+π)= 0.143± 0.062± 0.011.(9)Such measurements are the most precise to date and are compatible with previous re-sults [10]. In particular the suppressed mode B± → [Kπ]DK± (with D decaying throughthe doubly Cabibbo suppressed transition) is observed for the first time with a ∼ 10σstatistical significance. In addition AADS(K) and ACP+ show evidence of direct CP vi-olation with a statistical significance of 4.0σ and 4.5σ respectively. Finally, comparingthe maximum likelihood of the fit to that of the non-CP violation hypothesis, direct CPviolation in B± → DK± decays is observed with a 5.8σ statistical significance.Analyzing 0.32 fb−1 of data collected during 2011 LHCb measured ACP(B0 → Kπ) =−0.088± 0.011± 0.008 and ACP(B0s → πK) = 0.27± 0.08± 0.02 [7]. The former is themost precise measurement ever of this quantity [10] and the latter represents the firstevidence of direct CP violation in the B0s sector with a significance of more that 3σ andis compatible with previous measurements [11].Using 0.37 fb−1 LHCb observed for the first time the decay B0s → π+π− with a5.3σ significance and measured BR(B0 → K+K−) = (0.11+0.05−0.04 ± 0.06) × 10−6 andBR(B0s → π+π−) = (0.95+0.21−0.17 ± 0.13) × 10−6 (see figs. 1). Values are compatible withprevious results [11].Finally using 0.69 fb−1 of data LHCb measured the time-dependent CP asymmetriesof the B0 → π+π− and B0s → K+K− decays [9]:Adirππ = 0.11± 0.21± 0.03, Amixππ = −0.56± 0.17± 0.03,(10)AdirKK = 0.02± 0.18± 0.04, AmixKK = 0.17± 0.18± 0.05(11)with correlations ρ(Adirππ , Amixππ ) = −0.34 and ρ(AdirKK , AmixKK) = −0.10. Adirππ and Amixππ arecompatible with results from the B-Factories [12]; AdirKK and AmixKK are measured for thefirst time ever and are compatible with theoretical expectations [13].RECENT LHCB RESULTS RELATED TO THE MEASUREMENT OF THE CKM PHASE γ 21Fig. 1. – Invariant mass spectra for the mass hypotheses (a, b) K+K− and (c, d) π+π−. Plots (b)and (d) are the same as (a) and (c), respectively, but magnified to focus on the rare B0 → K+K−and B0s → π+π− signals. The results of the unbinned maximum-likelihood fits are overlaid. Themain components contributing to the fit model are also shown.REFERENCES[1] Atwood D., Dunietz I. and Soni A., Phys. Rev. Lett., 78 (1997) 3257; Atwood D.,Dunietz I. and Soni A., Phys. Rev. D, 63 (2001) 036005.[2] Gronau M. and London D., Phys. Lett. B, 253 (1991) 483; Gronau M. and WylerD., Phys. Lett. B, 265 (1991) 172.[3] Cabibbo N., Phys. Rev. Lett., 10 (1963) 531; Kobayashi M. and Maskawa T., Prog.Theor. Phys, 49 (1973) 652.[4] Fleischer R., Phys. Lett. B, 459 (1999) 306; Fleischer R., Eur. Phys. J. C, 52 (2007)267; Fleischer R. and Knegjens R., Eur. Phys. J. C, 71 (2011) 1532; Ciuchini M. etal.,, arXiv:1205.4948v1 [hep-ph].[5] Alves A. A. et al. (LHCb Collaboration), JINST, 3 (2008) S08005.[6] Aaij R. et al. (LHCb Collaboration), Phys. Lett. B, 712 (2012) 203.[7] Aaij R. et al. (LHCb Collaboration), Phys. Rev. Lett., 108 (2012) 201601.[8] Aaij R. et al. (LHCb Collaboration), arXiv:1206.2794v1 [hep-ex].[9] Aaij R. et al. (LHCb Collaboration), LHCb-CONF-2012-007.[10] Amhis Y. et al., arXiv:1207.1158v1 [hep-ex].[11] Aaltonen T. et al. (CDF Collaboration), Phys. Rev. Lett., 106 (2011) 181802.[12] Ishino H. et al. (Belle Collaboration), Phys. Rev. Lett., 98 (2007) 211801; AubertB. et al. (BaBar Collaboration), arXiv:0807.4226v2 [hep-ex].[13] Adeva B. et al. (LHCb Collaboration), arXiv:0912.4179v3 [hep-ex].

Recent LHCb results related to the measurement of the CKM phase γ

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Recent LHCb results related to the measurement of the CKM phase γ

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