Measurements of CP violation in B0s mesons decays provide an

excellent method to test the prediction of the Standard Model and search for effect of new physics beyond it. With the first 1 fb−1 √ of data collected in pp collisions at

√s = 7TeV the LHCb experiment performed the world best precision measurements of the CP-violating phase φs and on the decay width difference ΔΓs. The analysis of the B0s → φφ decay and the measurement of the B0s lifetime in decays to CP eigenstates are also presented

Calvi, M.

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DOI 10.1393/ncc/i2013-11395-7Colloquia: IFAE 2012IL NUOVO CIMENTO Vol. 36 C, N. 1 Gennaio-Febbraio 2013Recent results on CP violation B0s meson decaysM. CalviUniversita` di Milano-Bicocca and INFN, Sezione di Milano-Bicocca - Milano, Italyricevuto il 31 Agosto 2012Summary. — Measurements of CP violation in B0s mesons decays provide anexcellent method to test the prediction of the Standard Model and search for effectof new physics beyond it. With the first 1 fb−1 of data collected in pp collisions at√s = 7TeV the LHCb experiment performed the world best precision measurementsof the CP -violating phase φs and on the decay width difference ΔΓs. The analysisof the B0s → φφ decay and the measurement of the B0s lifetime in decays to CPeigenstates are also presented.PACS 12.15.Hh – Determination of Cabibbo-Kobayashi & Maskawa (CKM) matrixelements.PACS 13.25.Hw – Decays of bottom mesons.1. – CP violation in B0s→ J/ψφ and B0s→ J/ψπ+π− decaysFor a final state f equal to its CP conjugate (f = f¯), a CP -violating phase arisesfrom the interference between B0s decay to f either directly or via B0s -B0s oscillation,producing a difference in the proper-time distributions of B0s mesons and B0s mesons.The B0s → J/ψ(μμ)φ(KK) decay is considered the golden mode for measuring this typeof CP violation. Within the Standard Model (SM) this decay is dominated by b→ ccsquark level transitions, neglecting QCD penguin contribution the phase is φSMs = −2βs,where βs = arg(−VtsV ∗tb/VcsV ∗cb). Global fits to experimental data give a small andprecise value 2βs = (−0.036 ± 0.002) rad [1]. New particles could contribute to themixing box diagram modifying the SM prediction, adding a new phase [2].The measurement of φs was first performed at Tevatron in pp¯ collisions at√s =1.96TeV by the CDF and D0 experiments, they have recently updated their results on9.6 fb−1 [3] and 8 fb−1 [4] of data, respectively.The determination of φs is improved by the results of LHCb using about 1 fb−1 ofdata collected in 2011 at LHC in pp collisions at√s = 7TeV. A total of about 21200B0s→ J/ψφ signal events are selected, with a decay time t > 0.3 ps [5]. The time resolutionis measured on data with a fit to the J/ψ component of the prompt background. Theeffective decay time resolution is approximately 45 fs.c© Societa` Italiana di Fisica 56 M. CALVITable I. – Results for the physics parameters with the statistical and systematic uncertainties,respectively. LHCb preliminary results from 1 fb−1 [5].Parameter ResultΓs [ ps−1] 0.6580± 0.0054± 0.0066ΔΓs [ ps−1] 0.116± 0.018± 0.006|A⊥|2 0.246± 0.010± 0.013|A0|2 0.523± 0.007± 0.024FS 0.022± 0.012± 0.007φs [rad] −0.001± 0.101± 0.027The B0s → J/ψφ decay proceeds via a vector-vector intermediate state with contribu-tions from both CP -even and CP -odd amplitudes. In order to achieve good sensitivityto the two components the distribution of the final state decay angles in the transversitybasis is analysed. The decay can be described by three time-dependent amplitudes, att = 0: A0(0) and A‖(0) are CP -even while A⊥(0) is CP -odd. The differential decay ratedepends on several physics parameters: the B0s decay width Γs = (ΓL+ΓH)/2, the decaywidth difference between the light and heavy B0s mass eigenstates ΔΓs = (ΓL−ΓH), themixing frequency Δms = mBH −mBL , the CP -violating phase φs, the relative phasesand magnitudes of the three angular transversity amplitudes. A contribution from anS-wave component is also considered, with fraction FS .The flavour of the B meson at production is determined combining several “oppositeside” algorithms, namely high pT muons, electrons and kaons and the charge of aninclusively reconstructed secondary vertex. The algorithms have been optimized on datafor the maximum tagging power D2, where  is the tagging efficiency, D = 1 − 2ω isthe dilution and ω the mistag. The tagging power is enhanced by using in the fits aper-event mistag probability, which is calibrated on data, using B+ → J/ψK+ decays,and validated on B0→ J/ψK∗0 decays. The tagging power for B0s→ J/ψφ events is foundto be D2 = (2.29± 0.07± 0.26)%The results of the likelihood fit to the mass, decay time and angular distributions forthe main physics parameters are summarized in table I. In addition, the strong phasesare determined, they are all consistent with zero or π radians. This analysis provides themost precise measurement of φs so far and constitutes the first observation with morethan 5 sigmas of a non-zero lifetime difference. The values of φs and Γs are in goodagreement with the SM prediction. In fig. 1 the projection of the fitted PDF on thedecay time and the transversity angle distributions are shown.It should be noted that the B0s→ J/ψφ decay rates are unchanged under the transfor-mation (φs,ΔΓs, δ‖, δ⊥)↔ (π−φs,−ΔΓs,−δ‖, π− δ⊥). The values in table I correspondto the solution with ΔΓs > 0.This ambiguity is now solved by another LHCb analysis performed on the same dataset, but using an extended range in the K+K− invariant mass around the φ(1020) reso-nance [6]. As a consequence of the interference between the P - and S-wave componentsof the final state, a decreasing trend of the strong phase difference is expected, as afunction of mKK. The fit is repeated in four bins of mKK and the decreasing trend isobserved for the fit solution corresponding to the positive ΔΓs value, in agreement withthe SM prediction.RECENT RESULTS ON CP VIOLATION B0S MESON DECAYS 7Fig. 1. – Data points overlaid with fit projections for the B0s→ J/ψφ decay time and the transver-sity angle distributions for candidates with an invariant mass within ±20MeV/c2 around thenominal B0s mass. The signal component is shown as a solid blue line, the dashed and dottedblue lines represent the CP -even and CP -odd signal components, respectively. The S-wave com-ponent and the background component are shown by the solid pink and red line, respectively.LHCb preliminary [5].An independent measurement of φs is performed by LHCb in the B0s→ J/ψπ+π− decaychannel. A first LHCb measurement of φs from B0s → J/ψf0(980) events on about 0.4 fb−1of data was recently published [7]. A new analysis [8] is performed on the full 1 fb−1 datasample collected in 2011, using the final state J/ψπ+π− over a large range of π+π− masses,(775–1550)MeV/c2. This region has been shown to be 97.7% CP -odd eigenstate, at 95%CL [8]. As a consequence a time-dependent tagged analysis is performed, but no angularstudy is required. In fig. 2 the invariant-mass distribution of the selected J/ψπ+π−candidates and the mass distribution of the π+π− combinations for events in the B0s signalregion are shown. About 7400 B0s signal events are selected. In the likelihood fit the valuesof Γs and ΔΓs have been constrained to the result of the previous B0s → J/ψφ analysis.The measured value of the mixing phase is φs = −0.02±0.17±0.004 rad. Combining thisresult with the preliminary B0s→ J/ψφ measurement gives φs = −0.002±0.083±0.027 rad.2. – CP violation in B0s→ φφ decaysAnother interesting channel to search for new physics effects is B0s → φφ for whichthe CP -violating observables are predicted with low theoretical uncertainty in the SM.8 M. CALVI (MeV)-π+πψJ/m(          )5300 5400 5500Events / 5 MeV 0400800120016002000LHCb) (MeV)π πm(500 1000 1500 2000Events / 15 MeV0100200300400500600700800 LHCb+     -↓ ↓Fig. 2. – Left: mass distribution of the selected J/ψπ+π− combinations with 775 < mπ+π− <1550MeV/c2. The blue solid curve shows the result of a fit with a double Gaussian for thesignal (red solid curve) and several background components. Right: mass distribution of selectedπ+π− combinations shown as the (solid black) histogram for events in the B0s signal region. The(dashed red) line shows the background [7].In 1 fb−1 of data about 800 signal events have been selected, with φ→ K+K−. The datasample is not yet sufficient to perform a full time-dependent angular analysis of flavourtagged events, as required by a study of the CP -violating phase. However, evidenceof possible processes beyond the SM can be searched for in the measurement of tripleproducts asymmetry, that are T -odd quantities. There are two observable triple productsdenoted U = sin(2Φ)/2 and V = ± sin(Φ), where the positive sign is taken if the T -evenquantity cos θ1 cos θ2 ≥ 0 and the negative sign otherwise. These variables correspond tothe T -odd triple products sinΦ = (nˆ1 × nˆ2) · pˆ1 and sin(2Φ)/2 = (nˆ1 · nˆ2)(nˆ1 × nˆ2) · pˆ1where nˆi(i = 1, 2) is a unit vector perpendicular to the φi meson decay plane and pˆ1 isa unit vector in the direction of the φ1 meson momentum in the B0s rest frame.The asymmetry AU is defined as AU =N+−N−N++N−, where N+(N−) is the number ofevents with U > 0 (U < 0). Similarly AV =M+−M−M++M−, where M+(M−) is the number ofevents with V > 0 (V < 0). The measured values areAU = −0.055± 0.036(stat)± 0.018(syst), AV = 0.010± 0.036(stat)± 0.018(syst).Both values are in good agreement with those reported by the CDF Collaboration [10]and consistent with the hypothesis of CP conservation.3. – Lifetime measurements in CP eigenstatesAnother route to extract information about ΔΓs and φs is from a precise determi-nation of the B0s lifetime in decays to final states which are pure CP -even or CP -oddstates. If the initial flavour of the B0s meson is unknown, the decay time distribution isproportional to(1) Γ(t) ∝ (1−AΔΓs)e−ΓLt + (1 +AΔΓs)e−ΓHt.The parameter AΔΓs is defined as AΔΓs = −2Re(λ)/(1 + |λ|2), where λ = (q/p)(A¯/A),the complex coefficients p and q define the mass eigenstates of the B0s system in termsof the flavour eigenstates and A(A¯) is the amplitude for a B0s (B0s ) meson to decay toRECENT RESULTS ON CP VIOLATION B0S MESON DECAYS 9Fig. 3. – Left: invariant-mass distribution for B0(s) → h+h− events. The contribution fromB0s → K+K− is shown in striped (red). The background from B0→ K+π− and from partiallyreconstructed B mesons are shown in hatched (green) and shaded (blue), respectively. Right:Corresponding decay time distribution of B0s → K+K− signal candidates extracted from anunbinned log-likelihood fit to the data.the same final state. In the absence of CP violation AΔΓs = 1(−1) for CP -odd (CP -even) eigenstates. Examples are B0s → J/ψf0 and B0s → K+K− decay modes which aresensitive to the ΓH and ΓL components, respectively. If the decay time distribution givenby eq. (1) is fitted with a single exponential function the effective lifetime is given byτKK = τBs(1 +AΔΓsys +O(y2s)), where τBs = 1/Γs and ys = ΔΓs/2Γs.A measurement of B0s→ K+K− lifetime was performed at LHCb on 2010 data [11]. Inthat analysis B0s meson decay products that are significantly displaced from the B0s mesonproduction vertex were selected. As a consequence, B0s mesons with low decay times aresuppressed, introducing a bias to the decay time spectrum which must be corrected. Anew analysis has been performed on 1 fb−1 of data with a technique that explicitly avoidsa lifetime bias by using a neural network-based trigger and event selection [12].Figure 3 shows the B0s → K+K− selected candidates and the corresponding decaytime distribution. The preliminary measurement of the effective B0s→ K+K− lifetime isτKK = 1.468± 0.046(stat)± 0.006(syst) ps, in agreement with the previous measurementand with SM predictions.REFERENCES[1] Charles J. et al. (CKMfitter group), Phys. Rev. D, 84 (2011) 033005.[2] See, for example, Ligeti Z., Papucci M. and Perez G., Phys. Rev. Lett., 97 (2006)101801; Nierste U., Int. J. Mod. Phys., 22 (2008) 5986; Lenz A., Phys. Rev. D, 76(2007) 065006.[3] Aaltonen T. et al. (CDF Collaboration), arXiv:1112.1726; Sabato L., theseproceedings.[4] Abazov V. M. et al. (D0 Collaboration), Phys. Rev. D, 85 (2012) 032006.[5] Aaij R. et al. (LHCb Collaboration), LHCb-CONF-2012-002.[6] Aaij R. et al. (LHCb Collaboration), Phys. Rev. Lett., 108 (2012) 241801.[7] Aaij R. et al. (LHCb Collaboration), Phys. Lett. B, 707 (2012) 497.[8] Aaij R. et al. (LHCb Collaboration), Phys. Lett. B, 713 (2012) 378.[9] Aaij R. et al. (LHCb Collaboration), Phys. Lett. B, 713 (2012) 369.[10] Aaltonen T. et al. (CDF Collaboration), Phys. Rev. Lett., 107 (2011) 261802.[11] Aaij R. et al. (LHCb Collaboration), Phys. Lett. B, 707 (2012) 349.[12] Aaij R. et al. (LHCb Collaboration), LHCb-CONF-2012-001.

Recent results on CP violation B0s meson decays

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Recent results on CP violation B0s meson decays

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