The LHCb experiment is a single-arm spectrometer designed to

pursue an extensive study of CP violation in b and charm systems. In this contribution, three recent measurements are presented. First, the difference between CP asymmetries of D0 → K−K+ and D0 → π−π+ decays using 0.6 pb−1 of 2011

data is detailled. Then an important milestones towards the measurement of the γ angle is presented with the study of B

± → DK ± decays using 1 fb−1. Third, the measurement of the CP-violating phase φs in B0s → J/ψφ and B0s → J/ψf0 is

reported

Maurice, E.

English

Maurice, Emilie

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Study of CPV in $b$ and Charm Systems at LHCb

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DOI 10.1393/ncc/i2012-11363-9Colloquia: LaThuile12IL NUOVO CIMENTO Vol. 35 C, N. 6 Novembre-Dicembre 2012Study of CPV in b and charm systems at LHCbE. Maurice on behalf of the LHCb CollaborationCPPM, Universite´ Aix-Marseille, CNRS/IN2P3 - Francericevuto il 7 Settembre 2012Summary. — The LHCb experiment is a single-arm spectrometer designed topursue an extensive study of CP violation in b and charm systems. In this con-tribution, three recent measurements are presented. First, the difference betweenCP asymmetries of D0 → K−K+ and D0 → π−π+ decays using 0.6 pb−1 of 2011data is detailled. Then an important milestones towards the measurement of theγ angle is presented with the study of B± → DK± decays using 1 fb−1. Third,the measurement of the CP -violating phase φs in B0s → J/ψφ and B0s → J/ψf0 isreported.PACS 12.15.Ff – Quark and lepton masses and mixing.PACS 12.15.Hh – Determination of Cabibbo-Kobayashi & Maskawa (CKM) matrixelements.1. – IntroductionThe LHCb experiment is dedicated to the study of charm and beauty flavour physics.Precise measurements in these sectors allow to test the CKM paradigm of flavour struc-ture and CP violation. More precisely, LHCb investigate the possible New Physics effectsin the loop-mediated processes. In this document, we present three recent LHCb resultsabout CP violation in charm and beauty sectors. In sect. 2, we present the differencein time-integrated CP asymmetries between D0 → K−K+ and D0 → π−π+, using0.6 pb−1 of 2011 data. In sect. 3, we detail the CP violation in B± → DK±. In sect. 4,we report on the measurement of the CP -violating phase φs.2. – CP violation in charmIn the Standard Model, CP violation in charm sector is expected to be small [1, 2].New Physics could enhance the rate of CP violation [3]. LHCb measures the differencein time-integrated CP asymmetries between D0 → K−K+ and D0 → π−π+, using0.6 pb−1 of data collected in 2011 [4]. By requiring a D∗+ → D0π+ decay, the initialstate, D0 or D0, is tagged using the sign of the “slow” pion π±.c© CERN on behalf of the LHCb Collaboration under CC BY-NC 3.0 167168 E. MAURICE on behalf of the LHCb COLLABORATIONThe raw asymmetry for tagged D0 decays to a final state f is defined asAraw(f) =N(D∗+ → D0(f)π+)−N(D∗− → D0(f)π−)N(D∗+ → D0(f)π+) + N(D∗− → D0(f)π−),with N(X) the number of reconstructed events of decays X, and f the final state. Thisraw asymmetry can be written as the sum of various asymmetries,Araw(f) = ACP (f) + AD(f) + AD(πs) + AP (D∗+),where ACP (f) is the intrinsic physics CP asymmetry, AD(f) the asymmetry to selectthe D0 decay into the final state f , AD(πs) the detection asymmetry of the slow pioncoming from the D∗+ decay chain, and AP (D∗+) the production asymmetry for promptD∗+ mesons.For a two-body decay of a spin-0 particle to a self-conjugate final state there can beno D0 detection asymmetry: AD(K−K+) = AD(π−π+) = 0. At first order, AD(πs)and AP (D∗+) cancel out in the difference Araw(K−K+) − Araw(π−π+). Finally, themeasurement of ΔACP corresponds to the difference of physics asymmetries:ΔACP = ACP (K−K+)−ACP (π−π+) = Araw(K−K+)−Araw(π−π+).In order to minimize second-order effects, the analysis is done in bins of kinematicvariables, magnet polarity and running periods. In total, 216 independent measurementsare made for each decay mode. The χ2/ndf of these measurements is 211/215. The finaltime-integrated difference in CP asymmetry between D0 → K−K+ and D0 → π−π+decays is the weighted average over 216 bins:ΔACP = [−0.82± 0.21(stat)± 0.11(syst)]%.It is the first evidence of CP violation in charm sector, with a significance of 3.5σ. Thismeasurement is consistent with the current HFAG world average [5].ΔACP can be written at first order as the sum of CP asymmetries:ΔACP =(adirCP (K−K+)− adirCP (π−π+))+Δ〈t〉τaindCP ,with adirCP the asymmetry coming from direct CP violation for the decay, 〈Δt〉 the dif-ference in average decay time of the D0 mesons in the K−K+ and π−π+ sample, τ thetrue D0 lifetime, aindCP the asymmetry from CP violation in the mixing. Using this result,the HFAG world-average combination in the plan (ΔadirCP , aindCP ) represented by the fig. 1givesaindCP = (−0.019± 0.232)%, and ΔadirCP = (−0.645± 0.180)%.This combination is consistent with no CP violation at 0.128%. To understand this 3.5σeffect, further analyses are ongoing at LHCb, and more theoretical precision is needed.STUDY OF CPV IN b AND CHARM SYSTEMS AT LHCb 169Fig. 1. – HFAG combination of the ΔACP and AΓ measurements [5]. The bands represent ±1σintervals. No CP violation correspond to the black point at (0, 0), and the two dimensional 68%CL, 95% CL and 99.7% CL regions are the black ellipses.3. – Toward a measurement of the CKM angle γThe γ angle is the least accurately known parameter of the CKM unitarity triangle. Interms of CKM elements, γ is defined as: γ = arg(−VudV ubVcdV cb ). The indirect determinationvia global fits to experimental data gives γ = (67.1+4.6−3.7) [6]. One of the main goals of theLHCb experiment is to perform a precise direct measurement of this angle. It is extractedfrom the interference between b → u and b → c transitions. LHCb experiment has passedmany milestones towards γ measurement with [7]. This contribution focuses on the firstLHCb paper using the whole 2011 data: the direct CP violation in B± → D0K± decayswith 1 fb−1 [8].The analyses of direct CP violation in B± → D0K± are time-integrated measure-ments, using only the self-tagging modes. The interference between decays to the samefinal products (K−K+π−) by different intermediate states (D0K− or D0K−) gives ac-cess to γ. Depending of the D0 decay, different measurement methods are avaiblable.The sensitivity to γ is given by the asymmetries between the decay and its conjugate, A,and the ratio of the sum compared to the favoured control mode B− → D0h−, R.The GLW method was proposed by Gronau, Wyler and London [9, 10]. It is a theo-retically clean measurement of the angle γ from the rate and asymmetry measurement ofB− → DCPK− decays, where the D0 meson decays to CP eigenstates: D0 → K+K−,D0 → π+π−. This method benefits from the interference between the dominant b → cutransition with the doubly CKM-suppressed b → uc transition. The asymmetry andratio observables are defined byRCP+ = 2Γ(B− → D±K−) + Γ(B+ → D±K+)Γ(B− → D0K−) + Γ(B+ → D0K+) = 1 + r2B ± 2rB cos δB cos γ,ACP+ =Γ(B− → D±K−)− Γ(B+ → D±K+)Γ(B− → D±K−) + Γ(B+ → D±K+) = ±2rB sin δB sin γ/RCP+with the relative magnitude of suppressed amplitude rB = |A(b → u)/A(b → c)|, thestrong phase δB = arg(A(b → u)/A(b → c) and the weak phase γ.170 E. MAURICE on behalf of the LHCb COLLABORATIONFig. 2. – Invariant-mass distributions of selected B± → [K+K−]Dh± candidates [8]. The leftplots are B− candidates, B+ are on the right. In the top plots, the bachelor track is assignedto be a kaon, although in the bottom plots it is a pion. The dark (red) curve represents theB → DK± events, the light (green) curve is B → Dπ±. The shaded contribution are partiallyreconstructed events and the total PDF includes the combinatorial component. The contributionfrom Λb → Λ±c h∓ decays is indicated by the dashed line.The ADS method is a modification of the GLW approach, developed by Atwood,Dunietz and Soni [11]. In the B− → D0K− decays, the D0 meson decays to flavourspecific final states: D0 → Kπ. The favoured transition b → c is followed by the doublyCKM-suppressed D decay interfering with the supressed b → u transition followed bythe CKM-favoured D decay. The asymmetry and ratio are given byRADS =Γ(B− → D±K−) + Γ(B+ → D∓K+)Γ(B− → D∓K−) + Γ(B+ → D±K+) = r2B + r2D + 2rBrD cos(δB + δD) cos γ,AADS =Γ(B− → D±K−)− Γ(B+ → D∓K+)Γ(B− → D±K−) + Γ(B+ → D∓K+) =2rBrD sin(δB + δD) sin γRADS,with rB , δB, γ already defined for the GLW method, and rD, δD the correspondingamplitude ratio and strong phase difference of the D meson decay amplitudes.For this analysis, the strategy is to reconstruct every mass hypothesis combination,then to extract the ratios and asymmetries with a simultaneous fit. Figures 2 and 3show, respectively, the invariant-mass distributions of B± → DCPπ± and B± → D±π±.In fig. 2, there is a clear asymmetry in B± → [KK]DK±, but no asymmetry in B± →[KK]Dπ±.The measurements giveACP+ = 0.15± 0.03± 0.01 and RCP+ = 1.01± 0.04± 0.01,AADS(K) = −0.520± 0.150± 0.021 and RADS(K) = 0.0152± 0.0020± 0.0004,AADS(π) = 0.143± 0.062± 0.011 and RADS(π) = 0.0041± 0.0003± 0.0001.STUDY OF CPV IN b AND CHARM SYSTEMS AT LHCb 171Fig. 3. – Invariant-mass distribution of selection B± → [π±K∓]Dh± candidates [8]. See thecaption of 2 for a full description. The dashed line here represents the partially reconstructed,but Cabbibo-favoured, Bs → D0K−π+ and Bs → D0K+π− decays where the pions are lost.The pollution from favoured mode cross feed is drawn, but is too small to be seen.These measurements are an important contribution to a future extraction of the γangle. They are the most precise measurement and in good agreement with the Bfactories [5].4. – B0s mixing phase φsThe interference between B0s decays to J/ψφ either directly or via B0s -B0s oscillationgives rise to a CP -violating phase φs. In the Standard Model, this phase is predicted tobe  −2βs, where βs = arg(−VtsV ∗tb/VcsV ∗cb). The indirect determination via global fitsto experimental data gives 2βs = (0.0364 ± 0.0016) rad [6], within the Standard Model.φs is one of the CP observables with the smallest theoretical uncertainty in the StandardModel, neglecting the penguins contributions, and New Physics could significantly modifythis prediction.The decays B0s → J/ψφ are pseudo-scalar to vector-vector transitions. Morever, theK+K− non-resonant state, the S-wave, is taken into account. Thus the final state isa mixture of CP odd and CP even states. It is described by a four time-dependentdecay amplitudes corresponding to transitions in which the J/ψ and φ or K+K− have arelative orbital momentum L of 0, 1, or 2. In the transversity formalism [12], the initialamplitudes at time t = 0, A0(0) and A‖(0) describe the decays with L = 0, 2 while A⊥(0)and AS describes the L = 1 final states. The arguments of these complex amplitudes arestrong phases denoted δ0, δ‖, δ⊥ and δS .The measurement of φs phase requires a very good proper-time resolution to resolvethe fast B0s oscillations. It has been measured in B0s → J/ψφ channel: σt = 50 fs.Another key step towards the φs measurement is the tagging of the initial B-flavour.The tagging algorithm exploits charged tracks originating from the b-hadron oppositeto the signal B-meson (kaon, muon, electron and vertex charge) and also tracks closeto the signal B-meson (same-side tagging). The opposite side algorithm is optimizedusing B0 → D∗−μ+νμ and B+ → J/ψK+ events and calibrated using B+ → J/ψK+,172 E. MAURICE on behalf of the LHCb COLLABORATION  [MeV] BmEvents / 2 MeV05001000datasignalbackgroundsumLHCbdecay time  [ps] 2 4 6 8Events / 0.2 ps110210310 LHCbθcos-1 -0.5 0 0.5 1Events / 0.10200400600LHCb [rad] ϕ-2 0 2Events / 0.31 rad0200400600LHCbψcos-1 -0.5 0 0.5 1Events / 0.10200400600 LHCb5300 5350 5400Fig. 4. – Data points overlaid with fit projections for the reconstructed invariant mass, decaytime and transversity angle distributions of selected B0s → J/ψφ, candidates, in a mass range of±20MeV/c2 around the reconstructed B0s mass except for the invariant mass distribution [13].The total fit result is represented by the black line. The signal components are represented bythe red lines, and the background component by the blue line.B0 → J/ψK∗0 events [14]. An additional test is performed in [15], by measuring theB0s -B0s mixing frequency using B0s → D−s (3)π+ events using 370 pb−1:Δms = 17.725± 0.041(stat.)± 0.026(syst.) ps−1,which is compatible and competitive with the world best measurement [16].The physical parameters are extracted from a maximum likelihood fit to the mass,proper time and angles distributions of the fully reconstructed candidates as shown infig. 4. In 370 pb−1, 8 276±94B0s → J/ψφ signal events are extracted [13]. Two solutionsare available in the plan (ΔΓs, φs), due to the invariance of the differential decay rateunder the transformation (φs,ΔΓs, δ‖ − δ0, δ⊥ − δ0, δs − δ0) ↔ (π − φs,−ΔΓs, δ0 −δ‖, δ0 − δ⊥, δ0 − δs). LHCb has recently solved this ambiguity [17] by studying theinterferences between the S-wave and the P -wave, following similar method as BaBarcos(2β) measurement [18]. In the B0s → J/ψφ channel, we measure [13]:φs = 0.15± 0.18± 0.06 rad,Γs = 0.656± 0.009± 0.008 ps−1,ΔΓs = 0.123± 0.029± 0.011 ps−1,|A⊥(0)|2 = 0.237± 0.015± 0.012,|A0(0)|2 = 0.497± 0.013± 0.030,|AS |2 = 0.042± 0.015± 0.018,δ‖ = 2.95± 0.37± 0.12,δS = 2.98± 0.36± 0.12,STUDY OF CPV IN b AND CHARM SYSTEMS AT LHCb 173Fig. 5. – Artist’s view of the ΔΓs and φs measurements at the end of 2011. CDF update ismissing: φs ∈ [−π,−2.52] ∪ [−0.60, 0.12] ∪ [3.02, π] at 68% CL, with 9.6 pb−1 [19].where the first error is the statistical error from the fit and the second error is thesystematic uncertainty. It the first direct evidence of non-zero ΔΓs.The φs phase is extracted from the decay B0s → J/ψf0 too [20]. A simultaneous fitin these two channels with common φs, Γs, ΔΓs and Δms gives [21]:φs = 0.03± 0.16(stat)± 0.07(syst) rad.It is the most precise measurement of the φs phase, as illustrated by the artist’s view infig. 5. This value is compatible with the Standard Model, but there is still room for NewPhysics.5. – EpilogueWhile completing these proceedings, CDF has confirmed the ΔACP measurementfrom LHCb with: ΔACP = (−0.62±0.21±0.10)% [22]. LHCb has released its results onφs [23] using 1 fb−1: φs = −0.001± 0.101± 0.027 rad, still compatible with the StandardModel.6. – Conclusions2011 has been an excellent year for LHCb with many measurements related to CPviolation. Among all these measurements, LHCb made the first evidence of CP violationat 3.5σ in charm. Important milestones have been achieved to measure the CKM angleγ with the determination of RCP , ACP , RADS , AADS using 1 fb−1. In the B0s , system,LHCb made the first direct evidence of a non-zero ΔΓs, using 0.4 fb−1, and the CP -violating phase φs has been measured: φs = 0.03± 0.16(stat)± 0.07(syst) rad, consistentwith the Standard Model.∗ ∗ ∗I wish to thanks the organizers of the “Rencontres de Physique de la valle d’Aoste”, forthe nice atmosphere during the conference, and the LHCb collaboration for this beautifulopportunity.174 E. MAURICE on behalf of the LHCb COLLABORATIONREFERENCES[1] Grossman Y., Kagan A. L. and Nir Y., Phys. Rev. D, 75 (2007) 036008.[2] Bobrowski M., Lenz A., Riedl J. and Rohrwild J., JHEP, 03 (2010) 009.[3] Bigi I. I., Blanke M., Buras A. J. and Recksiegel S., JHEP, 07 (2009) 097.[4] The LHCb Collaboration, Phys. Rev. Lett., 108 (2012) 111602.[5] Asner D. et al. (Heavy Flavour Averaging Group), arXiv:1010.1589 [hep.ex].[6] Charles J. et al. (CKMfitter group), Eur. Phys. J. 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Study of CPV in b and charm systems at LHCb

The LHCb experiment is a single-arm spectrometer designed to pursue an extensive study of CP violation in $b$ and charm systems. In this contribution, three recent measurements are presented. First, the difference between CP asymmetries of $D^{o} \to K^-K+$ and $D^{o} \to \pi^{-}\pi^{+}$ decays using 0.6 pb$^{-1}$ of 2011 data is detailled. Then an important milestones towards the measuremnt of the $\gamma$ angle is presented with the study of $B^{\pm} \to DK^{\pm}$ decays using 1 fb$^{1}$. Third, the measurement of the CP-violatin phase $\phi_s$ in B${^0_s} \to J/\psi\phi$ and B${^0_s} \to J/\phi$f${_0}$ is reported

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Study of CPV in b and Charm Systems at LHCb

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Study of CPV in b and charm systems at LHCb

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