Flavour physics at LHCb

Flavour physics at LHCb B. Adeva1,a, on behalf of the LHCb Collaboration. 1University of Santiago de Compostela, Spain Abstract. Some selected results of the LHCb experiment, running at the LHC with pp collisions at 7 TeV and 8 TeV, are reported here, after operation with a total integrated luminosity of 3.0 fb−1(Run 1). We focus on the most recent analyses on flavour physics, that include measurements of the CKM invariant phases γand β, precision determination of the quark coupling strength Vub, observation of the very rare decays B0 (s)→μ+μ−, search for new physics in the anomalous branching ratio of B→D∗τ¯ν, and precision angular analysis of the rare decays B0→K∗0μ+μ−and B0 s→φμ+μ−. Detailed comparisons are performed in all cases with the predictions of the Standard Model, and a few interesting tensions are observed. 1 The LHCb experiment The LHCb detector [1, 2] is one of the four major detectors at the Large Hadron Collider. It is instrumented in a cone arround the proton beam axis, covering the angles between 10 and 250 mrad, where most bhadron decays produced in proton-proton collisions occur. The detector includes a high-precision tracking system with a dipole magnet, providing a measurement of momentum and impact parameter (IP), defined for charged particles as the minimum distance of a track to a primary pp interaction vertex (PV). Different types of charged particles are distinguished using information from two ring-imaging Cherenkov detectors, a calorimeter and a muon system. Simulated samples of specific signal and background decay modes of bhadrons are used at many stages throughout the analysis. These simulated events model the experimental conditions in full detail, including the pp collision, the decay of the particles, and the response of the detector [3–5]. Candidates of the different signal modes reported in this article are required to pass a trigger system [6] which reduces in real time the rate of recorded collisions from the readout clock of the LHC to approximately 4 KHz. For muon channels, a muon is typically selected with pT>1.48 GeV/c in the √s=7 TeV collision data (pT>1.76 GeV/c in the 8 TeV data), and at least one of the finalstate particles is required to have both pT>0.8 GeV/c and impact parameter larger than 100 μm with respect to all of the PVs in the event. For hadron channels, a multivariate algorithm is used for the identification of secondary vertices consistent with the decay of a bhadron [7]. In all cases, the tracks of two or more of the final-state particles are required to form a vertex that is significantly displaced from the PVs. ae-mail: Bernardo.Adev[email protected] DOI: 10.1051/ , 126 12602001 EPJ Web of Conferences epjconf/2016 02001 (2016) ICNFP 2015 © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). 2 Measurement of the γCKM phase In the Standard Model (SM), the Flavor Changing Charged Current processes of quarks are described by a unitary complex-valued Cabibbo-Kobayashi-Maskawa (CKM) mixing matrix [8], whose elements Vij, with i=u,c,tand j=d,s,b, quantify the relative i↔jcoupling strength. This matrix originates from the misalignment between up and down type quark couplings to the Higgs boson, and being unitary, it has only one independent phase. Every CP violation phenomenon (in the quark sector) should be related, in the SM, to this unique phase. However, up to four measurable phases can be formed from combinations of the type VαiV∗ αjVβjV∗ βi, and it is important to measure all of them because loop-level contributions from additional high-mass particles beyond the SM, may alter the unitarity relationships they must fulfil. The measurement of the phase β=arg −VcdV∗ cb/(VtdV∗ tb), that provided the first evidence for CPviolation in the b-quark sector [9], is a historical example. A new competitive measurement of this parameter has been provided this year by the LHCb experiment, which is reported in the next section. Yet another sensitive test of the SM comes from measurements of the phase βs=arg −VtsV∗ tb/(VcsV∗ cb), only accessible from B0 smeson decays, which is predicted to be very small in the SM. LHCb has initiated a series of measurements of this parameter using different channels, which we are not discussing here [10]. Within the precision attained so far, these measurements appear to confirm the SM prediction. In order to disentangle the nature of new physics contributions from quantum loops, in case a significant deviation from unitarity is found, it is particularly important to have a CP-observable that only receives contributions from tree-level diagrams, and can therefore be used as a test-bench for the SM. This is precisely the case with the phase γ=arg[−VudV∗ ub/(VcdV∗ cb)] [11], the least well-measured to date of those in the unitarity triangle VudV∗ ub +VcdV∗ cb +VtdV∗ tb =0. Figure 1. Mass distributions of B−→DXs−candidates using GLW selections , for B−→[K+K−]DXs−(left), B−→[K+K−]DXs+(center), and the suppressed ADS mode, B±→[K∓π±]DK±π∓π∓, sum of B+and B−(right). The phase γcan be probed by studying the interference between b→uand b→ctransitions, such as the interference between B−→D0K−and B−→¯ D0K−, when states accessible to both D0 and ¯ D0mesons are selected. A number of methods have been discussed in the literature, and are often grouped into three categories, depending on the D decay mode: (i) CP eigenstates, such as D→K+K− and D→π+π−(GLW) [12]; (ii) flavor-specific final states, such as the Cabibbo-favored and double Cabibbo suppressed D→K±π∓decays (ADS) [13]; and (iii) multi-body self-conjugate final states, such as D→K0 sπ+π−(GGSZ) [14]. Measurements of γhave been performed from averages over several decay modes from individual experiments, and the current experimental status is γ=(73+9 −10)◦by the LHCb collaboration [16], DOI: 10.1051/ , 126 12602001 EPJ Web of Conferences epjconf/2016 02001 (2016) ICNFP 2015 2 γ=(73+17 −16)◦by the BaBar collaboration [17], and γ=(68+15 −14)◦by the Belle collaboration [18]. The overall precision on γfrom a global fit is about 7◦[19]. In order to improve the overall precision, it is important to study a wide range of final states. It has been suggested that other multi-body final states of the recoiling strange quark system could be useful [15], due to their larger branching fractions, and potentially larger interference contribution. LHCb has performed the first ADS and GLW analyses of the decay B−→DX− s, where the Dmeson is observed through its decay to K±π∓,K+K−and π+π−, and a multibody final state X− s≡K−π+π− is defined for the recoil system [20]. Some invariant mass spectra for the B−→DX− sADS and GLW signal modes are shown in Fig.1. The analysis uses an integrated luminosity of 3.0 fb−1and includes the modes B−→DX− d, with lower sensitivity to γ, for normalization purposes. Significant signals are observed in the CP modes for both the favored and the suppressed B−decays, and first evidence is seen for the ADS DCS B−→D[K+π−]DK−π+π−decay. A fit for γis performed, from which γ=(74+20 −22)◦ is found with only B−→DX− smodes. Values of γbelow about 25◦and larger than approximately 165◦are not excluded by these modes, but are excluded when other modes are considered [16]. The sensitivity to γfrom this analysis makes it a promising channel for future studies. LHCb has also explored additional multibody final states leading to possible improvement of the global precision on the γparameter, and performed a measurement of CP observables from B±→Dh± decays, where D mesons are reconstructed in the ADS channel D→K∓π±π0and the quasi-GLW modes D→π+π−π0and D→K+K−π0[21]. In such cases, the interference effects that are sensitive to γvary over the phase space of the D decay, due to the role of strongly-decaying intermediate resonances, and the integration over the phase space in general dilutes the net sensitivity. For multi body ADS/GLW modes the dilution factor can be measured with D¯ Dpairs coherently produced at the ψ(3770) resonance [22]. Recent measurements of this type [23] indicate that the dilution effects in D→K∓π±π0and D→π+π−π0are rather small, making these decays particularly suitable for an inclusive analysis. In particular the latter is very close to being a CP-even eigenstate, and the interference terms suffer very little dilution. Figure 2. Invariant mass distributions of selected B∓→[π+π−π0]Dh∓candidates, separated by B hadron charge. B∓→DK∓ signal events are in the upper plots and B∓→Dπ∓in the lower plots.The red curve represents DK∓events and the green curve represents Dπ∓events. The grey shape indicates partially reconstructed B∓decays and the dotted red curve indicates wrongly reconstructed D decays. The blue line represents the total PDF As it is described in Ref. [21], twelve observables are measured by LHCb in total, including CP-asymmetries and amplitude ratios between Cabibbo favored and suppressed modes, for the above-mentioned ADS and quasi-GLW modes. Two-dimensional scans are performed for γvs. rB and γvs. δB, where the hadronic amplitude ratio rBand the phase difference δBare defined as: A(B−→¯ D0K−)/A(B−→D0K−)=rBei(δB−γ). The results are compatible with the values obtained from a global analysis of other LHCb measurements sensitive to γat tree level (also sensitive to rB and δB) [16]. No evidence of CP violation is seen with the current experimental precision. First evidence is obtained for the mode B∓→[K+K−π0]DK∓, and the channels B∓→[π∓K±π0]Dπ∓ B∓→[K+K−π0]Dπ∓have been observed for the first time. Some particular invariant mass spectra are DOI: 10.1051/ , 126 12602001 EPJ Web of Conferences epjconf/2016 02001 (2016) ICNFP 2015 3 shown in Fig.2, separated by the charge of the B∓candidate, as an indication of the signal significance for CP violating observables. When analysed in the context of the underlying physics parameters, the results exhibit good consistency with other LHCb measurements, and they will be valuable in improving knowledge of γin the unitarity triangle VudV∗ ub +VcdV∗ cb +VtdV∗ tb =0, when combined with results from B∓→DK∓ measurements using other D decay channels. 3 New precision measurement of the βCKM phase The violation of CP symmetry in processes involving Bmesons was first observed in the "golden mode" B0→J/ψK0 sby the BaBar and Belle experiments at the asymmetric e+e−colliders PEP-II and KEKB. Since then, measurements of CP violation in this decay mode have reached a precision at the level of 10−2[24]. LHCb has performed a new competitive measurement obtained at a hadron collider [25], where a reduced flavor tagging capability, as compared to B-factories, is compensated by a higher bhadron cross-section, with the integrated luminosity of 3.0 fb−1from Run 1. m(MeV/c2) 5240 5260 5280 5300 5320 Candidates / (1 MeV/c2) 0 500 1000 1500 2000 2500 3000 3500 LHCb (a) Figure 3. The distribution of the reconstructed mass of tagged B0→J/ψK0 s candidates is shown. The solid black line shows the fit projection, while the dashed (dotted) line shows the projection for the signal (background) components only. As the J/ψK0 sfinal state is common to both B0and ¯ B0meson decays, the interference between the amplitudes for the direct decay and for the decay after B0−¯ B0oscillation results in a decay-time dependent CP asymmetry as follows A(t)≡Γ(¯ B0(t)→J/ψK0 s)−Γ(B0(t)→J/ψK0 s) Γ(¯ B0(t)→J/ψK0 s)+Γ(B0(t)→J/ψK0 s)=Ssin(Δmt)−Ccos(Δmt) cosh(ΔΓt 2)+AΔΓ sinh(ΔΓt 2) where B0(t) and ¯ B0(t) indicate the flavor of the Bmeson at production, while tindicates the decay time. The parameters Δmand ΔΓ are the mass and decay width differences between the heavy and light mass eigenstates of the B0−¯ B0system, and S,Cand AΔΓ are CP observables. As ΔΓ is negligible for the B0−¯ B0system, the time dependent asymmetry simplifies to A(t)=Ssin(Δmt)−Ccos(Δmt). The B0→J/ψK0 sdecay is dominated by a ¯ b→c¯c¯stransition, and CP violation in the decay is expected to be negligible at the current level of experimental precision, giving C≈0, which allows to identify Swith sin(2β). t(ps) 51015 Signal yield asymmetry −0.4 −0.3 −0.2 −0.1 0 0.1 0.2 0.3 0.4 LHCb Figure 4. The time-dependent signal-yield asymmetry (N¯ B0−NB0)/(N¯ B0+NB0) is shown. Here NB0(N¯ B0) is the number of B0→J/ψK0 sdecays with a B0(¯ B0) flavor tag. The solid curve is the projection of the signal PDF. Compared to previous LHCb analysis, the effective tagging efficiency ef f has increased from 2.38% to 3.02%, mainly due to the inclusion of a same side pion tagger algorithm. The mass spectrum DOI: 10.1051/ , 126 12602001 EPJ Web of Conferences epjconf/2016 02001 (2016) ICNFP 2015 4 of the selected candidates is shown in Fig. 3, and the measured decay-time dependent signal-yield asymmetry is shown in Fig. 4. The CP observables Sand Care measured to be S=0.731 ±0.035 (stat) ±0.020 (syst) C=-0.038 ±0.032 (stat) ±0.005 (syst) with a statistical correlation ρ(S,C)=0.483. When Cis fixed to zero the measurement yields S=sin(2β)=0.746 ±0.030 (stat). This result represents the most precise time-dependent CP violation measurement at a hadron collider to date. Furthermore, it has a similar precision to, and is in good agreement with, previous measurements performed at the Belle and BaBar experiments at the KEKB and PEP-II colliders [24]. This result is in excellent agreement with expectations from other measurements and improves the consistency of the CKM sector of the Standard Model. Other measurements that constraint this angle of the unitarity triangle predict sin(2β)as0.771+0.017 −0.041 [26]. 4 Precision determination of the coupling strength |Vub| The Vub matrix element governs the most sensitive misalignment in the couplings between the up and down type quark flavors to the Higgs boson. Its apparent proportionality to the third power of λ(sine of the Cabibbo angle) remains unexplained, and it best quantifies the minimal flavor violation structure of the Standard Model. A precision measurement of the magnitude of Vub is naturally achieved via the semileptonic quark-level transition b→ul−¯νl, which minimizes hadronic uncertainties. There are two complementary methods to perform such measurement. The simplest is to measure the branching fraction of a specific (exclusive) decay such as ¯ B0→π+l−¯νor B−→π0l−¯ν, where the influence of the strong interaction in the decay, encompassed by the ¯ B0→π+form factor, is predicted by lattice QCD (LQCD) [28] or QCD sum rules [29]. The world average from Ref. [30] is Vub =(3.28 ±0.29) ×10−3, where the most precise inputs come from the Babar [31] and Belle [32] experiments. The uncertainty is dominated by the LQCD calculations, recently updated [33]. The alternative method is measure the differential decay rate in an inclusive way over all possible Bmeson decays containing the b→ul−¯νl quark level transition. This results in Vub =(4.41+0.15 −0.17)×10−3[34], where the second uncertainty comes from theoretical calculations. The above results are summarized in Fig.5 (left). Figure 5. Summary of the exclusive and inclusive measurements indicated in the text (left) and experimental constraints on left-handed coupling, VL ub and the fractional right-handed (RH) coupling R. While the overlap of the 68% CL bands for the inclusive and exclusive world averages suggested a RH coupling of significant magnitude, the inclusion of the LHCb measurement does not support this. The discrepancy between the exclusive and the inclusive Vub determinations has a significance of approximately 3σand has been a long standing puzzle in flavor physics. Several explanations have been proposed, such as the presence of a right-handed (V+A) W coupling [35]. LHCb has performed a measurement of the ratio of branching fractions of the Λ0 binto pμ−¯νμand Λ+ cμ−¯νμfinal states [27]. This has been done using pp collisions from the LHC, corresponding to 2.0 fb −1of integrated luminosity at 8 TeV. The b→utransition Λ0 b→pμ−¯νμcould not be considered before at B-factories, but becomes feasible at the LHC. DOI: 10.1051/ , 126 12602001 EPJ Web of Conferences epjconf/2016 02001 (2016) ICNFP 2015 5 Figure 6. Fits are made for Λ0 b→pμ−¯ν(left) and Λ0 b→Λ+ c(pK−π+)μ−¯ν(right) candidates. Data are represented by the black points, and the open boxes represent the statistical uncertainties from the finite size of the simulation samples used to model the mass shapes. There are no data above the nominal Λ0 bmass due to the removal of unphysical q2solutions. To facilitate Λ0 breconstruction at a hadron collider, LHCb introduces a corrected mass Mcorr =m2 hμ+p⊥2+p⊥2where hrepresents either the proton or the Λ+ ccandidate, mhμis the visible mass of the hμpair, and p⊥is the momentum of the hμpair transverse to the Λ0 bflight direction.The recoil squared mass q2(μν) can be determined from the above direction, up to a two-fold ambiguity. For kinematic reasons in the experiment, it is restricted to high values, precisely where the precision on the form factors is best. Secondary vertex isolation criteria are used, and from a detailed comparison between the Mcorr spectra shown in Fig. 6, a measurement of the ratio of branching ratios is achieved: B(Λ0 b→pμ−¯νμ)q2>15GeV/c2 B(Λ0 b→Λ+ cμ−¯νμ)q2>7GeV/c2 =(1.00 ±0.04 ±0.08) ×10−2 using the form factor information from [36] for the restricted q2regions, the measurement |Vub|/|Vcb|=0.083±0.004±0.004 is obtained, where the second uncertainty arises from the uncertainty in the LQCD prediction. When the exclusive world average is used for |Vcb|[34], the measurement is obtained |Vub|=(3.27 ±0.15 ±0.17 ±0.06) ×10−3 where the uncertainties indicate experimental, LQCD prediction and normalization to Vcb. The determination of |Vub|from the ratio of branching ratios depends on the size of a possible right-handed coupling [35], according to an effective Lagangian of the type Lef f =−4GF √2VL ub ¯uγμPLb+R¯uγμPRb(¯νγμPLl)+h.c. with PR,L=(1 ±γ5)/2. The sensitivity can be appreciated in Fig. 5 (right) which shows the experimental constraints on the left-handed coupling |VL ub|and the fractional right-handed coupling added to the SM, Rfor different measurements. Unlike the case for the pion in ¯ B0→π+l−¯νand B−→π0l−¯νdecays, the spin of the proton is non-zero, allowing an axial-vector current, which gives adifferent sensitivity to R. The overlap of the bands from the previous measurements suggested a significant right-handed coupling, but the inclusion of the LHCb |Vub|measurement does not support that assumption. In summary, the most precise measurement to date of |Vub|is reported using the exclusive decay mode Λ0 b→pμ−¯νμ. The measurement is in agreement with the exclusively measured world average [30], but disagrees with the inclusive measurement [34] at a significance level of 3.5σ. The measurement will have an important impact on the global fits to the parameters of the CKM matrix. DOI: 10.1051/ , 126 12602001 EPJ Web of Conferences epjconf/2016 02001 (2016) ICNFP 2015 6 5 Observation of the very rare decays B0 (s)→μ+μ− The decay of the B0 smeson into dimuons is suppressed strongly, to the level 10−9−10−10. It represents a powerful probe in testing new physics effects, because the suppression originates from essential features of the heavy particle spectrum of the SM, namely: the GIM mechanism, helicity suppression and the fact that the SM contributions involve an off-diagonal element of the CKM matrix. Since these features are of course not generally respected by generic extensions of the SM, the above decay has been searched for many years at most generations of accelerators. The Feynman diagrams are shown in Fig. 7, illustrating the suppression in the SM and the sensitivity to Supersymmetry models. The first evidence for the B0 s→μ+μ−decay was presented by the LHCb collaboration in 2012 [47]. The LHCb and CMS experiments have presented a joint analysis [48], in order to fully exploit the statistical power of the LHC data, and take into account the correlation between the physical quantities in common to the two analyses. The data correspond to total integrated luminosities of 25.0 fb−1and 3.0 fb−1for the CMS and LHCb experiments, respectively. This is equivalent to a total of approximately 1012B0 sand B0mesons produced in both experiments together, determined from the number of expected events assuming the SM branching fractions. The branching fractions of these two decays, accounting for higher order electromagnetic and strong interaction effects, are reliably calculated in the SM. The untagged time-integrated SM predictions are B(B0 s→μ+μ−)SM =(3.66 ±0.23) ×10−9and B(B0→μ+μ−)SM =(1.06 ±0.09) ×10−10 [37], which use the latest lattice QCD results to compute B0 sand B0meson decay constants [38]. Figure 7. Some Feynman diagrams for the B0 s→μ+μ−decay in (a) the SM and (b) Minimal Supersymmetric model. Many theories that seek to go beyond the SM (BSM) include new phenomena and particles [39], such as in the diagram shown in Fig. 8(b), that can considerably modify the SM branching fractions. In particular, theories with additional Higgs bosons [40] predict possible enhancements of the branching fractions. A significant deviation of either of the two measurements from the SM predictions would give insight on how the SM should be extended. Alternatively, a measurement compatible with the SM could provide strong constraints on BSM theories. The ratio of the branching fractions of the two decay modes additionally provides powerful discrimination among BSM theories [41]. In the SM it is predicted to be B(B0→μ+μ−)SM/B(B0 s→μ+μ−)SM =0.0295+0.0028 −0.0025 [44], [45], [46]. Notably, BSM theories with the property of minimal flavor violation [42] predict the same value as the SM for this ratio. The two experiments measure at different angular regions with respect to the LHC beams, according to their different design purposes. Their dimuon mass resolution are also different, namely ≈25 MeV/c2for LHCb, and ranging from 32-76 MeV/c2, depending on the pseudorapidity of the two tracks for CMS. The separation between genuine B0 s→μ+μ−decays and random combinations of two muons, most often from semi-leptonic decays of two different bhadrons, is achieved by means of boosted decision trees (BDTs) [43]. Each experiment selected the best set of discriminating variables in their respective BDT. One example is the decay length with respect to the PV. Having lifetimes of about 1.5 ps, B0 (s)mesons travel up to a few centimetres before they decay, with momenta between a few GeV/c and 100 GeV/c at the LHC. Candidates were categorized according to the value of the DOI: 10.1051/ , 126 12602001 EPJ Web of Conferences epjconf/2016 02001 (2016) ICNFP 2015 7 relevant BDT discriminant, to whether they were detected in CMS or LHCb, and to the kinematical configuration of both muons, in the case of CMS. 20 different categories were so defined, as described in Ref. [48]. A single dimuon mass distribution, extended over all categories, is shown in Fig. 8. Event candidates are weighted according to their values of S/(S+B), where Sis the expected number of B0 s signals and Bthe number of background events under the B0 speak in each category. Figure 8. Weighted distribution of the dimuon invariant mass mμ+μ− for all categories defined within the CMS and LHCb experiments. Superimposed on the black data points are the combined fit (solid blue line) and its components: the B0 s(yellow shaded area) and B0 (light blue shaded area) signal components, the combinatorial background (dashed-dotted green line); the sum of the semileptonic backgrounds (dotted salmon line); and the peaking backgrounds (dashed violet line). Likelihood contours for B(B0 s→μ+μ−) versus B(B0→μ+μ−) are shown in Fig. 8. Onedimensional likelihood scans for both decay modes are displayed in the same figure. A combined fit leads to the measurements B(B0 s→μ+μ−)=2.8+0.7 −0.6×10−9and B(B0→μ+μ−)=3.9+1.6 −1.4×10−10, where the uncertainties include both statistic and systematic sources. The statistical significance is computed to be 6.2σfor the B0 s→μ+μ−mode and 3.0σfor the B0→μ+μ−mode (we report the significance obtained with the FC method) . Figure 9. Likelihood contours in the B(B0 s→μ+μ−) versus B(B0→μ+μ−) plane. The (black) cross in (a) marks the best-fit central value. The SM expectation and its uncertainty is shown in the (red) marker. Each contour encloses a region corresponding to the reported confidence level. Variations of the test statistic -2lnL are also shown for B(B0 s→μ+μ−) (b) and B(B0→μ+μ−) (c). The dark and light (cyan) areas define the ±1σand ±2σ confidence intervals, respectively. The SM prediction and its uncertainty is denoted with the vertical (red) band. A fit for the ratios of the branching fractions relative to their SM predictions yields SB0 s SM =0.76+0.20 −0.18 and SB0 SM =3.7+1.6 −1.4. The ratio of branching ratios themselves yields R=0.14+0.08 −0.06 which is compatible with the SM at the 2.3σlevel. The combined analysis of the data from CMS and LHCb establishes conclusively the existence of the B0 s→μ+μ−decay and produces a 3σevidence for the B0→μ+μ−decay. For B0 s, this concludes a search that started more than three decades ago. A phase of even higher sensitivity and precision measurements is initiated for both decays. 6 Anomalous branching fraction of B→D∗τ¯ν Lepton universality, which is preserved within the Standard Model, requires equality of couplings between gauge bosons and the three families of leptons. Hints for lepton nonuniversal effects in DOI: 10.1051/ , 126 12602001 EPJ Web of Conferences epjconf/2016 02001 (2016) ICNFP 2015 8 B+→K+e+e−and B+→K+μ+μ−decays have been seen [50] , but no definitive observation of a deviation has yet been made. However, a large class of models that extend the SM contain additional interactions involving enhanced couplings to the third generation that would violate the above principle. Semileptonic decays of bhadrons to third generation leptons provide a sensitive probe for such effects. In particular, the presence of additional charged Higgs bosons, which are often required in these models, can have a significant effect on the rate of the semitauonic decay ¯ B0→D∗+τ−¯ντ[51]. Semitauonic decays have been observed by BaBar and Belle collaborations [52],[56]. Recently BaBar reported updated measurements [55],[56] of the ratios of branching fractions, R(D∗)≡B(¯ B0→D∗+τ−¯ντ)/B(¯ B0→D∗+μ−¯ντ) and R(D)≡B(¯ B0→D+τ−¯ντ)/B(¯ B0→D+μ−¯ντ), which show deviations of 2.7σand 2.0σ, respectively, from the SM predictions [57],[58]. These ratios have been calculated with high precision, owing to the cancelation of most of the uncertainties associated with the strong interaction in the Bto D(∗)transition. Within the SM they differ because of phase-space effects due to the differing charged lepton masses. LHCb has achieved a new measurement of R(D∗) using hadron collisions at the LHC with an integrated luminosity of 1.0 fb−1and 2.0 fb−1collected at pp center-of-mass energies of 7 TeV and 8 TeV, respectively [49]. The ¯ B0→D∗+τ−¯ντdecay with τ−→μ−¯νμντ(the signal channel) and the ¯ B0→D∗+μ−¯ντdecay (normalization channel) produce identical visible final-state topologies; consequently both are selected by a common reconstruction procedure. The selection identifies semileptonic ¯ B0decay candidates containing a muon candidate and a D∗+candidate through the decay chain D∗+→D0(→K−π+)π+. The selected sample contains contributions from the signal and the normalization channel, as well as several background processes from hadron collisions, which include partially reconstructed Bdecays and candidates from combinations of unrelated particles from different bhadron decays. The kinematic and topological properties of the various components are exploited to suppress the background contributions. The signal, the normalization component and the residual background are statistically disentangled with a multidimensional fit to the data, using template distributions derived from control samples, or from simulation validated against real data. Figure 10. Distributions of m2 miss (left) and E∗ μ(right) of one of the four q2bins of the signal data used in the fit (9.35<q2<12.60 GeV2/c4), overlaid with projections of the fit model with all normalization and shape parameters at their best-fit values. Below each pannel differences between the data and fit are shown, normalized by the Poisson uncertainty in the data. The bands give the 1σtemplate uncertainties. The separation of the signal from the normalization channel, as well as from background processes, is achieved by exploiting the distinct kinematic distributions resulting from the μ-τmass difference and the presence of extra neutrinos from the decay τ−→μ−¯νμντ. The most discriminating kinematic variables, computed in the Brest frame, are the following quantities: the muon energy E∗ μ; the missing mass squared, defined as m2 miss =(pμB−pμD−pμμ)2; and the squared four-momentum transfer to the lepton system, q2=(pμB−pμD)2, where Pμ B,Pμ D, and Pμ μare the four momenta of the B meson, the D∗+meson and the muon. The determination of the rest-frame variables requires knowlDOI: 10.1051/ , 126 12602001 EPJ Web of Conferences epjconf/2016 02001 (2016) ICNFP 2015 9