Lepton universality violation would represent a signal of physics beyond the Standard Model. Anomalies have been observed in the measurement of the branching ratios of semileptonic decays of beauty mesons in third-generation leptons. This contribution reports on the measurements of the observable R(D∗−) = B(B0 → D∗−τ +ντ )/B(B0 → D∗−μ+νμ) and R(J/ψ) = B(B+ c → J/ψτ +ντ )/B(B+ c → J/ψμ+νμ) performed at LHCb with data collected during the Run 1 of the LHC

Betti, F.

Lepton universality violation would represent a signal of physics beyond the Standard Model. Anomalies have been observed in the measurement of the branching ratios of semileptonic decays of beauty mesons in third-generation leptons. This contribution reports on the measurements of the observable $R(D^{\ast -}) = B(B^0 \rightarrow D^{\ast -} \tau^+ \nu _\tau)/B(B^0 \rightarrow D^\ast - \mu^+ \nu_\mu)$ and $R(J/\psi)$ = $B(B^+_c \rightarrow J/\psi \tau^+ \nu_\tau)/B(B^+_c \rightarrow J/\psi \mu^+ \nu_\mu)$ performed at LHCb with data collected during the Run 1 of the LHC

Betti, F

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Measurements of lepton universality at LHCb

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DOI 10.1393/ncc/i2019-19154-2Colloquia: IFAE 2018IL NUOVO CIMENTO 42 C (2019) 154Measurements of lepton universality at LHCbF. Betti(1)(2)(1) INFN, Sezione di Bologna - Bologna, Italy(2) Dipartimento di Fisica e Astronomia, Università di Bologna - Bologna, Italyreceived 31 January 2019Summary. — Lepton universality violation would represent a signal of physics be-yond the Standard Model. Anomalies have been observed in the measurement of thebranching ratios of semileptonic decays of beauty mesons in third-generation leptons.This contribution reports on the measurements of the observable R(D∗−) = B(B0 →D∗−τ+ντ )/B(B0 → D∗−μ+νμ) and R(J/ψ ) = B(B+c → J/ψτ+ντ )/B(B+c →J/ψμ+νμ) performed at LHCb with data collected during the Run 1 of the LHC.1. – IntroductionIn the Standard Model (SM) of particle physics the electroweak couplings of the gaugebosons to the leptons have the same value for all the lepton families. This propertyis known as lepton universality (LU), and if LU violation is observed, it will represent aclear signal of New Physics (NP).The branching fractions ratio, calculated with high precision in the SM [1],(1) R(D(∗)) = B(B0 → D(∗)−τ+ντ )B(B0 → D(∗)−μ+νμ),represents a sensitive probe for LU violation.BaBar and Belle measured R(D∗), reporting respectively discrepancy of 3.4σ andconsistency at 1σ level with the theoretical expectations [2, 3].The analogous ratio in the Bc sector,(2) R(J/ψ ) = B(B+c → J/ψτ+ντ )B(B+c → J/ψμ+νμ),has been calculated with less precision due to large uncertainties on the form factors [4],and it has never been measured so far.Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 F. BETTIThis document presents the measurements of R(D∗−) and R(J/ψ ) performed atLHCb with data collected during 2011 and 2012 at a centre-of-mass energy of 7 and8 TeV, corresponding to an integrated luminosity of 3 fb−1.2. – Measurement of R(D∗−)The first measurement of R(D∗−) at a hadron collider was performed by LHCb [5],using the muonic τ decay channel τ+ → μ+νμντ (1). The normalization mode B0 →D∗−μ+νμ is also the main background source for the signal B0 → D∗−τ+ντ . In orderto achieve a good separation between the two modes, three key kinematic observablesare computed in the B0 rest frame: the muon energy E∗μ, the squared missing massm2miss = (pB0 − pD∗ − pμ)2 and the squared four-momentum transfer q2 = (pB0 − pD∗)2,where pB0 , pD∗ and pμ are the four-momenta of the B0, the D∗− and the muon. Becauseof the presence of neutrinos in the decay, the exact determination of the B0 momentum inthe laboratory frame is not possible, only the momentum direction is determined from theB0 decay vertex and the associated primary vertex (PV). The three kinematic observablesare therefore computed by approximating the Lorentz boost of the B0 with the boost ofthe visible D∗−μ+ system, resulting in a resolution of about 15–20% on the rest-framevariables, sufficient to preserve the discrimination between the two modes. A binnedmultidimensional maximum-likelihood fit is performed to data on the distribution ofthe three observables, with templates for signal, normalization and background channelsextracted from simulation and data-driven control samples. The resultR(D∗−) = 0.336 ± 0.027 (stat) ± 0.030 (syst)is in good agreement with previous measurements and 2.1 standard deviations higherthan the SM expectation.A second measurement of R(D∗−) was performed in LHCb [6, 7] on an independentsample, with the τ lepton reconstructed through the τ+ → π+π−π+(π0)ντ decay. It isexperimentally more convenient to measure the ratio:(3) R(D∗−) = B(B0 → D∗−τ+ντ )B(B0 → D∗−π+π−π+) ×B(B0 → D∗−π+π−π+)B(B0 → D∗−μ+νμ),where the normalization channel B0 → D∗−π+π−π+ has been chosen in order to havemost of the systematic uncertainties cancelled out in the efficiency ratio. The secondterm of eq. (3) is obtained using externally measured branching fractions.The most dominant background, consisting of inclusive decays of b-hadrons to D∗3πX,is suppressed by requiring the τ vertex to be downstream, along the beam direction, withrespect to the B0 vertex with a 4σ significance. In order to discriminate from the signalthe remaining background, mainly due to double-charmed B decays, a set of variablesis used: variables computed with two partial reconstruction techniques, one in signalhypothesis and the other in background hypothesis; variables related to the 3π systemdynamics; isolation variables. All these variables are used as input to train a BoostedDecision Tree (BDT). Through the partial reconstruction in the signal, hypothesis q2(1) Charge conjugated decay modes are implied throughout the document.MEASUREMENTS OF LEPTON UNIVERSALITY AT LHCb 3and the τ decay time are computed with a sufficiently good resolution to maintain theseparation between signal and background.The signal yield is extracted by performing a multidimensional maximum-likelihoodtemplate fit on data in the high-BDT region, on three-dimensional shapes of q2, τ decaytime and BDT, derived from simulated and data-driven control samples. The normal-ization yield is obtained by fitting the D∗3π invariant-mass distribution in the B0 peakregion, after reverting the τ vertex requirement.The result of this analysis isR(D∗−) = 0.286 ± 0.019 (stat) ± 0.025 (syst) ± 0.021 (ext),where the last uncertainty originates from the uncertainties on the external branchingfractions [8-11]. This result is higher than the SM calculation, consistent with it withinone standard deviation, and compatible with all the previous measurements. The combi-nation of all available measurements of R(D∗) and R(D) shows a discrepancy from theSM prediction by 4.1σ [11].3. – Measurement of R(J/ψ )R(J/ψ ) has been measured in LHCb [12] reconstructing the τ from its muonic decayand the J/ψ in the J/ψ → μ+μ− mode. The adopted strategy is similar to the oneused for the muonic R(D∗−) measurement, adding to the list of discriminating variablesthe B+c decay time, since its distribution is significantly different between signal andbackground of semileptonic decays of lighter b-hadron decays. The measured value isR(J/ψ ) = 0.71 ± 0.17 (stat) ± 0.18 (syst),which is 2σ higher than the SM calculations.4. – ConclusionsLHCb performed three measurements of observables sensitive to LU violation in semi-tauonic decays with the data collected during the Run 1 of the LHC. Updates of thesemeasurements with the data collected during the Run 2 are currently ongoing, and at thesame time similar observables, like R(D), R(Ds), R(Λc) and R(Λ∗c), are under study, inorder to test the consistency of the anomalies in other decay modes.REFERENCES[1] Fajfer S., Kamenik J. F. and I. Nǐsandžić, Phys. Rev. D, 85 (2012) 094025.[2] The BaBar Collaboration (Lees J. P. et al.), Phys. Rev. D, 88 (2013) 072012.[3] Belle Collaboration (Huschle M. et al.), Phys. Rev. D, 92 (2015) 072014.[4] Ivanov M. A., Korner J. G. and P. Santorelli, Phys. Rev. D, 73 (2006) 054024.[5] LHCb Collaboration (Aaij R. et al.), Phys. Rev. Lett., 115 (2015) 111803.[6] LHCb Collaboration (Aaij R. et al.), Phys. Rev. Lett., 120 (2018) 171802.[7] LHCb Collaboration (Aaij R. et al.), Phys. Rev. D, 97 (2018) 072013.[8] LHCb Collaboration (Aaij R. et al.), Phys. Rev. D, 87 (2013) 092001.[9] The BaBar Collaboration (Lees J. P. et al.), Phys. Rev. D, 94 (2016) 091101.[10] Belle Collaboration (Majumder G. et al.), Phys. Rev. D, 70 (2004) 111103.[11] HFLAV Collaboration (Amhis Y. et al.), Eur. Phys. J. C, 77 (2017) 895.[12] LHCb Collaboration (Aaij R. et al.), Phys. Rev. Lett., 120 (2018) 121801.

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Measurements of lepton universality at LHCb

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