Search

We report the first observation of a baryonic $B_s^0$ decay, $B_s^0\to p\overline{\mathrm{\Lambda }}{K}^{-}$, using proton-proton collision data recorded by the LHCb experiment at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$. The branching fraction is measured to be $\mathcal{B}(B_s^0\to p\overline{\mathrm{\Lambda }}{K}^{-})+\mathcal{B}(B_s^0\to \overline{p}\mathrm{\Lambda }{K}^{+})=[5.46\pm 0.61\pm 0.57\pm 0.50(\mathcal{B})\pm 0.32(f_s/f_d)]\times {10}^{-6}$, where the first uncertainty is statistical and the second systematic, the third uncertainty accounts for the experimental uncertainty on the branching fraction of the $B^0\to p\overline{\mathrm{\Lambda }}{\pi }^{-}$ decay used for normalization, and the fourth uncertainty relates to the knowledge of the ratio of $b$-quark hadronization probabilities $f_s/f_d$.

The $B^{+}\to D^{+}{K}^{+}{\pi }^{-}$ decay is observed in a data sample corresponding to $3.0{\mathrm{fb}}^{-1}$ of $pp$ collision data recorded by the LHCb experiment during 2011 and 2012. The signal significance is $8\sigma$ and the branching fraction is measured to be $\mathcal{B}(B^{+}\to D^{+}{K}^{+}{\pi }^{-})=(5.31\pm 0.90\pm 0.48\pm 0.35)\times {10}^{-6}$, where the uncertainties are statistical, systematic and due to the normalization mode $B^{+}\to D^{-}{K}^{+}{\pi }^{+}$, respectively. The Dalitz plot appears to be dominated by broad structures. Angular distributions are exploited to search for quasi-two-body contributions from $B^{+}\to D_2^{*}(2460{)}^0{K}^{+}$ and $B^{+}\to D^{+}{K}^{*}(892{)}^0$ decays. No significant signals are observed and upper limits are set on their branching fractions.

A search for the rare decay of a $B^0$ or $B_s^0$ meson into the final state $J/\psi \gamma$ is performed, using data collected by the LHCb experiment in $pp$ collisions at $\sqrt{s}=7$ and 8 TeV, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$. The observed number of signal candidates is consistent with a background-only hypothesis. Branching fraction values larger than $1.5\times 10^{-6}$ for the $B^0\to J/\psi \gamma$ decay mode are excluded at 90% confidence level. For the $B_s^0\to J/\psi \gamma$ decay mode, branching fraction values larger than $7.3\times 10^{-6}$ are excluded at 90% confidence level; this is the first branching fraction limit for this decay.

The associated production of a $W$ boson with a jet originating from either a light parton or heavy-flavor quark is studied in the forward region using proton-proton collisions. The analysis uses data corresponding to integrated luminosities of 1.0 and $2.0{\mathrm{fb}}^{-1}$ collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV, respectively. The $W$ bosons are reconstructed using the $W\to \mu \nu$ decay and muons with a transverse momentum, $p_{\mathrm{T}}$, larger than 20 GeV in the pseudorapidity range $2.0<\eta <4.5$. The partons are reconstructed as jets with $p_{\mathrm{T}}>20\mathrm{GeV}$ and $2.2<\eta <4.2$. The sum of the muon and jet momenta must satisfy $p_{\mathrm{T}}>20\mathrm{GeV}$. The fraction of $W+\text{jet}$ events that originate from beauty and charm quarks is measured, along with the charge asymmetries of the $W+b$ and $W+c$ production cross sections. The ratio of the $W+\text{jet}$ to $Z+\text{jet}$ production cross sections is also measured using the $Z\to \mu \mu$ decay. All results are in agreement with Standard Model predictions.

Using a $pp$ collision data sample corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$, collected by the LHCb detector, we present the first search for the strangeness-changing weak decay ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-}$. No $b$ hadron decay of this type has been seen before. A signal for this decay, corresponding to a significance of 3.2 standard deviations, is reported. The relative rate is measured to be $$\frac{f_{{\mathrm{\Xi }}_b^{-}}}{f_{{\mathrm{\Lambda }}_b^0}}\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-})=(5.7\pm 1.8_{-0.9}^{+0.8})\times {10}^{-4},$$where $f_{{\mathrm{\Xi }}_b^{-}}$ and $f_{{\mathrm{\Lambda }}_b^0}$ are the $b\to {\mathrm{\Xi }}_b^{-}$ and $b\to {\mathrm{\Lambda }}_b^0$ fragmentation fractions, and $\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-})$ is the branching fraction. Assuming $f_{{\mathrm{\Xi }}_b^{-}}/f_{{\mathrm{\Lambda }}_b^0}$ is bounded between 0.1 and 0.3, the branching fraction $\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-})$ would lie in the range from $(0.57\pm 0.21)\%$ to $(0.19\pm 0.07)\%$.

Violations of $CPT$ symmetry and Lorentz invariance are searched for by studying interference effects in $B^0$ mixing and in $B_s^0$ mixing. Samples of $B^0\to J/\psi K_S^0$ and $B_s^0\to J/\psi K^{+}{K}^{-}$ decays are recorded by the LHCb detector in proton-proton collisions at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$. No periodic variations of the particle-antiparticle mass differences are found, consistent with Lorentz invariance and $CPT$ symmetry. Results are expressed in terms of the standard model extension parameter $\mathrm{\Delta }{a}_{\mu }$ with precisions of $\mathcal{O}({10}^{-15})$ and $\mathcal{O}({10}^{-14})\mathrm{GeV}$ for the $B^0$ and $B_s^0$ systems, respectively. With no assumption on Lorentz (non)invariance, the $CPT$-violating parameter $z$ in the $B_s^0$ system is measured for the first time and found to be $\mathcal{R}e(z)=-0.022\pm 0.033\pm 0.005$ and $\mathcal{I}m(z)=0.004\pm 0.011\pm 0.002$, where the first uncertainties are statistical and the second systematic.

Decays of the ${\mathrm{\Xi }}_b^{-}$ and ${\mathrm{\Omega }}_b^{-}$ baryons to the charmless final states $p{h}^{-}{h}^{\prime -}$, where $h^{(\prime )}$ denotes a kaon or pion, are searched for with the LHCb detector. The analysis is based on a sample of proton-proton collision data collected at center-of-mass energies $\sqrt{s}=7$ and 8 TeV, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$. The decay ${\mathrm{\Xi }}_b^{-}\to p{K}^{-}{K}^{-}$ is observed with a significance of 8.7 standard deviations, and evidence at the level of 3.4 standard deviations is found for the ${\mathrm{\Xi }}_b^{-}\to p{K}^{-}{\pi }^{-}$ decay. Results are reported, relative to the $B^{-}\to K^{+}{K}^{-}{K}^{-}$ normalization channel, for the products of branching fractions and $b$-hadron production fractions. The branching fractions of ${\mathrm{\Xi }}_b^{-}\to p{K}^{-}{\pi }^{-}$ and ${\mathrm{\Xi }}_b^{-}\to p{\pi }^{-}{\pi }^{-}$ relative to ${\mathrm{\Xi }}_b^{-}\to p{K}^{-}{K}^{-}$ decays are also measured.

The time-dependent $CP$ asymmetry in $B_s^0\to J/\psi K^{+}{K}^{-}$ decays is measured using $pp$ collision data, corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$, collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV. In a sample of 96 000 $B_s^0\to J/\psi K^{+}{K}^{-}$ decays, the $CP$-violating phase ${\varphi }_s$ is measured, as well as the decay widths ${\mathrm{\Gamma }}_{\mathrm{L}}$ and ${\mathrm{\Gamma }}_{\mathrm{H}}$ of the light and heavy mass eigenstates of the $B_s^0–{\overline{B}}_s^0$ system. The values obtained are ${\varphi }_s=-0.058\pm 0.049\pm 0.006\mathrm{rad}$, ${\mathrm{\Gamma }}_s\equiv ({\mathrm{\Gamma }}_{\mathrm{L}}+{\mathrm{\Gamma }}_{\mathrm{H}})/2=0.6603\pm 0.0027\pm 0.0015{\mathrm{ps}}^{-1}$, and $\mathrm{\Delta }{\mathrm{\Gamma }}_s\equiv {\mathrm{\Gamma }}_{\mathrm{L}}-{\mathrm{\Gamma }}_{\mathrm{H}}=0.0805\pm 0.0091\pm 0.0032{\mathrm{ps}}^{-1}$, where the first uncertainty is statistical and the second, systematic. These are the most precise single measurements of those quantities to date. A combined analysis with $B_s^0\to J/\psi {\pi }^{+}{\pi }^{-}$ decays gives ${\varphi }_s=-0.010\pm 0.039\mathrm{rad}$. All measurements are in agreement with the standard model predictions. For the first time, the phase ${\varphi }_s$ is measured independently for each polarization state of the $K^{+}{K}^{-}$ system and shows no evidence for polarization dependence.

The first observation of the $B_s^0\to {\overline{D}}^{*0}\varphi$ decay is reported, with a significance of more than seven standard deviations, from an analysis of $pp$ collision data corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$, collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV. The branching fraction is measured relative to that of the topologically similar decay $B^0\to {\overline{D}}^0{\pi }^{+}{\pi }^{-}$ and is found to be $\mathcal{B}(B_s^0\to {\overline{D}}^{*0}\varphi )=(3.7\pm 0.5\pm 0.3\pm 0.2)\times 10^{-5}$, where the first uncertainty is statistical, the second systematic, and the third from the branching fraction of the $B^0\to {\overline{D}}^0{\pi }^{+}{\pi }^{-}$ decay. The fraction of longitudinal polarization in this decay is measured to be $f_{\mathrm{L}}=(73\pm 15\pm 4)\%$. The most precise determination of the branching fraction for the $B_s^0\to {\overline{D}}^0\varphi$ decay is also obtained, $\mathcal{B}(B_s^0\to {\overline{D}}^0\varphi )=(3.0\pm 0.3\pm 0.2\pm 0.2)\times 10^{-5}$. An upper limit, $\mathcal{B}(B^0\to {\overline{D}}^0\varphi )<2.0$ $(2.3)\times 10^{-6}$ at 90% (95%) confidence level is set. A constraint on the $\omega -\varphi$ mixing angle $\delta$ is set at $|\delta |<5.2°$ (5.5°) at 90% (95%) confidence level.

The first measurement of the production rate of ${\mathrm{\Xi }}_b^{-}$ baryons in $pp$ collisions relative to that of ${\mathrm{\Lambda }}_b^0$ baryons is reported, using data samples collected by the LHCb experiment, and corresponding to integrated luminosities of 1, 2 and $1.6{\mathrm{fb}}^{-1}$ at $\sqrt{s}=7$, 8 and 13 TeV, respectively. In the kinematic region $2<\eta <6$ and $p_{\mathrm{T}}<20\mathrm{GeV}/c$, we measure $\frac{f_{{\mathrm{\Xi }}_b^{-}}}{f_{{\mathrm{\Lambda }}_b^0}}\frac{\mathcal{B}({\mathrm{\Xi }}_b^{-}\to J/\psi {\mathrm{\Xi }}^{-})}{\mathcal{B}({\mathrm{\Lambda }}_b^0\to J/\psi \mathrm{\Lambda })}=(10.8\pm 0.9\pm 0.8)\times {10}^{-2}$ [$\sqrt{s}=7,8\mathrm{TeV}$], $\frac{f_{{\mathrm{\Xi }}_b^{-}}}{f_{{\mathrm{\Lambda }}_b^0}}\frac{\mathcal{B}({\mathrm{\Xi }}_b^{-}\to J/\psi {\mathrm{\Xi }}^{-})}{\mathcal{B}({\mathrm{\Lambda }}_b^0\to J/\psi \mathrm{\Lambda })}=(13.1\pm 1.1\pm 1.0)\times {10}^{-2}$ [$\sqrt{s}=13\mathrm{TeV}$], where $f_{{\mathrm{\Xi }}_b^{-}}$ and $f_{{\mathrm{\Lambda }}_b^0}$ are the fragmentation fractions of $b$ quarks into ${\mathrm{\Xi }}_b^{-}$ and ${\mathrm{\Lambda }}_b^0$ baryons, respectively; $\mathcal{B}$ represents branching fractions; and the uncertainties are due to statistical and experimental systematic sources. The values of $f_{{\mathrm{\Xi }}_b^{-}}/f_{{\mathrm{\Lambda }}_b^0}$ are obtained by invoking SU(3) symmetry in the ${\mathrm{\Xi }}_b^{-}\to J/\psi {\mathrm{\Xi }}^{-}$ and ${\mathrm{\Lambda }}_b^0\to J/\psi \mathrm{\Lambda }$ decays. Production asymmetries between ${\mathrm{\Xi }}_b^{-}$ and ${\overline{\mathrm{\Xi }}}_b^{+}$ baryons are also reported. The mass of the ${\mathrm{\Xi }}_b^{-}$ baryon is also measured relative to that of the ${\mathrm{\Lambda }}_b^0$ baryon, from which it is found that $m({\mathrm{\Xi }}_b^{-})=5796.70\pm 0.39\pm 0.15\pm 0.17\mathrm{MeV}/c^2$, where the last uncertainty is due to the precision on the known ${\mathrm{\Lambda }}_b^0$ mass. This result represents the most precise determination of the ${\mathrm{\Xi }}_b^{-}$ mass.

The production of charged hadrons within jets recoiling against a $Z$ boson is measured in proton-proton collision data at $\sqrt{s}=8\mathrm{TeV}$ recorded by the LHCb experiment. The charged-hadron structure of the jet is studied longitudinally and transverse to the jet axis for jets with transverse momentum $p_T>20\mathrm{GeV}$ and in the pseudorapidity range $2.5<\eta <4$. These are the first measurements of jet hadronization at these forward rapidities and also the first where the jet is produced in association with a $Z$ boson. In contrast to previous hadronization measurements at the Large Hadron Collider, which are dominated by gluon jets, these measurements probe predominantly light-quark jets which are found to be more longitudinally and transversely collimated with respect to the jet axis when compared to the previous gluon dominated measurements. Therefore, these results provide valuable information on differences between quarks and gluons regarding nonperturbative hadronization dynamics.

The Dalitz plot distribution of $B^0\to {\overline{D}}^0{K}^{+}{\pi }^{-}$ decays is studied using a data sample corresponding to $3.0{\mathrm{fb}}^{-1}$ of $pp$ collision data recorded by the LHCb experiment during 2011 and 2012. The data are described by an amplitude model that contains contributions from intermediate $K^{*}(892{)}^0$, $K^{*}(1410{)}^0$, $K_2^{*}(1430{)}^0$ and $D_2^{*}(2460{)}^{-}$ resonances. The model also contains components to describe broad structures, including the $K_0^{*}(1430{)}^0$ and $D_0^{*}(2400{)}^{-}$ resonances, in the $K\pi$ S-wave and the $D\pi$ S- and P-waves. The masses and widths of the $D_0^{*}(2400{)}^{-}$ and $D_2^{*}(2460{)}^{-}$ resonances are measured, as are the complex amplitudes and fit fractions for all components included in the amplitude model. The model obtained will be an integral part of a future determination of the angle $\gamma$ of the Cabibbo-Kobayashi-Maskawa quark mixing matrix using $B^0\to D{K}^{+}{\pi }^{-}$ decays.

A search for $CP$ violation in $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$ decays is performed using $pp$ collision data, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$, collected using the LHCb detector at center-of-mass energies of 7 and 8 TeV. The flavor of the charm meson is inferred from the charge of the pion in $D^{*+}\to D^0{\pi }^{+}$ and $D^{*-}\to {\overline{D}}^0{\pi }^{-}$ decays. The difference between the $CP$ asymmetries in $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$ decays, $\mathrm{\Delta }{A}_{CP}\equiv A_{CP}(K^{-}{K}^{+})-A_{CP}({\pi }^{-}{\pi }^{+})$, is measured to be $[-0.10\pm 0.08(\mathrm{stat})\pm 0.03(\mathrm{syst})]\%$. This is the most precise measurement of a time-integrated $CP$ asymmetry in the charm sector from a single experiment.

The doubly charmed baryon decay ${\mathrm{\Xi }}_{cc}^{++}\to {\mathrm{\Xi }}_c^{+}{\pi }^{+}$ is observed for the first time, with a statistical significance of $5.9\sigma$, confirming a recent observation of the baryon in the ${\mathrm{\Lambda }}_c^{+}{K}^{-}{\pi }^{+}{\pi }^{+}$ final state. The data sample used corresponds to an integrated luminosity of $1.7{\mathrm{fb}}^{-1}$, collected by the LHCb experiment in $pp$ collisions at a center-of-mass energy of 13 TeV. The ${\mathrm{\Xi }}_{cc}^{++}$ mass is measured to be $3620.6\pm 1.5(\mathrm{stat})\pm 0.4(\mathrm{syst})\pm 0.3({\mathrm{\Xi }}_c^{+})\mathrm{MeV}/c^2$ and is consistent with the previous result. The ratio of branching fractions between the decay modes is measured to be $[\mathcal{B}({\mathrm{\Xi }}_{cc}^{++}\to {\mathrm{\Xi }}_c^{+}{\pi }^{+})\times \mathcal{B}({\mathrm{\Xi }}_c^{+}\to p{K}^{-}{\pi }^{+})]/[\mathcal{B}({\mathrm{\Xi }}_{cc}^{++}\to {\mathrm{\Lambda }}_c^{+}{K}^{-}{\pi }^{+}{\pi }^{+})\times \mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+})]=0.035\pm 0.009(\mathrm{stat})\pm 0.003(\mathrm{syst})$.

Observations of exotic structures in the $J/\psi p$ channel, which we refer to as charmonium-pentaquark states, in ${\mathrm{\Lambda }}_b^0\to J/\psi K^{-}p$ decays are presented. The data sample corresponds to an integrated luminosity of $3{\mathrm{fb}}^{-1}$ acquired with the LHCb detector from 7 and 8 TeV $pp$ collisions. An amplitude analysis of the three-body final state reproduces the two-body mass and angular distributions. To obtain a satisfactory fit of the structures seen in the $J/\psi p$ mass spectrum, it is necessary to include two Breit-Wigner amplitudes that each describe a resonant state. The significance of each of these resonances is more than 9 standard deviations. One has a mass of $4380\pm 8\pm 29\mathrm{MeV}$ and a width of $205\pm 18\pm 86\mathrm{MeV}$, while the second is narrower, with a mass of $4449.8\pm 1.7\pm 2.5\mathrm{MeV}$ and a width of $39\pm 5\pm 19\mathrm{MeV}$. The preferred $J^P$ assignments are of opposite parity, with one state having spin $3/2$ and the other $5/2$.

The polarization of photons produced in radiative $B_s^0$ decays is studied for the first time. The data are recorded by the LHCb experiment in $pp$ collisions corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$ at center-of-mass energies of 7 and 8 TeV. A time-dependent analysis of the $B_s^0\to \varphi \gamma$ decay rate is conducted to determine the parameter ${\mathcal{A}}^{\mathrm{\Delta }}$, which is related to the ratio of right- over left-handed photon polarization amplitudes in $b\to s\gamma$ transitions. A value of ${\mathcal{A}}^{\mathrm{\Delta }}=-0.9{8_{-0.52}^{+0.46}}_{-0.20}^{+0.23}$ is measured. This result is consistent with the standard model prediction within 2 standard deviations.

The first full amplitude analysis of $B^{+}\to J/\psi \varphi K^{+}$ with $J/\psi \to {\mu }^{+}{\mu }^{-}$, $\varphi \to K^{+}{K}^{-}$ decays is performed with a data sample of $3{\mathrm{fb}}^{-1}$ of $pp$ collision data collected at $\sqrt{s}=7$ and 8 TeV with the LHCb detector. The data cannot be described by a model that contains only excited kaon states decaying into $\varphi K^{+}$, and four $J/\psi \varphi$ structures are observed, each with significance over 5 standard deviations. The quantum numbers of these structures are determined with significance of at least 4 standard deviations. The lightest has mass consistent with, but width much larger than, previous measurements of the claimed $X(4140)$ state. The model includes significant contributions from a number of expected kaon excitations, including the first observation of the $K^{*}(1680{)}^{+}\to \varphi K^{+}$ transition.

The first measurements of the forward-backward asymmetry of the dimuon pair ($A_{\mathrm{FB}}$), the triple-product asymmetry ($A_{2\varphi }$), and the charge-parity-conjugation asymmetry ($A_{CP}$), in $D^0\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ and $D^0\to K^{+}{K}^{-}{\mu }^{+}{\mu }^{-}$ decays are reported. They are performed using data from proton-proton collisions collected with the LHCb experiment from 2011 to 2016, corresponding to a total integrated luminosity of $5{\mathrm{fb}}^{-1}$. The asymmetries are measured to be $A_{\mathrm{FB}}(D^0\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-})=(3.3\pm 3.7\pm 0.6)\%$, $A_{2\varphi }(D^0\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-})=(-0.6\pm 3.7\pm 0.6)\%$, $A_{CP}(D^0\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-})=(4.9\pm 3.8\pm 0.7)\%$, $A_{\mathrm{FB}}(D^0\to K^{+}{K}^{-}{\mu }^{+}{\mu }^{-})=(0\pm 11\pm 2)\%$, $A_{2\varphi }(D^0\to K^{+}{K}^{-}{\mu }^{+}{\mu }^{-})=(9\pm 11\pm 1)\%$, $A_{CP}(D^0\to K^{+}{K}^{-}{\mu }^{+}{\mu }^{-})=(0\pm 11\pm 2)\%$, where the first uncertainty is statistical and the second systematic. The asymmetries are also measured as a function of the dimuon invariant mass. The results are consistent with the standard model predictions.

The ratio of branching fractions $\mathcal{R}(D^{*-})\equiv \mathcal{B}(B^0\to D^{*-}{\tau }^{+}{\nu }_{\tau })/\mathcal{B}(B^0\to D^{*-}{\mu }^{+}{\nu }_{\mu })$ is measured using a data sample of proton-proton collisions collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$. The $\tau$ lepton is reconstructed with three charged pions in the final state. A novel method is used that exploits the different vertex topologies of signal and backgrounds to isolate samples of semitauonic decays of $b$ hadrons with high purity. Using the $B^0\to D^{*-}{\pi }^{+}{\pi }^{-}{\pi }^{+}$ decay as the normalization channel, the ratio $\mathcal{B}(B^0\to D^{*-}{\tau }^{+}{\nu }_{\tau })/\mathcal{B}(B^0\to D^{*-}{\pi }^{+}{\pi }^{-}{\pi }^{+})$ is measured to be $1.97\pm 0.13\pm 0.18$, where the first uncertainty is statistical and the second systematic. An average of branching fraction measurements for the normalization channel is used to derive $\mathcal{B}(B^0\to D^{*-}{\tau }^{+}{\nu }_{\tau })=(1.42\pm 0.094\pm 0.129\pm 0.054)\%$, where the third uncertainty is due to the limited knowledge of $\mathcal{B}(B^0\to D^{*-}{\pi }^{+}{\pi }^{-}{\pi }^{+})$. A test of lepton flavor universality is performed using the well-measured branching fraction $\mathcal{B}(B^0\to D^{*-}{\mu }^{+}{\nu }_{\mu })$ to compute $\mathcal{R}(D^{*-})=0.291\pm 0.019\pm 0.026\pm 0.013$, where the third uncertainty originates from the uncertainties on $\mathcal{B}(B^0\to D^{*-}{\pi }^{+}{\pi }^{-}{\pi }^{+})$ and $\mathcal{B}(B^0\to D^{*-}{\mu }^{+}{\nu }_{\mu })$. This measurement is in agreement with the Standard Model prediction and with previous measurements.

A search for the rare decays $B_s^0\to {\tau }^{+}{\tau }^{-}$ and $B^0\to {\tau }^{+}{\tau }^{-}$ is performed using proton–proton collision data collected with the LHCb detector. The data sample corresponds to an integrated luminosity of $3{\mathrm{fb}}^{-1}$ collected in 2011 and 2012. The $\tau$ leptons are reconstructed through the decay ${\tau }^{-}\to {\pi }^{-}{\pi }^{+}{\pi }^{-}{\nu }_{\tau }$. Assuming no contribution from $B^0\to {\tau }^{+}{\tau }^{-}$ decays, an upper limit is set on the branching fraction $\mathcal{B}(B_s^0\to {\tau }^{+}{\tau }^{-})<6.8\times {10}^{-3}$ at the 95% confidence level. If instead no contribution from $B_s^0\to {\tau }^{+}{\tau }^{-}$ decays is assumed, the limit is $\mathcal{B}(B^0\to {\tau }^{+}{\tau }^{-})<2.1\times {10}^{-3}$ at the 95% confidence level. These results correspond to the first direct limit on $\mathcal{B}(B_s^0\to {\tau }^{+}{\tau }^{-})$ and the world’s best limit on $\mathcal{B}(B^0\to {\tau }^{+}{\tau }^{-})$.