Search

A sample of $pp$ collision data, corresponding to an integrated luminosity of $5.5{\mathrm{fb}}^{-1}$ and collected by the LHCb experiment during LHC Run 2, is used to measure the ratio of the lifetime of the ${\mathrm{\Xi }}_b^{-}$ baryon to that of the ${\mathrm{\Lambda }}_b^0$ baryon, $r_{\tau }\equiv {\tau }_{{\mathrm{\Xi }}_b^{-}}/{\tau }_{{\mathrm{\Lambda }}_b^0}$. The value $r_{\tau }=1.076\pm 0.013\pm 0.006$ is obtained, where the first uncertainty is statistical and the second systematic. This value is averaged with the corresponding value from Run 1 to obtain $r_{\tau }^{\mathrm{Run}1,2}=1.078\pm 0.012\pm 0.007$. Multiplying by the world-average value of the ${\mathrm{\Lambda }}_b^0$ lifetime yields ${\tau }_{{\mathrm{\Xi }}_b^{-}}^{\mathrm{Run}1,2}=1.578\pm 0.018\pm 0.010\pm 0.011\mathrm{ps}$, where the uncertainties are statistical, systematic, and due to the limited knowledge of the ${\mathrm{\Lambda }}_b^0$ lifetime. This measurement improves the precision of the current world average of the ${\mathrm{\Xi }}_b^{-}$ lifetime by about a factor of 2, and is in good agreement with the most recent theoretical predictions.

A search for the lepton-flavor violating decays $B_s^0\to \varphi {\mu }^{\pm }{\tau }^{\mp }$ is presented, using a sample of proton-proton collisions at center-of-mass energies of 7, 8, and 13 TeV, collected with the LHCb detector and corresponding to a total integrated luminosity of $9{\mathrm{fb}}^{-1}$. The $\tau$ leptons are selected using decays with three charged pions. No significant excess is observed, and an upper limit on the branching fraction is determined to be $\mathcal{B}(B_s^0\to \varphi {\mu }^{\pm }{\tau }^{\mp })<1.0\times {10}^{-5}$ at 90% confidence level.

Flow harmonic coefficients, $v_n$, which are the key to studying the hydrodynamics of the quark-gluon plasma (QGP) created in heavy-ion collisions, have been measured in various collision systems and kinematic regions and using various particle species. The study of flow harmonics in a wide pseudorapidity range is particularly valuable to understand the temperature dependence of the shear viscosity to entropy density ratio of the QGP. This paper presents the first LHCb results of the second- and the third-order flow harmonic coefficients of charged hadrons as a function of transverse momentum in the forward region, corresponding to pseudorapidities between 2.0 and 4.9, using the data collected from PbPb collisions in 2018 at a center-of-mass energy of 5.02 $\text{TeV}$. The coefficients measured using the two-particle angular correlation analysis method are smaller than the central-pseudorapidity measurements at ALICE and ATLAS from the same collision system but share similar features.

The production of prompt $D_s^{+}$ and $D^{+}$ mesons is measured by the LHCb experiment in proton-lead ($p\mathrm{Pb}$) collisions in both the forward ($1.5<y^{*}<4.0$) and backward ($-5.0<y^{*}<-2.5$) rapidity regions at a nucleon-nucleon center-of-mass energy of $\sqrt{s_{\mathrm{NN}}}=8.16\mathrm{TeV}$. The nuclear modification factors of both $D_s^{+}$ and $D^{+}$ mesons are determined as a function of transverse momentum, $p_{\mathrm{T}}$, and rapidity. In addition, the $D_s^{+}$ to $D^{+}$ cross section ratio is measured as a function of the primary charged particle multiplicity in the event. An enhanced $D_s^{+}$ to $D^{+}$ production in high-multiplicity events is observed for the whole measured $p_{\mathrm{T}}$ range, in particular at low $p_{\mathrm{T}}$ and backward rapidity, where the significance exceeds six standard deviations. This constitutes the first observation of strangeness enhancement in charm quark hadronization in high-multiplicity $p\mathrm{Pb}$ collisions. The results are also qualitatively consistent with the presence of quark coalescence as an additional charm quark hadronization mechanism in high-multiplicity proton-lead collisions.

The collinear and transverse-momentum-dependent jet fragmentation function and the radial profile for $B^{\pm }$ mesons in jets are measured. The $B^{\pm }$ mesons are reconstructed through the $J/\psi (\to {\mu }^{+}{\mu }^{-})K^{\pm }$ decay channel using proton-proton collision data collected during 2016–2018 with the LHCb detector at a center-of-mass (c.m.) energy of $\sqrt{s}=13\mathrm{TeV}$, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$. The results complement recent measurements of jet fragmentation functions for heavy-flavor hadrons and suggest a growing contribution of gluon fragmentation to $B^{\pm }$ mesons as the jet transverse momentum increases.

A flavor-tagged time-dependent analysis of $B^0\to K_{\mathrm{S}}^0{\mu }^{+}{\mu }^{-}$ decays is performed across the full dimuon mass range excluding the $J/\psi$ and $\psi (2S)$ resonance regions. The analysis uses proton-proton collision data collected by the LHCb experiment in 2011–2018 at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The $CP$ violation parameters are determined to be $C=-0.13\pm 0.32\pm 0.04$, $S=+0.82\pm 0.29\pm 0.05$, where the first uncertainties are statistical and the second are systematic. The results are consistent with the standard model prediction. This is the first experimental study of time-dependent $CP$ violation in $b\to s{\ell }^{+}{\ell }^{-}$ processes.

An analysis of the $B^0\to K^{*0}(\to K^{+}{\pi }^{-}){\mu }^{+}{\mu }^{-}$ decay is presented using proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of $8.4{\mathrm{fb}}^{-1}$. The full set of $CP$-averaged and $CP$-asymmetric angular observables is determined in bins of the invariant mass squared of the dimuon system, as well as the branching fraction relative to the $B^0\to J/\psi (\to {\mu }^{+}{\mu }^{-})K^{+}{\pi }^{-}$ decay. For the first time, the full set of observables pertaining to the $K^{+}{\pi }^{-}$ S-wave contribution to the final state are presented and consideration is given to effects arising from the mass of the muons. The extracted $CP$-asymmetry observables show no significant deviations from zero. The measurements of the $CP$-averaged observables and the branching fractions continue to exhibit the pattern of tensions with the standard model predictions that have been seen in previous analyses that use part of the dataset considered in this Letter.

The first evidence for the decay $B^{-}\to D^{**0}{\tau }^{-}{\overline{\nu }}_{\tau }$ is obtained using proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$, at centre-of-mass energies of 7, 8, and 13 TeV. Here, the $D^{**0}$ meson represents any of the three excited charm mesons $D_1(2420{)}^0$, $D_2^{*}(2460{)}^0$, and $D_1^{\prime }(2400{)}^0$. The $B^{-}\to D^{**0}{\tau }^{-}{\overline{\nu }}_{\tau }$ signal is measured with a significance of $3.5\sigma$, including systematic uncertainties. The combined branching fraction $\mathcal{B}(B^{-}\to D_{1,2}^{**0}{\tau }^{-}{\overline{\nu }}_{\tau })\times \mathcal{B}(D_{1,2}^{**0}\to D^{*+}{\pi }^{-})$, where $D_{1,2}^{**0}$ denotes both $D_1(2420{)}^0$ and $D_2^{*}(2460{)}^0$ contributions, is measured to be $[0.051\pm 0.013(\mathrm{stat})\pm 0.006(\mathrm{syst})\pm 0.009(\mathrm{ext})]\%$, where the last uncertainty reflects that of the branching fraction of the normalization channel $B^{-}\to D_{1,2}^{**0}{D_s^{-}}^{(*)}$. The ratio between the tauonic and muonic semileptonic $B$ decays, with the latter taken from world average values, is also determined and found to be $\mathcal{R}(D_{1,2}^{**0})=0.13\pm 0.03(\mathrm{stat})\pm 0.01(\mathrm{syst})\pm 0.02(\mathrm{ext})$.

A search for the rare decay $B^{+}\to \overline{\mathrm{\Lambda }}p{\mu }^{+}{\mu }^{-}$ is performed using proton-proton collision data recorded by the LHCb experiment at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$. An excess of events is found with respect to the background-only expectation, with a signal significance of 3.5 standard deviations, in the low invariant-mass region of $m(\overline{\mathrm{\Lambda }}p)<2.8\mathrm{GeV}/c^2$. The branching fraction is measured to be ${\mathcal{B}}_{\mathrm{low}}(B^{+}\to \overline{\mathrm{\Lambda }}p{\mu }^{+}{\mu }^{-})=(1.70_{-0.56}^{+0.65}(\mathrm{stat})\pm 0.17(\mathrm{syst})\pm 0.14(\mathrm{ext}))\times {10}^{-8}$, where the last uncertainty is due to external inputs on $\mathcal{B}(B^{+}\to J/\psi \overline{\mathrm{\Lambda }}p)\times \mathcal{B}(J/\psi \to {\mu }^{+}{\mu }^{-})$. With no significant signal observed in the high $m(\overline{\mathrm{\Lambda }}p)$ region above $2.8\mathrm{GeV}/c^2$, an upper limit is set to be ${\mathcal{B}}_{\mathrm{high}}(B^{+}\to \overline{\mathrm{\Lambda }}p{\mu }^{+}{\mu }^{-})<2.8(3.7)\times {10}^{-9}$ at the 90% (95%) confidence level.

The first evidence for the decay $B^{+}\to {\pi }^{+}{e}^{+}{e}^{-}$ is reported using proton-proton collision data recorded by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. A signal excess with a significance of $3.2\sigma$ is observed and the branching fraction is measured to be $\mathcal{B}(B^{+}\to {\pi }^{+}{e}^{+}{e}^{-})=({2.4}_{-0.8}^{+0.9}{}_{-0.2}^{+0.4})\times {10}^{-8}$, where the first set of uncertainties is statistical and the second is systematic. The result is consistent with the Standard Model expectation.

A sample of $pp$ collision data, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$ and collected by the LHCb experiment during LHC Run 2, is used to measure the ratio of the lifetime of the ${\mathrm{\Xi }}_b^0$ baryon to that of the ${\mathrm{\Lambda }}_b^0$ baryon, $r_{\tau }\equiv {\tau }_{{\mathrm{\Xi }}_b^0}/{\tau }_{{\mathrm{\Lambda }}_b^0}$. The value $r_{\tau }^{\mathrm{Run}2}=1.004\pm 0.009\pm 0.006$ is obtained, where the first uncertainty is statistical and the second systematic. This value is averaged with the corresponding value from Run 1 to obtain $r_{\tau }=1.004\pm 0.008\pm 0.005$. Multiplying by the known value of the ${\mathrm{\Lambda }}_b^0$ lifetime yields ${\tau }_{{\mathrm{\Xi }}_b^0}=1.475\pm 0.012\pm 0.008\pm 0.009\mathrm{ps}$, where the last uncertainty is due to the limited knowledge of the ${\mathrm{\Lambda }}_b^0$ lifetime. This measurement improves the precision of the current world average of the ${\mathrm{\Xi }}_b^0$ lifetime by about a factor of two, and is in good agreement with the most recent theoretical predictions.

A search for the charmless baryonic decay $B^{+}\to \overline{\mathrm{\Lambda }}p\overline{p}p$ is performed using proton-proton collision data recorded by the LHCb experiment, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$. The branching fraction for this decay is measured for the first time relative to that of the topologically similar decay $B^{+}\to J/\psi K^{+}$, with $J/\psi \to \overline{\mathrm{\Lambda }}p{K}^{-}$. The branching fraction is measured to be $\mathcal{B}(B^{+}\to \overline{\mathrm{\Lambda }}p\overline{p}p)=(2.15\pm 0.35\pm 0.12\pm 0.28)\times {10}^{-7}$, where the first uncertainty is statistical, the second is systematic, and the third arises from the uncertainty in the normalization channel branching fraction. The $CP$ asymmetry is measured to be ${\mathcal{A}}_{CP}=(5.4\pm 15.6\pm 2.4)\%$, where the uncertainties are statistical and systematic. The background-subtracted invariant-mass distributions of $\overline{\mathrm{\Lambda }}p$ and $\overline{p}p$ pairs exhibit pronounced enhancements at both kinematic thresholds, in contrast to a uniform phase-space distribution.

A time- and flavor-integrated amplitude analysis of $B^0$ and $B_s^0$ decays to the $(K^{+}{\pi }^{-})(K^{-}{\pi }^{+})$ final state in the $K^{*}(892{)}^0{\overline{K}}^{*}(892{)}^0$ region is presented, using $pp$ collision data recorded with the LHCb detector in 2011–2018, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The branching fractions of the $B^0$ and $B_s^0$ decays are measured relative to the $B^0\to D^{-}{\pi }^{+}$ and $B_s^0\to D_s^{-}{\pi }^{+}$ modes, respectively. The corresponding longitudinal polarization fractions are found to be $f_L^d=0.600\pm 0.022\pm 0.017$ and $f_L^s=0.159\pm 0.010\pm 0.007$, where the uncertainties are statistical and systematic, respectively. The theory-motivated ratio of the squared $B_s^0$ to $B^0$ longitudinally polarized decay amplitudes is found to be $L_{K^{*0}{\overline{K}}^{*0}}=4.92\pm 0.55\pm 0.48\pm 0.02\pm 0.10$, where the uncertainties are statistical, systematic, due to uncertainty of external mass and lifetime measurements, and due to knowledge of the fragmentation fraction ratio, respectively. This confirms the previously reported tension between experimental determinations and theoretical predictions of longitudinal polarization in $B\to VV$ decays at the level of 4.4 standard deviations.

A measurement of the ratio of semileptonic branching fractions $\mathcal{R}(J/\psi )$, defined as $\mathcal{R}(J/\psi )\equiv \mathcal{B}(B_c^{+}\to J/\psi {\tau }^{+}{\nu }_{\tau })/\mathcal{B}(B_c^{+}\to J/\psi {\mu }^{+}{\nu }_{\mu })$, is reported using a sample of proton-proton collision data corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$ recorded by the LHCb experiment in 2016–2018 at a center-of-mass energy of 13 TeV. The measured value is found to be $\mathcal{R}(J/\psi )=0.51\pm 0.12(\mathrm{stat})\pm 0.08(\mathrm{syst})$, which is within 1.8 standard deviations of the predictions from the standard model assuming lepton flavor universality.

The ${\mathrm{\Xi }}_c^{+}{\pi }^{-}{\pi }^{+}$ spectrum is investigated using proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$, collected by the LHCb experiment during 2016–2018. Four states are observed with high significance, and their masses and widths are measured to be $m[{\mathrm{\Xi }}_c(2815{)}^{+}]=2816.65\pm 0.03\pm 0.03\pm 0.23\mathrm{MeV}$, $\mathrm{\Gamma }[{\mathrm{\Xi }}_c(2815{)}^{+}]=2.07\pm 0.08\pm 0.12\mathrm{MeV}$, $m[{\mathrm{\Xi }}_c(2923{)}^{+}]=2922.8\pm 0.3\pm 0.5\pm 0.2\mathrm{MeV}$, $\mathrm{\Gamma }[{\mathrm{\Xi }}_c(2923{)}^{+}]=5.3\pm 0.9\pm 1.4\mathrm{MeV}$, $m[{\mathrm{\Xi }}_c(2970{)}^{+}]=2968.6\pm 0.5\pm 0.5\pm 0.2\mathrm{MeV}$, $\mathrm{\Gamma }[{\mathrm{\Xi }}_c(2970{)}^{+}]=31.7\pm 1.7\pm 1.9\mathrm{MeV}$, $m[{\mathrm{\Xi }}_c(3080{)}^{+}]=3076.8\pm 0.7\pm 1.3\pm 0.2\mathrm{MeV}$, $\mathrm{\Gamma }[{\mathrm{\Xi }}_c(3080{)}^{+}]=6.8\pm 2.3\pm 0.9\mathrm{MeV}$, where the uncertainties are statistical, systematic, and due to the limited precision on the ${\mathrm{\Xi }}_c^{+}$ mass, respectively. The ${\mathrm{\Xi }}_c(2923{)}^{+}$ baryon is observed for the first time, and is consistent with being the isospin partner of the previously observed ${\mathrm{\Xi }}_c(2923{)}^0$ state. Most of the measured parameters are more precise than existing world averages.

The production of $\eta$ and ${\eta }^{\prime }$ mesons is studied in proton-proton and proton-lead collisions collected with the LHCb detector. Proton-proton collisions are studied at center-of-mass energies of $5.02$ and $13\mathrm{TeV}$ and proton-lead collisions are studied at a center-of-mass energy per nucleon of $8.16\mathrm{TeV}$. The studies are performed in center-of-mass (c.m.) rapidity regions $2.5<y_{\mathrm{c}.\mathrm{m}.}<3.5$ (forward rapidity) and $-4.0<y_{\mathrm{c}.\mathrm{m}.}<-3.0$ (backward rapidity) defined relative to the proton beam direction. The $\eta$ and ${\eta }^{\prime }$ production cross sections are measured differentially as a function of transverse momentum for $1.5<p_{\mathrm{T}}<10\mathrm{GeV}$ and $3<p_{\mathrm{T}}<10\mathrm{GeV}$, respectively. The differential cross sections are used to calculate nuclear modification factors. The nuclear modification factors for $\eta$ and ${\eta }^{\prime }$ mesons agree at both forward and backward rapidity, showing no significant evidence of mass dependence. The differential cross sections of $\eta$ mesons are also used to calculate $\eta /{\pi }^0$ cross-section ratios, which show evidence of a deviation from the world average. These studies offer new constraints on mass-dependent nuclear effects in heavy-ion collisions, as well as $\eta$ and ${\eta }^{\prime }$ meson fragmentation.

The production rate of ${\mathrm{\Lambda }}_b^0$ baryons relative to $B^0$ mesons in $pp$ collisions at a center-of-mass energy $\sqrt{s}=13\mathrm{TeV}$ is measured by the LHCb experiment. The ratio of ${\mathrm{\Lambda }}_b^0$ to $B^0$ production cross sections shows a significant dependence on both the transverse momentum and the measured charged-particle multiplicity. At low multiplicity, the ratio measured at LHCb is consistent with the value measured in $e^{+}{e}^{-}$ collisions, and increases by a factor of $\sim 2$ with increasing multiplicity. At relatively low transverse momentum, the ratio of ${\mathrm{\Lambda }}_b^0$ to $B^0$ cross sections is higher than what is measured in $e^{+}{e}^{-}$ collisions, but converges with the $e^{+}{e}^{-}$ ratio as the momentum increases. These results imply that the evolution of heavy $b$ quarks into final-state hadrons is influenced by the density of the hadronic environment produced in the collision. Comparisons with several models and implications for the mechanisms enforcing quark confinement are discussed.

A measurement of time-dependent $CP$ violation in $D^0\to {\pi }^{+}{\pi }^{-}{\pi }^0$ decays using a $pp$ collision data sample collected by the LHCb experiment in 2012 and from 2015 to 2018, corresponding to an integrated luminosity of $7.7{\mathrm{fb}}^{-1}$, is presented. The initial flavor of each $D^0$ candidate is determined from the charge of the pion produced in the $D^{*}(2010{)}^{+}\to D^0{\pi }^{+}$ decay. The decay $D^0\to K^{-}{\pi }^{+}{\pi }^0$ is used as a control channel to validate the measurement procedure. The gradient of the time-dependent $CP$ asymmetry $\mathrm{\Delta }Y$ in $D^0\to {\pi }^{+}{\pi }^{-}{\pi }^0$ decays is measured to be $\mathrm{\Delta }Y=(-1.3\pm 6.3\pm 2.4)\times {10}^{-4},$ where the first uncertainty is statistical and the second is systematic, which is compatible with $CP$ conservation.

A search for hidden-charm pentaquark states decaying to a range of ${\mathrm{\Sigma }}_c\overline{D}$ and ${\mathrm{\Lambda }}_c^{+}\overline{D}$ final states, as well as doubly charmed pentaquark states to ${\mathrm{\Sigma }}_{c}D$ and ${\mathrm{\Lambda }}_c^{+}D$, is made using samples of proton-proton collision data corresponding to an integrated luminosity of $5.7{\mathrm{fb}}^{-1}$ recorded by the LHCb detector at $\sqrt{s}=13\mathrm{TeV}$. Since no significant signals are found, upper limits are set on the pentaquark yields relative to that of the ${\mathrm{\Lambda }}_c^{+}$ baryon in the ${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+}$ decay mode. The known pentaquark states are also investigated, and their signal yields are found to be consistent with zero in all cases.

Lepton flavor universality in rare $b\to s$ transitions is tested for the first time using $B_s^0$ meson decays. The measurements are performed using $pp$ collision data collected by the LHCb experiment between 2011 and 2018, corresponding to a total integrated luminosity of $9{\mathrm{fb}}^{-1}$. Branching fraction ratios between the $B_s^0\to \varphi e^{+}{e}^{-}$ and $B_s^0\to \varphi {\mu }^{+}{\mu }^{-}$ decays are measured in three regions of dilepton mass squared, $q^2$, with $0.1<q^2<1.1$, $1.1<q^2<6.0$, and $15<q^2<19{\mathrm{GeV}}^2/c^4$. The results agree with the standard model expectation of lepton flavor universality.