Search

The transverse polarization of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ hyperons is measured in $p\mathrm{Pb}$ collisions collected by the LHCb experiment at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The polarization is averaged over hyperon transverse momentum in the range $0.15<p_{\mathrm{T}}<6.00\mathrm{GeV}/c$, and Feynman-$x$ in the ranges $0.005<x_{\mathrm{F}}<0.040$ (forward region) and $-0.10<x_{\mathrm{F}}<-0.01$ (backward region) defined relative to the proton beam direction. The transverse polarization is found to be compatible with zero for both $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ hyperons. The results are also measured as a function of $p_{\mathrm{T}}$ and $x_{\mathrm{F}}$ with no significant dependence on these variables observed. The results are compared with previous experimental measurements at different center-of-mass energies and collision environments.

A search for $K_{\mathrm{S}(\mathrm{L})}^0\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ decays is performed using proton-proton collision data collected by the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$. No $K_{\mathrm{S}(\mathrm{L})}^0\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ signals are found and upper limits are set for the first time on the branching fractions $\mathcal{B}(K_{\mathrm{S}}^0\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-})<1.4\times {10}^{-9}$ and $\mathcal{B}(K_{\mathrm{L}}^0\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-})<6.6\times {10}^{-7}$, at the 90% confidence level.

The first observation of the doubly charmed baryon ${\mathrm{\Xi }}_{cc}^{+}$ is reported through its decay to the ${\mathrm{\Lambda }}_c^{+}{K}^{-}{\pi }^{+}$ final state, with a statistical significance exceeding seven standard deviations. The observation is made using proton-proton collision data collected in 2024 with the LHCb Run 3 detector at a center-of-mass energy of 13.6 TeV, corresponding to a total integrated luminosity of $6.9{\mathrm{fb}}^{-1}$. The ${\mathrm{\Xi }}_{cc}^{+}$ mass is measured to be $3619.97\pm 0.83\pm {0.26}_{-1.30}^{+1.90}\mathrm{MeV}/c^2$, where the first uncertainty is statistical, the second is systematic, and the third is due to the unknown ${\mathrm{\Xi }}_{cc}^{+}$ lifetime, which is assumed to lie in the range 15–160 fs with a baseline value of 45 fs. The difference between the masses of the ${\mathrm{\Xi }}_{cc}^{+}$ and ${\mathrm{\Xi }}_{cc}^{++}$ baryons is determined to be $-1.77\pm 0.84\pm {0.15}_{-1.30}^{+1.90}\mathrm{MeV}/c^2$. This is the first observation of a new particle made with the LHCb Run 3 detector.

The onset of nuclear effects in light-ion collisions is studied by measuring the ratio of $D^0$ meson production between NeNe and OO collisions at a center-of-mass energy per nucleon pair of 5.36 TeV recorded by the LHCb detector. The $D^0$ meson yields are measured differentially in transverse momentum ($p_{\mathrm{T}}$) for $0.5<p_{\mathrm{T}}<20\mathrm{GeV}$ in the rapidity ($y$) region $2.0<y<4.5$, and are normalized to the total number of recorded inelastic nucleus-nucleus collisions in each data sample. The resulting production ratio shows evidence of variation as a function of $p_{\mathrm{T}}$, which is inconsistent with predictions based on nuclear modification of nucleon structure alone. This measurement is consistent with the onset of quark-gluon plasma production with increasing nucleus size in high-energy nuclear collisions.

A search is performed for lepton-number-violating $B^{-}\to D^{(*)+}{\mu }^{-}{\mu }^{-}$ decays, using data collected by the LHCb experiment in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$. No significant signal is observed, and upper limits are set on the branching fractions, $\mathcal{B}(B^{-}\to D^{+}{\mu }^{-}{\mu }^{-})<4.6\times {10}^{-8}$ and $\mathcal{B}(B^{-}\to D^{*+}{\mu }^{-}{\mu }^{-})<5.9\times {10}^{-8}$, at the 95% confidence level.

The first observation of the ${\chi }_{c1}(3872)\to J/\psi {\mu }^{+}{\mu }^{-}$ decay is reported using proton-proton collision data recorded with the LHCb detector corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The decay mode is observed for the first time, with a significance of $6.5\sigma$. Its branching fraction is measured relative to the ${\chi }_{c1}(3872)\to J/\psi {\pi }^{+}{\pi }^{-}$ decay mode $\frac{\mathcal{B}({\chi }_{c1}(3872)\to J/\psi {\mu }^{+}{\mu }^{-})}{\mathcal{B}({\chi }_{c1}(3872)\to J/\psi {\pi }^{+}{\pi }^{-})}=(1.68\pm 0.32\pm 0.05)\times {10}^{-3}$, where the first uncertainty includes both statistical contributions and systematic contributions which are uncorrelated between data-taking periods, and the second represents the systematic contributions that are correlated between data-taking periods.

The first dedicated $Z$-boson mass measurement at the LHC with $Z\to {\mu }^{+}{\mu }^{-}$ decays is reported. The dataset uses proton-proton collisions at a center-of-mass energy of 13 TeV, recorded in 2016 by the LHCb experiment, and corresponds to an integrated luminosity of $1.7{\mathrm{fb}}^{-1}$. A template fit to the ${\mu }^{+}{\mu }^{-}$ mass distribution yields the following result for the $Z$-boson mass: $m_Z=91,185.7\pm 8.3\pm 3.9\mathrm{MeV}$, where the first uncertainty is statistical and the second systematic. This result is consistent with previous measurements and predictions from global electroweak fits.

The study of a wide peaking structure in the $B_c^{+}\gamma$ mass spectrum is reported using a data sample of proton-proton collisions collected by the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The observed structure is consistent with the lowest excited $P$-wave $B_c^{+}$ states and exhibits a statistical significance exceeding 7 standard deviations relative to the background-only hypothesis. A two-peak model serves as an effective description of the data, with various theory-constrained models further explored to provide physical interpretation. Based on the predictions for the $B_c(1P{)}^{+}$ spectrum, the relative production cross section of the overall $B_c(1P{)}^{+}$ states with respect to the $B_c^{+}$ ground state with the transverse momentum $p_{\mathrm{T}}$ and rapidity $y$ of $B_c^{+}$ mesons in the regions $p_{\mathrm{T}}<20\mathrm{GeV}/c$ and $2.0<y<4.5$ at $\sqrt{s}=13\mathrm{TeV}$ is measured to be $0.20\pm 0.03\pm 0.02\pm 0.03$, where the uncertainty terms represent statistical, systematic, and uncertainties related to the choice of theoretical models, respectively. The results provide a test of theoretical models and deepen our understanding of quantum chromodynamics.

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.

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 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.

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.

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 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 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.

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.

The first observation of the ${\mathrm{\Sigma }}^{+}\to p{\mu }^{+}{\mu }^{-}$ decay is reported with high significance using proton-proton collision data, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$, collected with the LHCb detector at a center-of-mass energy of 13 TeV. A yield of $237\pm 16{\mathrm{\Sigma }}^{+}\to p{\mu }^{+}{\mu }^{-}$ decays is obtained, where the uncertainty is statistical only. A branching fraction of $(1.08\pm 0.17)\times {10}^{-8}$ is measured, where the uncertainty includes statistical and systematic sources. No evidence of resonant structures is found in the dimuon invariant-mass distribution. All results are compatible with standard model expectations. This represents the rarest decay of a baryon ever observed.

The substructure of jets in quantum chromodynamics (QCD) has garnered significant attention with the advent of infrared- and collinear-safe clustering algorithms and observables. A key question emerging from these studies is how in-jet emissions at soft and hard energy scales, across collinear and wide angles relative to the emitter, differ with the mass of the emitting parton. The Lund jet plane (LJP) is a perturbatively well-defined substructure observable that maps the radiation pattern of jets onto a plane, visually distinguishing emissions with different kinematic properties. Comparing LJP for jets containing hadrons of low versus high mass enables the testing of QCD splitting functions from first-principles calculations across both soft and hard regimes and at different radiation angles. This article presents the first measurement of the LJP for light-quark-enriched and beauty-initiated jets at a center-of-mass energy of 13 TeV at LHCb. This marks the first direct observation of the dead-cone effect in beauty-quark jets, measured in the collinear region of the LJP.

The ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{D}_s^{-}{K}^{+}{K}^{-}$ decay is observed for the first time using the data sample from proton-proton collisions recorded at a center-of-mass energy of 13 TeV with the LHCb detector, corresponding to an integrated luminosity of $6{\mathrm{fb}}^{-1}$. The ratio of branching fraction to that of ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{D}_s^{-}$ decays is measured as $0.0141\pm 0.0019\pm 0.0012$, where the first uncertainty is statistical and the second systematic. A search for hidden-charm pentaquarks with strangeness is performed in the ${\mathrm{\Lambda }}_c^{+}{D}_s^{-}$ system. No evidence is found, and upper limits on the production ratio of $P_{c\overline{c}s}(4338{)}^0$ and $P_{c\overline{c}s}(4459{)}^0$ pentaquarks relative to the ${\mathrm{\Lambda }}_c^{+}{D}_s^{-}$ final state are set at the 95% confidence level as 0.12 and 0.20, respectively.