Search

A measurement of the flavor-untagged decay-time-integrated $CP$ asymmetry in the flavor-specific decay $B_s^0\to D_s^{-}{\pi }^{+}$, $⟨A_{\text{untagged}}^s⟩$, is performed using proton-proton collision data collected by the LHCb experiment between 2016 and 2018 at a center-of-mass energy of 13 TeV, corresponding to a total integrated luminosity of $5.4{\mathrm{fb}}^{-1}$. The $CP$ asymmetry is measured in two $D_s^{-}$ meson decay modes, $D_s^{-}\to K^{-}{K}^{+}{\pi }^{-}$ and $D_s^{-}\to {\pi }^{-}{\pi }^{+}{\pi }^{-}$. The combined result, $⟨A_{\text{untagged}}^s⟩=(-1.4\pm 5.9(\mathrm{stat})\pm 1.1(\mathrm{syst}))\times {10}^{-3}$, is consistent with the Standard Model expectation and provides a direct constraint on new physics in tree-level $b$-hadron decays.

The first measurement of the polarization of charm baryons by the LHCb experiment recorded in fixed-target mode is presented. The polarization of ${\mathrm{\Lambda }}_c$ baryons is studied in collisions of protons, at an energy of 2.51 TeV, incident on a gaseous target of neon, at a nucleon-nucleon center-of-mass energy of 68.6 GeV. The world’s first measurement of separate-charge polarizations for ${\mathrm{\Lambda }}_c^{+}$ and ${\overline{\mathrm{\Lambda }}}_c^{-}$ baryons is performed, determining $P_{{\mathrm{\Lambda }}_c^{+}}=(24\pm 9\pm 2)\%$, $P_{{\overline{\mathrm{\Lambda }}}_c^{-}}=(-8\pm 12\pm 3)\%$, where the first uncertainty is statistical and the second systematic. The polarization is also measured in intervals of baryon transverse momentum and the Feynman-$x$ variable.

Searches are presented for $B_c^{+}\to D{h}^{+}{h}^{-}$ decays, where $D$ is a charmed meson and $h^{\pm }$ is a charged pion or kaon, using $pp$ collision data collected by the LHCb experiment corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The decays $B_c^{+}\to D^{+}{K}^{+}{\pi }^{-}$, $B_c^{+}\to D^{*+}{K}^{+}{\pi }^{-}$, and $B_c^{+}\to D_s^{+}{K}^{+}{K}^{-}$ are observed for the first time. Their branching fractions, expressed as ratios relative to that of the $B_c^{+}\to B_s^0{\pi }^{+}$ decay, are determined to be $\mathcal{R}(B_c^{+}\to D^{+}{K}^{+}{\pi }^{-})=(1.96\pm 0.23\pm 0.08\pm 0.10)\times {10}^{-3}$, $\mathcal{R}(B_c^{+}\to D^{*+}{K}^{+}{\pi }^{-})=(3.67\pm 0.55\pm 0.24\pm 0.20)\times {10}^{-3}$, $\mathcal{R}(B_c^{+}\to D_s^{+}{K}^{+}{K}^{-})=(1.61\pm 0.35\pm 0.13\pm 0.07)\times {10}^{-3}$, where the first uncertainty is statistical, the second is systematic, and the third is due to the limited precision on the $D$-meson branching fractions. The decay channels proceed primarily through excited $K^0$ or $D^0$ resonances or $\varphi$ mesons, and open a new avenue for studies of charge-parity violation in beauty mesons.

The time-dependent $CP$ asymmetry in $B^0\to J/\psi \rho (770{)}^0$ decays is measured using proton-proton collision data corresponding to an integrated luminosity of $6{\mathrm{fb}}^{-1}$, collected with the LHCb detector at a center-of-mass energy of 13 TeV during the years 2015–2018. The $CP$-violation parameters for this process are determined to be $2{\beta }_{c\overline{c}d}^{\mathrm{eff}}=0.710\pm 0.084\pm 0.051\mathrm{rad}$ and $|\lambda |=1.019\pm 0.034\pm 0.024$, where the first uncertainty is statistical and the second systematic. This constitutes the first observation of time-dependent $CP$ violation in $B^0\to J/\psi \rho (770{)}^0$ decays. Assuming approximate SU(3) flavor symmetry, these results are combined with the previous consistent LHCb measurement to set the most stringent constraint on the penguin contribution, $\mathrm{\Delta }{\varphi }_s$, to the $CP$-violating phase ${\varphi }_s$ in $B_s^0\to J/\psi \varphi (1020)$ decays, yielding $\mathrm{\Delta }{\varphi }_s=5.0\pm 4.6\mathrm{mrad}$.

A precision measurement of the muon charge asymmetry from $W$-boson decays in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ is presented. The analysis utilizes data corresponding to an integrated luminosity of $5.1{\mathrm{fb}}^{-1}$, recorded by the LHCb detector during 2016, 2017, and 2018. The asymmetry is measured for muons with transverse momentum between 25 and 55 GeV and pseudorapidity between 2.0 and 4.5. This result represents the most precise determination of the muon charge asymmetry in the forward region to date, exhibiting excellent agreement with next-to-next-to-leading-order predictions in perturbative quantum chromodynamics. The measured asymmetries are corrected for experimental efficiencies, resolution, and final-state photon radiation.

A search for $CP$ violation in the ${\overline{B}}^0\to {\overline{K}}^{*0}{\mu }^{+}{\mu }^{-}$ decay is performed using proton-proton collision data collected by the LHCb experiment during Run 1 and Run 2, corresponding to an integrated luminosity of $8.4{\mathrm{fb}}^{-1}$. The analysis exploits the full angular distribution of the decay, providing sensitivity to $CP$-violating effects in both vector and axial-vector contributions to this flavor-changing neutral-current process. The complex Wilson coefficients are determined within the weak effective theory through an unbinned maximum-likelihood fit to the angular observables, incorporating nonlocal hadronic amplitudes across the full dimuon mass spectrum. The precision of the $CP$-violation observables is improved by an order of magnitude relative to previous measurements, with the imaginary parts of the Wilson coefficients now determined more precisely than the real parts. No significant $CP$ violation is observed, and the results are consistent with the standard model.

The first measurements of the top- and antitop-quark differential production cross sections and the top-quark charge asymmetry in the forward region are presented, 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 $5.4{\mathrm{fb}}^{-1}$. The total production cross sections of top and antitop quarks are also determined. Measurements are performed using the $\mu +b\text{-jet}$ final state within a fiducial region defined by $b$-jet $p_{\mathrm{T},\text{jet}}>50\mathrm{GeV}$ and pseudorapidity $2.2<{\eta }_{\text{jet}}<4.0$, with the muon from the $W$-boson decay required to have $p_{\mathrm{T},\mu }>25\mathrm{GeV}$ and $2.0<{\eta }_{\mu }<4.5$. The muon and $b$-jet system must satisfy $p_{\mathrm{T}}(\mu +\text{jet})>20\mathrm{GeV}$. The measured integrated production cross sections for the top and antitop quarks are ${\sigma }_t=0.95\pm 0.04\pm 0.08\pm 0.02\mathrm{pb}$, ${\sigma }_{\overline{t}}=0.81\pm 0.03\pm 0.07\pm 0.02\mathrm{pb}$, where the first uncertainty is statistical, the second systematic, and the third accounts for the luminosity uncertainty. The top-quark charge asymmetry is measured to be $A_C^t=0.08\pm 0.03\pm 0.01$, where the first uncertainty is statistical and the second is systematic. These results are consistent with next-to-leading order Standard Model predictions.

The first four-dimensional amplitude analysis of the $B^{+}\to \psi (2S)K_{\mathrm{S}}^0{\pi }^{+}$ decay is performed with proton-proton collision data collected by the LHCb experiment at $\sqrt{s}=13\mathrm{TeV}$, corresponding to an integrated luminosity of $5.4\mathrm{f}{\mathrm{b}}^{-1}$. The data cannot be fully explained by $B^{+}\to \psi (2S)K^{*+}$ contributions alone. A significantly better description of the data is obtained by adding a $T_{c\overline{c}}^{+}$ contribution decaying to $\psi (2S){\pi }^{+}$. The properties of the $T_{c\overline{c}}^{+}$ structure are consistent with the exotic state $T_{c\overline{c}1}(4430{)}^{+}$ reported in the isospin-related ${\overline{B}}^0\to \psi (2S)K^{-}{\pi }^{+}$ decay. Effects of a possible $T_{c\overline{c}1}(4430{)}^{+}\to {\overline{D}}_1^{*}(2600{)}^0{D}^{+}$ decay mode on the $T_{c\overline{c}1}(4430{)}^{+}\to \psi (2S){\pi }^{+}$ mass distribution are investigated through a Flatté parametrization, providing constraints on the relative decay strength. A description of the $T_{c\overline{c}1}(4430{)}^{+}$ structure using the triangle singularity mechanism is studied and also found to be consistent with the data.

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.

A precision measurement of the $W$-boson production cross section is performed using the $W\to \mu \nu$ decay channel, based on a sample of proton-proton collision data collected by the LHCb experiment at $\sqrt{s}=13\mathrm{TeV}$ and corresponding to an integrated luminosity of $5.1{\mathrm{fb}}^{-1}$. The cross section is measured for muons with transverse momentum between 25 and 55 GeV and pseudorapidity between 2.0 and 4.5. The total production cross sections of $W$ bosons are measured to be ${\sigma }_{W^{+}\to {\mu }^{+}\nu }=1754.2\pm 1.5\pm 11.8\pm 35.1\mathrm{pb}$ ${\sigma }_{W^{-}\to {\mu }^{-}\overline{\nu }}=1178.1\pm 1.3\pm 9.7\pm 23.6\mathrm{pb}$ where uncertainties are statistical, systematic, and due to the luminosity determination, respectively. Differential distributions $\mathrm{d}{\sigma }_{W^{\pm }\to {\mu }^{\pm }\nu }/\mathrm{d}{\eta }^{{\mu }^{\pm }}$ are also presented. The total and differential cross sections are in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. This measurement is significantly more precise than previous results in this kinematic regime.

A search is presented for the two-body charmed baryonic decays, ${\overline{B}}_{(s)}^0\to {\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-}$, using a data sample collected by the LHCb experiment during 2011–2012 and 2015–2018, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The first observation of the ${\overline{B}}_s^0\to {\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-}$ decay is reported with $6.2\sigma$ significance along with $4.3\sigma$ evidence for the ${\overline{B}}^0\to {\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-}$ decay. The branching fractions are measured to be $\mathcal{B}({\overline{B}}^0\to {\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-})=(1.01_{-0.28}^{+0.27}\pm 0.08\pm 0.15)\times {10}^{-5}$ and $\mathcal{B}({\overline{B}}_s^0\to {\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-})=(5.0\pm 1.3\pm 0.5\pm 0.8)\times {10}^{-5}$, where the first uncertainty is statistical, the second systematic, and the third due to external inputs. These results provide novel experimental inputs for the theoretical framework describing two-body baryonic decays of $B$ mesons via $W$-emission and $W$-exchange mechanisms.

The decay $B^0\to {\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-}{K}_{\mathrm{S}}^0$ is studied at LHCb for the first time 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 $5.4{\mathrm{fb}}^{-1}$. The branching ratio relative to the decay $B^{+}\to {\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-}{K}^{+}$ is measured to be [$\mathcal{B}(B^0\to {\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-}{K}_{\mathrm{S}}^0)]/[\mathcal{B}(B^{+}\to {\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-}{K}^{+})]=0.53\pm 0.05\pm 0.05$ where the first uncertainty is statistical and the second is systematic. Evidence is found for contributions from two resonant states, ${\mathrm{\Xi }}_c(2923{)}^{+}$ and ${\mathrm{\Xi }}_c(2939{)}^{+}$, in the ${\mathrm{\Lambda }}_c^{+}{K}_{\mathrm{S}}^0$ system. The two states show a significance of $3.9\sigma$ relative to the nonresonant hypothesis. These two ${\mathrm{\Xi }}_c^{+}$ states are consistent with being the isospin partners of the states observed in the ${\mathrm{\Lambda }}_c^{+}{K}^{-}$ system.

Muon-electron universality is tested in $B^0\to K^{*0}{\ell }^{+}{\ell }^{-}$ decays, in the dilepton-invariant-mass region above the $\psi (2S)$ resonance. The analysis uses beauty mesons produced in proton-proton collisions recorded 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 ratio of branching fractions between the muon and electron channels $R_{K^{*0}}$ is measured to be ${1.08}_{-0.12}^{+0.14}(\mathrm{stat})\pm 0.07(\mathrm{syst})$ for a dilepton-invariant-mass squared above $14.0{\mathrm{GeV}}^2/c^4$, consistent with the standard model prediction. This result represents the most precise measurement of $R_{K^{*0}}$ in this region and the first such measurement performed at a hadron collider.

The first search for the lepton-flavor violating decays $B^{+}\to {\pi }^{+}{\mu }^{\pm }{e}^{\mp }$ in proton-proton collisions is presented, using data collected by the LHCb experiment between 2011 and 2018, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. No significant signal is observed and an upper limit on the branching fraction is set at $\mathcal{B}(B^{+}\to {\pi }^{+}{\mu }^{\pm }{e}^{\mp })<1.8\times {10}^{-9}$ at the 90% confidence level, 2 orders of magnitude more restrictive than the most stringent upper limit to date. This is the first constraint on lepton-flavor violating $b\to d$ quark transitions at the LHC and also sets the most stringent upper limits to date on $b\to d{\mu }^{\pm }{e}^{\mp }$ transitions. Limits on left-handed and scalar scenarios beyond the standard model are also reported.

An amplitude analysis of the $B^{+}\to ({\pi }^{+}{\pi }^{-})(K_{\mathrm{S}}^0{\pi }^{+})$ decay is performed in the mass regions $0.30<m_{{\pi }^{+}{\pi }^{-}}<1.10\mathrm{GeV}/c^2$ and $0.75<m_{K_{\mathrm{S}}^0{\pi }^{+}}<1.20\mathrm{GeV}/c^2$, using $pp$ collision data recorded with the LHCb detector corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The polarization fractions and $CP$ asymmetries for $B^{+}\to \rho (770{)}^0{K}^{*}(892{)}^{+}$ decays are measured. Violation of the $CP$ symmetry in the decay $B^{+}\to \rho (770{)}^0{K}^{*}(892{)}^{+}$ is observed for the first time, with a significance exceeding 9 standard deviations. The $CP$ asymmetry is measured to be ${\mathcal{A}}_{CP}=0.507\pm 0.062(\mathrm{stat})\pm 0.024(\mathrm{syst})$ and the $CP$-averaged longitudinal polarization fraction of $f_L=0.720\pm 0.028(\mathrm{stat})\pm 0.009(\mathrm{syst})$. The measurements help to shed light on the polarization puzzle of $B$ mesons decaying to two vector mesons.

The first observation of the decay $B^{+}\to p\overline{\mathrm{\Lambda }}$ is presented using proton-proton collision data collected by the LHCb experiment between 2016 and 2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$. The signal significance exceeds seven standard deviations. Using the $B^{+}\to K_{\mathrm{S}}^0{\pi }^{+}$ decay as a normalization channel, the branching fraction is measured and combined with previous LHCb results based on data collected at 7 and 8 TeV in 2011 and 2012, yielding $\mathcal{B}(B^{+}\to p\overline{\mathrm{\Lambda }})=(1.24\pm 0.17\pm 0.05\pm 0.03)\times {10}^{-7},$ where the first uncertainty is statistical, the second is systematic, and the third comes from the uncertainty on the branching fraction of the normalization channel. The $B^{+}\to p\overline{\mathrm{\Lambda }}$ weak decay parameter is measured to be ${\alpha }_B=0.87_{-0.29}^{+0.26}\pm 0.09$, indicating the presence of comparable S-wave and P-wave decay amplitudes.

The decay $B^{\pm }\to K_{\mathrm{S}}^0{\pi }^{\pm }$, with a $CP$ asymmetry expected to be close to zero in the standard model, is theoretically clean and sensitive to potential new physics. An analysis of the decays $B^{\pm }\to K_{\mathrm{S}}^0{\pi }^{\pm }$ and $B^{\pm }\to K_{\mathrm{S}}^0{K}^{\pm }$ 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 $5.4{\mathrm{fb}}^{-1}$. The $CP$ asymmetries are determined to be ${\mathcal{A}}^{CP}(B^{\pm }\to K_{\mathrm{S}}^0{\pi }^{\pm })=-0.028\pm 0.009\pm 0.009$ and ${\mathcal{A}}^{CP}(B^{\pm }\to K_{\mathrm{S}}^0{K}^{\pm })=0.118\pm 0.062\pm 0.031$, and the branching fraction ratio is measured to be $\mathcal{B}(B^{\pm }\to K_{\mathrm{S}}^0{K}^{\pm })/\mathcal{B}(B^{\pm }\to K_{\mathrm{S}}^0{\pi }^{\pm })=0.055\pm 0.004\pm 0.002$, where the first uncertainties are statistical and the second are systematic. These results are the most precise measurements of these quantities to date. A search for the rare decay $B_c^{\pm }\to K_{\mathrm{S}}^0{K}^{\pm }$ is also performed. No significant signal is observed, and the upper limit on the product of the branching fraction ratio $\mathcal{B}(B_c^{\pm }\to K_{\mathrm{S}}^0{K}^{\pm })/\mathcal{B}(B^{\pm }\to K_{\mathrm{S}}^0{\pi }^{\pm })$ and the fragmentation-fraction ratio $f_c/f_u$ is set to be 0.015 (0.016) at the 90% (95%) confidence level.

The first search for the $B_s^0\to K^{-}{\pi }^{+}\gamma$ decay in the range $796<m(K^{-}{\pi }^{+})<1800\mathrm{MeV}/c^2$ is performed using data from proton-proton collisions collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The photons are reconstructed through their conversion into an electron-positron pair, which significantly improves the mass resolution of the reconstructed decays with respect to decays with an unconverted photon. A signal excess with a significance of 3.5 standard deviations is measured, constituting the first experimental evidence for this decay. In the range $796<m(K^{-}{\pi }^{+})<996\mathrm{MeV}/c^2$, the ratio $\mathcal{R}$ between the branching fractions of the signal decay and the favored ${\overline{B}}^0\to K^{-}{\pi }^{+}\gamma$ decay is measured to be $\mathcal{R}=(3.7\pm 1.2\pm 0.4)\times {10}^{-2}$ where the first uncertainty is statistical and the second is systematic. This measurement is consistent with the value predicted in the Standard Model. In the range $996<m(K^{-}{\pi }^{+})<1800\mathrm{MeV}/c^2$, the ratio $\mathcal{R}=(0.2\pm 2.7\pm 1.3)\times {10}^{-2}$ is measured.

The first searches for $B^0\to K^{+}{\pi }^{-}{\tau }^{+}{\tau }^{-}$ and $B_s^0\to K^{+}{K}^{-}{\tau }^{+}{\tau }^{-}$ decays at the LHCb experiment are conducted with $pp$ collision data corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$. The tau leptons are reconstructed using the ${\tau }^{+}\to {\mu }^{+}{\overline{\nu }}_{\tau }{\nu }_{\mu }$ decay and the results are presented in bins of $K^{+}{\pi }^{-}$ or $K^{+}{K}^{-}$ mass. No signal is observed and upper limits are set on the branching fractions. The searches result in the first upper limits for $B^0\to K^{+}{\pi }^{-}{\tau }^{+}{\tau }^{-}$ decays outside the $K^{*}(892{)}^0$ region in $K^{+}{\pi }^{-}$ mass and the first limits for $B_s^0\to K^{+}{K}^{-}{\tau }^{+}{\tau }^{-}$ decays. The searches are recast into limits on the decays $B^0\to K^{*}(892{)}^0{\tau }^{+}{\tau }^{-}$ and $B_s^0\to \varphi (1020){\tau }^{+}{\tau }^{-}$, yielding $2.8\times {10}^{-4}$ ($2.5\times {10}^{-4}$) and $4.7\times {10}^{-4}$ ($4.1\times {10}^{-4}$) at the 95% (90%) confidence level, respectively. For the decay $B^0\to K^{*}(892{)}^0{\tau }^{+}{\tau }^{-}$, this result improves on the current best upper limit by an order of magnitude.

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.