Search

A study of $B_c^{+}\to K^{+}{K}^{-}{\pi }^{+}$ decays is performed for the first time using data corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$ collected by the LHCb experiment in $pp$ collisions at center-of-mass energies of 7 and 8 TeV. Evidence for the decay $B_c^{+}\to {\chi }_{c0}(\to K^{+}{K}^{-}){\pi }^{+}$ is reported with a significance of 4.0 standard deviations, giving $\frac{\sigma (B_c^{+})}{\sigma (B^{+})}\times \mathcal{B}(B_c^{+}\to {\chi }_{c0}{\pi }^{+})=(9.8_{-3.0}^{+3.4}(\mathrm{stat})\pm 0.8(\mathrm{syst}))\times {10}^{-6}$. Here $\mathcal{B}$ denotes a branching fraction while $\sigma (B_c^{+})$ and $\sigma (B^{+})$ are the production cross sections for $B_c^{+}$ and $B^{+}$ mesons. An indication of $\overline{b}c$ weak annihilation is found for the region $m(K^{-}{\pi }^{+})<1.834\mathrm{GeV}/c^2$, with a significance of 2.4 standard deviations.

Decays of $B^0$ and $B_s^0$ mesons to the charmless baryonic final states $p\overline{p}{h}^{+}{h}^{\prime -}$, where $h$ and $h^{\prime }$ each denote a kaon or a pion, are searched for using the LHCb detector. The analysis is based on a sample of proton-proton collision data collected at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$. Four-body charmless baryonic $B_s^0$ decays are observed for the first time. The decays $B_s^0\to p\overline{p}{K}^{+}{K}^{-}$, $B_s^0\to p\overline{p}{K}^{\pm }{\pi }^{\mp }$, $B^0\to p\overline{p}{K}^{\pm }{\pi }^{\mp }$ and $B^0\to p\overline{p}{\pi }^{+}{\pi }^{-}$ are observed with a significance greater than 5 standard deviations; evidence at 4.1 standard deviations is found for the $B^0\to p\overline{p}{K}^{+}{K}^{-}$ decay and an upper limit is set on the branching fraction for $B_s^0\to p\overline{p}{\pi }^{+}{\pi }^{-}$. Branching fractions in the kinematic region $m(p\overline{p})<2850\mathrm{MeV}/c^2$ are measured relative to the $B^0\to J/\psi (\to p\overline{p})K^{*}(892{)}^0$ channel.

The first observation of the decay $B^0\to D^0{\overline{D}}^0{K}^{+}{\pi }^{-}$ is reported using proton-proton collision data corresponding to an integrated luminosity of $4.7{\mathrm{fb}}^{-1}$ collected by the LHCb experiment in 2011, 2012 and 2016. The measurement is performed in the full kinematically allowed range of the decay outside of the $D^{*-}$ region. The ratio of the branching fraction relative to that of the control channel $B^0\to D^{*-}{D}^0{K}^{+}$ is measured to be $\mathcal{R}=(14.2\pm 1.1\pm 1.0)\%$, where the first uncertainty is statistical and the second is systematic. The absolute branching fraction of $B^0\to D^0{\overline{D}}^0{K}^{+}{\pi }^{-}$ decays is thus determined to be $\mathcal{B}(B^0\to D^0{\overline{D}}^0{K}^{+}{\pi }^{-})=(3.50\pm 0.27\pm 0.26\pm 0.30)\times {10}^{-4}$, where the third uncertainty is due to the branching fraction of the control channel. This decay mode is expected to provide insights to spectroscopy and the charm-loop contributions in rare semileptonic decays.

A baryonic decay of the $B_c^{+}$ meson, $B_c^{+}\to J/\psi p\overline{p}{\pi }^{+}$, is observed for the first time, with a significance of 7.3 standard deviations, in $pp$ collision data collected with the LHCb detector and corresponding to an integrated luminosity of $3.0{\text{fb}}^{-1}$ taken at center-of-mass energies of 7 and 8 TeV. With the $B_c^{+}\to J/\psi {\pi }^{+}$ decay as the normalization channel, the ratio of branching fractions is measured to be $\mathcal{B}(B_c^{+}\to J/\psi p\overline{p}{\pi }^{+})/\mathcal{B}(B_c^{+}\to J/\psi {\pi }^{+})=0.143_{-0.034}^{+0.039}(\text{stat})\pm 0.013(\text{syst})$. The mass of the $B_c^{+}$ meson is determined as $M(B_c^{+})=6274.0\pm 1.8(\text{stat})\pm 0.4(\text{syst})\mathrm{MeV}/c^2$, using the $B_c^{+}\to J/\psi p\overline{p}{\pi }^{+}$ channel.

Using a proton-proton collision data sample corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$ collected by LHCb at center-of-mass energies of 7 and 8 TeV, about 3800 ${\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}_c^{+}{\pi }^{-}$, ${\mathrm{\Xi }}_c^{+}\to p{K}^{-}{\pi }^{+}$ signal decays are reconstructed. From this sample, the first measurement of the ${\mathrm{\Xi }}_b^0$ baryon lifetime is made, relative to that of the ${\mathrm{\Lambda }}_b^0$ baryon. The mass differences $M({\mathrm{\Xi }}_b^0)-M({\mathrm{\Lambda }}_b^0)$ and $M({\mathrm{\Xi }}_c^{+})-M({\mathrm{\Lambda }}_c^{+})$ are also measured with precision more than 4 times better than the current world averages. The resulting values are $$\begin{gathered} \frac{{\tau }_{{\mathrm{\Xi }}_b^0}}{{\tau }_{{\mathrm{\Lambda }}_b^0}}=1.006\pm 0.018\pm 0.010, \\ M({\mathrm{\Xi }}_b^0)-M({\mathrm{\Lambda }}_b^0)=172.44\pm 0.39\pm 0.17\mathrm{MeV}/c^2, \\ M({\mathrm{\Xi }}_c^{+})-M({\mathrm{\Lambda }}_c^{+})=181.51\pm 0.14\pm 0.10\mathrm{MeV}/c^2, \end{gathered}$$where the first uncertainty is statistical and the second is systematic. The relative rate of ${\mathrm{\Xi }}_b^0$ to ${\mathrm{\Lambda }}_b^0$ baryon production is measured to be $$\frac{f_{{\mathrm{\Xi }}_b^0}}{f_{{\mathrm{\Lambda }}_b^0}}\frac{\mathcal{B}({\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}_c^{+}{\pi }^{-})}{\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-})}\frac{\mathcal{B}({\mathrm{\Xi }}_c^{+}\to p{K}^{-}{\pi }^{+})}{\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+})}=(1.88\pm 0.04\pm 0.03)\times 10^{-2},$$where the first factor is the ratio of fragmentation fractions, $b\to {\mathrm{\Xi }}_b^0$ relative to $b\to {\mathrm{\Lambda }}_b^0$. Relative production rates as functions of transverse momentum and pseudorapidity are also presented.

A search for charge-parity ($CP$) violation in Cabibbo-suppressed $D_s^{+}\to K_{\mathrm{S}}^0{\pi }^{+}$, $D^{+}\to K_{\mathrm{S}}^0{K}^{+}$, and $D^{+}\to \varphi {\pi }^{+}$ decays is reported using proton-proton collision data, corresponding to an integrated luminosity of $3.8{\mathrm{fb}}^{-1}$, collected at a center-of-mass energy of 13 TeV with the LHCb detector. High-yield samples of kinematically and topologically similar Cabibbo-favored $D_{(s)}^{+}$ decays are analyzed to subtract nuisance asymmetries due to production and detection effects, including those induced by $CP$ violation in the neutral kaon system. The results are $$\begin{gathered} {\mathcal{A}}_{CP}(D_s^{+}\to K_{\mathrm{S}}^0{\pi }^{+})=(1.3\pm 1.9\pm 0.5)\times {10}^{-3}, \\ {\mathcal{A}}_{CP}(D^{+}\to K_{\mathrm{S}}^0{K}^{+})=(-0.09\pm 0.65\pm 0.48)\times {10}^{-3}, \\ {\mathcal{A}}_{CP}(D^{+}\to \varphi {\pi }^{+})=(0.05\pm 0.42\pm 0.29)\times {10}^{-3}, \end{gathered}$$where the first uncertainties are statistical and the second systematic. They are the most precise measurements of these quantities to date, and are consistent with $CP$ symmetry. A combination with previous LHCb measurements, based on data collected at 7 and 8 TeV, is also reported.

Using $pp$ collision data collected by LHCb at center-of-mass energies $\sqrt{s}=7\mathrm{TeV}$ and 8 TeV, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$, the ratio of the branching fraction of the $B_c^{+}\to \psi (2S){\pi }^{+}$ decay relative to that of the $B_c^{+}\to J/\psi {\pi }^{+}$ decay is measured to be $0.268\pm 0.032(\mathrm{stat})\pm 0.007(\mathrm{syst})\pm 0.006(\mathrm{BF})$. The first uncertainty is statistical, the second is systematic, and the third is due to the uncertainties on the branching fractions of the $J/\psi \to {\mu }^{+}{\mu }^{-}$ and $\psi (2S)\to {\mu }^{+}{\mu }^{-}$ decays. This measurement is consistent with the previous LHCb result, and the statistical uncertainty is halved.

The first measurement that relates semileptonic and hadronic decay rates of the $B_c^{+}$ meson is performed using proton-proton collision data corresponding to $1.0{\mathrm{fb}}^{-1}$ of integrated luminosity collected with the LHCb detector. The measured value of the ratio of branching fractions, $\mathcal{B}(B_c^{+}\to J/\psi {\pi }^{+})/\mathcal{B}(B_c^{+}\to J/\psi {\mu }^{+}{\nu }_{\mu })=0.0469\pm 0.0028(\text{stat})\pm 0.0046(\text{syst})$, is at the lower end of available theoretical predictions.

Using $(106\pm 4)\times {10}^6$ $\psi (3686)$ events accumulated with the BESIII detector at the BEPCII $e^{+}{e}^{-}$ collider, we present the first measurement of decays of ${\chi }_{c1}$ to vector meson pairs $\varphi \varphi$, $\omega \omega$, and $\omega \varphi$. The branching fractions are measured to be $(4.4\pm 0.3\pm 0.5)\times {10}^{-4}$, $(6.0\pm 0.3\pm 0.7)\times {10}^{-4}$, and $(2.2\pm 0.6\pm 0.2)\times {10}^{-5}$, for ${\chi }_{c1}\to \varphi \varphi$, $\omega \omega$, and $\omega \varphi$, respectively, which indicates that the hadron helicity selection rule is significantly violated in ${\chi }_{\mathrm{cJ}}$ decays. In addition, the measurement of ${\chi }_{\mathrm{cJ}}\to \omega \varphi$ provides the first indication of the rate of doubly OZI-suppressed ${\chi }_{\mathrm{cJ}}$ decay. Finally, we present improved measurements for the branching fractions of ${\chi }_{c0}$ and ${\chi }_{c2}$ to vector meson pairs.

The decays ${\chi }_{cJ}\to \gamma V$ ($V=\varphi$, ${\rho }^0$, $\omega$) are studied with a sample of radiative ${\psi }^{\prime }\to \gamma {\chi }_{cJ}$ events in a sample of $(1.06\pm 0.04)\times {10}^8{\psi }^{\prime }$ events collected with the BESIII detector. The branching fractions are determined to be: $\mathcal{B}({\chi }_{c1}\to \gamma \varphi )=(25.8\pm 5.2\pm 2.3)\times {10}^{-6}$, $\mathcal{B}({\chi }_{c1}\to \gamma {\rho }^0)=(228\pm 13\pm 22)\times {10}^{-6}$, and $\mathcal{B}({\chi }_{c1}\to \gamma \omega )=(69.7\pm 7.2\pm 6.6)\times {10}^{-6}$. The decay ${\chi }_{c1}\to \gamma \varphi$ is observed for the first time. Upper limits at the 90% confidence level on the branching fractions for ${\chi }_{c0}$ and ${\chi }_{c2}$ decays into these final states are determined. In addition, the fractions of the transverse polarization component of the vector meson in ${\chi }_{c1}\to \gamma V$ decays are measured to be ${0.29}_{-0.12-0.09}^{+0.13+0.10}$ for ${\chi }_{c1}\to \gamma \varphi$, $0.158\pm {0.034}_{-0.014}^{+0.015}$ for ${\chi }_{c1}\to \gamma {\rho }^0$, and ${0.247}_{-0.087-0.026}^{+0.090+0.044}$ for ${\chi }_{c1}\to \gamma \omega$, respectively. The first errors are statistical and the second ones are systematic.

From samples of $pp$ collision data collected by the LHCb experiment at $\sqrt{s}=7$, 8 and 13 TeV, corresponding to integrated luminosities of 1.0, 2.0 and $1.5{\mathrm{fb}}^{-1}$, respectively, a peak in both the ${\mathrm{\Lambda }}_b^0{K}^{-}$ and ${\mathrm{\Xi }}_b^0{\pi }^{-}$ invariant mass spectra is observed. In the quark model, radially and orbitally excited ${\mathrm{\Xi }}_b^{-}$ resonances with quark content $bds$ are expected. Referring to this peak as ${\mathrm{\Xi }}_b(6227{)}^{-}$, the mass and natural width are measured to be $m_{{\mathrm{\Xi }}_b(6227{)}^{-}}=6226.9\pm 2.0\pm 0.3\pm 0.2\mathrm{MeV}/c^2$ and ${\mathrm{\Gamma }}_{{\mathrm{\Xi }}_b(6227{)}^{-}}=18.1\pm 5.4\pm 1.8\mathrm{MeV}/c^2$, where the first uncertainty is statistical, the second is systematic, and the third, on $m_{{\mathrm{\Xi }}_b(6227{)}^{-}}$, is due to the knowledge of the ${\mathrm{\Lambda }}_b^0$ baryon mass. Relative production rates of the ${\mathrm{\Xi }}_b(6227{)}^{-}\to {\mathrm{\Lambda }}_b^0{K}^{-}$ and ${\mathrm{\Xi }}_b(6227{)}^{-}\to {\mathrm{\Xi }}_b^0{\pi }^{-}$ decays are also reported.

The cross section for prompt antiproton production in collisions of protons with an energy of 6.5 TeV incident on helium nuclei at rest is measured with the LHCb experiment from a data set corresponding to an integrated luminosity of $0.5{\mathrm{nb}}^{-1}$. The target is provided by injecting helium gas into the LHC beam line at the LHCb interaction point. The reported results, covering antiproton momenta between 12 and $110\mathrm{GeV}/c$, represent the first direct determination of the antiproton production cross section in $p\text{−}\mathrm{He}$ collisions, and impact the interpretation of recent results on antiproton cosmic rays from space-borne experiments.

The production of $J/\psi$ mesons in jets is studied in the forward region of proton-proton collisions using data collected with the LHCb detector at a center-of-mass energy of 13 TeV. The fraction of the jet transverse momentum carried by the $J/\psi$ meson, $z(J/\psi )\equiv p_{\mathrm{T}}(J/\psi )/p_{\mathrm{T}}(\text{jet})$, is measured using jets with $p_{\mathrm{T}}(\text{jet})>20\mathrm{GeV}$ in the pseudorapidity range $2.5<\eta (\text{jet})<4.0$. The observed $z(J/\psi )$ distribution for $J/\psi$ mesons produced in $b$-hadron decays is consistent with expectations. However, the results for prompt $J/\psi$ production do not agree with predictions based on fixed-order nonrelativistic QCD. This is the first measurement of the $p_{\mathrm{T}}$ fraction carried by prompt $J/\psi$ mesons in jets at any experiment.

A search for the decays $B_c^{+}\to J/\psi D^{(*)0}{K}^{+}$ and $B_c^{+}\to J/\psi D^{(*)+}{K}^{*0}$ is performed with data collected at the LHCb experiment corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$. The decays $B_c^{+}\to J/\psi D^0{K}^{+}$ and $B_c^{+}\to J/\psi D^{*0}{K}^{+}$ are observed for the first time, while first evidence is reported for the $B_c^{+}\to J/\psi D^{*+}{K}^{*0}$ and $B_c^{+}\to J/\psi D^{+}{K}^{*0}$ decays. The branching fractions of these decays are determined relative to the $B_c^{+}\to J/\psi {\pi }^{+}$ decay. The $B_c^{+}$ mass is measured, using the $J/\psi D^0{K}^{+}$ final state, to be $6274.28\pm 1.40(\text{stat})\pm 0.32(\text{syst})\mathrm{MeV}/c^2$. This is the most precise single measurement of the $B_c^{+}$ mass to date.

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.

A search for a long-lived scalar particle $\chi$ is performed, looking for the decay $B^{+}\to K^{+}\chi$ with $\chi \to {\mu }^{+}{\mu }^{-}$ in $pp$ collision data corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$, collected by the LHCb experiment at center-of-mass energies of $\sqrt{s}=7$ and 8 TeV. This new scalar particle, predicted by hidden sector models, is assumed to have a narrow width. The signal would manifest itself as an excess in the dimuon invariant mass distribution over the Standard Model background. No significant excess is observed in the accessible ranges of mass $250<m(\chi )<4700\mathrm{MeV}/c^2$ and lifetime $0.1<\tau (\chi )<1000\mathrm{ps}$. Upper limits on the branching fraction $\mathcal{B}(B^{+}\to K^{+}\chi ({\mu }^{+}{\mu }^{-}))$ at 95% confidence level are set as a function of $m(\chi )$ and $\tau (\chi )$, varying between $2\times {10}^{-10}$ and ${10}^{-7}$. These are the most stringent limits to date. The limits are interpreted in the context of a model with a light inflaton particle.

A search for the rare decays $B_s^0\to {\mu }^{+}{\mu }^{-}$ and $B^0\to {\mu }^{+}{\mu }^{-}$ is performed at the LHCb experiment using data collected in $pp$ collisions corresponding to a total integrated luminosity of $4.4{\mathrm{fb}}^{-1}$. An excess of $B_s^0\to {\mu }^{+}{\mu }^{-}$ decays is observed with a significance of 7.8 standard deviations, representing the first observation of this decay in a single experiment. The branching fraction is measured to be $\mathcal{B}(B_s^0\to {\mu }^{+}{\mu }^{-})=(3.0\pm 0.6_{-0.2}^{+0.3})\times {10}^{-9}$, where the first uncertainty is statistical and the second systematic. The first measurement of the $B_s^0\to {\mu }^{+}{\mu }^{-}$ effective lifetime, $\tau (B_s^0\to {\mu }^{+}{\mu }^{-})=2.04\pm 0.44\pm 0.05\mathrm{ps}$, is reported. No significant excess of $B^0\to {\mu }^{+}{\mu }^{-}$ decays is found, and a 95% confidence level upper limit, $\mathcal{B}(B^0\to {\mu }^{+}{\mu }^{-})<3.4\times {10}^{-10}$, is determined. All results are in agreement with the standard model expectations.

A search is presented for hidden-sector bosons, $\chi$, produced in the decay $B^0\to K^{*}(892{)}^0\chi$, with $K^{*}(892{)}^0\to K^{+}{\pi }^{-}$ and $\chi \to {\mu }^{+}{\mu }^{-}$. The search is performed using $pp$-collision data corresponding to $3.0{\mathrm{fb}}^{-1}$ collected with the LHCb detector. No significant signal is observed in the accessible mass range $214\le m(\chi )\le 4350\mathrm{MeV}$, and upper limits are placed on the branching fraction product $\mathcal{B}\mathbf{(}{B}^0\to K^{*}(892{)}^0\chi \mathbf{)}\times \mathcal{B}(\chi \to {\mu }^{+}{\mu }^{-})$ as a function of the mass and lifetime of the $\chi$ boson. These limits are of the order of $10^{-9}$ for $\chi$ lifetimes less than 100 ps over most of the $m(\chi )$ range, and place the most stringent constraints to date on many theories that predict the existence of additional low-mass bosons.

A search for charge-parity ($CP$) violation in $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$ decays is reported, using $pp$ collision data corresponding to an integrated luminosity of $5.9{\mathrm{fb}}^{-1}$ collected at a center-of-mass energy of 13 TeV with the LHCb detector. The flavor of the charm meson is inferred from the charge of the pion in $D^{*}(2010{)}^{+}\to D^0{\pi }^{+}$ decays or from the charge of the muon in $\overline{B}\to D^0{\mu }^{-}{\overline{\nu }}_{\mu }X$ decays. The difference between the $CP$ asymmetries in $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$ decays is measured to be $\mathrm{\Delta }{A}_{CP}=[-18.2\pm 3.2(\mathrm{stat})\pm 0.9(\mathrm{syst})]\times {10}^{-4}$ for $\pi$-tagged and $\mathrm{\Delta }{A}_{CP}=[-9\pm 8(\mathrm{stat})\pm 5(\mathrm{syst})]\times {10}^{-4}$ for $\mu$-tagged $D^0$ mesons. Combining these with previous LHCb results leads to $\mathrm{\Delta }{A}_{CP}=(-15.4\pm 2.9)\times {10}^{-4}$, where the uncertainty includes both statistical and systematic contributions. The measured value differs from zero by more than 5 standard deviations. This is the first observation of $CP$ violation in the decay of charm hadrons.

The first measurement of heavy-flavor production by the LHCb experiment in its fixed-target mode is presented. The production of $J/\psi$ and $D^0$ mesons is studied with beams of protons of different energies colliding with gaseous targets of helium and argon with nucleon-nucleon center-of-mass energies of $\sqrt{s_{\mathrm{NN}}}=86.6$ and 110.4 GeV, respectively. The $J/\psi$ and $D^0$ production cross sections in $p\mathrm{He}$ collisions in the rapidity range [2, 4.6] are found to be ${\sigma }_{J/\psi }=652\pm 33(\text{stat})\pm 42(\text{syst})\mathrm{nb}/\text{nucleon}$ and ${\sigma }_{D^0}=80.8\pm 2.4(\text{stat})\pm 6.3(\text{syst})\mu \mathrm{b}/\text{nucleon}$, where the first uncertainty is statistical and the second is systematic. No evidence for a substantial intrinsic charm content of the nucleon is observed in the large Bjorken-$x$ region.