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LHCb collaboration

Publications and source records attributed to LHCb collaboration.

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arXiv2024

Measurement of $J/ψ$-pair production in $pp$ collisions at $\sqrt{s}=13$ TeV and study of gluon transverse-momentum dependent PDFs

The production cross-section of $J/ψ$ pairs in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV is measured using a data sample corresponding to an integrated luminosity of 4.2 fb$^{-1}$ collected by the LHCb experiment. The measurement is performed with both $J/ψ$ mesons in the transverse momentum range $0<p_{\text{T}}<14$ GeV/$c$ and rapidity range $2.0<y<4.5$. The cross-section of this process is measured to be 16.36$\pm$0.28(stat)$\pm$0.88(syst) nb. The contributions from single-parton scattering and double-parton scattering are separated based on the dependence of the cross-section on the absolute rapidity difference $Δy$ between the two $J/ψ$ mesons. The effective cross-section of double-parton scattering is measured to be $σ_{\text{eff}}=$13.1$\pm$1.8(stat)$\pm$2.3(syst) mb. The distribution of the azimuthal angle $ϕ_{\text{CS}}$ of one of the $J/ψ$ mesons in the Collins-Soper frame and the $p_{\text{T}}$-spectrum of the $J/ψ$ pairs are also measured for the study of the gluon transverse-momentum dependent distributions inside protons. The extracted values of $\langle\cos2ϕ_{\text{CS}}\rangle$ and $\langle\cos4ϕ_{\text{CS}}\rangle$ are consistent with zero, but the presence of azimuthal asymmetry at a few percent level is allowed.

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arXiv2024

Search for $\textit{CP}$ violation in the phase space of $D^{0} \rightarrow K_{S}^{0} K^{\pm} π^{\mp} $ decays with the energy test

A search for $\textit{CP}$ violation in $D^{0} \rightarrow K_{S}^{0} K^{+} π^{-} $ and $D^{0} \rightarrow K_{S}^{0} K^{-} π^{+} $ decays is reported. The search is performed using an unbinned model-independent method known as the energy test that probes local $\textit{CP}$ violation in the phase space of the decays. The data analysed correspond to an integrated luminosity of $5.4~$fb$^{-1}$ collected in proton-proton collisions by the LHCb experiment at a centre-of-mass energy of $\sqrt{s}=13$~TeV, amounting to approximately 950 thousand and 620 thousand signal candidates for the $D^{0} \rightarrow K_{S}^{0} K^{-} π^{+} $ and $D^{0} \rightarrow K_{S}^{0} K^{+} π^{-} $ modes, respectively. The method is validated using $D^{0} \rightarrow K^{-} π^{+} π^{-} π^{+} $ and $D^{0} \rightarrow K_{S}^{0} π^{+} π^{-} $ decays, where $\textit{CP}$-violating effects are expected to be negligible, and using background-enhanced regions of the signal decays. The results are consistent with $\textit{CP}$ symmetry in both the $D^{0} \rightarrow K_{S}^{0} K^{-} π^{+} $ and the $D^{0} \rightarrow K_{S}^{0} K^{+} π^{-} $ decays, with $p$-values for the hypothesis of no $\textit{CP}$ violation of 70% and 66%, respectively.

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arXiv2024

Search for $CP$ violation in the phase space of $D^0 \to π^-π^+π^0$ decays with the energy test

A search for $CP$ violation in $D^0 \to π^-π^+π^0$ decays is reported, using $pp$ collision data collected by the LHCb experiment from 2015 to 2018 corresponding to an integrated luminosity of 6$fb^{-1}$. An unbinned model-independent approach provides sensitivity to local $CP$ violation within the two-dimensional phase space of the decay. The method is validated using the Cabibbo-favoured channel $\D^0 \to \K^-π^+π^0$ and background regions of the signal mode. The results are consistent with $CP$ symmetry in this decay.

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arXiv2024

$Λ^+_\mathrm{c}$ polarimetry using the dominant hadronic mode

The polarimeter vector field for multibody decays of a spin-half baryon is introduced as a generalisation of the baryon asymmetry parameters. Using a recent amplitude analysis of the $Λ^+_\mathrm{c} \to p K^- π^+$ decay performed at the LHCb experiment, we compute the distribution of the kinematic-dependent polarimeter vector for this process in the space of Mandelstam variables to express the polarised decay rate in a model-agnostic form. The obtained representation can facilitate polarisation measurements of the $Λ^+_\mathrm{c}$ baryon and eases inclusion of the $Λ^+_\mathrm{c} \to p K^- π^+$ decay mode in hadronic amplitude analyses.

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arXiv2024

Measurement of the $Z$ boson production cross-section in $pp$ collisions at $\sqrt{s} = 5.02$ TeV

The first measurement of the $Z$ boson production cross-section at centre-of-mass energy $\sqrt{s} = 5.02\,$TeV in the forward region is reported, using $pp$ collision data collected by the LHCb experiment in year 2017, corresponding to an integrated luminosity of $100 \pm 2\,\rm{pb^{-1}}$. The production cross-section is measured for final-state muons in the pseudorapidity range $2.0<η<4.5$ with transverse momentum $p_{\rm{T}}> 20\,\rm{GeV/}\it{c}$. The integrated cross-section is determined to be \[ σ_{Z \rightarrow μ^{+}μ^{-}} = 39.6 \pm 0.7\,(\rm{stat}) \pm 0.6\,(\rm{syst}) \pm 0.8\,(\rm{lumi}) \ \rm{pb} \] for the di-muon invariant mass in the range $60<M_{μμ}<120\,\rm{GeV/}\it{c^{2}}$. This result and the differential cross-section results are in good agreement with theoretical predictions at next-to-next-to-leading order in the strong coupling. Based on a previous LHCb measurement of the $Z$ boson production cross-section in $p$Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV, the nuclear modification factor $R_{p\rm{Pb}}$ is measured for the first time at this energy. The measured values are $1.2^{+0.5}_{-0.3}\,(\rm{stat}) \pm 0.1\,(\rm{syst})$ in the forward region ($1.53<y^*_μ<4.03$) and $3.6^{+1.6}_{-0.9}\,(\rm{stat}) \pm 0.2\,(\rm{syst})$ in the backward region ($-4.97<y^*_μ<-2.47$), where $y^*_μ$ represents the muon rapidity in the centre-of-mass frame.

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arXiv2024

Study of $B_c^+ \rightarrow χ_c π^+$ decays

A study of $B_c^+ \rightarrow χ_c π^+$ decays is reported using proton-proton collision data, collected with the LHCb detector at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9fb$^{-1}$. The decay $B_c^+ \rightarrow χ_{c2} π^+$ is observed for the first time, with a significance exceeding seven standard deviations. The relative branching fraction with respect to the $B_c^+ \rightarrow J/ψπ^+$ decay is measured to be $$ \frac{\mathcal{B}_{B_c^+ \rightarrow χ_{c2} π^+}} {\mathcal{B}_{B_c^+ \rightarrow J/ψπ^+}} = 0.37 \pm 0.06 \pm 0.02 \pm 0.01 , $$ where the first uncertainty is statistical, the second is systematic, and the third is due to the knowledge of the $χ_c \rightarrow J/ψγ$ branching fraction. No significant $B_c^+ \rightarrow χ_{c1} π^+$ signal is observed and an upper limit for the relative branching fraction for the $B_c^+ \rightarrow χ_{c1} π^+$ and $B_c^+ \rightarrow χ_{c2} π^+$ decays of $$ \frac{\mathcal{B}_{B_c^+ \rightarrow χ_{c1} π^+}} {\mathcal{B}_{B_c^+ \rightarrow χ_{c2} π^+}} < 0.49 $$ is set at the 90\% confidence level.

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arXiv2024

A search for rare $B \rightarrow D μ^+ μ^-$ decays

A search for rare $B \rightarrow D μ^+ μ^-$ decays is performed using proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of 9 fb$^{-1}$. No significant signals are observed in the non-resonant $μ^+μ^-$ modes, and upper limits of $\mathcal{B}(B^0 \rightarrow \overline{D}^0 μ^+ μ^-) < 5.1 \times 10^{-8}$, $\mathcal{B}(B^+ \rightarrow D_s^+ μ^+ μ^-) < 3.2 \times 10^{-8}$, $\mathcal{B}(B_s^0 \rightarrow \overline{D}^0 μ^+ μ^-) < 1.6 \times 10^{-7}$ and $f_c/f_u \cdot \mathcal{B}(B_c^+ \rightarrow D_s^+ μ^+ μ^-) < 9.6 \times 10^{-8}$ are set at the 95\% confidence level, where $f_c$ and $f_u$ are the fragmentation fractions of a $B$ meson with a $c$ and $u$ quark respectively in proton-proton collisions. Each result is either the first such measurement or an improvement by three orders of magnitude on an existing limit. Separate upper limits are calculated when the muon pair originates from a $J/ψ\rightarrow μ^+ μ^-$ decay. The branching fraction of $B_c^+ \rightarrow D_s^+ J/ψ$ multiplied by the fragmentation-fraction ratio is measured to be $f_c/f_u \cdot \mathcal{B}(B_c^+ \rightarrow D_s^+ J/ψ) = (1.63 \pm 0.15 \pm 0.13) \times 10^{-5}$, where the first uncertainty is statistical and the second systematic.

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arXiv2024

Observation of Cabibbo-Suppressed Two-Body Hadronic Decays and Precision Mass Measurement of the $Ω_{c}^{0}$ Baryon

The first observation of the singly Cabibbo-suppressed $Ω_{c}^{0}\toΩ^{-}K^{+}$ and $Ω_{c}^{0}\toΞ^{-}π^{+}$ decays is reported, using proton-proton collision data at a center-of-mass energy of $13\,{\rm TeV}$, corresponding to an integrated luminosity of $5.4\,{\rm fb}^{-1}$, collected with the LHCb detector between 2016 and 2018. The branching fraction ratios are measured to be $\frac{\mathcal{B}(Ω_{c}^{0}\toΩ^{-}K^{+})}{\mathcal{B}(Ω_{c}^{0}\toΩ^{-}π^{+})}=[6.08\pm0.51({\rm stat})\pm0.40({\rm syst})]\%$, $\frac{\mathcal{B}(Ω_{c}^{0}\toΞ^{-}π^{+})}{\mathcal{B}(Ω_{c}^{0}\toΩ^{-}π^{+})}=[15.81\pm0.87({\rm stat})\pm0.44({\rm syst})\pm0.16({\rm ext})]\%$. In addition, using the $Ω_{c}^{0}\toΩ^{-}π^{+}$ decay channel, the $Ω_{c}^{0}$ baryon mass is measured to be $M(Ω_{c}^{0})=2695.28\pm0.07({\rm stat})\pm0.27({\rm syst})\pm0.30({\rm ext})\,{\rm MeV}$, improving the precision of the previous world average by a factor of 4.

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arXiv2024

A model-independent measurement of the CKM angle $γ$ in partially reconstructed $B^{\pm} \to D^{*} h^{\pm}$ decays with $D \to K_{S}^{0} h^{+}h^{-}$ $(h=π, K)$

A measurement of $C\!P$-violating observables in $B^{\pm} \to D^{*} K^{\pm}$ and $B^{\pm} \to D^{*} π^{\pm}$ decays is made where the photon or neutral pion from the $D^{*} \to Dγ$ or $D^{*} \to Dπ^{0}$ decay is not reconstructed. The $D$ meson is reconstructed in the self-conjugate decay modes, $D \to K_{S}^{0} π^{+} π^{-}$ or $D \to K_{S}^{0} K^{+} K^{-}$. The distribution of signal yields in the $D$ decay phase space is analysed in a model-independent way. The measurement uses a data sample collected in proton-proton collisions at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to a total integrated luminosity of approximately 9 fb$^{-1}$. The $B^{\pm} \to D^{*} K^{\pm}$ and $B^{\pm} \to D^{*} π^{\pm}$ $C\!P$-violating observables are interpreted in terms of hadronic parameters and the CKM angle $γ$, resulting in a measurement of $γ= (92^{+21}_{-17})^{\circ}$. The total uncertainty includes the statistical and systematic uncertainties, and the uncertainty due to external strong-phase inputs.

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arXiv2024

Enhanced production of $Λ_{b}^{0}$ baryons in high-multiplicity $pp$ collisions at $\sqrt{s} = 13$ TeV

The production rate of $Λ_{b}^{0}$ baryons relative to $B^{0}$ mesons in $pp$ collisions at a center-of-mass energy $\sqrt{s} = 13$ TeV is measured by the LHCb experiment. The ratio of $Λ_{b}^{0}$ 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 $\sim2$ with increasing multiplicity. At relatively low transverse momentum, the ratio of $Λ_{b}^{0}$ 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.

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arXiv2024

Improved measurement of $CP$ violation parameters in $B^0_s\to J/ψK^{+}K^{-}$ decays in the vicinity of the $ϕ(1020)$ resonance

The decay-time-dependent $CP$ asymmetry in $B^0_s\to J/ψ(\to μ^{+}μ^{-}) K^{+}K^{-}$ decays is measured using proton-proton collision data, corresponding to an integrated luminosity of 6 $fb^{-1}$, collected with the LHCb detector at a center-of-mass energy of 13 TeV. Using a sample of approximately 349 000 $B^{0}_{s}$ signal decays with an invariant $K^{+}K^{-}$ mass in the vicinity of the $ϕ(1020)$ resonance, the $CP$-violating phase $ϕ_s$ is measured, along with the difference in decay widths of the light and heavy mass eigenstates of the $B^0_s$-$\bar{B}^0_s$ system, $ΔΓ_s$, and the difference of the average $B^0_s$ and $B^0$ meson decay widths, $Γ_s-Γ_d$. The values obtained are $ϕ_s = \ -0.039 \pm 0.022 \pm 0.006$ rad, $ΔΓ_s = 0.0845 \pm 0.0044 \pm 0.0024 $ ps$^{-1}$ and $Γ_s-Γ_d = -0.0056 ^{+ 0.0013}_{-0.0015} \pm 0.0014$ ps$^{-1}$, where the first uncertainty is statistical and the second systematic. These are the most precise single measurements to date and are consistent with expectations based on the Standard Model and with the previous LHCb analyses of this decay. These results are combined with previous independent LHCb measurements. The phase $ϕ_s$ is also measured independently for each polarization state of the $K^{+}K^{-}$ system and shows no evidence for polarization dependence.

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arXiv2024

Open charm production and asymmetry in $p$Ne collisions at $\sqrt{s_{\scriptscriptstyle\rm NN}} =$ 68.5 GeV

A measurement of $D^0$ meson production by the LHCb experiment in its fixed-target configuration is presented. The production of $D^0$ mesons is studied with a beam of 2.5 TeV protons colliding on a gaseous neon target at rest, corresponding to a nucleon-nucleon centre-of-mass energy of $\sqrt{s_{\rm NN}}$ = 68.5 GeV. The sum of the $D^0$ and ${\overline D^0}$ production cross-section in $p$Ne collisions in the centre-of-mass rapidity range $y^{\star}\in [-2.29, 0]$ is found to be $σ_{D^{0}}^{y^\star \in [-2.29, 0]} = 48.2 \pm 0.3 \pm 4.5 \,μ\textrm{b/nucleon}$ where the first uncertainty is statistical and the second is systematic. The $D^0-{\overline D^0}$ production asymmetry is also evaluated and suggests a trend towards negative values at large negative $y^{\star}$. The considered models do not account precisely for all the features observed in the LHCb data, but theoretical predictions including 1$\%$ intrinsic charm and 10$\%$ recombination contributions better describe the data than the other models considered.

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arXiv2024

Study of the $B^{-} \to Λ_{c}^{+} \barΛ_{c}^{-} K^{-}$ decay

The decay $B^{-} \to Λ_{c}^{+} \barΛ_{c}^{-} K^{-}$ is studied in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13$ TeV using data corresponding to an integrated luminosity of 5 $\mathrm{fb}^{-1}$ collected by the LHCb experiment. In the $Λ_{c}^+ K^{-}$ system, the $Ξ_{c}(2930)^{0}$ state observed at the BaBar and Belle experiments is resolved into two narrower states, $Ξ_{c}(2923)^{0}$ and $Ξ_{c}(2939)^{0}$, whose masses and widths are measured to be $$ m(Ξ_{c}(2923)^{0}) = 2924.5 \pm 0.4 \pm 1.1 \,\mathrm{MeV}, \\ m(Ξ_{c}(2939)^{0}) = 2938.5 \pm 0.9 \pm 2.3 \,\mathrm{MeV}, \\ Γ(Ξ_{c}(2923)^{0}) = \phantom{000}4.8 \pm 0.9 \pm 1.5 \,\mathrm{MeV},\\ Γ(Ξ_{c}(2939)^{0}) = \phantom{00}11.0 \pm 1.9 \pm 7.5 \,\mathrm{MeV}, $$ where the first uncertainties are statistical and the second systematic. The results are consistent with a previous LHCb measurement using a prompt $Λ_{c}^{+} K^{-}$ sample. Evidence of a new $Ξ_{c}(2880)^{0}$ state is found with a local significance of $3.8\,σ$, whose mass and width are measured to be $2881.8 \pm 3.1 \pm 8.5\,\mathrm{MeV}$ and $12.4 \pm 5.3 \pm 5.8 \,\mathrm{MeV}$, respectively. In addition, evidence of a new decay mode $Ξ_{c}(2790)^{0} \to Λ_{c}^{+} K^{-}$ is found with a significance of $3.7\,σ$. The relative branching fraction of $B^{-} \to Λ_{c}^{+} \barΛ_{c}^{-} K^{-}$ with respect to the $B^{-} \to D^{+} D^{-} K^{-}$ decay is measured to be $2.36 \pm 0.11 \pm 0.22 \pm 0.25$, where the first uncertainty is statistical, the second systematic and the third originates from the branching fractions of charm hadron decays.

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arXiv2024

Momentum scale calibration of the LHCb spectrometer

For accurate determination of particle masses accurate knowledge of the momentum scale of the detectors is crucial. The procedure used to calibrate the momentum scale of the LHCb spectrometer is described and illustrated using the performance obtained with an integrated luminosity of $1.6~ fb^{-1}$ collected during 2016 in $pp$ running. The procedure uses large samples of $J/ψ\rightarrow μ^+ μ^-$ and $B^+ \rightarrow J/ψK^+$ decays and leads to a relative accuracy of $3 \times 10^{-4}$ on the momentum scale.

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arXiv2024

Helium identification with LHCb

The identification of helium nuclei at LHCb is achieved using a method based on measurements of ionisation losses in the silicon sensors and timing measurements in the Outer Tracker drift tubes. The background from photon conversions is reduced using the RICH detectors and an isolation requirement. The method is developed using $pp$ collision data at $\sqrt{s}=13\,{\rm TeV}$ recorded by the LHCb experiment in the years 2016 to 2018, corresponding to an integrated luminosity of $5.5\,{\rm fb}^{-1}$. A total of around $10^5$ helium and antihelium candidates are identified with negligible background contamination. The helium identification efficiency is estimated to be approximately $50\%$ with a corresponding background rejection rate of up to $\mathcal O(10^{12})$. These results demonstrate the feasibility of a rich programme of measurements of QCD and astrophysics interest involving light nuclei.

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arXiv2024

Measurement of prompt $D^+$ and $D^+_{s}$ production in $p\mathrm{Pb}$ collisions at $\sqrt {s_{\mathrm{NN}}}=5.02\,$TeV

The production of prompt $D^+$ and $D^+_{s}$ mesons is studied in proton-lead collisions at a centre-of-mass energy of $\sqrt {s_{\mathrm{NN}}}=5.02\,$TeV. The data sample corresponding to an integrated luminosity of $(1.58\pm0.02)\mathrm{nb}^{-1}$ is collected by the LHCb experiment at the LHC. The differential production cross-sections are measured using $D^+$ and $D^+_{s}$ candidates with transverse momentum in the range of $0<p_{\mathrm{T}} <14\,\mathrm{GeV}/c$ and rapidities in the ranges of $1.5<y^*<4.0$ and $-5.0<y^*<-2.5$ in the nucleon-nucleon centre-of-mass system. For both particles, the nuclear modification factor and the forward-backward production ratio are determined. These results are compared with theoretical models that include initial-state nuclear effects. In addition, measurements of the cross-section ratios between $D^+$, $D^+_{s}$ and $D^0$ mesons are presented, providing a baseline for studying the charm hadronization in lead-lead collisions at LHC energies.

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arXiv2024

Measurement of CP violation in $B^0\toψ(\to\ell^+\ell^-)K^0_S(\toπ^+π^-)$ decays

A measurement of time-dependent CP violation in the decays of $B^0$ and $\overline{B}^0$ mesons to the final states $J/ψ(\toμ^+μ^-)K^0_S$, $ψ(2S)(\toμ^+μ^-)K^0_S$ and $J/ψ(\to e^+e^-)K^0_S$ with $K^0_S\toπ^+π^-$ is presented. The data correspond to an integrated luminosity of 6 fb${}^{-1}$ collected at a center-of-mass energy of $\sqrt{s}=13$ TeV with the LHCb detector. The CP-violation parameters are measured to be \begin{align*} S_{ψK^0_S} &= 0.717 \pm 0.013 (\text{stat}) \pm 0.008 (\text{syst}), \\ C_{ψK^0_S} &= 0.008 \pm 0.012 (\text{stat}) \pm 0.003 (\text{syst}). \end{align*} This measurement of $S_{ψK^0_S}$ represents the most precise single measurement of the CKM angle $β$ to date and is more precise than the current world average. In addition, measurements of the CP-violation parameters of the individual channels are reported and a combination with the LHCb Run 1 measurements is performed.

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arXiv2023

Observation of the $B^0_s\rightarrow χ_{c1}(3872)π^+π^-$ decay

The first observation of the $B^0_s \rightarrow \left( χ_{c1}(3872) \rightarrow J/ψπ^+π^-\right) π^+ π^-$ decay is reported using proton-proton collision data, corresponding to integrated luminosities of 1, 2 and 6fb$^{-1}$, collected by the LHCb experiment at centre-of-mass energies of 7, 8 and 13TeV, respectively. The ratio of branching fractions relative to the $B^0_s \rightarrow \left( ψ(2S) \rightarrow Jψπ^+π^- \right) π^+ π^-$ decay is measured to be $$ \frac{ \mathcal{B} \left( B^0_s \rightarrow χ_{c1}(3872) π^+π^-\right) \times \mathcal{B} \left( χ_{c1}(3872) \rightarrow Jψπ^+π^-\right)} { \mathcal{B} \left( B^0_s \rightarrow ψ(2S) π^+ π^- \right) \times \mathcal{B} \left( ψ(2S) \rightarrow Jψπ^+π^-\right) } = \left( 6.8 \pm 1.1 \pm 0.2 \right) \times 10^{-2} , $$ where the first uncertainty is statistical and the second systematic. The mass spectrum of the $π^+π^-$ system recoiling against the $χ_{c1}(3872)$ meson exhibits a large contribution from $B^0_s \rightarrow χ_{c1}(3872) \left( f_0(980) \rightarrow π^+ π^-\right)$ decays.

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