arXiv subjects

G. Bassi

Publications and source records attributed to G. Bassi.

At least 37 records · Page 2Linked to original sources

arXiv2025

Search for resonance-enhanced $CP$ and angular asymmetries in the $Λ^+_{c}\to pμ^+μ^-$ decay at LHCb

The first measurement of the $CP$ asymmetry of the decay rate ($A_{CP}$) and the $CP$ average ($ΣA_{\text{FB}}$) and $CP$ asymmetry ($ΔA_{\text{FB}}$) of the forward-backward asymmetry in the muon system of $\mathitΛ^+_c\to pμ^+μ^-$ decays is reported. The measurement is performed using a data sample of proton-proton collisions, recorded by the LHCb experiment from 2016 to 2018 at a center-of-mass energy of 13$\text{ TeV}$, which corresponds to an integrated luminosity of 5.4$\text{ fb}^{-1}$. The asymmetries are measured in two regions of dimuon mass near the $ϕ$-meson mass peak. The dimuon-mass integrated results are \begin{align*} A_{CP} &= (-1.1 \pm 4.0 \pm 0.5)\%,\\ ΣA_{\text{FB}} &= (\phantom{-}3.9 \pm 4.0 \pm 0.6)\%,\\ ΔA_{\text{FB}} &= (\phantom{-}3.1 \pm 4.0 \pm 0.4)\%, \end{align*} where the first uncertainty is statistical and the second systematic. The results are consistent with the conservation of $CP$ symmetry and the Standard Model expectations.

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arXiv2025

Comprehensive analysis of local and nonlocal amplitudes in the $B^0\rightarrow K^{*0}μ^+μ^-$ decay

A comprehensive study of the local and nonlocal amplitudes contributing to the decay $B^0\rightarrow K^{*0}(\to K^+π^-) μ^+μ^-$ is performed by analysing the phase-space distribution of the decay products. The analysis is based on $pp$ collision data corresponding to an integrated luminosity of 8.4fb$^{-1}$ collected by the LHCb experiment. This measurement employs for the first time a model of both one-particle and two-particle nonlocal amplitudes, and utilises the complete dimuon mass spectrum without any veto regions around the narrow charmonium resonances. In this way it is possible to explicitly isolate the local and nonlocal contributions and capture the interference between them. The results show that interference with nonlocal contributions, although larger than predicted, only has a minor impact on the Wilson Coefficients determined from the fit to the data. For the local contributions, the Wilson Coefficient $C_9$, responsible for vector dimuon currents, exhibits a $2.1σ$ deviation from the Standard Model expectation. The Wilson Coefficients $C_{10}$, $C_{9}'$ and $C_{10}'$ are all in better agreement than $C_{9}$ with the Standard Model and the global significance is at the level of $1.5σ$. The model used also accounts for nonlocal contributions from $B^{0}\to K^{*0}\left[τ^+τ^-\to μ^+μ^-\right]$ rescattering, resulting in the first direct measurement of the $b sττ$ vector effective-coupling $C_{9τ}$.

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arXiv2025

Search for $D^0$ meson decays to $π^+ π^- e^+ e^-$ and $K^+ K^- e^+ e^-$ final states

A search for $D^0$ meson decays to the $π^+π^-e^+e^-$ and $K^+K^-e^+e^-$ final states is reported using a sample of proton-proton collisions collected by the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 6 fb$^{-1}$. The decay $D^0 \rightarrow π^+π^-e^+e^-$ is observed for the first time when requiring that the two electrons are consistent with coming from the decay of a $ϕ$ or $ρ^0/ω$ meson. The corresponding branching fractions are measured relative to the $D^0 \rightarrow K^-π^-[e^+e^-]_{ρ^0/ω}$ decay, where the two electrons are consistent with coming from the decay of a $ρ^0$ or $ω$ meson. No evidence is found for the $D^0 \rightarrow K^+K^-e^+e^-$ decay and world-best limits are set on its branching fraction. The results are compared to, and found to be consistent with, the branching fractions of the $D^0 \rightarrow π^+π^-μ^+μ^-$ and $D^0 \rightarrow K^+K^-μ^+μ^-$ decays recently measured by LHCb and confirm lepton universality at the current precision.

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arXiv2025

Study of light-meson resonances decaying to $K^0_{\rm S} K π$ in the $B \to (K^0_{\rm S} K π) K$ channels

A study is presented of $B^+ \to K^0_{\rm S} K^- π^+ K^-$ and $B^+ \to K^0_{\rm S} K^+ π^- K^+$ decays based on the analysis of 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 $9 fb^{-1}$. The $K^0_{\rm S} K π$ invariant-mass distributions of both $B^+$ decay modes show, in the $m(K^0_{\rm S} K π)<1.85$ GeV mass region, large activity which is resolved using an amplitude analysis. A simple model, were $J^{PC}$ amplitudes are described by multiple Breit--Wigner functions with appropriate angular distributions provides a good description of the experimental data. In this approach a complex mixture of $J^{PC}=0^{-+}, 1^{++}$ and $1^{+-}$ amplitudes is observed that is dominated by $η(1405)$, $η(1470)$, $η(1760)$, $f_1(1285)$, $f_1(1420)$ and $h_1(1405)$ resonances. The $K^0_{\rm S} K π$ Dalitz plots are dominated by asymmetric crossing $K^* \bar{K}$ bands which are different for the two $B^+$ decay modes. This is due to a different interference pattern between the $1^{++}$ and $1^{+-}$ amplitudes in the two channels. Branching fractions are measured for each resonant contribution.

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arXiv2025

Measurement of the multiplicity dependence of $\mitΥ$ production ratios in $pp$ collisions at $\sqrt{s}=13$ TeV

The $\mitΥ(\mathrm{2}S)$ and $\mitΥ(\mathrm{3}S)$ production cross-sections are measured relative to that of the $\mitΥ(\mathrm{1}S)$ meson, as a function of charged-particle multiplicity in proton-proton collisions at a centre-of-mass energy of $13$ TeV. The measurement uses data collected by the LHCb experiment in 2018 corresponding to an integrated luminosity of 2 $\text{fb}^{-1}$. Both the $\mitΥ(\mathrm{2}S)$-to-$\mitΥ(\mathrm{1}S)$ and $\mitΥ(\mathrm{3}S)$-to-$\mitΥ(\mathrm{1}S)$ cross-section ratios are found to decrease significantly as a function of event multiplicity, with the $\mitΥ(\mathrm{3}S)$-to-$\mitΥ(\mathrm{1}S)$ ratio showing a steeper decline towards high multiplicity. This hierarchy is qualitatively consistent with the comover model predictions, indicating that final-state interactions play an important role in bottomonia production in high-multiplicity events.

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arXiv2025

Branching fraction measurement of the decay $B^+ \to ψ(2S) ϕ(1020) K^+$

The branching fraction of the decay $B^+\to ψ(2S)ϕ(1020)K^+$, relative to the topologically similar decay $B^+\to J/ψϕ(1020) K^+$, is measured using proton-proton collision data 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 ratio is found to be $0.061 \pm 0.004 \pm 0.009$, where the first uncertainty is statistical and the second systematic. Using the world-average branching fraction for $B^+ \to J/ψϕ(1020) K^+$, the branching fraction for the decay $B^+\to ψ(2S) ϕ(1020) K^+$ is found to be $ (3.0 \pm 0.2 \pm 0.5 \pm 0.2) \times 10^{-6}$, where the first uncertainty is statistical, the second systematic, and the third is due to the branching fraction of the normalization channel.

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arXiv2025

Test of lepton flavour universality with $B^+ \to K^+π^+π^-\ell^+\ell^-$ decays

The first test of lepton flavor universality between muons and electrons using $B^+ \to K^+π^+π^-\ell^+\ell^-$ ($\ell=e,μ$) decays is presented. The measurement is performed with 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 ratio of branching fractions between $B^+ \to K^+π^+π^-e^+e^-$ and $B^+ \to K^+π^+π^-μ^+μ^-$decays is measured in the dilepton invariant-mass-squared range $1.1 < q^2 < 7.0~\mathrm{GeV}^2/c^4$ and is found to be $R_{Kππ}^{-1} = 1.31^{+0.18}_{-0.17} \;(\mathrm{stat})\;^{+0.12}_{-0.09} \;(\mathrm{syst})$, in agreement with the standard model prediction. The first observation of the $B^+ \to K^+π^+π^-e^+e^-$ decay is also reported.

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arXiv2025

Measurement of $CP$ asymmetries in $Λ_b^0\to ph^{-}$ decays

A search for $CP$ violation in $Λ_b^0\rightarrow pK^-$ and $Λ_b^0\rightarrow pπ^-$ decays is presented using the full Run 1 and Run 2 data samples of $pp$ collisions collected with the LHCb detector, corresponding to an integrated luminosity of 9 $\mathrm{fb}^{-1}$ at center-of-mass energies of 7, 8, and 13 TeV. For the Run 2 data sample, the $CP$-violating asymmetries are measured to be $A_{CP}^{pK^-} = (-1.4 \pm 0.7 \pm 0.4)\%$ and $A_{CP}^{pπ^-} = (0.4 \pm 0.9 \pm 0.4)\%$, where the first uncertainty is statistical and the second is systematic. Following significant improvements in the evaluation of systematic uncertainties compared to the previous LHCb measurement, the Run 1 dataset is reanalyzed to update the corresponding results. When combining the Run 2 and updated Run 1 measurements, the final results are found to be $A_{CP}^{pK^-} = (-1.1 \pm 0.7 \pm 0.4)\%$ and $A_{CP}^{pπ^-} = (0.2 \pm 0.8 \pm 0.4)\%$, constituting the most precise measurements of these asymmetries to date.

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arXiv2025

Amplitude analysis of $B^+ \to ψ(2S) K^+ π^+ π^-$ decays

The first full amplitude analysis of $B^+ \to ψ(2S) K^+ π^+ π^-$ decays is performed using proton-proton collision data corresponding to an integrated luminosity of $9\,\text{fb}^{-1}$ recorded with the LHCb detector. The rich $K^+ π^+ π^-$ spectrum is studied and the branching fractions of the resonant substructure associated with the prominent $K_1(1270)^+$ contribution are measured. The data cannot be described by conventional strange and charmonium resonances only. An amplitude model with 53 components is developed comprising 11 hidden-charm exotic hadrons. New production mechanisms for charged charmonium-like states are observed. Significant resonant activity with spin-parity $J^P = 1^+$ in the $ψ(2S) π^+$ system is confirmed and a multi-pole structure is demonstrated. The spectral decomposition of the $ψ(2S) π^+ π^-$ invariant-mass structure, dominated by $X^0 \to ψ(2S) ρ(770)^0$ decays, broadly resembles the $J/ψϕ$ spectrum observed in $B^+ \to J/ψϕK^+$ decays. Exotic $ψ(2S) K^+ π^-$ resonances are observed for the first time.

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arXiv2025

Test of lepton flavour universality with $B_s^0 \rightarrow ϕ\ell^+\ell^-$ decays

Lepton flavour universality in rare $b\rightarrow s$ 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$\,{\rm fb}^{-1}$. Branching fraction ratios between the $B_s^0 \rightarrow ϕe^+e^-$ and $B_s^0 \rightarrow ϕμ^+μ^-$ 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 $15 < q^2 < 19\,{\rm GeV}^2/c^4$. The results agree with the Standard Model expectation of lepton flavour universality.

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arXiv2025

Observation of the open-charm tetraquark candidate $T_{cs 0}^{*}(2870)^0$ in the $B^- \rightarrow D^- D^0 K_\mathrm{S}^0$ decay

An amplitude analysis of $B^-\rightarrow D^- D^0 K_\mathrm{S}^0$ decays is performed using proton-proton collision data, corresponding to an integrated luminosity of $9\,\text{fb}^{-1}$, collected with the LHCb detector at center-of-mass energies of 7, 8, and 13$\mathrm{\,Te\kern -0.1em V}$. A resonant structure of spin-parity $0^+$ is observed in the $D^0 K_\mathrm{S}^0$ invariant-mass spectrum with a significance of $5.3\,σ$. The mass and width of the state, modeled with a Breit$-$Wigner lineshape, are determined to be $2883\pm11\pm8\mathrm{\,Me\kern -0.1em V\!/}c^2$ and $87_{-47}^{+22}\pm17\mathrm{\,Me\kern -0.1em V}$ respectively, where the first uncertainties are statistical and the second systematic. These properties and the quark content are consistent with those of the open-charm tetraquark candidate $T_{cs 0}^{*}(2870)^0$ observed previously in the $D^+ K^-$ final state of the $B^-\rightarrow D^- D^+ K^-$ decay. This result confirms the existence of the $T_{cs 0}^{*}(2870)^0$ state in a new decay mode. The $T_{cs1}^{*}(2900)^0$ state, reported in the $B^-\rightarrow D^- D^+ K^-$ decay, is also searched for in the $D^0 K_\mathrm{S}^0$ invariant-mass spectrum of the $B^- \rightarrow D^- D^0 K_\mathrm{S}^0$ decay, without finding evidence for it.

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arXiv2025

First determination of the spin-parity of $Ξ_{c}(3055)^{+,0}$ baryons

The ${Ξ_{b}^{0(-)}\toΞ_{c}(3055)^{+(0)}(\to D^{+(0)}Λ)π^{-}}$ decay chains are observed, and the spin-parity of $Ξ_{c}(3055)^{+(0)}$ baryons is determined for the first time. The measurement is performed using proton-proton collision data at a center-of-mass energy of $\sqrt{s}=13\,\text{TeV}$, corresponding to an integrated luminosity of $5.4\,\text{fb}^{-1}$, recorded by the~$\text{LHCb}$ experiment between 2016 and 2018. The spin-parity of the $Ξ_{c}(3055)^{+(0)}$ baryons is determined to be $3/2^{+}$ with a significance of more than $6.5σ$ ($3.5σ$) compared to all other tested hypotheses. The up-down asymmetries of the ${Ξ_{b}^{0(-)}\toΞ_{c}(3055)^{+(0)}π^{-}}$ transitions are measured to be $-0.92\pm0.10\pm0.05$ ($-0.92\pm0.16\pm0.22$), consistent with maximal parity violation, where the first uncertainty is statistical and the second is systematic. These results support the hypothesis that the $Ξ_{c}(3055)^{+(0)}$ baryons correspond to the first $D$-wave $λ$-mode excitation of the $Ξ_{c}$ flavor triplet.

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arXiv2025

Measurement of $ϕ(1020)$ meson production in fixed-target $\textit{p}$Ne collisions at $\sqrt{s_{NN}}$ = 68.5 GeV

The first measurement of $ϕ(1020)$ meson production in fixed-target $p$Ne collisions at $\sqrt{s_{NN}}=68.5$ GeV is presented. The $ϕ(1020)$ mesons are reconstructed in their $K^{+}K^{-}$ decay in a data sample consisting of proton collisions on neon nuclei at rest, corresponding to an integrated luminosity of $21.7 \pm 1.4$ nb$^{-1}$, collected by the LHCb detector at CERN. The $ϕ(1020)$ production cross-section in the centre-of-mass rapidity range of $-1.8<y^*<0$ and transverse momentum range of $800<p_{T}<6500$ MeV/c is found to be $σ=182.7\pm2.7~\text{(stat.)}\pm14.1~\text{(syst)}~μ$b/nucleon. A double-differential measurement of the cross-section is also provided in four regions of rapidity and six regions of transverse momentum of the $ϕ(1020)$ meson and compared with the predictions from Pythia and EPOS4, which are found to underestimate the experimental values.

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arXiv2025

First evidence for direct CP violation in beauty to charmonium decays

The $C\!P$ asymmetry and branching fraction of the CKM-suppressed decay $B^+\!\to J\mskip -3mu/\mskip -2muψ\,π^+$ are precisely measured relative to the favoured decay $B^+\!\to J\mskip -3mu/\mskip -2muψ\,K^+$, using a sample of proton-proton collision data corresponding to an integrated luminosity of $5.4~\mathrm{fb}^{-1}$ recorded at center-of-mass energy of $13~\mathrm{TeV}$ during 2016--2018. The results of the $C\!P$ asymmetry difference and branching fraction ratio are \begin{align*} Δ\mathcal{A}^{C\!P} &\equiv \mathcal{A}^{C\!P}(B^+ \to J\mskip -3mu/\mskip -2muψ\,π^+) - \mathcal{A}^{C\!P}(B^+ \to J\mskip -3mu/\mskip -2muψ\,K^+) = (1.29 \pm 0.49 \pm 0.08) \times 10^{-2}, \end{align*} \begin{equation*} \mathcal{R}_{π/K} \equiv \frac{\mathcal{B}(B^+ \!\to J\mskip -3mu/\mskip -2muψ\,π^+)}{\mathcal{B}(B^+ \!\to J\mskip -3mu/\mskip -2muψ\,K^+)} = (3.852 \pm 0.022 \pm 0.018) \times 10^{-2}. \end{equation*} where the first uncertainties are statistical and the second systematic. A combination with previous LHCb results based on data collected at $7$ and $8~\mathrm{TeV}$ in 2011 and 2012 yields $Δ\mathcal{A}^{C\!P} = (1.42 \pm 0.43 \pm 0.08) \times 10^{-2}$ and $\mathcal{R}_{π/K} = (3.846 \pm 0.018 \pm 0.018) \times 10^{-2}$. The combined $Δ\mathcal{A}^{C\!P}$ value deviates from zero by 3.2 standard deviations, providing the first evidence for direct $C\!P$ violation in the amplitudes of beauty decays to charmonium final states.

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arXiv2025

Constraints on the photon polarisation in $b \to s γ$ transitions using $B_s^0 \rightarrow ϕe^+e^-$ decays

An angular analysis of the $B_s^0 \rightarrow ϕe^+e^-$ decay is performed using the proton-proton collision dataset collected between 2011 and 2018 by the LHCb experiment, corresponding to an integrated luminosity of $9\,{\rm fb}^{-1}$ at centre-of-mass energies of 7, 8 and $13\,{\rm TeV}$. The analysis is performed in the very low dielectron invariant mass-squared region between $0.0009$ and $0.2615\,{\rm GeV}^2\!/c^4$. The longitudinal polarisation fraction of the $ϕ$ meson is measured to be less than $11.5\%$ at $90\%$ confidence level. The $A_{\mathrm{T}}^{\mathcal{R}e C\!P}$ observable, which is related to the lepton forward-backward asymmetry, is measured to be $0.116 \pm 0.155 \pm 0.006$, where the first uncertainty is statistical and the second systematic. The transverse asymmetries, $A_{\mathrm{T}}^{(2)}$ and $A_{\mathrm{T}}^{\mathcal{I}m C\!P}$, which are sensitive to the virtual photon polarisation, are found to be $-0.045 \pm 0.235 \pm 0.014$ and $0.002 \pm 0.247 \pm 0.016$, respectively. The results are consistent with Standard Model predictions.

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arXiv2025

Measurement of the CKM angle $γ$ in $B^{\pm} \to D K^*(892)^{\pm}$ decays

Measurements of $CP$ observables and the CKM angle $γ$ are performed in $B^{\pm} \to D K^*(892)^{\pm}$ decays, where $D$ represents a superposition of $D^0$ and $\overline{D}{}^0$ states, using the LHCb dataset collected during Run 1 (2011-2012) and Run 2 (2015-2018). A study of this channel is presented with the $D$ meson reconstructed in two-body final states $K^{\pm}π^{\mp}$, $K^+K^-$ and $π^+π^-$; four-body final states $K^{\pm}π^{\mp}π^{\pm}π^{\mp}$ and $π^+π^-π^+π^-$; and three-body final states $K^0_{S} π^+π^-$ and $K^0_{S} K^+ K^-$. This analysis includes the first observation of the suppressed $B^{\pm} \to [π^{\pm}K^{\mp}]_D K^{*\pm}$ and $B^{\pm} \to [π^{\pm}K^{\mp}π^{\pm}π^{\mp}]_D K^{*\pm}$ decays. The combined result gives $γ=(63\pm 13)^\circ$.

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arXiv2025

Measurement of exclusive $J/ψ$ and $ψ(2S)$ production at $\sqrt{s}=13$ TeV

Measurements are presented of the cross-section for the central exclusive production of $J/ψ\toμ^+μ^-$ and $ψ(2S)\toμ^+μ^-$ processes in proton-proton collisions at $\sqrt{s} = 13 $ TeV with 2016-2018 data. They are performed by requiring both muons to be in the LHCb acceptance (with pseudorapidity $2<η_{μ^\pm} < 4.5$) and mesons in the rapidity range $2.0 < y < 4.5$. The integrated cross-section results are \begin{equation*} σ_{J/ψ\toμ^+μ^-}(2.0<y_{J/ψ}<4.5,2.0<η_{μ^\pm} < 4.5) = 400 \pm 2 \pm 5 \pm 12 \,{\rm pb}\,, \end{equation*} \begin{equation*} σ_{ψ(2S)\toμ^+μ^-}(2.0<y_{ψ(2S)}<4.5,2.0<η_{μ^\pm} < 4.5) = 9.40 \pm 0.15 \pm 0.13 \pm 0.27 \,{\rm pb}\,, \end{equation*} where the uncertainties are statistical, systematic and due to the luminosity determination. In addition, a measurement of the ratio of $ψ(2S)$ and $J/ψ$ cross-sections, at an average photon-proton centre-of-mass energy of 1 TeV, is performed, giving \begin{equation*} \frac{σ_{ψ(2S)}}{σ_{J/ψ}} = 0.1763 \pm 0.0029 \pm 0.0008 \pm 0.0039 \,, \end{equation*} where the first uncertainty is statistical, the second systematic and the third due to the knowledge of the involved branching fractions. For the first time, the dependence of the $J/ψ$ and $ψ(2S)$ cross-sections on the total transverse momentum transfer is determined in $pp$ collisions and is found consistent with the behaviour observed in electron-proton collisions.

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arXiv2025

Long-lived particle reconstruction downstream of the LHCb magnet

Charged-particle trajectories are usually reconstructed with the LHCb detector using combined information from the tracking devices placed upstream and downstream of the 4\,T\,m dipole magnet. Trajectories reconstructed using only information from the tracker downstream of the dipole magnet, which are referred to as T tracks, have not been used for physics analysis to date. The challenges of the reconstruction of long-lived particles using T tracks for use in physics analyses are discussed and solutions are proposed. The feasibility and the tracking performance are studied using samples of long-lived $\varLambda$ and $K_S^0$ hadrons decaying between 6.0 and 7.6 metres downstream of the proton-proton collision point, thereby traversing most of the magnetic field region and providing maximal sensitivity to magnetic and electric dipole moments. The reconstruction can be expanded upstream to about 2.5 meters for use in direct searches of exotic long-lived particles. The data used in this analysis have been recorded between 2015 and 2018 and correspond to an integrated luminosity of 6\,fb$^{-1}$. The results obtained demonstrate the possibility to further extend the decay volume and the physics reach of the LHCb experiment.

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