LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2025). Amplitude analysis of B+ → ψ(2S)K+ π+ π- decays. J. High Energy Phys., 01(1), 054–69pp.
Abstract: The first full amplitude analysis of B+ -> psi(2S)K+pi(+)pi(-) decays is performed using proton-proton collision data corresponding to an integrated luminosity of 9 fb(-1) recorded with the LHCb detector. The rich K+pi(+)pi(-) 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 psi(2S)pi(+) system is confirmed and a multi-pole structure is demonstrated. The spectral decomposition of the psi(2S)pi(+)pi(-) invariant-mass structure, dominated by X-0 -> psi(2S)rho(770)(0) decays, broadly resembles the J/psi phi spectrum observed in B+ -> J/psi phi K+ decays. Exotic psi(2S)K+pi(-) resonances are observed for the first time.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2024). Observation of muonic Dalitz decays of chib mesons and precise spectroscopy of hidden-beauty states. J. High Energy Phys., 10(10), 122–28pp.
Abstract: The decays of the chi(b1)(1P), chi(b2)(1P), chi(b1)(2P) and chi(b2)(2P) mesons into the Upsilon(1S)mu(+)mu(-) final state are observed with a high significance using proton-proton collision data collected with the LHCb detector and corresponding to an integrated luminosity of 9 fb(-1). The newly observed decays together with the Upsilon(2S) -> Upsilon(1S)pi(+)pi(-) and Upsilon(3S) -> Upsilon(2S)pi(+)pi(-) decay modes are used for precision measurements of the mass and mass splittings for the hidden-beauty states.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2024). Probing the nature of the χc1(3872) state using radiative decays. J. High Energy Phys., 11(11), 121–31pp.
Abstract: The radiative decays chi(c1)(3872) -> (2S) gamma and chi(c1)(3872) -> J/gamma are used to probe the nature of the chi(c1)(3872) state using proton-proton collision data collected with the LHCb detector, corresponding to an integrated luminosity of 9 fb(-1). Using the B+ -> chi(c1)(3872)K+ decay, the chi(c1)(3872) -> psi(2S) gamma process is observed for the first time and the ratio of its partial width to that of the chi(c1)(3872) -> J/psi gamma decay is measured to be Gamma(chi c1(3872))->psi(2S)gamma/Gamma(chi c1(3872)) -> J/psi gamma = 1.67 +/- 0.21 +/- 0.12 +/- 0.04, where the first uncertainty is statistical, the second systematic and the third is due to the uncertainties on the branching fractions of the psi(2S) and J/psi mesons. The measured ratio makes the interpretation of the chi(c1)(3872) state as a pure D-0(D) over bar (*0) + (D) over bar D-0(*0) molecule questionable and strongly indicates a sizeable compact charmonium or tetraquark component within the chi(c1)(3872) state.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2024). Study of the rare decay J/ψ → μ+μ-μ+μ-. J. High Energy Phys., 12(12), 062–25pp.
Abstract: The rare electromagnetic J/psi -> mu(+)mu(-)mu(+)mu(-) decay is observed with a significance greatly exceeding the discovery threshold, using proton-proton collision data collected by the LHCb experiment during 2016-2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb(-1). The rate of this decay is measured relative to that of the J/psi -> mu(+)mu(-) mode. Using the QED model for the four-muon decay in the efficiency estimation, its branching fraction is determined to be B(J/psi -> mu(+)mu(-)mu(+)mu(-)) = (1.13 +/- 0.10 +/- 0.05 +/- 0.01) x 10(-6), where the uncertainties are statistical, systematic and due to the uncertainty on the branching fraction of the J/psi -> mu(+)mu(-) decay.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2024). Study of b-hadron decays to Λc+h-h′- final states. J. High Energy Phys., 08(8), 132–32pp.
Abstract: Decays of Xi(-)(b) and Omega(-)(b) baryons to Lambda(+)(c)h(-)h(-) final states, with h(-)h '(-) being pi(-)pi(-), K-pi(-) and K-K- meson pairs, are searched for using data collected with the LHCb detector. The data sample studied corresponds to an integrated luminosity of 8.7 fb(-1) of pp collisions collected at centre-of-mass energies root s = 7, 8 and 13 TeV. The products of the relative branching fractions and fragmentation fractions for each signal mode, relative to the B- -> Lambda(+)(c)(p) over bar pi(-) mode, are measured, with Xi(-)(b) -> Lambda K-+(c)-pi(-), Xi(-)(b) -> Lambda+cK-K- and Omega(-)(b) -> Lambda+cK-K- decays being observed at over 5 sigma significance. The Xi(-)(b) -> Lambda K-+(c)-pi(-) mode is also used to measure the Xi(-)(b) production asymmetry, which is found to be consistent with zero. In addition, the B- -> Lambda(+)(c)(p) over barK(-) decay is observed for the first time, and its branching fraction is measured relative to that of the B- -> Lambda(+)(c)(p) over bar pi(-) mode.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2024). Measurement of the effective leptonic weak mixing angle. J. High Energy Phys., 12(12), 026–28pp.
Abstract: Using pp collision data at root s = 13TeV, recorded by the LHCb experiment between 2016 and 2018 and corresponding to an integrated luminosity of 5.4 fb-1, the forward-backward asymmetry in the pp -> Z/gamma* -> mu(+) mu(-) process is measured. The measurement is carried out in ten intervals of the difference between the muon pseudorapidities, within a fiducial region covering dimuon masses between 66 and 116 GeV, muon pseudorapidities between 2.0 and 4.5 and muon transverse momenta above 20 GeV. These forward-backward asymmetries are compared with predictions, at next-to-leading order in the strong and electroweak couplings. The measured effective leptonic weak mixing angle is sin(2) theta(l)(eff) = 0.23147 +/- 0.00044 +/- 0.00005 +/- 0.00023, where the first uncertainty is statistical, the second arises from systematic uncertainties associated with the asymmetry measurement, and the third arises from uncertainties in the fit model used to extract sin(2) theta(l)(eff) from the asymmetry measurement. This result is based on an arithmetic average of results using the CT18, MSHT20, and NNPDF31 parameterisations of the proton internal structure, and is consistent with previous measurements and with predictions from the global electroweak fit.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2024). Measurement of CP Violation Observables in D+ → K- K+ π+ Decays. Phys. Rev. Lett., 133(25), 251801–13pp.
Abstract: A search for violation of the charge-parity (CP) symmetry in the D+ -> K- K+ pi(+) decay is presented, with proton-proton collision data corresponding to an integrated luminosity of 5.4 fb(-1), collected at a center-of-mass energy of 13 TeV with the LHCb detector. A novel model-independent technique is used to compare the D+ and D- phase-space distributions, with instrumental asymmetries subtracted using the D-s(+) -> K- K+ pi(+) decay as a control channel. The p value for the hypothesis of CP conservation is 8.1%. The CP asymmetry observables A(CP vertical bar S)(phi pi+) = (0.95 +/- 0.43(stat) +/- 0.26(syst)) x 10(-3) and A(CP vertical bar S)((K) over bar *0 K+) = (-0.26 +/- 0.56(stat) +/- 0.18(syst)) x 10(-3) are also measured. These results show no evidence of CP violation and represent the most sensitive search performed through the phase space of a multibody decay.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2024). Measurement of Λ0b, Λ+c, and Λ Decay Parameters Using Λ0b →Λ+c h- Decays. Phys. Rev. Lett., 133(26), 261804–15pp.
Abstract: A comprehensive study of the angular distributions in the bottom-baryon decays Lambda(0)(b) ->Lambda(+)(c)h(-) (h = pi,K) , followed by Lambda(0)(c) ->Lambda h(+) with Lambda -> p pi(-) or Lambda(+)(c)-> pK(S)(0) decays, is performed using a data sample of proton-proton collisions corresponding to an integrated luminosity of 9 fb(-1) collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV. The decay parameters and the associated charge-parity (CP) asymmetries are measured, with no significant CP violation observed. For the first time, Lambda(0)(b) ->Lambda(+)(c)h(-) decay parameters are measured. The most precise measurements of the decay parameters alpha, ss, and are obtained for Lambda(+)(c) decays and an independent measurement of the decay parameters for the strange-baryon Lambda decay is provided. The results deepen our understanding of weak decay dynamics in baryon
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2025). Long-lived particle reconstruction downstream of the LHCb magnet. Eur. Phys. J. C, 85(1), 7–30pp.
Abstract: 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 with T tracks for physics use are discussed and solutions are proposed. The feasibility and the tracking performance are studied using samples of long-lived Lambda and K-S(0) hadrons decaying between 6.0 and 7.6 m 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 m 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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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2025). Search for charge-parity violation in semileptonically tagged D0 → K+ π- decays. J. High Energy Phys., 03(3), 149–30pp.
Abstract: An analysis of the flavour oscillations of the charmed neutral meson is presented. The ratio of D-0 -> K+ pi(-) and D-0 -> K- pi(+) decay rates is measured as a function of the decay time of the D-0 meson and compared with the charge-conjugated system to search for charge-parity violation. The meson flavour at production is double-tagged by the charges of the muon and pion in the preceding (B) over bar -> D*(2010)(+) mu(-) X and D*(2010)(+) -> D-0 pi(+) decays, respectively. These decays are selected from proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of 13TeV and corresponding to an integrated luminosity of 5.4 fb(-1). The flavour oscillation parameters, relating to the differences in mass and width of the mass eigenstates, are found to be y' = (5.8 +/- 1.6) x10(-3) and (x')(2) = (0.0 +/- 1.2) x10(-4). No evidence for charge-parity violation is seen either in the flavour oscillations or in the decay, where the direct charge-parity asymmetry is measured to be A(D) = (2.3 +/- 1.7)%.
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