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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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2025). Measurement of CP asymmetry in Bs0 → Ds∓ K± decays. J. High Energy Phys., 03(3), 139–33pp.
Abstract: A measurement of the CP-violating parameters in B-s(0) -> D-s(-/+) K-+/- decays is reported, based on the analysis of proton-proton collision data collected by the LHCb experiment corresponding to an integrated luminosity of 6 fb(-1) at a centre-of-mass energy of 13TeV. The measured parameters are obtained with a decay-time dependent analysis yielding C-f = 0.791 +/- 0.061 +/- 0.022, A(f)(Delta Gamma) = -0.051 +/- 0.134 +/- 0.058, A((f) over bar)(Delta Gamma) = -0.303 +/- 0.125 +/- 0.055, S-f = -0.571 +/- 0.084 +/- 0.023 and S-(f) over bar = -0.503 +/- 0.084 +/- 0.025, where the first uncertainty is statistical and the second systematic. This corresponds to CP violation in the interference between mixing and decay of about 8.6 sigma. Together with the value of the B-s(0) mixing phase -2 beta s, these parameters are used to obtain a measurement of the CKM angle gamma equal to (74 +/- 12)degrees modulo 180 degrees, where the uncertainty contains both statistical and systematic contributions. This result is combined with the previous LHCb measurement in this channel using 3 fb(-1) resulting in a determination of gamma = (81(-11)(+12))degrees.
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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). Measurement of φ(1020) meson production in fixed-target pNe collisions at √sNN=68.5 GeV. J. High Energy Phys., 03(3), 151–21pp.
Abstract: The first measurement of phi(1020) meson production in fixed-target pNe collisions at root s(NN) = 68.5 GeV is presented. The phi(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 +/- 1.4 nb(-1), collected by the LHCb detector at CERN. The phi(1020) production cross-section in the centre-of-mass rapidity range of -1.8 < y* < 0 and transverse momentum range of 800 < p(T) < 6500MeV/ c is found to be sigma = 182.7 +/- 2.7 (stat.) +/- 14.1 (syst) μb/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 phi(1020) meson and compared with the predictions from Pythia and EPOS4, which are found to underestimate the experimental values.
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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). Measurement of CP violation in B0 → D+D- and B0s→ D+ s D- s decays. J. High Energy Phys., 01(1), 061–26pp.
Abstract: A time-dependent, flavour-tagged measurement of CP violation is performed with B-0 -> D+D- and B-s(0)-> D+ (s) D- (s)decays, using data collected by the LHCb detector in proton-proton collisions at a centre-of-mass energy of 13TeV corresponding to an integrated luminosity of 6 fb(-1). In B-0 -> D+D- decays the CP-violation parameters are measured to be S-D(+) (-)(D) = -0.552 +/- 0.100 (stat) +/- 0.010 (syst), C-D(+) (-)(D) = 0.128 +/- 0.103 (stat) +/- 0.010 (syst). In B-s(0)-> D+ (s) D- (s) decays the CP-violating parameter formulation in terms of phi(s) and |lambda| results in phi s = -0.086 +/- 0.106 (stat) +/- 0.028 (syst) rad, |lambda (+)(D) (-)(s D) (s) | = 1.145 +/- 0.126 (stat) +/- 0.031 (syst). These results represent the most precise single measurement of the CP-violation parameters in their respective channels. For the first time in a single measurement, CP symmetry is observed to be violated in B-0 -> D+D- decays with a significance exceeding six standard deviations.
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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). Measurement of the CKM angle γ in B± → DK*(892)± decays. J. High Energy Phys., 02(2), 113–37pp.
Abstract: Measurements of CP observables and the CKM angle gamma are performed in B-+/- -> DK*(892)(+/-) decays, where D represents a superposition of D-0 and (D) over bar (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-+/- pi(+/-), K+ K- and pi(+)pi(-); four-body final states K-+/-pi(-/+) pi(+/-) pi(-/+) and pi(+) pi(-) pi(+)pi(-); and threebody final states K-S(0)pi(+) pi(-) and K-S(0) K+ K-. This analysis includes the first observation of the suppressed B +/-. [pi(+/-) K-+/-](D)K*(+/-) and B +/-. [pi(+/-) K--/+ pi(+/-)pi(-/+)](D)K*(+/-) decays. The combined result gives gamma = (63 +/- 13)degrees.
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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). First Determination of the Spin-Parity of Ξc(3055) +,0 Baryons. Phys. Rev. Lett., 134(8), 081901–13pp.
Abstract: The Xi(0(-))(b) -> Xi(c)(3055)(+(0))(-> D+(0)Lambda)pi(-) decay chains are observed, and the spin-parity of Xi(c)(3055)(+(0)) baryons is determined for the first time. The measurement is performed using protonproton collision data at a center-of-mass energy of root s = 13 TeV, corresponding to an integrated luminosity of 5.4 fb(-1), recorded by the LHCb experiment between 2016 and 2018. The spin-parity of the Xi(c)(3055)(+(0)) baryons is determined to be 3/2(+) with a significance of more than 6.5 sigma (3.5 sigma) compared to all other tested hypotheses. The up-down asymmetries of the Xi(0(-))(b) -> Xi(c)(3055)(+(0))pi(-) transitions are measured to be -0.92 +/- 0.10 +/- 0.05 (-0.92 +/- 0.16 +/- 0.22), consistent with maximal parity violation, where the first uncertainty is statistical and the second is systematic. These results support the hypothesis that the Xi(c)(3055)(+(0)) baryons correspond to the first D-wave lambda-mode excitation of the Xi(c) flavor triplet.
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