First Determination of the Spin-Parity of Ξ_{c}(3055)^{+,0} Baryons

The Ξ_{b}^{0(-)}→Ξ_{c}(3055)^{+(0)}(→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  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 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(-)}→Ξ_{c}(3055)^{+(0)}π^{-} 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 Ξ_{c}(3055)^{+(0)} baryons correspond to the first D-wave λ-mode excitation of the Ξ_{c} flavor triplet.

Study of Λ_{b}^{0} and Ξ_{b}^{0} Decays to Λh^{+}h^{'-} and Evidence for CP Violation in Λ_{b}^{0}→ΛK^{+}K^{-} Decays

A study of Λ_{b}^{0} and Ξ_{b}^{0} decays to Λh^{+}h^{'-} (h^{(')}=π,K) is performed using pp collision data collected by the LHCb experiment during LHC Runs 1-2, corresponding to an integrated luminosity of 9  fb^{-1}. The branching fractions for these decays are measured using the Λ_{b}^{0}→Λ_{c}^{+}(→Λπ^{+})π^{-} decay as a control channel. The decays Λ_{b}^{0}→Λπ^{+}π^{-} and Ξ_{b}^{0}→ΛK^{-}π^{+} are observed for the first time. For decay modes with sufficient signal yields, CP asymmetries are measured in the full and localized regions of the final-state phase space. Evidence is found for CP violation in the Λ_{b}^{0}→ΛK^{+}K^{-} decay, interpreted as originating primarily from an asymmetric Λ_{b}^{0}→N^{*+}K^{-} decay amplitude. The measured CP asymmetries for other decays are compatible with zero.

Measurement of Λ_{b}^{0}, Λ_{c}^{+}, and Λ Decay Parameters Using Λ_{b}^{0}→Λ_{c}^{+}h^{-} Decays

A comprehensive study of the angular distributions in the bottom-baryon decays Λ_{b}^{0}→Λ_{c}^{+}h^{-}(h=π,K), followed by Λ_{c}^{+}→Λh^{+} with Λ→pπ^{-} or Λ_{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, the Λ_{b}^{0}→Λ_{c}^{+}h^{-} decay parameters are measured. The most precise measurements of the decay parameters α, β, and γ are obtained for Λ_{c}^{+} decays and an independent measurement of the decay parameters for the strange-baryon Λ decay is provided. The results deepen our understanding of weak decay dynamics in baryon decays.

Observation of the Open-Charm Tetraquark Candidate T_{cs0}^{*}(2870)^{0} in the B^{-}→D^{-}D^{0}K_{S}^{0} Decay

An amplitude analysis of B^{-}→D^{-}D^{0}K_{S}^{0} decays is performed using proton-proton collision data, corresponding to an integrated luminosity of 9  fb^{-1}, collected with the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV. A resonant structure of spin-parity 0^{+} is observed in the D^{0}K_{S}^{0} invariant-mass spectrum with a significance of 5.3σ. The mass and width of the state, modeled with a Breit-Wigner line shape, are determined to be 2883±11±8  MeV/c^{2} and 87_{-47}^{+22}±17  MeV, 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_{cs0}^{*}(2870)^{0} observed previously in the D^{+}K^{-} final state of the B^{-}→D^{-}D^{+}K^{-} decay. This result confirms the existence of the T_{cs0}^{*}(2870)^{0} state in a new decay mode. The T_{cs1}^{*}(2900)^{0} state, reported in the B^{-}→D^{-}D^{+}K^{-} decay, is also searched for in the D^{0}K_{S}^{0} invariant-mass spectrum of the B^{-}→D^{-}D^{0}K_{S}^{0} decay, without finding evidence for it.

First Evidence for Direct CP Violation in Beauty to Charmonium Decays

The CP asymmetry and branching fraction of the Cabibbo-Kobayashi-Maskawa-suppressed decay B^{+}→J/ψπ^{+} are precisely measured relative to the favored decay B^{+}→J/ψK^{+} using a sample of proton-proton collision data corresponding to an integrated luminosity of 5.4  fb^{-1} recorded at a center-of-mass energy of 13 TeV during 2016-2018. The results of the CP asymmetry difference and branching fraction ratio are ΔA^{CP}≡A^{CP}(B^{+}→J/ψπ^{+})-A^{CP}(B^{+}→J/ψK^{+})=(1.29±0.49±0.08)×10^{-2}, R_{π/K}≡[B(B^{+}→J/ψπ^{+})/B(B^{+}→J/ψK^{+})]=(3.852±0.022±0.018)×10^{-2}, where the first uncertainties are statistical and the second are systematic. A combination with previous LHCb results based on data collected at 7 and 8 TeV in 2011 and 2012 yields ΔA^{CP}=(1.42±0.43±0.08)×10^{-2} and R_{π/K}=(3.846±0.018±0.018)×10^{-2}. The combined ΔA^{CP} value deviates from zero by 3.2 standard deviations, providing the first evidence for direct CP violation in the amplitudes of beauty decays to charmonium final states.

Study of Ds1(2460)+→Ds+π+π- in B→D¯(*)Ds+π+π- decays

An amplitude analysis of the Ds1(2460)+→Ds+π+π- transition is performed simultaneously in B0→D-Ds+π+π-, B+→D¯0Ds+π+π-, and B0→D*-Ds+π+π- decays. The study is based on a data sample of proton-proton collisions recorded with the LHCb detector at centre-of-mass energies of s=7,8, and 13TeV, corresponding to a total integrated luminosity of 9fb-1. A clear double-peak structure is observed in the m(π+π-) spectrum of the Ds1(2460)+→Ds+π+π- decay. The data can be described either with a model including f0500,f0980, and f2(1270) resonances, in which the contributions of f0(980) and f2(1270) are unexpectedly large, or with a model including f0(500), a doubly charged open-charm tetraquark state Tcs¯++ and its isospin partner Tcs¯0. If the former is considered implausible, the Tcs¯ states are observed with high significance, and the data are consistent with isospin symmetry. When imposing isospin constraints between the two Tcs¯ states, their mass and width are determined to be (2327±13±13) MeV and (96±16-23+170) MeV, respectively, where the first uncertainty is statistical and the second is systematic. The mass is slightly below the DK threshold, and a spin-parity of 0+ is favoured with high significance.