Search for new decays of the Λ0b baryon at the LHCb experiment

High statistics collected by the LHCb experiment between 2011 and 2018 in proton-proton (pp) collisions at the Large Hadron Colllider (LHC), jointly with the large cross-section of bb pairs production provides unique possibility for the search and study new decays of beauty hadrons. The results of search for new decays of the Λb baryons are presented. In particular the decays Λb → Λ+c ppπ−, Λb → J/ψΛ, Λb → ψ(2S)Λ, Λb → ψ(2S)pπ−, Λb → χc1(3872)pK and Λb → Λγ are found and branching fraction ratios measurement for these decays are done. Also the study of the Λbπ +π− systems led to the observation of two new excited Λb states, denoted as Λb(6146) 0 and Λb(6152), are found.


Introduction
fact makes the study of beauty baryons not only an interesting but also a promising task for 4 the LHCb collaboration. 5 The LHCb experiment [1] is one of the four main experiments at the LHC. It is designed to 6 search for indirect manifestations of New Physics beyond the Standard Model (SM) through 7 the study of CP-symmetry violation in decays of particles containing b-and c-quarks. Be-8 sides, there are researches in such areas like measurement of the unitarity triangle angles, 9 search for rare decays of beauty hadrons, study of heavy hadrons properties and search for 14 2 Observation of the Λ 0 b → Λ + c ppπ − decay 15 The study of the decay channel Λ 0 b → Λ + c ppπ − , Λ + c → pK − π + is of high interest due to pos-16 sible charmed dibaryon resonant states contributions. As discussed in reference [2], such  The decay Λ 0 b → Λ + c ppπ − is observed for the first time using pp collision data collected with the LHCb detector at centre-of-mass energies of √ s = 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb −1 . The decay of Λ 0 b → Λ + c π − is used as the normalization channel. In those two decays, the Λ + c baryon is reconstructed as a combination of pK − π + particles. The fit to the mass distribution for the signal channel are shown in figure 1 (left). The signal yields are determined to be 926 ± 43 and (167.00 ± 0.50) × 10 3 for the Λ 0 b → Λ + c ppπ − and Λ 0 b → Λ + c π − decay modes, respectively. The ratio of branching fractions between Λ 0 b → Λ + c ppπ − and Λ 0 b → Λ + c π − decays is measured to be where the first uncertainty is statistical and the second is systematic [3].

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Two resonant structures are observed in the Λ + c π − mass spectrum of the Λ 0 b → Λ + c ppπ − decays, corresponding to the Σ c (2455) 0 and Σ * c (2520) 0 states (see figure 1 (right)) with yields of 59 ± 10 and 104 ± 17, respectively [3]. The ratios of branching fractions with respect to the decay In all of the above results, the first uncertainty is statistical and the second is systematic. The 26 phase space is also examined for the presence of dibaryon resonances, but no evidence for 27 such resonances is found. 28 3 Observation of the Λ 0 b → J/ψ Λ and Λ 0 b → ψ(2S)Λ decays 29 The LHCb collaboration has observed several Λ 0 b → J/ψ X [4][5][6][7][8][9] and Λ 0 b → ψ(2S)X 30 decays [7,10], where X indicates a final-state particle system. The ratios of final-state particle system) provide useful information on the production of charmo- 33 nia in b-hadron decays. These ratios can be used to test factorisation of ampli-34 tudes. The ATLAS collaboration has previously measured the ratio of the branching 36 where in both decays J/ψ and ψ(2S) mesons are reconstructed using µ + µ − fi-37 nal state and Λ hyperon is reconstructed using pπ − final state. This result 38 differs by 2.8σ from a theoretical prediction in the framework of the covari-39 ant quark model, [12,13], and with simi- . 42 The ratio of branching fraction of Λ 0 b → ψ(2S)Λ decay relative to the branching fraction of Λ 0 b → J/ψ Λ decay cancels most experimental uncertainties. A measurement with improved precision helps to better understand this possible discrepancy and sets new constraints on the available form-factor models [13]. This study is based on two data samples containing long and downstream tracks. These two types of tracks are defined as tracks including hits from the vertex detector (long) and tracks that do not include vertex detector hits (downstream) [1]. Using pp collisions corresponding to 3 fb −1 integrated luminosity, recorded by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV, the ratio of branching fractions is determined to be where the first uncertainty is statistical, the second is systematic and the third is related to the 43 uncertainties of the known branching fractions of ψ(2S) and J/ψ mesons to two leptons [15].

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This measurement is compatible within one standard deviation with the measurement from 45 the ATLAS collaboration [11] and has a better precision. It confirms the discrepancy with the 46 covariant quark model theory predictions [12,13] and sets additional constraints on available models. 48 4 Observation of the Λ 0 b → ψ(2S)pπ − decay 49 The Cabibbo-suppressed decay Λ 0 b → ψ(2S)pπ − is observed for the first time using a data sample collected by the LHCb experiment in pp collisions corresponding to 1.0, 2.0 and 1.9 fb −1 of integrated luminosity at centre-of-mass energies of 7, 8 and 13 TeV, respectively. The ψ(2S) mesons are reconstructed in the µ + µ − final state. The branching fraction with respect to that of the Λ 0 b → ψ(2S)pK − decay mode is measured to be where the first uncertainty is statistical and the second is systematic [10]. The mass  constructed using J/ψπ + π − final state. The J/ψ meson is reconstructed using J/ψ → µ + µ − 77 decay mode. Projections of the two-dimensional fit to the J/ψπ + π − pK − and J/ψπ + π − mass 78 distributions for the interval of 3.80 < m J/ψπ + π − < 3.95 GeV/c 2 are shown in figure 3. The 79 signal yields are determined to be 55 ± 11 and 610 ± 30 for the Λ 0 b → χ c1 (3872)pK − and 80 Λ 0 b → ψ(2S)pK − decay modes, respectively. The statistical significance of the observed 81 Λ 0 b → χ c1 (3872)pK − signal is estimated to be 7.2σ.

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Using a data sample of pp collisions corresponding to an integrated luminosity of 1.7 fb −1 collected by the LHCb experiment at a center-of-mass energy of 13 TeV the radiative decay Λ 0 b → Λγ is observed for the first time. The decay B 0 → K * (892) 0 γ is used as the normalization channel. The signal yields are found to be 65 ± 13 (with significance of 5.6σ) and 32670 ± 290 for Λ 0 b → Λγ and B 0 → K * (892) 0 γ decays, respectively. The branching fraction of the Λ 0 b → Λγ decay is measured exploiting the normalization mode and is found to be where the quoted uncertainties are statistical, systematic and systematic from external inputs, 95 respectively [40]. This is the first observation of the radiative decay of beauty baryon. In the constituent quark model [41,42], baryons containing a beauty quark form multiplets 98 according to the internal symmetries of flavour, spin, and parity [43]. Beyond the Λ 0 b baryon, 99 which is the lightest beauty baryon, a rich spectrum of radially and orbitally excited states

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The observation of a new structure in the Λ 0 b π + π − spectrum using the full LHCb data set of pp collisions, corresponding to an integrated luminosity of 9 fb −1 , collected at √ s = 7, 8 and 13 TeV is reported. In summary, a new structure with high statistical significance is observed in the Λ 0 b π + π − mass spectrum using Λ 0 b → Λ + c π − , Λ + c → pK − π + decays, and confirmed using a sample of Λ 0 b baryons reconstructed through the Λ 0 b → J/ψ pK − , J/ψ → µ + µ − decay. The mass distributions for selected Λ 0 b π + π − candidates are shown in figure 4 (left). An analysis of the Λ 0 b π + π − mass spectra for the regions enriched by the Σ ( * )± b resonances suggests the interpretation of the structure as two almost degenerate narrow states, denoted as Λ b (6146) 0 and Λ b (6152) 0 . The Λ 0 b π + π − mass spectra in these three regions are shown in figure 4 (right). The masses and natural widths of these states are measured to be where the first uncertainty is statistical, the second systematic and the third for the mass measurements due to imprecise knowledge of the mass of the Λ 0 b baryon [54]. The mass differences with respect to the Λ 0 b mass are measured to be