Observation of excited Lambda_b0 baryons

Using pp collision data corresponding to 1.0 fb − 1 integrated luminosity collected by the LHCb detector, two narrow states are observed in the Λ 0 b π + π − spectrum with masses 5911 . 97 ± 0 . 12(stat) ± 0 . 02(syst) ± 0 . 66( Λ 0 b mass) MeV /c 2 and 5919 . 77 ± 0 . 08(stat) ± 0 . 02(syst) ± 0 . 66( Λ 0 b mass) MeV /c 2 . The signiﬁcances of the observations are 5 . 2 and 10 . 2 standard deviations, respectively. These states are interpreted as the orbitally excited Λ 0 b baryons, Λ ∗ 0 b (5912) and Λ ∗ 0 b (5920).

The system of baryons containing a b quark (beauty baryons) remains largely unex- have so far been observed via their decay to Λ 0 b π ± final states [7,8]. 10 None of the quantum numbers of beauty baryons have been measured.

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The quark model predicts the existence of two orbitally excited Λ 0 b states, Λ * 0 b , with 12 the quantum numbers J P = 1/2 − and 3/2 − , respectively, that should decay to Λ 0 b π + π − or  The LHCb detector [16] is a single-arm forward spectrometer covering the pseudo-24 rapidity range 2 < η < 5, designed for the study of particles containing b or c quarks.

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The detector includes a high precision tracking system consisting of a silicon-strip vertex  scale has been calibrated using J/ψ → µ + µ − decays, and its accuracy has been quantified 67 with other two-body resonance decays (  retained. From the simulation study, this requirement is optimal for the observation of a 92 narrow state near the kinematic threshold with signal-to-background ratio around one.

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The fit of the Λ + c π − mass spectrum ( Fig. 1) indicates the presence of the background Alternatively, its rate can be estimated from the ratio of B + → D 0 K + and B + → D 0 π + 96 decays that equals to 8% [18]. Due to the Λ 0 b mass constraint in the kinematic fit, the 97 Λ 0 b π + π − invariant mass distribution for this mode is biased by less than 0.1 MeV/c 2 if 98 reconstructed under the Λ + c π − mass hypothesis, and has a resolution only a factor of two 99 worse than that with the Λ + c π − signal. After the kinematic fit quality requirement, the to 8%. This mode is thus not treated separately, and its effect is taken into account as a 102 part of the systematic uncertainty due to the signal shape.

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Combinations of Λ 0 b candidates with both opposite-sign and same-sign slow pions are 104 selected in data. The latter are used to constrain the background shape coming from 105 random combinations of Λ 0 b baryon and two tracks. The assumption that the shape of the 106 background in Λ 0 b π + π − and Λ 0 b π ± π ± modes is the same is validated with simulation. The 107 Λ 0 b π + π − and Λ 0 b π ± π ± invariant mass spectra are shown in Fig. 2; two narrow structures 108 with masses around 5912 and 5920 MeV/c 2 are evident in the Λ 0 b π + π − spectrum. They are interpreted as the orbitally excited Λ 0 b states, and are denoted hereafter as Λ * 0 b (5912) 110 and Λ * 0 b (5920).

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The invariant mass of the two pions, M (π + π − ), in the is shown in Fig. 3. The background is subtracted using the sWeights procedure [27].

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The weights are calculated from the fit to Λ 0 b π + π − invariant mass distribution, which is 144 practically uncorrelated with M (π + π − ). The M (π + π − ) distribution is consistent with   and taking the largest difference from the nominal fit result as systematic uncertainty.

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The effect of the momentum scale correction is evaluated by varying the scale coefficient 164 by its relative uncertainty 5 × 10 −4 in simulated signal samples.

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The significance of the observation of the two states is evaluated with simulated  In summary, we report the observation of two narrow states in the Λ 0 b π + π − mass