Observation of new baryons in the $\Xi_b^-\pi^+\pi^-$ and $\Xi_b^0\pi^+\pi^-$ systems

The first observation and study of two new baryonic structures in the final state $\Xi_b^0\pi^+\pi^-$ and the confirmation of the $\Xi_b(6100)^-$ state in the $\Xi_b^-\pi^+\pi^-$ decay mode are reported using proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of 9$\mathrm{fb}^{-1}$. In addition, the properties of the known $\Xi_b^{*0}$, $\Xi_b^{'-}$ and $\Xi_b^{*-}$ resonances are measured with improved precision. The new decay mode of the $\Xi_b^0$ baryon to the $\Xi_c^+\pi^-\pi^+\pi^-$ final state is observed and exploited for the first time in these measurements.

shown in the supplemental material [34].The decay mode Ξ 0 b → Ξ + c π − π + π − is observed for the first time experimentally.The selection requirement on the BDT classifier output is optimized for the observation of new states and retains 96% of the Ξ − b and 92% of the Ξ 0 b signal candidates.Additional vetoes are imposed, as described in Ref. [35], to suppress other abundant processes with displaced vertices e.g.those coming from D 0 , D + , D + s and Λ + c decays with a misidentified particle.The Ξ baryon candidates constrained to their known values [30] and the Ξ (−,0) b flight direction to originate from the PV [36].2), together with the results of the fit.All signal components are modeled using relativistic Breit-Wigner distributions [37] including Blatt-Weisskopf form factors [38] with a radius of 3 GeV −1 .The orbital angular momentum between the Ξ (−,0) b baryons and the pions is assumed according to the expected spin assignment.The relativistic Breit-Wigner distributions are convolved with functions parameterizing the detector resolution.These resolution models are determined from simulation samples and are consistent with a resolution that scales as √ Q. Simulation shows that for each resonance, the resolution is comparable to or smaller than the measured natural widths of the peaks, with the exception of the Ξ ′− b baryon.The background contribution is parameterized as (Q − d) n , where d and n parameters vary freely in the fit.This function, which is validated using wrong-charge Ξ − b π − and Ξ 0 b π + candidates, is sufficient to describe the smooth background coming from random track combinations.Additional components are included in the fit to the Ξ 0 b π − spectrum to describe partially reconstructed candidates coming from higher mass resonances.These components are referred to as reflections in the rest of this Letter.The reflections of the newly observed states in the Ξ b ππ mode to the Ξ b π spectrum are studied with simulation and dedicated components are included in the fit, modeled as Gaussian functions with power-law tails [39].The means of the reflection components vary freely in the fits to data and their fitted values are consistent with expectations from simulated backgrounds and cross-checks in data.The fit confirms the presence of partially reconstructed Ξ b (6100) − → Ξ * 0 b (Ξ 0 b π 0 )π − decays and shows hints of a contribution from the decay chain , where neither the Ξ b (1P, 1/2) − state, the expected lighter resonance equivalent to that found in the neutral system, nor the Ξ ′0 b state has been observed experimentally to date (Fig. 2).This component has been validated simulating different mass and width hypotheses for the two particles involved, taking into account expected isospin splittings given the masses of their charged partners.However, a precise estimation of the Ξ b (1P, 1/2) − and Ξ ′0 b state  Different sources of systematic uncertainties are considered in the determination of the resonance parameters.All these systematic uncertainties are summarized in Table 1.One of the most important contributions to the mass measurements comes from the knowledge of the momentum scale at LHCb.The associated systematic uncertainty is assigned as the larger of the changes in the measured parameters when the momentum scale is changed by its uncertainty, which is estimated to be 3 × 10 −4 [40].An additional uncertainty arises from the empirical description of the background shapes and is estimated by modeling them with alternative functions.A third uncertainty is assigned by varying the description of the reflection components and their properties, obtained either from simulation or   from data, where relevant.Further sources of uncertainty on the measurement of the natural widths are included to describe the known differences in resolution between data and simulated events.Differences are expected to be within 5%, based on previous studies [41][42][43][44], therefore uncertainties are estimated by varying the resolution function width and the parametrization of the mass resolution function by that amount.Possible uncertainties can arise from the assumed relativistic Breit-Wigner distributions and their parameters.Lower mass states are assumed to decay with angular momentum l = 0, while higher mass states with l = 1.For the newly observed states, the hypotheses assuming l = 0, 2, 3 are tested and the largest shifts of the fitted parameters with respect to the default fit are assigned as systematic uncertainties.A further uncertainty on the baryon mass m 0 is assigned due to the limited knowledge of the Ξ − b and Ξ 0 b baryon masses [30].The numerical results are summarized in Table 2.The properties of the Ξ * 0 b , Ξ ′− b and Ξ * − b baryons are measured with world-leading precision.For the narrow Ξ ′− b state, its natural width is compatible with zero once the systematic uncertainties are considered, and an upper limit < 0.05 MeV is estimated at 90% confidence level.
In summary, the first observation of two new baryons Ξ b (6087) 0 and Ξ b (6095) 0 , with quark content bsu, is reported in the Ξ 0 b π + π − final state.Additionally, this Letter confirms the observation of the Ξ b (6100) − charged state by the CMS collaboration [18], with improved significance and sensitivity on its physical parameters.This measurement uses final states with up to nine tracks, most of which are pions, showing excellent performance of the LHCb tracking, reconstruction and PID systems.Finally, the decay mode respective m Ξ b π mass windows [34].These patterns closely resemble those observed in the Ξ 0 c and Ξ + c baryon systems [30].An interpretation would be that the new states are P -wave states (l = 1 between the b quark and the qs diquark) coupling to the b quark to give a pair of states with J P = 1 GridPP (United Kingdom), CSCS (Switzerland), IFIN-HH (Romania), CBPF (Brazil), Polish WLCG (Poland) and NERSC (USA).We are indebted to the communities behind the multiple open-source software packages on which we depend.Individual groups or members have received support from ARC and ARDC (Australia); Minciencias (Colombia); AvH Foundation (Germany); EPLANET, Marie Sk lodowska-Curie Actions, ERC and NextGenerationEU (European Union); A*MIDEX, ANR, IPhU and Labex P2IO, and Région Auvergne-Rhône-Alpes (France); Key Research Program of Frontier Sciences of CAS, CAS PIFI, CAS CCEPP, Fundamental Research Funds for the Central Universities, and Sci.& Tech.Program of Guangzhou (China); GVA, XuntaGal, GENCAT, Inditex, InTalent and Prog.Atracción Talento, CM (Spain); SRC (Sweden); the Leverhulme Trust, the Royal Society and UKRI (United Kingdom).n Università di Milano Bicocca, Milano, Italy o Università di Padova, Padova, Italy p Università di Perugia, Perugia, Italy q Scuola Normale Superiore, Pisa, Italy r Università di Pisa, Pisa, Italy s Università della Basilicata, Potenza, Italy t Università di Roma Tor Vergata, Roma, Italy u Università di Urbino, Urbino, Italy v Universidad de Alcalá, Alcalá de Henares , Spain w Universidade da Coruña, Coruña, Spain † Deceased

Supplemental material
within a ±75 MeV window around the peak position are combined with one charged pion (two pions) to investigate the Ξ − b π + and Ξ 0 b π − (Ξ − b π + π − and Ξ 0 b π − π + ) mass spectra.In order to improve the mass resolution, the obtained candidates are refitted with the masses of the Ξ Additional requirements are applied to the Ξ b π − π + candidates, where the Ξ * 0 b , Ξ ′− b and Ξ * − b intermediate states are selected according to their observed widths and known mass values [30].Signal mass windows for the Ξ b π intermediate resonances are defined as |m(Ξ b π) − m Ξ * 0 b | < 3 MeV, |m(Ξ b π) − m Ξ ′− b | < 1.25 MeV and |m(Ξ b π) − m Ξ * − b | < 5 MeV, each corresponding to 2.5 σ of the observed experimental peak.The signal yields and lineshape parameters of the signal resonances are determined with an extended unbinned maximum-likelihood fit to the Q-value distributions defined as m Ξ b π − m Ξ b − m π and m Ξ b ππ − m Ξ b − 2m π for Ξ b π and Ξ b ππ decays, respectively.The mass distributions of the Ξ − b π + and Ξ 0 b π − (Ξ − b π + π − and Ξ 0 b π − π + ) samples are shown in Fig. 1 (Fig.
observed for the first time.The properties of the Ξ * 0 b , Ξ ′− b and Ξ * − b baryons are measured with high precision.Determination of the spin and parity for the new baryons is not possible given the low signal yields.However, data indicate that the Ξ b (6100) − baryon decays mainly through the Ξ * 0 b π − state, the Ξ b (6087) 0 baryon mainly through the Ξ ′− b π + state, and the Ξ b (6095) 0 baryon mainly through the Ξ * − b π + state, with no significant contributions to the signals from events outside their

4 and 5
show the mass-difference (Q = m Ξ b ππ −m Ξ b −2m π ) distributions inside and outside the mass windows corresponding to the known Ξ b π intermediate resonances.Simultaneous fits are performed using the fit models presented in the Letter, with the resonance parameters fixed to their nominal values.Yields are allowed to vary in the fit.The observed suppression of decays not proceeding through the respective intermediate resonances confirms the decay pattern described in the text.

Figure 4 :
Figure 4: Distributions of the mass difference Q = m Ξ b ππ − m Ξ b − 2m π for selected Ξ − b π + π − candidates, (a) in the Ξ * 0 b mass window and (b) outside the Ξ * 0 b mass window.The results of the simultaneous fit are superimposed.

Figure 5 :
Figure 5: Distributions of the mass difference Q = m Ξ b ππ − m Ξ b − 2m π for selected Ξ 0 b π + π − candidates, (a) in the Ξ ′− b mass window, (b) in the Ξ * − b mass window and (c) outside the mass windows of the intermediate resonances.The results of the simultaneous fit are superimposed.

Table 1 :
Systematic uncertainties ( MeV) on the measured physical properties.The parameters Q 0 and Γ are the mean and the width of the Breit-Wigner distribution, respectively.

Table 2 :
[30]es and widths of the states considered in this Letter.The first uncertainty is statistical, the second systematic.The third uncertainty on m 0 is due to limited knowledge of the Ξ − b and Ξ 0 b baryon masses[30].