EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-EP-2020-233 LHCb-PAPER-2020-039 18 December 2020
Evidence of a J/ψΛ structure
and observation of excited Ξ
−
states
in the Ξ
b
−
→ J/ψΛK
−
decay
LHCb collaboration† Abstract
First evidence of a structure in the J/ψΛ invariant mass distribution is obtained
from an amplitude analysis of Ξb−→ J/ψΛK− decays. The observed structure is
consistent with being due to a charmonium pentaquark with strangeness. Its mass
and width are determined to be 4458.8 ± 2.9+4.7−1.1MeV and 17.3 ± 6.5+8.0−5.7MeV, where
the quoted uncertainties are statistical and systematic, respectively. The structure is also consistent with being due to two resonances. In addition, the narrow excited
Ξ− states, Ξ(1690)− and Ξ(1820)−, are seen for the first time in a Ξb− decay, and
their masses and widths are measured with improved precision. The analysis is performed using pp collision data corresponding to a total integrated luminosity of
9 fb−1, collected with the LHCb experiment at centre-of-mass energies of 7, 8 and
13 TeV.
Submitted to Science Bulletin
© 2020 CERN for the benefit of the LHCb collaboration. CC BY 4.0 licence. †Authors are listed at the end of this paper.
The existence of pentaquark states, comprising four quarks and an antiquark, has been anticipated since the establishment of the quark model [1]. The first observation of pentaquark states has been reported by the LHCb experiment [2]. In the analysis of Λ0
b → J/ψ(→ µ+µ
−)pK− decays, a narrow structure in the J/ψp mass spectrum indicated
a possible pentaquark contribution [2, 3]. (Charge conjugation is implied and natural units with ~ = c = 1 are used throughout this Letter.) An amplitude analysis showed that the data could be best described by the presence of two pentaquark states, the Pc(4380)+ and
Pc(4450)+. With the inclusion of additional data and an improved selection strategy, it
was found that the Pc(4450)+ could be resolved into two narrow states, the Pc(4440)+ and
Pc(4457)+. In addition, a new narrow state, the Pc(4312)+was discovered [4]. The [uudc¯c]
valence quark content is attributed to these pentaquark states. Their strange counterparts, denoted Pcs0, with [udsc¯c] valence quark content, are predicted in Refs. [5–10] and it has been suggested to search for them in Ξb− → J/ψΛK− decays [7, 11].
The Ξb− → J/ψΛK− decay also provides the opportunity to study excited Ξ−
reso-nances (denoted collectively as Ξ∗−) in a mass range of [1.61, 2.70] GeV, where only a small number of Ξ∗− states have been observed, with a typical uncertainty of more than 5 MeV on their masses and widths [12]. Five Ξ∗− states have been established experimentally, the Ξ(1530)−, Ξ(1690)−, Ξ(1820)−, Ξ(1950)− and Ξ(2030)−. Recently, several results on the Ξ(1690)− and Ξ(1820)− states have been reported by the BESIII collaboration [13, 14]. For the neutral partners, the Belle collaboration observed a Ξ(1620)0 resonance and found
evidence for a Ξ(1690)0 state in Ξ+
c → Ξ−π+π+ decays [15]. More studies of excited Ξ
states will improve our understanding of the Ξ spectrum and of the structure of baryon resonances.
In this Letter, an amplitude analysis of the Ξb−→ J/ψΛK− decay is performed using
proton-proton (pp) collision data collected at centre-of-mass energies of√s = 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb−1 (Run 1) and at √s = 13 TeV, corresponding to 6 fb−1 (Run 2). The LHCb collaboration first observed the de-cay Ξb−→ J/ψΛK− using Run 1 data and measured the production rate of Ξ−
b with
Ξb− → J/ψΛK− decays relative to Λ0
b→ J/ψΛ decays [16].
The LHCb detector [17, 18] is a single-arm forward spectrometer covering the pseudo-rapidity range 2 < η < 5, designed for the study of particles containing b or c quarks. The detector includes a silicon-strip vertex detector surrounding the proton-proton interaction region, tracking stations on either side of a dipole magnet, ring-imaging Cherenkov (RICH) detectors, calorimeters and muon chambers. Simulation is required to evaluate the detector acceptance in the full phase space of Ξb−→ J/ψΛK− decays and the efficiency of signal
selection. In the simulation, pp collisions are generated using Pythia [19] with a specific LHCb configuration [20]. Decays of unstable particles are described by EvtGen [21], in which final-state radiation is generated using Photos [22]. The interaction of the generated particles with the detector, and its response, are implemented using the Geant4 toolkit [23] as described in Ref. [24]. The production kinematics of Ξb− baryons in the simulation is corrected using a sample of Λ0
b → J/ψΛ decays selected from data.
The Ξb− → J/ψ(→ µ+µ−)Λ(→ pπ−)K− signal candidates are first required to pass an
online event selection performed by a trigger [25], consisting of a hardware stage that is based on information from the calorimeters and the muon system, followed by two software stages that perform a partial event reconstruction. At the hardware stage, events are required to have a muon with a high momentum component transverse to the beam direction (pT) or a hadron, photon or electron with a high transverse energy. In the
first software stage, the event is required to have either two well-identified oppositely charged muons with large invariant mass, or at least one muon with pT > 1 GeV and a
large impact-parameter significance with respect to any primary pp collision vertex (PV). In the second stage, events containing a µ+µ− pair with invariant mass consistent with
the known J/ψ mass [12], and with a vertex significantly displaced from any PV, are selected. Candidate Λ → pπ− decays are reconstructed in two different categories: long involving Λ baryons with a flight distance short enough for the proton and pion to have the decay vertex reconstructed in the vertex detector; and downstream containing Λ baryons decaying later such that the track segments of the proton and pion cannot be formed in the vertex detector and are reconstructed only in the tracking stations. The candidates in the long category have better mass, momentum and decay vertex resolution than those in the downstream category. A Ξb− candidate is then reconstructed by combining the J/ψ, Λ and a well identified K− candidate, which are required to form a good-quality vertex.
Candidates are required to pass a set of selection criteria and are then further filtered using a multivariate classifier based on a gradient boosted decision tree (BDTG) [26, 27]. The selection criteria are almost identical to those used in the previous analysis [16], except those on the pT of the Λ decay products and on the χ2IP of the kaon candidate
which are relaxed. The χ2IP is defined as the difference in the vertex-fit χ2 of a given PV reconstructed with and without the track considered. In total, 15 variables are combined to train the BDTG classifier. The requirement of the BDTG response is selected to maximise the signal significance, separately for four categories (long and downstream Λ
[MeV] − K Λ ψ J/ m 5700 5800 5900 Candidates / (5 MeV) 0 20 40 60 80 LHCb 1 − Data 3 fb Total fit Signal Comb. bkg − K 0 Σ ψ J/ → − b Ξ [MeV] − K Λ ψ J/ m 5700 5800 5900 Candidates / (5 MeV) 0 20 40 LHCb 1 − Data 3 fb Total fit Signal Comb. bkg − K 0 Σ ψ J/ → − b Ξ [MeV] − K Λ ψ J/ m 5700 5800 5900 Candidates / (5 MeV) 0 100 200 300 LHCb 1 − Data 6 fb Total fit Signal Comb. bkg − K 0 Σ ψ J/ → − b Ξ [MeV] − K Λ ψ J/ m 5700 5800 5900 Candidates / (5 MeV) 0 50 100 150 200 LHCb 1 − Data 6 fb Total fit Signal Comb. bkg − K 0 Σ ψ J/ → − b Ξ
Figure 1: Invariant mass distributions of selected Ξb−→ J/ψΛK− candidates in the (top) Run 1
and (bottom) Run 2, (left) downstream and (right) long samples. The data are overlaid by the result of the fit described in the text.
candidates in Run 1 and 2). In the selected sample, less than 0.5% of the events contain more than one Ξb− candidate, which are all retained.
A kinematic fit [28] is applied to the Ξb− decay to improve the mass resolution where the J/ψ and Λ candidate masses are constrained to their known values [12], and the Ξb− candidate is constrained to originate from a primary vertex. The resulting J/ψΛK− invariant mass spectra for the four categories are shown in Fig. 1. The total signal yield is 1750 ± 50, determined by an unbinned extended maximum-likelihood fit to the J/ψΛK− invariant mass spectra for each of the four categories. The signal is described by a Hypatia function [29], while combinatorial background is modelled by an exponential function. Partially reconstructed Ξb− → J/ψΣ0(→ Λγ)K−
decays form a specific background at masses below the known Ξb− mass. The shape of this background is determined using a non-parametric model from simulation. Its yield varies freely in the fit. Weights [30] are assigned to the candidates to statistically subtract background contributions by weighting each candidate depending on their invariant J/ψΛK− mass. A kinematic fit is performed to improve the momentum resolution of final-state particles by further constraining the Ξb− candidate mass to its known value [12]. The resulting Dalitz plot for candidates within ±15 MeV of the Ξb− peak position is shown in Fig. 2. As expected, significant Ξ∗− → ΛK− contributions, in particular from the Ξ(1690)− and Ξ(1820)− resonances,
are observed. The J/ψΛ mass spectrum will be further explored in this Letter.
An amplitude analysis is carried out to measure the properties of the Ξ(1690)− and Ξ(1820)− resonances, and to examine a possible contribution from any P0
cs pentaquark
states decaying into J/ψΛ. The amplitude fits minimise an unbinned negative log-likelihood, L, constructed in a six-dimensional phase space of the data [2]. The six dimensions
]
2[GeV
− K Λ 2m
4
6
]
2[GeV
Λ ψ J/ 2m
18
20
22
24
26
28
LHCb
1 −9 fb
Figure 2: Dalitz plot for all candidates within ±15 MeV of the known Ξb− mass. The yellow area
correspond to the ΛK− mass and five angular observables θΞ− b , θΞ
−, θJ/ψ, φΛ, φµ, where θ and φ denote the polar and azimuthal angles, respectively. The probability distribution function only comprises the contribution from signal Ξb− decays since the background has been subtracted [30, 31]. The efficiency on the six-dimensional phase space is folded into the signal probability density function. The amplitude analysis follows a similar strategy to that of Λ0b → J/ψpK− decays in Ref. [2], with the Λ0
b baryon and proton in the Λ 0 b
decay replaced by the Ξb− and Λ baryons, respectively. However, a cross-check with the Dalitz-plot decomposition method proposed in Ref. [32] indicates that the method used in Ref. [2] has to be modified in two aspects to properly align the helicity state of the spin-half Λ baryon in the Ξ∗− and Pcs0 decay chains [33]. First, in the Ξ∗− → ΛK− decay, the Λ particle is used to define the two decay angles of the Ξ∗− system. The definition of the remaining angles is the same as in Ref. [2]. Secondly, the Euler rotation in the Ξ∗− → ΛK− decay aligns the spin axis along the Λ momentum, while the rotation in the P0
cs → J/ψΛ frame aligns the spin axis in the direction opposite to the Λ momentum. An
additional rotation to align the z-axis between the Pcs0 and Ξ∗− chains generates a term (−1)JΛ−λPcsΛ in the amplitude of the P0
cs chain, where JΛ and λPΛcs are the spin and the
helicity of the Λ particle in the Pcs0 rest frame, respectively. This term is the particle-two convention factor [34].
Table 1 lists the possible contributions from well established Ξ∗− states according to the PDG [12]. The states constitute a default description of the ΛK− invariant mass spectrum, which is also assumed to include a nonresonant (NR) contribution. As the spin-parities of these resonances are unknown except for that of the Ξ(1820)− baryon, combinations of different JP of these states are examined in the amplitude fit. Due to limited sample size, each Ξ∗− resonance is described by 3 or 4 independent LS couplings, where L stands for the decay orbital angular momentum, and S is the sum of spins of the decay products. Couplings corresponding to higher L are expected to be suppressed by the angular momentum barrier, so the contributions are chosen in increasing order of L. For all Ξ∗− resonances, relativistic Breit-Wigner functions [2] are used to model their line shape and phase variation as a function of the invariant mass of the ΛK− system, mΛK−. The masses and widths of the Ξ(1690)− and Ξ(1820)− resonances are free fit parameters, while those of other Ξ∗− resonances are constrained by their known uncertainties [12].
Table 1: The components in the amplitude fit used to describe the ΛK− system. The JP,
masses (M0) and widths (Γ0) of the Ξ− states are taken from the PDG [12]. The numbers
of LS couplings used in the default fit are listed, together with the total number of the LS
couplings associated to the ΛK− component, given in parentheses. The Ξ(1820)− coupling of
lowest LS is set to (1,0) for reference. Multiple JP assignments are considered for states where
this assignment has not been previously established. A nonresonant S-wave ΛK− contribution,
labelled as NR, is also considered in the fit model.
State M0 [MeV] Γ0 [MeV] LS couplings JP examined
Ξ(1690)− 1690 ± 10 < 30 4 (6) (1/2, 3/2)± Ξ(1820)− 1823 ± 5 24+15−10 3 (6) 3/2− Ξ(1950)− 1950 ± 15 60 ± 20 3 (6) (1/2, 3/2, 5/2)±
Ξ(2030)− 2025 ± 5 20+15−5 3 (6) 5/2±
The contribution of the NR S-wave component to the mΛK− spectrum is described with a function that is inversely proportional to m2
ΛK− [35]. Alternative descriptions of the NR component are considered to estimate systematic uncertainties on the model. The ΛK− and J/ψΛ mass spectra are shown in Fig. 3 with the projections of amplitude fit overlaid.
After the determination of the amplitude model with ΛK−-only contributions, a Pcs0 state is added to the amplitude model, with spin hypotheses ranging from 1/2 to 5/2 and parity hypotheses of both −1 and +1. Only the smallest allowed L is considered due to the suppression of higher values of L. A J/ψΛ mass resolution of 2.6 MeV obtained from simulation is taken into account by smearing the Pcs0 Breit-Wigner amplitude accordingly. The fits show a significant improvement when adding the Pcs0 state. The largest improvement on −2 ln L when adding a single P0
cs contribution is
found to be ∆2 ln L = 43, for an addition of 6 parameters. This fit, which includes the ΛK− resonances in Table 1, a NR ΛK− component and a single Pcs0 state each with their favoured JP assignment is referred to below as the default fit. The improvement in
−2 ln L corresponds to a statistical significance of 4.3 standard deviations (σ). This is estimated using pseudoexperiments where the look-elsewhere effect is taken into account. The difference of the −2 ln L obtained using fit models with and without the contribution of the Pcs0 state is used as the test statistic to evaluate the p-value of the null hypothesis, where several alternative Ξ∗− models are used to describe the contributions from the ΛK− resonances. The p-value is estimated by fitting the distribution of the test statistic from 10 000 pseudoexperiments for the model based on the results from the fit to data fit, generated with the null hypothesis. To take into account the look-elsewhere effect for each pseudoexperiment, the global maximum of 2 ln L is obtained by scanning the values of the mass and width of P0
cs state in the kinematically allowed region, instead of
limiting their values to be consistent with that of the data fit. When including systematic uncertainties discussed below, the p-value is determined to be 0.2% by counting the fraction of pseudoexperiments with the ∆2 ln L value exceeding the smallest ∆2 ln L value from data. This p-value corresponds to the signal significance of 3.1σ with a two-sided Gaussian test for the P0
cs state, providing the first evidence for a charmonium pentaquark
candidate with strangeness.
As shown in Fig. 3, the projections of the full amplitude fit onto the ΛK− and J/ψΛ invariant mass spectra match the data distributions well. A test of the fit quality is
[GeV] − K Λ m 2 2.5 Yield / (10 MeV) 1 10 2 10 3 10 Data 9 fb−1 cs P − (1690) Ξ − (1820) Ξ − (1950) Ξ − (2030) Ξ NR LHCb [GeV] Λ ψ J/ m 4.5 5 Yield / (20 MeV) 20 40 60 LHCb 1 − 9 fb
Figure 3: The (left) mΛK− and (right) mJ/ψΛ distributions of selected candidates compared to
[GeV] Λ ψ J/ m 4.4 4.45 4.5 Yield / (6 MeV) 0 5 10 15 > 2.2 GeV − K Λ m LHCb Data 9 fbPcs −1 cs P Fit without cs P Fit with cs P θ cos 1 − −0.5 0 0.5 1 Yield / 0.25 0 5 10 15 20 cs P Γ | < cs P -M Λ ψ J/ m | LHCb 1 − 9 fb
Figure 4: (Left) Projection of mJ/ψΛ in the range of [4.36, 4.54] GeV with the m(ΛK−) > 2.2 GeV
requirement. (Right) Projection of cos θPcs for candidates having mJ/ψΛ within one natural
width from the fitted Pcs0 resonance mass. The red dashed lines show the result of the fit in
Ξ∗-only hypothesis and the blue solid lines show the fit accounting for both Ξ∗ and Pcs0 states.
performed by comparing the default fit of the Dalitz plot with the data distribution. The data is divided into 64 bins containing approximately the same number of decays. The χ2 is calculated to be 77 for these bins, indicating a reasonably good description of the data. The P0
cs state is determined to have a mass of 4458.8 ± 2.9 (stat) MeV and a width
of 17.3 ± 6.5 (stat) MeV, and hereafter is denoted as Pcs(4459)0. Figure 4 highlights the
Pcs(4459)0 contribution by comparing the fits to the mJ/ψΛ and cos θPcs distributions with and without the Pcs(4459)0 state included, where θPcs is the helicity angle of the J/ψΛ
4.5 5 20 40 60 1.6 GeV < mΛK− < 1.9 GeV 4.5 5 20 40 60 LHCb 1 − Data 9 fb cs P − (1690) Ξ − (1820) Ξ − (1950) Ξ − (2030) Ξ NR 4.5 5 0 10 20 < 2.2 GeV − K Λ m 1.9 GeV < 4.5 5 0 10 20 < 2.8 GeV − K Λ m 2.2 GeV < [GeV] Λ ψ J/ m Yield / (20 MeV) Yield / (20 MeV)
Figure 5: Projections of mJ/ψΛ in intervals of (top left) mΛK− < 1.9 GeV (bottom left) 1.9 <
mΛK−< 2.2 GeV and (bottom right) mΛK−> 2.2 GeV based on the default fit, superimposed
Table 2: Mass (M0), width (Γ0) and fit fraction (FF) of the components involved in the default
fit. The masses and widths of the Pcs0, Ξ(1690)−, and Ξ(1820)− resonances are free parameters,
while those of the other Ξ∗− resonances are constrained by the known uncertainties [12]. The
quoted uncertainties are statistical and systematic. When only one uncertainty is given, it is statistical.
State M0 [MeV] Γ0 [MeV] FF (%)
Pcs(4459)0 4458.8 ± 2.9+4.7−1.1 17.3 ± 6.5+ 8.0− 5.7 2.7+ 1.9 + 0.7− 0.6 − 1.3 Ξ(1690)− 1692.0 ± 1.3+ 1.2− 0.4 25.9 ± 9.5+ 14.0− 13.5 22.1+ 6.2 + 6.7− 2.6 − 8.9 Ξ(1820)− 1822.7 ± 1.5+ 1.0− 0.6 36.0 ± 4.4+ 7.8− 8.2 32.9+ 3.2 + 6.9− 6.2 − 4.1 Ξ(1950)− 1910.6 ± 18.4 105.7 ± 23.2 11.5+ 5.8 + 49.9− 3.5 − 9.4 Ξ(2030)− 2022.8 ± 4.7 68.2 ± 8.5 7.3+ 1.8 + 3.8− 1.8 − 4.1 NR − − 35.8+ 4.6 +10.3− 6.4 −11.2
system, defined as the angle between the direction of the J/ψ particle and the opposite direction of the K− particle in the J/ψΛ rest frame. The Pcs(4459)0 state is more visible
when the dominant contributions from Ξ∗− with low masses are suppressed by requiring mΛK− > 2.2 GeV. As shown in Fig. 4 (right), a significant improvement of the fit quality is also found in the cos θPcs distribution when including the Pcs(4459)
0 state.
No evidence for any other P0
cs state is found in the considered mass range. This is also
clear when examining the mJ/ψΛ projections in three intervals of mΛK−, shown in Fig. 5. The measured mass, width and fit fraction (FF) of all components involved in the default fit are shown in Table 2. Systematic uncertainties on these results are discussed below.
The measured Pcs(4459)0 mass is about 19 MeV below the Ξc0D∗0 threshold. In this
region, Ref. [10] predicts two states with JP = 1/2− and 3/2− and a mass difference of
6 MeV. This is similar to the two Pc(4440)+ and Pc(4457)+ pentaquark states, which
are just below the Σc+D∗0 threshold. Thus the hypothesis of a two-peak structure with the predicted JP values is tested. The fit provides a good description of the data. The
result is shown for the P0
cs enhanced region in Fig. 6. The masses and widths of the two
states are 4454.9 ± 2.7 MeV and 7.5 ± 9.7 MeV, and 4467.8 ± 3.7 MeV and 5.2 ± 5.3 MeV, respectively, where the uncertainties are statistical only. The fit improves 2 ln L by 4.8 units for 4 additional free parameters, compared to the fit using one Breit-Wigner function to model the structure. Therefore, the analysis of the current data sample cannot confirm or refute the two-peak hypothesis.
Systematic uncertainties are summarised in Table 3. Due to the limited number of candidates, JP for the Pcs0 state and for the Ξ(1690)−, Ξ(1950)− and Ξ(2030)− states cannot be reliably determined. To estimate the systematic uncertainty related to the choice of JP assignments, all possible JP assignments for these states resulting
in −2 ln L differences less than 9 units are compared to that from the default fit. The largest variation, with respect to the values in the default fit, is taken as an asymmetric systematic uncertainty. The uncertainty related to the fit model is estimated by: varying the hadron-size parameter in the Blatt-Weisskopf barrier factor [2] between 1.0 and 5.0 GeV−1; modifying the orbital angular momenta L in Ξb− decays that are used in the treatment of the Ξ∗− resonances by one or two units; using all allowed couplings, instead of a limited number of L couplings, for Ξ∗− or Pcs(4459)0 states; using alternative models
to describe the nonresonant ΛK− component, including an exponential function or a function inversely proportional to m2
ΛK−+ m2NR, where mNR is a free parameter in the fit;
considering the effects of Ξb− polarisation, which are found to be consistent with zero in the analysis and neglected in the default fit; using an extended Ξ∗− model, which includes two more Ξ∗− states, in which the mass and width constraints on the Ξ∗− states are removed, and all allowed couplings for all Ξ∗− states are used. The largest value among all model variations is taken as systematic uncertainty for this source. The other systematic sources are estimated by: including the Λ → pπ− decay angles instead of taking the Λ baryon as a stable final-state particle; determining the sWeights by either splitting Ξ∗− helicity angles into bins or removing partially reconstructed physical background from the Ξb− → J/ψΣ0(→ Λγ)K− decays in the low J/ψΛK− mass sideband; and varying
the efficiency due to imperfect simulation. The significance for the Pcs(4459)0 state is
conservatively taken as the smallest significance found when combining different sources of systematic uncertainty together, and is equal to 3.1σ. It corresponds to varying the hadron-size parameter in the extended Ξ∗− model with full couplings for the considered Pcs(4459)0 state.
The negative systematic uncertainty for the Pcs(4459)0 fit fraction, −1.3%, is obtained
from an alternative value of JP used for the Pcs(4459)0 state. In such a fit, the significance
of the Pcs(4459)0 state is 4.1σ, even though the fit fraction is 1.4%. This is because
the significance has contributions from two sources, the fit fraction and the interference fraction involving the Pcs(4459)0 state. The interference fraction is about +1.3% in this
alternative JP fit, while it is almost 0 in the default fit. The systematic uncertainty of the Ξ(1950)− fit fraction is +49.9%, most of which originates from an alternative fit where its mass and width are floated in the extended model, rather than constrained to the known values [12], while the second largest one, from other sources considered in the estimation of systematic uncertainty, is +5.9%. Considering this large value, the fit fractions for all components involved in the extended model and their interference fractions are checked. A large interference fraction of −60.3% between Ξ(1950)− and NR is found in the extended model, and a large width of Ξ(1950)− of about 350 MeV is found. This could indicate the NR description in the default model is not perfect. Therefore several other NR models discussed before, or the NR contribution replaced by a broad Breit-Wigner function are tested; all of these variations don’t change the conclusion on the Pcs(4459)0 result.
[GeV] Λ ψ J/ m 4.4 4.45 4.5 Yield / (6 MeV) 0 5 10 15 > 2.2 GeV − K Λ m LHCb 1 − Data 9 fb cs P cs P Fit without cs P Fit with
Figure 6: Projection of mJ/ψΛ with a mΛK−> 2.2 GeV requirement applied, overlaid by the fit
Table 3: Summary of absolute systematic uncertainties for the fit parameters. The units for
masses (M0) and widths (Γ0) are MeV. The fit fraction in percent is denoted FF.
Source Pcs(4459)0 Ξ(1690)− Ξ(1820)− Ξ∗− Ξ∗− N R (1950) (2030) M0 Γ0 FF M0 Γ0 FF M0 Γ0 FF FF FF FF JP +4.7−0.3 +0.0−5.7 +0.1−1.3 +1.2−0.1 +14.0− 0.9 −0.3+6.7 +0.8−0.2 +1.4−0.5 +4.2−0.3 + 0.2− 9.4 +0.0−4.1 + 0.9−11.2 Model +0.7−1.1 +8.0−2.0 +0.7−0.5 +0.5−0.4 + 1.8−13.5 −8.9+1.9 +1.0−0.6 +7.8−8.2 +6.9−4.1 +49.9− 5.4 +3.8−1.6 +10.3− 6.4 Λ decay +0.0−0.7 +0.0−4.7 +0.0−0.3 +0.0−0.4 + 0.2− 0.0 −0.8+0.0 +0.0−0.5 +0.0−7.2 +0.0−4.1 + 2.4− 0.0 +0.0−1.3 + 3.9− 0.0 sWeights +0.0−0.2 +0.3−0.0 +0.1−0.0 +0.1−0.1 + 3.1− 0.2 −0.0+1.4 +0.2−0.2 +2.2−1.5 +1.6−0.5 + 0.7− 1.6 +0.0−0.2 + 0.0− 2.7 Efficiency +0.0−0.1 +0.0−0.5 +0.0−0.1 +0.1−0.2 + 2.1− 1.5 −1.3+0.8 +0.0−0.2 +1.1−0.2 +0.5−0.7 + 2.3− 1.0 +0.3−0.2 + 1.1− 0.9 Final +4.7−1.1 +8.0−5.7 +0.7−1.3 +1.2−0.4 +14.0−13.5 −8.9+6.7 +1.0−0.6 +7.8−8.2 +6.9−4.1 +49.9− 9.4 +3.8−4.1 +10.3−11.2
In conclusion, an amplitude analysis of Ξb− → J/ψΛK− decays is performed using
approximately 1750 candidates, and a structure in the J/ψΛ mass spectrum around 4459 MeV is seen. This structure can be explained by including a pentaquark candidate with strangeness in the amplitude model. Its significance exceeds 3σ after considering all systematic uncertainties. The mass and width of this new exotic state are measured to be 4458.8 ± 2.9+ 4.7− 1.1MeV and 17.3 ± 6.5+ 8.0− 5.7MeV, respectively. The Pcs(4459)0 state has
a mass only about 19 MeV below the Ξc0D∗0 threshold and a narrow width. Motivated by this fact, the hypothesis of two resonances contributing to the enhancement is tested. The data cannot confirm or refute the two-peak hypothesis. Furthermore, two Ξ∗− states, Ξ(1690)− and Ξ(1820)−, are observed for the first time in Ξb− decays. Using the full amplitude analysis, their masses and widths are measured to be
M (Ξ(1690)−) = 1692.0 ± 1.3+ 1.2− 0.4MeV, Γ(Ξ(1690)−) = 25.9 ± 9.5+ 14.0− 13.5MeV, M (Ξ(1820)−) = 1822.7 ± 1.5+ 1.0− 0.6MeV, Γ(Ξ(1820)−) = 36.0 ± 4.4+ 7.8− 8.2MeV.
These results are consistent with the average values reported in the PDG [12] and recent results from the BESIII experiment [13, 14]. The mass determinations are much more precise than those listed in the PDG. Due to limited signal yield, the JP of the P
cs(4459)0
and Ξ(1690)− states are not determined at this stage.
Acknowledgements
The numerical calculations for this paper have been supported by the Supercomputing Center of Wuhan University. We express our gratitude to our colleagues in the CERN accelerator departments for the excellent performance of the LHC. We thank the technical and administrative staff at the LHCb institutes. We acknowledge support from CERN and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); MOST and NSFC (China); CNRS/IN2P3 (France); BMBF, DFG and MPG (Germany); INFN (Italy); NWO (Netherlands); MNiSW and NCN (Poland); MEN/IFA (Romania); MSHE (Russia); MICINN (Spain); SNSF and SER (Switzerland); NASU (Ukraine); STFC (United Kingdom); DOE NP and NSF (USA). We acknowledge the computing resources that are provided by CERN, IN2P3 (France), KIT and DESY (Germany), INFN (Italy),
SURF (Netherlands), PIC (Spain), GridPP (United Kingdom), RRCKI and Yandex LLC (Russia), CSCS (Switzerland), IFIN-HH (Romania), CBPF (Brazil), PL-GRID (Poland) and OSC (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 AvH Foundation (Germany); EPLANET, Marie Sk lodowska-Curie Actions and ERC (European Union); A*MIDEX, ANR, Labex P2IO and OCEVU, and R´egion Auvergne-Rhˆone-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); RFBR, RSF and Yandex LLC (Russia); GVA, XuntaGal and GENCAT (Spain); the Royal Society and the Leverhulme Trust (United Kingdom).
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S. Schael14, M. Schellenberg15, M. Schiller59, H. Schindler48, M. Schmelling16, B. Schmidt48,
O. Schneider49, A. Schopper48, M. Schubiger32, S. Schulte49, M.H. Schune11, R. Schwemmer48,
B. Sciascia23, A. Sciubba31, S. Sellam46, A. Semennikov39, M. Senghi Soares33, A. Sergi53,48,
N. Serra50, L. Sestini28, A. Seuthe15, P. Seyfert48, D.M. Shangase85, M. Shapkin44,
I. Shchemerov81, L. Shchutska49, T. Shears60, L. Shekhtman43,u, Z. Shen4, V. Shevchenko80,
E.B. Shields25,i, E. Shmanin81, J.D. Shupperd68, B.G. Siddi21, R. Silva Coutinho50, G. Simi28,
S. Simone19,c, I. Skiba21,f, N. Skidmore62, T. Skwarnicki68, M.W. Slater53, J.C. Smallwood63,
J.G. Smeaton55, A. Smetkina39, E. Smith14, M. Smith61, A. Snoch32, M. Soares20,
L. Soares Lavra9, M.D. Sokoloff65, F.J.P. Soler59, A. Solovev38, I. Solovyev38,
F.L. Souza De Almeida2, B. Souza De Paula2, B. Spaan15, E. Spadaro Norella26,n, P. Spradlin59,
F. Stagni48, M. Stahl65, S. Stahl48, P. Stefko49, O. Steinkamp50,81, S. Stemmle17,
O. Stenyakin44, H. Stevens15, S. Stone68, M.E. Stramaglia49, M. Straticiuc37, D. Strekalina81,
S. Strokov83, F. Suljik63, J. Sun27, L. Sun73, Y. Sun66, P. Svihra62, P.N. Swallow53,
K. Swientek35, A. Szabelski36, T. Szumlak35, M. Szymanski48, S. Taneja62, F. Teubert48,
E. Thomas48, K.A. Thomson60, M.J. Tilley61, V. Tisserand9, S. T’Jampens8, M. Tobin6,
S. Tolk48, L. Tomassetti21,f, D. Torres Machado1, D.Y. Tou13, M. Traill59, M.T. Tran49,
E. Trifonova81, C. Trippl49, G. Tuci29,m, A. Tully49, N. Tuning32, A. Ukleja36, D.J. Unverzagt17,
E. Ursov81, A. Usachov32, A. Ustyuzhanin42,82, U. Uwer17, A. Vagner83, V. Vagnoni20,
A. Valassi48, G. Valenti20, N. Valls Canudas45, M. van Beuzekom32, M. Van Dijk49,
E. van Herwijnen81, C.B. Van Hulse18, M. van Veghel78, R. Vazquez Gomez46,
A. Venkateswaran68, M. Veronesi32, M. Vesterinen56, D. Vieira65, M. Vieites Diaz49,
H. Viemann76, X. Vilasis-Cardona84, E. Vilella Figueras60, P. Vincent13, G. Vitali29,
A. Vollhardt50, D. Vom Bruch13, A. Vorobyev38, V. Vorobyev43,u, N. Voropaev38, R. Waldi76,
J. Walsh29, C. Wang17, J. Wang3, J. Wang73, J. Wang4, J. Wang6, M. Wang3, R. Wang54,
Y. Wang7, Z. Wang50, H.M. Wark60, N.K. Watson53, S.G. Weber13, D. Websdale61,
C. Weisser64, B.D.C. Westhenry54, D.J. White62, M. Whitehead54, D. Wiedner15,
G. Wilkinson63, M. Wilkinson68, I. Williams55, M. Williams64,69, M.R.J. Williams58,
F.F. Wilson57, W. Wislicki36, M. Witek34, L. Witola17, G. Wormser11, S.A. Wotton55, H. Wu68,
K. Wyllie48, Z. Xiang5, D. Xiao7, Y. Xie7, A. Xu4, J. Xu5, L. Xu3, M. Xu7, Q. Xu5, Z. Xu5,
Z. Xu4, D. Yang3, S. Yang5, Y. Yang5, Z. Yang3, Z. Yang66, Y. Yao68, L.E. Yeomans60, H. Yin7,
J. Yu71, X. Yuan68, O. Yushchenko44, E. Zaffaroni49, K.A. Zarebski53, M. Zavertyaev16,b,
M. Zdybal34, O. Zenaiev48, M. Zeng3, D. Zhang7, L. Zhang3, S. Zhang4, Y. Zhang4, Y. Zhang63,
A. Zhelezov17, Y. Zheng5, X. Zhou5, Y. Zhou5, X. Zhu3, V. Zhukov14,40, J.B. Zonneveld58,
S. Zucchelli20,d, D. Zuliani28, G. Zunica62.
1Centro Brasileiro de Pesquisas F´ısicas (CBPF), Rio de Janeiro, Brazil 2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil 3Center for High Energy Physics, Tsinghua University, Beijing, China
4School of Physics State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing, China
5University of Chinese Academy of Sciences, Beijing, China 6Institute Of High Energy Physics (IHEP), Beijing, China
7Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China 8Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IN2P3-LAPP, Annecy, France 9Universit´e Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France
10Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France 11Universit´e Paris-Saclay, CNRS/IN2P3, IJCLab, Orsay, France 12Laboratoire Leprince-ringuet (llr), Palaiseau, France
13LPNHE, Sorbonne Universit´e, Paris Diderot Sorbonne Paris Cit´e, CNRS/IN2P3, Paris, France 14I. Physikalisches Institut, RWTH Aachen University, Aachen, Germany
15Fakult¨at Physik, Technische Universit¨at Dortmund, Dortmund, Germany 16Max-Planck-Institut f¨ur Kernphysik (MPIK), Heidelberg, Germany
17Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany 18School of Physics, University College Dublin, Dublin, Ireland
19INFN Sezione di Bari, Bari, Italy 20INFN Sezione di Bologna, Bologna, Italy 21INFN Sezione di Ferrara, Ferrara, Italy 22INFN Sezione di Firenze, Firenze, Italy
23INFN Laboratori Nazionali di Frascati, Frascati, Italy 24INFN Sezione di Genova, Genova, Italy
25INFN Sezione di Milano-Bicocca, Milano, Italy 26INFN Sezione di Milano, Milano, Italy
27INFN Sezione di Cagliari, Monserrato, Italy
28Universita degli Studi di Padova, Universita e INFN, Padova, Padova, Italy 29INFN Sezione di Pisa, Pisa, Italy
30INFN Sezione di Roma Tor Vergata, Roma, Italy 31INFN Sezione di Roma La Sapienza, Roma, Italy
32Nikhef National Institute for Subatomic Physics, Amsterdam, Netherlands
33Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, Netherlands
34Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krak´ow, Poland 35AGH - University of Science and Technology, Faculty of Physics and Applied Computer Science, Krak´ow, Poland
36National Center for Nuclear Research (NCBJ), Warsaw, Poland
38Petersburg Nuclear Physics Institute NRC Kurchatov Institute (PNPI NRC KI), Gatchina, Russia 39Institute of Theoretical and Experimental Physics NRC Kurchatov Institute (ITEP NRC KI), Moscow, Russia
40Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia
41Institute for Nuclear Research of the Russian Academy of Sciences (INR RAS), Moscow, Russia 42Yandex School of Data Analysis, Moscow, Russia
43Budker Institute of Nuclear Physics (SB RAS), Novosibirsk, Russia
44Institute for High Energy Physics NRC Kurchatov Institute (IHEP NRC KI), Protvino, Russia, Protvino, Russia
45ICCUB, Universitat de Barcelona, Barcelona, Spain
46Instituto Galego de F´ısica de Altas Enerx´ıas (IGFAE), Universidade de Santiago de Compostela, Santiago de Compostela, Spain
47Instituto de Fisica Corpuscular, Centro Mixto Universidad de Valencia - CSIC, Valencia, Spain 48European Organization for Nuclear Research (CERN), Geneva, Switzerland
49Institute of Physics, Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Lausanne, Switzerland 50Physik-Institut, Universit¨at Z¨urich, Z¨urich, Switzerland
51NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine
52Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine 53University of Birmingham, Birmingham, United Kingdom
54H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom 55Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom 56Department of Physics, University of Warwick, Coventry, United Kingdom 57STFC Rutherford Appleton Laboratory, Didcot, United Kingdom
58School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom 59School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom 60Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom 61Imperial College London, London, United Kingdom
62Department of Physics and Astronomy, University of Manchester, Manchester, United Kingdom 63Department of Physics, University of Oxford, Oxford, United Kingdom
64Massachusetts Institute of Technology, Cambridge, MA, United States 65University of Cincinnati, Cincinnati, OH, United States
66University of Maryland, College Park, MD, United States
67Los Alamos National Laboratory (LANL), Los Alamos, United States 68Syracuse University, Syracuse, NY, United States
69School of Physics and Astronomy, Monash University, Melbourne, Australia, associated to56
70Pontif´ıcia Universidade Cat´olica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil, associated to2 71Physics and Micro Electronic College, Hunan University, Changsha City, China, associated to 7 72Guangdong Provencial Key Laboratory of Nuclear Science, Institute of Quantum Matter, South China Normal University, Guangzhou, China, associated to 3
73School of Physics and Technology, Wuhan University, Wuhan, China, associated to 3
74Departamento de Fisica , Universidad Nacional de Colombia, Bogota, Colombia, associated to 13 75Universit¨at Bonn - Helmholtz-Institut f¨ur Strahlen und Kernphysik, Bonn, Germany, associated to 17 76Institut f¨ur Physik, Universit¨at Rostock, Rostock, Germany, associated to 17
77INFN Sezione di Perugia, Perugia, Italy, associated to 21
78Van Swinderen Institute, University of Groningen, Groningen, Netherlands, associated to 32 79Universiteit Maastricht, Maastricht, Netherlands, associated to 32
80National Research Centre Kurchatov Institute, Moscow, Russia, associated to39
81National University of Science and Technology “MISIS”, Moscow, Russia, associated to39 82National Research University Higher School of Economics, Moscow, Russia, associated to42 83National Research Tomsk Polytechnic University, Tomsk, Russia, associated to39
84DS4DS, La Salle, Universitat Ramon Llull, Barcelona, Spain, associated to45 85University of Michigan, Ann Arbor, United States, associated to68
aUniversidade Federal do Triˆangulo Mineiro (UFTM), Uberaba-MG, Brazil
bP.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia cUniversit`a di Bari, Bari, Italy
eUniversit`a di Cagliari, Cagliari, Italy fUniversit`a di Ferrara, Ferrara, Italy gUniversit`a di Firenze, Firenze, Italy hUniversit`a di Genova, Genova, Italy iUniversit`a di Milano Bicocca, Milano, Italy jUniversit`a di Roma Tor Vergata, Roma, Italy
kAGH - University of Science and Technology, Faculty of Computer Science, Electronics and Telecommunications, Krak´ow, Poland
lUniversit`a di Padova, Padova, Italy mUniversit`a di Pisa, Pisa, Italy
nUniversit`a degli Studi di Milano, Milano, Italy oUniversit`a di Urbino, Urbino, Italy
pUniversit`a della Basilicata, Potenza, Italy qScuola Normale Superiore, Pisa, Italy
rUniversit`a di Modena e Reggio Emilia, Modena, Italy sUniversit`a di Siena, Siena, Italy
tMSU - Iligan Institute of Technology (MSU-IIT), Iligan, Philippines uNovosibirsk State University, Novosibirsk, Russia
![Table 1 lists the possible contributions from well established Ξ ∗− states according to the PDG [12]](https://thumb-eu.123doks.com/thumbv2/123doknet/13984235.454515/6.892.176.713.988.1125/table-lists-possible-contributions-established-states-according-pdg.webp)
![Figure 4: (Left) Projection of m J/ψΛ in the range of [4.36, 4.54] GeV with the m(ΛK − ) > 2.2 GeV requirement](https://thumb-eu.123doks.com/thumbv2/123doknet/13984235.454515/8.892.115.773.113.317/figure-left-projection-ψλ-range-gev-λk-requirement.webp)
