Evidence of a J Λ resonance states Ξb J K decays DRAFT...Evidence of a J/ψΛ resonance and...
Transcript of Evidence of a J Λ resonance states Ξb J K decays DRAFT...Evidence of a J/ψΛ resonance and...
DRAFTEvidence of a J/ψΛ resonanceand observation of excited Ξ− states
in Ξ−b → J/ψΛK− decays
LHCb-PAPER-2020-039 in preparation
Bo FangWuhan University
The 6th China LHC Physics WorshopNovember 6, 2020
DRAFTOutline
1 Introdution
2 Analysis strategy
3 The LHCb detector
4 Dataset and Selections
5 Signal extraction
6 Amplitude Analysis
7 Systematic uncertainty
8 Conclusion
B.Fang Wuhan Univ. CLHCP 2020 November 6, 2020 2 / 16
DRAFTObservation of pentaquark states in Λ0
b → J/ψpK− decaysIn 2015, the LHCb collaboration reported the first observation of twopentaquark states(Pc(4380)+ and Pc(4450)+) in the decays ofΛ0
b → J/ψpK−.(Phys. Rev. Lett. 115, 072001)In 2019, a new narrow pentaquark state(Pc(4312)+) and two-peakstructure of the Pc(4450)+ were observed with the inclusion of RunIIdata at LHCb.(Phys. Rev. Lett. 122, 222001)
4200 4250 4300 4350 4400 4450 4500 4550 4600 [MeV]pψ/Jm
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ghte
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datatotal fitbackground
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+(4312)cP+(4440)cP +(4457)cP
0*D +cΣ
0D +
cΣ
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DRAFTMotivationWhy Ξ−
b → J/ψΛK−?With u quark changed to s quark,Ξ−
b → J/ψΛK− is an ideal channel tosearch for hidden-charm pentaquarkstate with strangness S = -1.
previous analysis in Ξ−b → J/ψΛK−:
This decay (Ξ−b → J/ψΛK−) was
observed and its branching ratio hasbeen measured with RunI data inLHCb. (Phys. Lett. B772 (2017) 265)
5700 5800 5900
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40DataTotal fit
signal-bΞ
-K0ΣψJ/Comb. bkg
LHCb
LL
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4−2−024
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DD
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]2c) [MeV/−KΛψm(J/)2 c
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about 310 signal decaysB.Fang Wuhan Univ. CLHCP 2020 November 6, 2020 4 / 16
DRAFTAnalysis strategy
Reconstruction and selections:Similar to the previous analysis in RunI.Some selection criteria improved.
6-D amplitude analysis:Similar to the analysis of Λ0
b → J/ψpK− in 2015.Λ taken as a final-state particle.Formula cross checked with the Dalitz-Plot Decomposition formulaand updated.
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DRAFTThe LHCb detector
A forward spectrometer : 2 < η < 5Dedicated to precise study of b, c particles.Excellent tracking and vertexing.Good PID performance.
B.Fang Wuhan Univ. CLHCP 2020 November 6, 2020 6 / 16
DRAFTDatasetData sample:
Lumi = 9.0 fb−1. ( 3.0fb−1 for RunI, 6.0fb−1 for RunII).Selections:
Ξ−b → J/ψΛK−, J/ψ → µ+µ−, Λ → pπ−
Λ candidates are reconstructed into two categories:two daughters with Long track(LL).two daughters with Downstream track(DD).
Selections are similar to the one used in previous analysis, except:remove transverse momentum cuts and PID cuts for pπ−(low Q value).loosen the χ2
IP of the associated K−(included in MVA).
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DRAFTMass spectrum fit for Ξ−
b candidates
) [MeV]−KΛψJ/(m5700 5800 5900
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preliminary
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Total fit
Signal
Comb. bkg
−K0ΣψJ/→−bΞ
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Comb. bkg
−K0ΣψJ/→−bΞ
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preliminary
Data
Total fit
Signal
Comb. bkg
−K0ΣψJ/→−bΞ
) [MeV]−KΛψJ/(m5700 5800 5900
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100
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preliminary
Data
Total fit
Signal
Comb. bkg
−K0ΣψJ/→−bΞ
RunI DD(top left) RunI LL(top right)RunII DD(bottom left) RunII LL(bottom right)
sig: Hypatia functioncomb bkg:exponential functionphys bkg:Ξ−
b → J/ψΣ0K− withΣ0 → Λγ with missingγ, generator level MCconvoluted withresolution Gaussian.
In total, we obtain about1750 signal decays.Background takes 22.6% in±2σ signal region(about±15 MeV).
B.Fang Wuhan Univ. CLHCP 2020 November 6, 2020 8 / 16
DRAFTDalitz plots
Two excited Ξ− states
]2 [GeV−KΛ2m
4 6
]2 [
GeV
Λψ
J/2m
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28LHCb
Possible P0cs state?
Amplitude analysis required.
Invariant mass squared of ΛK−
versus J/ψΛ for candidateswithin ±2σ of the Ξ−
b mass.The yellow area shows thekinematically allowed region.Ξ−
b , J/ψ and Λ are constrainedto PDG mass, Ξ−
b constrainedto point back to primary vertex.
B.Fang Wuhan Univ. CLHCP 2020 November 6, 2020 9 / 16
DRAFTAmplitude Analysis Strategy
Amplitude analysis in Ξ−b → J/ψΛK− is performed similar to
Pentaquark analysis of Λ0b → J/ψpK− in 2015 . Here we take Λ as a final state
particle, since Λ has JP the same as proton, so the code for Λ0b → J/ψpK− is
directly used.Formula cross checked with the Dalitz-Plot Decomposition formulaand updated.Ξ∗ resonances listed in PDG with a rank higher than 2 star are considered.Ξ∗ spectroscopy is not well measured, JP of many states are not determined.Here we examine different JP combinations.
State JP M(MeV) Γ (MeV) Couplings used(max) JP examinedΞ(1690)− ?? 1690±10 <30 4(4) (1/2, 3/2)±
Ξ(1820)− 3/2− 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)− 5/2? 2025±5 20+15−5 3(6) 5/2±
NR 1/2− - - 4(4) 1/2−
P0cs ?? ? ? 1(4) (1/2, 3/2, 5/2)±
PDG errors of masses and widths are used as Gaussian constraints for Ξ(1950)−
and Ξ(2030)−. We measure masses, widths and fit fractions of Ξ(1690)− andΞ(1820)−, which are totally float.
B.Fang Wuhan Univ. CLHCP 2020 November 6, 2020 10 / 16
DRAFTResults of amplitude analysis
Clear peaks for two Ξ− states:Ξ(1690)−, Ξ(1820)−.
[GeV]−KΛm2 2.5
Yie
ld/ (
10 M
eV)
1
10
210csP(1690)Ξ(1820)Ξ(1950)Ξ(2030)Ξ
NR
LHCbpreliminary
[GeV]−KΛm2 2.5
5−05
[GeV]ΛψJ/m4.5 5
Yie
ld/ (
20 M
eV)
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60LHCbpreliminary
[GeV]ΛψJ/m4.5 5
5−05
Adding a P0cs state improves
−2lnL by 43,corresponding to a significanceof 4.3σ(stat. only).
B.Fang Wuhan Univ. CLHCP 2020 November 6, 2020 11 / 16
DRAFTResults of amplitude analysis
Projection of m(J/ψΛ) and cosθPcs zoomed in P0cs region:
[GeV]ΛψJ/m4.4 4.45 4.5
Yie
ld/ (
3 M
eV)
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4
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> 2.2 GeVKΛm
LHCbpreliminary
data
Fit NoPcs
Fit + Pcs
[GeV]ΛψJ/m4.4 4.45 4.5
5−05
csPθcos1− 0.5− 0 0.5 1
Yie
ld/ 0
.25
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20 PcsΓ | < Pcs-MΛψJ/m| LHCbpreliminary
csPθcos1− 0.5− 0 0.5 1
5−05
Statistics are not enough to determine JP of P0cs and Ξ(1690)−.
B.Fang Wuhan Univ. CLHCP 2020 November 6, 2020 12 / 16
DRAFTSystematic uncertaintiesSystematic uncertainties are evaluated for the fit fractions, masses andwidths of Pcs(4459)0, Ξ(1690)− and Ξ(1820)−, as well as the significanceof P0
cs, the largest variation is taken as final systematic uncertainty:Change to other possible JP of the states used.The modeling uncertainty includes:
varying hadron-size parameter in the Blatt-Weisskopf barrier factor.changing the orbital angular momenta L in Ξ−
b decay.using full numbers of couplings for Ξ− or P0
cs resonances.polarization of Ξ−
b .Extended model: two more Ξ− state fixed at 2.25 GeV (3/2−) and2.5 GeV (1/2−), removing constraints for Ξ(1950)−, Ξ(2030)−, andusing full couplings for all Ξ−.Alternative NR ΛK− models: constant NR + Ξ(1620)−, Exp,1/(m2
ΛK + m20).
Including Λ → pπ decay in the amplitude fit.The uncertainty due to efficiency.sWeights: physical background in low sideband of m(J/ψΛK),correlations between fit variables and sw.
B.Fang Wuhan Univ. CLHCP 2020 November 6, 2020 13 / 16
DRAFTMeasured results and significance
LHCb preliminaryState M0 (MeV/c2) Γ (MeV/c2) Fit fraction (%)
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
Final significance of the Pcs(4459)0 state is estimated by pseudoexperiments.After syst. uncertainty considered, the significance of the Pcs(4459)0
state is finally determined to be 3.1σ(LHCb preliminary).
B.Fang Wuhan Univ. CLHCP 2020 November 6, 2020 14 / 16
DRAFTConclusion
Conclusion:With RunI and RunII data(Lumi=9fb−1), about 1750 Ξ−
b → J/ψΛK−
decays are observed.A full amplitude analysis is performed, and we get these results:
An evidence of a new pentaquark candidate Pcs(4459)0 in J/ψΛsystem:
Including syst. uncertainty, significance is 3.1σ. (LHCb preliminary)M0(Pcs) = 4458.8 ± 2.9+4.7
−1.1 MeV (LHCb preliminary)Γ(Pcs) = 17.3 ± 6.5+8.0
−5.7 MeV (LHCb preliminary)fit fraction is (2.7+1.9
−0.6+0.7−1.3)% (LHCb preliminary)
This state is only 19 MeV below Ξ0c D̄∗0 mass threshold.
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DRAFTConclusion
Conclusion:Some other results from amplitude analysis:
Two narrow excited Ξ− states, Ξ(1690)− and Ξ(1820)−, areobserved:
M0(Ξ−1690) = 1692.0 ± 1.3+1.2
−0.4 MeV, Γ(Ξ−1690) = 25.9 ± 9.5+14.0
−13.5 MeV(LHCb preliminary)M0(Ξ−
1820) = 1822.7 ± 1.5+1.0−0.6 MeV, Γ(Ξ−
1820) = 36.0 ± 4.4+7.8−8.2 MeV
(LHCb preliminary)Fit fractions are (22.1+6.2
−2.6+6.7−8.9)%, (32.9+3.2
−6.2+6.9−4.1)% for Ξ(1690)− and
Ξ(1820)−, respectively. (LHCb preliminary)Masses and widths of Ξ(1690)− and Ξ(1820)− are consistent withPDG value, with improved precision.
Due to limited statistics, JP of Pcs(4459)0 and Ξ(1690)− states arenot determined.
Thank you for your attention!
B.Fang Wuhan Univ. CLHCP 2020 November 6, 2020 16 / 16