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

0

200

400

600

800

1000

1200

Wei

ghte

d ca

ndid

ates

/(2

MeV

)

datatotal fitbackground

LHCb

+(4312)cP+(4440)cP +(4457)cP

0*D +cΣ

0D +

cΣ


https://arxiv.org/abs/1507.03414v2


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

20

40DataTotal fit

signal-bΞ

-K0ΣψJ/Comb. bkg

LHCb

LL

5700 5800 5900

4−2−024

5700 5800 5900

20

40

60

80LHCb

DD

5700 5800 5900

4−2−024

]2c) [MeV/−KΛψm(J/)2 c

Can

dida

tes/

(6 M

eV/

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.


https://journals.aps.org/prd/abstract/10.1103/PhysRevD.101.034033

DRAFTThe LHCb detector

A forward spectrometer : 2 < η < 5Dedicated to precise study of b, c particles.Excellent tracking and vertexing.Good PID performance.


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).



DRAFTMass spectrum fit for Ξ−

b candidates

) [MeV]−KΛψJ/(m5700 5800 5900

Can

dida

tes/

(5 M

eV)

0

20

40

60

80

LHCb

preliminary

Data

Total fit

Signal

Comb. bkg

−K0ΣψJ/→−bΞ

) [MeV]−KΛψJ/(m5700 5800 5900

Can

dida

tes/

(5 M

eV)

0

20

40

LHCb

preliminary

Data

Total fit

Signal

Comb. bkg


) [MeV]−KΛψJ/(m5700 5800 5900

Can

dida

tes/

(5 M

eV)

0

100

200

300 LHCb

preliminary

Data

Total fit

Signal

Comb. bkg


) [MeV]−KΛψJ/(m5700 5800 5900

Can

dida

tes/

(5 M

eV)

0

50

100

150

200 LHCb

preliminary

Data

Total fit

Signal

Comb. bkg


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).


DRAFTDalitz plots

Two excited Ξ− states

]2 [GeV−KΛ2m

4 6

]2 [

GeV

Λψ

J/2m

18

20

22

24

26

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.


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.



https://journals.aps.org/prd/abstract/10.1103/PhysRevD.101.034033

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)

20

40

60LHCbpreliminary

[GeV]ΛψJ/m4.5 5

5−05

Adding a P0cs state improves

−2lnL by 43,corresponding to a significanceof 4.3σ(stat. only).


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)

2

4

6

8

> 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

5

10

15

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)−.


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.


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).


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.


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!


Evidence of a J Λ resonance states Ξb J K decays DRAFT...Evidence of a J/ψΛ resonance and...

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