Observation of the decay Lambda(+)(c) -> Sigma(-)pi(+)pi(+)pi(0)

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Observation

of

the

decay



+

_c

→ 

−

π

+

π

+

π

0

BESIII

Collaboration

M. Ablikim

a

,

M.N. Achasov

i

,

1

,

S. Ahmed

n

,

M. Albrecht

d

,

A. Amoroso

bc

,

be

,

F.F. An

a

,

Q. An

az

,

2

,

J.Z. Bai

a

,

O. Bakina

z

,

R. Baldini Ferroli

t

,

Y. Ban

ah

,

D.W. Bennett

s

,

J.V. Bennett

e

,

N. Berger

y

,

M. Bertani

t

,

D. Bettoni

v

,

J.M. Bian

ax

,

F. Bianchi

bc

,

be

,

E. Boger

z

,

3

,

I. Boyko

z

,

R.A. Briere

e

,

H. Cai

bg

,

X. Cai

a

,

2

,

O. Cakir

ar

,

A. Calcaterra

t

,

G.F. Cao

a

,

S.A. Cetin

as

,

J. Chai

be

,

J.F. Chang

a

,

2

,

G. Chelkov

z

,

3

,

4

,

G. Chen

a

,

H.S. Chen

a

,

J.C. Chen

a

,

M.L. Chen

a

,

2

,

S.J. Chen

af

,

X.R. Chen

ac

,

Y.B. Chen

a

,

X.K. Chu

ah

,

G. Cibinetto

v

,

H.L. Dai

a

,

2

,

J.P. Dai

ak

,

5

,

A. Dbeyssi

n

,

D. Dedovich

z

,

Z.Y. Deng

a

,

A. Denig

y

,

I. Denysenko

z

,

M. Destefanis

bc

,

be

,

F. De Mori

bc

,

be

,

Y. Ding

ad

,

C. Dong

ag

,

J. Dong

a

,

2

,

L.Y. Dong

a

,

M.Y. Dong

a

,

2

,

O. Dorjkhaidav

x

,

Z.L. Dou

af

,

S.X. Du

bi

,

P.F. Duan

a

,

J. Fang

a

,

2

,

S.S. Fang

a

,

X. Fang

az

,

2

,

Y. Fang

a

,

R. Farinelli

v

,

w

,

L. Fava

bd

,

be

,

S. Fegan

y

,

F. Feldbauer

y

,

G. Felici

t

,

C.Q. Feng

az

,

2

,

E. Fioravanti

v

,

M. Fritsch

n

,

y

,

C.D. Fu

a

,

Q. Gao

a

,

X.L. Gao

az

,

2

,

Y. Gao

aq

,

Y.G. Gao

f

,

Z. Gao

az

,

2

,

I. Garzia

v

,

K. Goetzen

j

,

L. Gong

ag

,

W.X. Gong

a

,

2

,

W. Gradl

y

,

M. Greco

bc

,

be

,

M.H. Gu

a

,

2

,

S. Gu

o

,

Y.T. Gu

l

,

A.Q. Guo

a

,

L.B. Guo

ae

,

R.P. Guo

a

,

Y.P. Guo

y

,

Z. Haddadi

ab

,

S. Han

bg

,

X.Q. Hao

o

,

F.A. Harris

aw

,

K.L. He

a

,

X.Q. He

ay

,

F.H. Heinsius

d

,

T. Held

d

,

Y.K. Heng

a

,

2

,

T. Holtmann

d

,

Z.L. Hou

a

,

C. Hu

ae

,

H.M. Hu

a

,

T. Hu

a

,

2

,

Y. Hu

a

,

G.S. Huang

az

,

2

,

J.S. Huang

o

,

X.T. Huang

aj

,

X.Z. Huang

af

,

Z.L. Huang

ad

,

T. Hussain

bb

,

W. Ikegami Andersson

bf

,

Q. Ji

a

,

Q.P. Ji

o

,

X.B. Ji

a

,

X.L. Ji

a

,

2

,

X.S. Jiang

a

,

2

,

X.Y. Jiang

ag

,

J.B. Jiao

aj

,

Z. Jiao

q

,

D.P. Jin

a

,

2

,

S. Jin

a

,

T. Johansson

bf

,

A. Julin

ax

,

N. Kalantar-Nayestanaki

ab

,

X.L. Kang

a

,

X.S. Kang

ag

,

M. Kavatsyuk

ab

,

B.C. Ke

e

,

T. Khan

az

,

2

,

P. Kiese

y

,

R. Kliemt

j

,

L. Koch

aa

,

O.B. Kolcu

as

,

6

,

B. Kopf

d

,

M. Kornicer

aw

,

M. Kuemmel

d

,

M. Kuhlmann

d

,

A. Kupsc

bf

,

W. Kühn

aa

,

J.S. Lange

aa

,

M. Lara

s

,

P. Larin

n

,

L. Lavezzi

be

,

a

,

H. Leithoff

y

,

C. Leng

be

,

C. Li

bf

,

Cheng Li

az

,

2

,

D.M. Li

bi

,

F. Li

a

,

2

,

F.Y. Li

ah

,

G. Li

a

,

H.B. Li

a

,

H.J. Li

a

,

J.C. Li

a

,

Jin Li

ai

,

K. Li

m

,

K. Li

aj

,

Lei Li

c

,

∗

,

P.L. Li

az

,

2

,

P.R. Li

g

,

av

,

Q.Y. Li

aj

,

T. Li

aj

,

W.D. Li

a

,

W.G. Li

a

,

X.L. Li

aj

,

X.N. Li

a

,

2

_,

_{X.Q. Li}

ag

_,

_{Z.B. Li}

ap

_,

_{H. Liang}

az

,

2

_,

Y.F. Liang

am

,

Y.T. Liang

aa

,

G.R. Liao

k

,

D.X. Lin

n

,

B. Liu

ak

,

5

,

B.J. Liu

a

,

C.X. Liu

a

,

D. Liu

az

,

2

,

F.H. Liu

al

,

Fang Liu

a

,

Feng Liu

f

,

H.B. Liu

l

,

H.H. Liu

p

,

H.H. Liu

a

,

H.M. Liu

a

,

J.B. Liu

az

,

2

,

J.P. Liu

bg

,

J.Y. Liu

a

,

K. Liu

aq

,

K.Y. Liu

ad

,

Ke Liu

f

,

L.D. Liu

ah

,

P.L. Liu

a

,

2

,

Q. Liu

av

,

S.B. Liu

az

,

2

,

X. Liu

ac

,

Y.B. Liu

ag

,

Y.Y. Liu

ag

,

Z.A. Liu

a

,

2

,

Zhiqing Liu

y

,

Y.F. Long

ah

,

X.C. Lou

a

,

2

,

7

,

H.J. Lu

q

,

J.G. Lu

a

,

2

,

Y. Lu

a

,

Y.P. Lu

a

,

2

,

C.L. Luo

ae

,

M.X. Luo

bh

,

T. Luo

aw

,

X.L. Luo

a

,

2

,

X.R. Lyu

av

,

F.C. Ma

ad

,

H.L. Ma

a

,

L.L. Ma

aj

,

M.M. Ma

a

,

Q.M. Ma

a

,

T. Ma

a

,

X.N. Ma

ag

,

X.Y. Ma

a

,

2

,

Y.M. Ma

aj

,

F.E. Maas

n

,

M. Maggiora

bc

,

be

_,

_{Q.A. Malik}

bb

_,

_{Y.J. Mao}

ah

_,

_{Z.P. Mao}

a

_,

S. Marcello

bc

,

be

,

J.G. Messchendorp

ab

,

G. Mezzadri

w

,

J. Min

a

,

2

,

T.J. Min

a

,

R.E. Mitchell

s

,

X.H. Mo

a

,

2

,

Y.J. Mo

f

,

C. Morales Morales

n

,

G. Morello

t

,

N.Yu. Muchnoi

i

,

1

,

H. Muramatsu

ax

,

P. Musiol

d

,

A. Mustafa

d

,

Y. Nefedov

z

,

F. Nerling

j

,

I.B. Nikolaev

i

,

1

,

Z. Ning

a

,

2

,

S. Nisar

h

,

S.L. Niu

a

,

2

,

X.Y. Niu

a

,

S.L. Olsen

ai

,

Q. Ouyang

a

,

2

,

S. Pacetti

u

,

Y. Pan

az

,

2

,

P. Patteri

t

,

M. Pelizaeus

d

,

J. Pellegrino

bc

,

be

,

H.P. Peng

az

,

2

,

K. Peters

j

,

8

,

J. Pettersson

bf

,

J.L. Ping

ae

,

*

Correspondingauthor.

http://dx.doi.org/10.1016/j.physletb.2017.06.065

0370-2693/©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

R.G. Ping

a

,

R. Poling

ax

,

V. Prasad

ao

,

az

,

H.R. Qi

b

,

M. Qi

af

,

S. Qian

a

,

2

,

C.F. Qiao

av

,

J.J. Qin

av

,

N. Qin

bg

,

X.S. Qin

a

,

Z.H. Qin

a

,

2

,

J.F. Qiu

a

,

K.H. Rashid

bb

,

C.F. Redmer

y

,

M. Richter

d

,

M. Ripka

y

,

G. Rong

a

,

Ch. Rosner

n

,

X.D. Ruan

l

,

A. Sarantsev

z

,

9

,

M. Savrié

w

,

C. Schnier

d

,

K. Schoenning

bf

,

W. Shan

ah

,

M. Shao

az

,

2

,

C.P. Shen

b

,

P.X. Shen

ag

,

X.Y. Shen

a

,

H.Y. Sheng

a

,

J.J. Song

aj

,

X.Y. Song

a

,

S. Sosio

bc

,

be

,

C. Sowa

d

,

S. Spataro

bc

,

be

,

G.X. Sun

a

,

J.F. Sun

o

,

S.S. Sun

a

,

X.H. Sun

a

,

Y.J. Sun

az

,

2

,

Y.K. Sun

az

,

2

,

Y.Z. Sun

a

,

Z.J. Sun

a

,

2

,

Z.T. Sun

s

,

C.J. Tang

am

,

G.Y. Tang

a

,

X. Tang

a

,

I. Tapan

at

,

M. Tiemens

ab

,

B.T. Tsednee

x

,

I. Uman

au

,

G.S. Varner

aw

,

B. Wang

a

,

B.L. Wang

av

,

D. Wang

ah

,

D.Y. Wang

ah

,

Dan Wang

av

,

K. Wang

a

,

2

,

L.L. Wang

a

,

L.S. Wang

a

,

M. Wang

aj

,

P. Wang

a

,

P.L. Wang

a

,

W.P. Wang

az

,

2

,

X.F. Wang

aq

,

Y.D. Wang

n

,

Y.F. Wang

a

,

2

,

Y.Q. Wang

y

,

Z. Wang

a

,

2

,

Z.G. Wang

a

,

2

,

Z.H. Wang

az

,

2

,

Z.Y. Wang

a

,

Z.Y. Wang

a

,

T. Weber

y

,

D.H. Wei

k

,

P. Weidenkaff

y

,

S.P. Wen

a

,

U. Wiedner

d

,

M. Wolke

bf

,

L.H. Wu

a

,

L.J. Wu

a

,

Z. Wu

a

,

2

,

L. Xia

az

,

2

,

Y. Xia

r

,

D. Xiao

a

,

H. Xiao

ba

,

Y.J. Xiao

a

,

Z.J. Xiao

ae

,

Y.G. Xie

a

,

2

,

Y.H. Xie

f

,

X.A. Xiong

a

,

Q.L. Xiu

a

,

2

,

G.F. Xu

a

,

J.J. Xu

a

,

L. Xu

a

,

Q.J. Xu

m

,

Q.N. Xu

av

,

X.P. Xu

an

,

L. Yan

bc

,

be

,

W.B. Yan

az

,

2

,

W.C. Yan

az

,

2

,

Y.H. Yan

r

,

H.J. Yang

ak

,

5

,

H.X. Yang

a

,

L. Yang

bg

,

Y.H. Yang

af

,

Y.X. Yang

k

,

M. Ye

a

,

2

,

M.H. Ye

g

,

J.H. Yin

a

,

Z.Y. You

ap

,

B.X. Yu

a

,

2

,

C.X. Yu

ag

,

J.S. Yu

ac

,

C.Z. Yuan

a

,

Y. Yuan

a

,

A. Yuncu

as

,

10

,

A.A. Zafar

bb

,

Y. Zeng

r

,

Z. Zeng

az

,

2

,

B.X. Zhang

a

,

B.Y. Zhang

a

,

2

,

C.C. Zhang

a

,

D.H. Zhang

a

,

H.H. Zhang

ap

,

H.Y. Zhang

a

,

2

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_{J. Zhang}

a

_,

_{J.L. Zhang}

a

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a

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_{J.W. Zhang}

a

,

2

_,

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a

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_{J.Z. Zhang}

a

_,

K. Zhang

a

,

L. Zhang

aq

,

S.Q. Zhang

ag

,

X.Y. Zhang

aj

,

Y. Zhang

a

,

Y. Zhang

a

,

Y.H. Zhang

a

,

2

,

Y.T. Zhang

az

,

2

,

Yu Zhang

av

,

Z.H. Zhang

f

,

Z.P. Zhang

az

,

Z.Y. Zhang

bg

,

G. Zhao

a

,

J.W. Zhao

a

,

2

,

J.Y. Zhao

a

,

J.Z. Zhao

a

,

2

,

Lei Zhao

az

,

2

,

Ling Zhao

a

,

M.G. Zhao

ag

,

Q. Zhao

a

,

S.J. Zhao

bi

,

T.C. Zhao

a

,

Y.B. Zhao

a

,

2

,

Z.G. Zhao

az

,

2

,

A. Zhemchugov

z

,

3

,

B. Zheng

ba

,

J.P. Zheng

a

,

2

,

W.J. Zheng

aj

,

Y.H. Zheng

av

,

B. Zhong

ae

,

L. Zhou

a

,

2

,

X. Zhou

bg

,

X.K. Zhou

az

,

2

,

X.R. Zhou

az

,

2

,

X.Y. Zhou

a

,

Y.X. Zhou

l

,

2

,

K. Zhu

a

,

K.J. Zhu

a

,

2

,

S. Zhu

a

,

S.H. Zhu

ay

,

X.L. Zhu

aq

,

Y.C. Zhu

az

,

2

,

Y.S. Zhu

a

,

Z.A. Zhu

a

,

J. Zhuang

a

,

2

_,

_{L. Zotti}

bc

,

be

_,

_{B.S. Zou}

a

_,

_{J.H. Zou}

a

a_{InstituteofHighEnergyPhysics,Beijing100049,People’sRepublicofChina} b_{BeihangUniversity,Beijing100191,People’sRepublicofChina}

c_{BeijingInstituteofPetrochemicalTechnology,Beijing102617,People’sRepublicofChina} d_{BochumRuhr-University,D-44780Bochum,Germany}

e_{CarnegieMellonUniversity,Pittsburgh,PA 15213,USA}

f_{CentralChinaNormalUniversity,Wuhan430079,People’sRepublicofChina}

g_{ChinaCenterofAdvancedScienceandTechnology,Beijing100190,People’sRepublicofChina}

h_{COMSATSInstituteofInformationTechnology,Lahore,DefenceRoad,OffRaiwindRoad,54000 Lahore,Pakistan} i_{G.I.BudkerInstituteofNuclearPhysicsSBRAS(BINP),Novosibirsk630090,Russia}

j_{GSIHelmholtzcentreforHeavyIonResearchGmbH,D-64291Darmstadt,Germany} k_{GuangxiNormalUniversity,Guilin541004,People’sRepublicofChina}

l_{GuangxiUniversity,Nanning530004,People’sRepublicofChina}

m_{HangzhouNormalUniversity,Hangzhou310036,People’sRepublicofChina} n_{HelmholtzInstituteMainz,Johann-Joachim-Becher-Weg45,D-55099Mainz,Germany} o_{HenanNormalUniversity,Xinxiang453007,People’sRepublicofChina}

p_{HenanUniversityofScienceandTechnology,Luoyang471003,People’sRepublicofChina} q_{HuangshanCollege,Huangshan245000,People’sRepublicofChina}

r_{HunanUniversity,Changsha410082,People’sRepublicofChina} s_{IndianaUniversity,Bloomington,IN 47405,USA}

t_{INFNLaboratoriNazionalidiFrascati,I-00044,Frascati,Italy} u_{INFNandUniversityofPerugia,I-06100,Perugia,Italy} v_{INFNSezionediFerrara,I-44122,Ferrara,Italy} w_{UniversityofFerrara,I-44122,Ferrara,Italy}

x_{InstituteofPhysicsandTechnology,PeaceAve.54B,Ulaanbaatar13330,Mongolia}

y_{JohannesGutenbergUniversityofMainz,Johann-Joachim-Becher-Weg45,D-55099Mainz,Germany} z_{JointInstituteforNuclearResearch,141980Dubna,Moscowregion,Russia}

aa_{Justus-Liebig-UniversitaetGiessen,II.PhysikalischesInstitut,Heinrich-Buff-Ring16,D-35392Giessen,Germany} ab_{KVI-CART,UniversityofGroningen,NL-9747AAGroningen,TheNetherlands}

ac_{LanzhouUniversity,Lanzhou730000,People’sRepublicofChina} ad_{LiaoningUniversity,Shenyang110036,People’sRepublicofChina} ae_{NanjingNormalUniversity,Nanjing210023,People’sRepublicofChina} af_{NanjingUniversity,Nanjing210093,People’sRepublicofChina} ag_{NankaiUniversity,Tianjin300071,People’sRepublicofChina} ah_{PekingUniversity,Beijing100871,People’sRepublicofChina} ai_{SeoulNationalUniversity,Seoul,151-747,RepublicofKorea} aj_{ShandongUniversity,Jinan250100,People’sRepublicofChina}

ak_{ShanghaiJiaoTongUniversity,Shanghai200240,People’sRepublicofChina} al_{ShanxiUniversity,Taiyuan030006,People’sRepublicofChina}

am_{SichuanUniversity,Chengdu610064,People’sRepublicofChina} an_{SoochowUniversity,Suzhou215006,People’sRepublicofChina}

ao_{StateKeyLaboratoryofParticleDetectionandElectronics,Beijing 100049,Hefei 230026,People’sRepublicofChina} ap_{SunYat-SenUniversity,Guangzhou510275,People’sRepublicofChina}

aq_{TsinghuaUniversity,Beijing100084,People’sRepublicofChina} ar_{AnkaraUniversity,06100Tandogan,Ankara,Turkey} as_{IstanbulBilgiUniversity,34060Eyup,Istanbul,Turkey} at_{UludagUniversity,16059Bursa,Turkey}

au_{NearEastUniversity,Nicosia,NorthCyprus,Mersin 10,Turkey}

av_{UniversityofChineseAcademyofSciences,Beijing100049,People’sRepublicofChina} aw_{UniversityofHawaii,Honolulu,HI 96822,USA}

ax_{UniversityofMinnesota,Minneapolis,MN 55455,USA}

ay_{UniversityofScienceandTechnologyLiaoning,Anshan114051,People’sRepublicofChina} az_{UniversityofScienceandTechnologyofChina,Hefei230026,People’sRepublicofChina} ba_{UniversityofSouthChina,Hengyang421001,People’sRepublicofChina}

bb_{UniversityofthePunjab,Lahore-54590,Pakistan} bc_{UniversityofTurin,I-10125,Turin,Italy}

bd_{UniversityofEasternPiedmont,I-15121,Alessandria,Italy} be_{INFN,I-10125,Turin,Italy}

bf_{UppsalaUniversity,Box516,SE-75120Uppsala,Sweden} bg_{WuhanUniversity,Wuhan430072,People’sRepublicofChina} bh

ZhejiangUniversity,Hangzhou310027,People’sRepublicofChina

bi_{ZhengzhouUniversity,Zhengzhou450001,People’sRepublicofChina}

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Articlehistory: Received31May2017 Accepted26June2017 Availableonline28June2017 Editor:W.-D.Schlatter Keywords: Branchingfraction Charmedbaryon Weakdecays e+e−annihilation BESIII

We reportthefirstobservationofthedecay+c → −

π

+

π

+

π

0,basedondata obtainedine+e−

an-nihilations withanintegratedluminosity of567 pb−1 _at√_{s=4.6 GeV. Thedatawerecollectedwith}

theBESIIIdetectorattheBEPCIIstoragerings.TheabsolutebranchingfractionB(c+→ −

π

+

π

+

π

0)is

determinedtobe(2.11±0.33(stat.)±0.14(syst.))%.Inaddition,animprovedmeasurementofB(+c → −

π

+

π

+)isdeterminedas(1.81_±0.17(stat.)±0.09(syst.))%.

©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

Thestudyofhadronicdecaysofcharmedbaryonsprovides im-portant informationtounderstand both thestrongand theweak interactions[1].Italsoprovidesessentialinputtounderstand back-groundcontributionsinthe studyofb-baryon physics,as

b

de-cays dominantly to



+_c. More than 30 years have passed since the



+_c baryon was first observed in e+e− annihilations by the Mark IIexperiment[2]andtheknowledge of



+c decaysremains verypoorcomparedtothatforcharmedmesons.Sofar,measured decaymodesaccountforonlyabout60%[3]ofall



c+decays, pri-marilyconsistingofmodeswitha

()

hyperonoraprotoninthe finalstate.Decaystothe



−hyperonareCabibbo-allowedandare expectedtohavelarge rates.However,no experimental

measure-E-mailaddress:lilei2014@bipt.edu.cn(LeiLi).

1 _{AlsoattheNovosibirskStateUniversity,Novosibirsk,630090,Russia.} 2 _{Also at State Key Laboratory of Particle Detection and Electronics, Beijing} 100049,Hefei230026,People’sRepublicofChina.

3 _{AlsoattheMoscowInstituteofPhysicsandTechnology,Moscow141700,Russia.} 4 _{Alsoatthe FunctionalElectronicsLaboratory,Tomsk StateUniversity,Tomsk,} 634050,Russia.

5 _{AlsoatKeyLaboratoryforParticlePhysics,AstrophysicsandCosmology,} Min-istryofEducation;Shanghai KeyLaboratoryfor ParticlePhysicsand Cosmology; Institute ofNuclearand Particle Physics,Shanghai 200240, People’sRepublic of China.

6 _{AlsoatIstanbulArelUniversity,34295Istanbul,Turkey.} 7 _{AlsoatUniversityofTexasatDallas,Richardson,TX 75083,USA.} 8 _{AlsoatGoetheUniversityFrankfurt,60323FrankfurtamMain,Germany.} 9 _{AlsoattheNRC“KurchatovInstitute”,PNPI,188300,Gatchina,Russia.} 10 _{AlsoatBogaziciUniversity,34342Istanbul,Turkey.}

ments exist except for



+c

→ 

−

π

+

π

+ [3]. Therefore,searching for additional decaymodes with



− in the final state is impor-tant to build up knowledge on



+c decays.In thispaper, we re-portthefirstobservationoftheso-farundetermined,butexpected to be large, decay of



+_c

→ 

−

π

+

π

+

π

0_.11 _{Inaddition, we} per-formthefirstabsolutemeasurement ofthebranchingfractionfor



+c

→ 

−

π

+

π

+.

Thedataanalyzedinthisworkcorrespondstoanintegrated lu-minosityof567 pb−1 _[4]ofe+_e− _{annihilationsatcenter-of-mass}

energy (c.m.)

√

s

=

4

.

6 GeV by the BEPCII collider andcollected withtheBESIII detector[5].The c.m. energyis slightlyabove the threshold forthe production of



+c

¯

−c, so



+c

¯

−c pairs are pro-ducedwith noadditional hadrons. Theanalysis technique inthis work, which was first applied in the Mark III experiment [6], is optimized for measuring charm hadron pairs produced near threshold. First, we select the subset of our events in which a

¯

−

c is reconstructed in an exclusive hadronic decay mode, des-ignated as the single-tag (ST) sample. Events in this ST sample are then searched for the signal channel



+_c

→ 

−

π

+

π

+

(

π

0

₎

in the system recoiling against the ST to select double tag (DT) events. In the final states of



+c

→ 

−

π

+

π

+

(

π

0

)

, the



− hy-peron is detected through



−

→

n

π

−. Asthe neutronis not re-constructedinthisanalysis,wededuceitskinematicpropertiesby four-momentumconservation.Theabsolutebranchingfraction(BF) of



+_c

→ 

−

π

+

π

+

(

π

0

₎

_{isderivedfromtheprobabilityof} detect-ingtheDTsignalsintheSTsample.Hence,thismethodprovidesa

11 _{Throughoutthispaper,chargedconjugatemodesareimpliedunlessexplicitly} statedotherwise.

cleanandstraightforwardBFmeasurementthatisindependentof thenumberof



+_c

¯

−_c eventsproduced.

2. BESIIIdetectorandMonteCarlosimulation

BESIII[5]isacylindricaldetectorwithacoverageof93%ofthe full 4

π

solid angle.It consists ofa Helium-gas based main drift chamber (MDC),a plasticscintillator time-of-flight (TOF) system, a CsI (Tl) electromagnetic calorimeter (EMC), a superconducting solenoidproviding a 1.0 T magnetic field, anda muon detection systeminthe iron flux returnof the magnet. The charged parti-cle momentum resolution is 0.5% at a transverse momentum of 1 GeV

/

c. The photon energy resolution at 1 GeV is 2.5% in the centralbarrel region and5.0% inthe two endcaps. More details about the design and performance of the detector are given in Ref.[5].

A GEANT4-based [7] Monte Carlo (MC) simulation package, whichincludesthe geometricdescription ofthedetectorandthe detector response, is used to determine the detection efficiency andtoestimatethepotentialbackgrounds.MCsamplesofthe sig-nalmode



c+

→ 

−

π

+

π

+

(

π

0

)

,together witha

¯

−c decayingto specifiedSTmodes,aregeneratedwithKKMC[8]andEVTGEN[9], takingintoaccountinitial-stateradiation(ISR)[10]andfinal-state radiation[11]effects.The



+c

→ 

−

π

+

π

+

(

π

0

)

decayissimulated by reweightingthe phase-space-generatedMC events to approxi-mateobservedkinematicdistributionsindata.Tounderstand po-tentialbackgroundcontributions, aninclusiveMCsample isused. Itincludesgeneric



+c

¯

−c events,D

(∗)

(s)D

¯

(∗)

(s)

+

X production,ISR

re-turn to the charmonium states at lower masses and continuum qq processes.

¯

Previouslymeasureddecaymodesofthe

c

,

ψ

and D(s) aresimulatedwithEVTGEN,usingBFsfromtheParticleData

Group(PDG)[3].Theunknown decaysofthe

ψ

statesare gener-atedwithLUNDCHARM[12].

3. Analysis

The ST and DT selection technique that is used in our anal-ysisfollows closely theone used anddescribed inRef. [13]. We reconstructthe

¯

−_c baryonsin theeleven hadronicdecay modes listedinTable 1.Intermediateparticlesare reconstructedthrough theirdecays K0_S

→

π

+

π

−,

¯ → ¯

p

π

+,

¯

0

→

γ

¯

with

¯ → ¯

p

π

+,

¯

−

_{→ ¯}

_p

_π

0_,and

_π

0

_→

_{γ γ}

_{.The selection criteria forthe proton,} kaon,pion,

π

0_,K0

S and

¯

candidatesusedinthereconstructionof theSTsignalsaredescribedinRef.[13].

The ST

¯

−c signals are identified using the beam-energy-constrained mass, MBC

=



E2_beam

− |

p_¯− c

|

2_{, where E} beam is the beam energy and



p_¯−

c is the momentum of the

¯

−

c candidate in the rest frame of the initial e+e− system.12 To improve the signal purity, the energy difference



E

=

Ebeam

−

E¯−_c for each candidateis requiredto bewithin approximately

±

3

σ

ofthe



E signal peak position, where

σ

is the



E resolution and E_¯−

c is

thereconstructed

¯

−c energy.Table 1showsthemode-dependent



E requirements and the ST yields in the MBC signal region

(

2

.

280

,

2

.

296

)

GeV

/

c2_{,whichareobtainedbyfitstothe M}

BC dis-tributions. See Ref. [13] for more details. The total ST yield is Ntot

¯−

c

=

14415

±

159,wheretheuncertaintyisstatisticalonly. Candidates for the decay



+c

→ 

−

π

+

π

+

(

π

0

)

with



−

→

n

π

−arereconstructedfromthetracksnot usedintheST

¯

_c− re-construction.Itisrequiredthatthereareonlythreechargedtracks

12 _{Allkinematicquantitiespresentedinthispaperareevaluatedintherestframe} oftheinitiale+e−system.

Table 1

RequirementsonE andSTyieldsN_¯−

c fortheelevenSTmodes.Theuncertainties

arestatisticalonly.

Mode E (GeV) N_¯− c ¯ p K0 S [−0.025,0.028] 1066±33 ¯ p K+π− [−0.019,0.023] 5692±88 ¯ p K0 Sπ 0 _[−0 .035,0.049] 593±41 ¯ p K+π−π0 _{[−0.044,0.052] 1547±61} ¯ p K0 Sπ+π− [−0.029,0.032] 516±34 ¯π− [−0.033,0.035] 593±25 ¯π−π0 _{[−0.037,0.052] 1864±56} ¯π−π+π− [−0.028,0.030] 674±36 ¯0_π− _{[−0.029,0.032] 532±30} ¯−_π0 _[−0 .038,0.062] 329±28 ¯−_π+_π− _{[−0.049,0.054] 1009±57} in the system recoiling against the

¯

−_c satisfying

|

cos

θ

|

<

0

.

93, where

θ

is the polar angle with respect to the beam direction. Forthetwo

π

+ candidatesfromthe



+c,thedistancesofclosest approach to the interaction point mustbe within

±

10 cm along the beamdirection andwithin 1 cm inthe perpendicular plane, while the

π

− candidatefrom



− decayis not subjected to this requirement.Identificationofchargedtracksisperformedby com-biningthedE

/

dx informationfromtheMDCandthetimeofflight measuredintheTOFtoobtaintheprobability

Lh

foreachhadron type h.Thethreechargedpionsmustsatisfy

L

π

>

LK

.Photon can-didatesarereconstructedfromisolatedclustersintheEMCinthe regions

|

cos

θ

|

≤

0

.

80 (barrel)and0

.

86

≤ |

cos

θ

|

≤

0

.

92 (endcap). The depositedenergyofa neutralcluster isrequiredto belarger than25 (50) MeVinthebarrel(endcap)region,andtheangle be-tweenthephotoncandidateandthenearestchargedtrackmustbe largerthan 10◦.Tosuppresselectronicnoise andenergydeposits unrelatedtothe event,thedifferencebetweentheEMCtime and theeventstarttimeisrequiredtobewithin

(

0

,

700

)

ns.To recon-struct

π

0_{candidates,theinvariantmassofphotonpairsisrequired} tobe within

(

0

.

110

,

0

.

155

)

GeV

/

c2 and,asasecond step,a kine-maticfitisimplementedtoconstrainthe

γ γ

invariantmasstothe nominal

π

0_mass[3].

Thekinematicvariable

Mn

=



(

Ebeam

−

Eπ+π+π−(π0₎

)

2

− |

→−p_+ c

−

− →_p π+π+π−(π0₎

|

2 is computed to characterize the reconstructed mass of the undetected neutron, where _Eπ+_π+_π−_(π0₎ is the energy of the

π

+

π

+

π

−

(

π

0

₎

_{combination and}→−_pπ

+π+π−(π0₎ is the three-mo-mentumofthe

π

+

π

+

π

−

(

π

0

₎

_{combination.Theexpected} momen-tum



p_+ c ofthe



+ c iscalculated by



pc+

= − ˆ

ptag



E2_beam

−

m2 +c , where p

ˆ

tag isthedirectionofthe momentumoftheST

¯

−c can-didateandm_+

c isthe mass ofthe



+

c taken fromthe PDG[3]. Similarly,wecanconstructthevariable

Mnπ−

=



(

Ebeam

−

Eπ+π+(π0₎

)

2

− |

→−p_+ c

−

− →_p π+π+(π0₎

|

2 torepresentthereconstructedmassofthe



−.

Thedistributions ofMn versus Mnπ− forthe



+c

→ 

−

π

+

π

+ and



+_c

→ 

−

π

+

π

+

π

0_{candidatesindataareshowninFigs. 1(a)} and(b),respectively,whereclusterscorrespondingtosignaldecays areevident. Toimprovetheresolutionofthesignal mass,aswell astobetter handlethebackgrounds aroundthe



− andneutron massregions,wedeterminethesignalyieldsfromthedistribution ofthemassdifference Mnπ−

−

Mn,sinceMnπ− andMnarehighly correlated.BasedonastudyoftheinclusiveMCsamples,no peak-ingbackgroundsareexpectedforthesetwochannels.Weperform anunbinnedmaximumlikelihoodfittotheMnπ−

−

Mnspectra,as showninFigs. 1(c)and(d).Inthefits,thesignalsaredescribedby

Fig. 1. Scatter plots of Mn versus Mnπ− for candidates in data for (a) +c →

−π+π+and(b)+c → −π+π+π0.Alsoshownarefitstothedistributionsof Mnπ−−Mnfor(c)+c → −π+π+and(d)+c → −π+π+π0indata.Solidlines aretheresultsofacompletefitwhiledashedlinesreflectthebackground compo-nents.

non-parametricfunctionsextractedfromthesignalMCconvoluted witha Gaussianfunction accountingforthe resolutiondifference betweendataandMC,whilethebackgroundshapesaredescribed withasecond-orderpolynomial function.Thewidthofthe Gaus-sian is left free in the fit, while its mean is fixed to zero. From the fits, we find the DT signal yields Nobs

−π+π+

=

161

±

15 and Nobs

−π+π+π0

=

88

±

14,wheretheuncertaintiesarestatisticalonly. Backgroundsfromnon-



+c decaysareestimatedbyexaminingthe STcandidatesintheMBC sideband

(

2

.

252

,

2

.

272

)

GeV

/

c2 indata. Thebackgroundsfromnon-



+c decaysarefoundtobenegligible.

TheabsoluteBFsfor



+c

→ 

−

π

+

π

+ and



+c

→ 

−

π

+

π

+

π

0 aredeterminedby

B

(

+_c

→ 

−

π

+

π

+

(

π

0

))

=

N obs −π+π+(π0₎ Ntot ¯− c

·

ε

−π+π+(π 0₎

·

B

(

−

→

n

π

−

)

,

(1)

where

ε

−π+π+(π0₎ is the detection efficiency for the



+_c

→



−

π

+

π

+

(

π

0

₎

_{decay with}

_

−

_→

_n

_π

−_{. The intermediate decay} branching fraction of



−

→

n

π

− is included in the denomina-tor of Eq. (1). For each ST mode i, the efficiency

ε

i

−π+π+(π0₎ is obtained by dividing the DT efficiency

ε

i

tag,−π+π+(π0₎ by the ST efficiency

ε

i

tag.After weighting

ε

i_−π+π+(π0₎ by the mode-by-mode ST yields in data, we find the overall average efficiencies

ε

−π+π+

= (

61

.

8

±

0

.

4

)

% and

ε

−π+π+π0

= (

29

.

0

±

0

.

2

)

%, where thebranching fractionfor

π

0

_→

_{γ γ}

_{is included.Substitutingthe} valuesofN_obs₋ π+π+(π0₎,N tot ¯− c,

ε

−π+π+(π 0₎ and

B(

−

→

n

π

−

)

in Eq.(1),weobtain

B(

_c+

→ 

−

π

+

π

+

)

= (

1

.

81

±

0

.

17

±

0

.

09

)

% and

B(

+

c

→ 

−

π

+

π

+

π

0

)

= (

2

.

11

±

0

.

33

±

0

.

14

)

%, where the first uncertaintiesare statistical,andthesecondare systematic,as de-scribedbelow.

With the DT technique, the BF measurement is insensitive to uncertainty in the ST efficiencies. The systematic uncertain-tiesinmeasuring

B(

+c

→ 

−

π

+

π

+

)

and

B(

+c

→ 

−

π

+

π

+

π

0

)

mainly arise from the efficiencies of

π

detection and identifica-tion, fits to the Mnπ−

−

Mn distributions and the signal mod-elling in the MC simulation. The systematicuncertainties in the

π

± tracking and identification are both determined to be 1.0% by studying a set of samples of e+e−

→

π

+

π

−

π

+

π

−, e+e−

→

K+K−

π

+

π

−ande+e−

→

pp

¯

π

+

π

−obtainedfromdatawithc.m. energy above 4.0 GeV. The

π

0 _{reconstruction efficiency is} val-idated by analyzing DT events with D

¯

0

_→

_K+

_π

− _{or K}+

_π

−

_π

0

Table 2

Summaryofthe relativesystematicuncertaintiessyst_−_π+_π+ andsyst_−π+π+π0 in

B(+

c → −π+π+)andB(c+→ −π+π+π0),respectively.

Source syst_−π+π+[%] syst_−π+π+π0[%]

π±tracking 3.0 3.0 π±identification 3.0 3.0 π0_{reconstruction · · · 2.0} Fit to Mn−Mnπ− 2.0 3.6 Signal modelling 2.0 2.0 MC statistics 0.6 0.7 Ntot ¯− c 1.0 1.0 Total 5.2 6.4

versus D0

→

K−

π

+

π

0 _{[14].The differenceofthe}

_π

0 reconstruc-tionefficienciesbetweendataandMCsimulationsisestimatedto be 2.0%.Theuncertaintyfromthefit tothe Mnπ−

−

Mn distribu-tionisevaluatedbycheckingtherelativechangesofN_obs₋

π+π+(π0₎ with different choices for signal shapes (double Gaussian func-tion), background shapes (first-order polynomial function, third-order polynomial function and a MC-derived background shape) andfitranges(

(

0

.

19

,

0

.

34

)

GeV

/

c2).The uncertaintyinmodelling the signal process is obtained by varying the reweighting fac-tors of the observed kinematic variables within their statistical uncertainties and extracting the difference of the resultant effi-ciencies. The difference is estimated to be 2.0% for the studied channels and is taken as the systematic uncertainty due to the signal modelling. In addition, there are systematic uncertainties in obtaining Ntot_¯−

c evaluated by using alternative signal shapesin

the fits to the MBC spectra [13], resulting in an uncertainty of 1.0%, and in the statistical limitation of the MC samples, which isestimatedtobe0.6 (0.7)%for



+_c

→ 

−

π

+

π

+

(

π

0

₎

_.The uncer-taintiesfromtheBFsof



−

→

n

π

− and

π

0

_→

_{γ γ}

_{arenegligible.} All ofthe above systematic uncertainties are summarized in Ta-ble 2,andthetotaluncertaintiesareevaluatedtobe5.2%and6.4% for

B(

+_c

→ 

−

π

+

π

+

)

and

B(

+_c

→ 

−

π

+

π

+

π

0

₎

_{,respectively,} bycombiningallitemsinquadrature.

4. Summary

Based on an e+e− collision data sample with an integrated luminosity of 567 pb−1 taken at

√

s

=

4

.

6 GeV with the BE-SIII detector,we report the first observation ofthe decay



+_c

→



−

π

+

π

+

π

0 _{and the first absolute BF measurement for}

_

+

c

→



−

π

+

π

+.The results are

B(

+_c

→ 

−

π

+

π

+

)

= (

1

.

81

±

0

.

17

±

0

.

09

)

% and

B(

+_c

→ 

−

π

+

π

+

π

0

₎

_{= (}

₂

_.

₁₁

_±

₀

_.

₃₃

_±

₀

_.

₁₄

₎

_%,where thefirstuncertaintiesarestatisticalandthesecondaresystematic. Ourresultfor

B(

+_c

→ 

−

π

+

π

+

)

isconsistentwithandmore precise than the previous result [3]. BESIII measured the BF of the isospin symmetric channel

B(

+_c

→ 

+

π

+

π

−

)

= (

4

.

25

±

0

.

24

±

0

.

20

)

%[15].Thisallowsustodeterminetheratio

B(

+_c

→



−

π

+

π

+

)/

B(

_c+

→ 

+

π

+

π

−

)

=

0

.

42

±

0

.

05

±

0

.

02,where the first uncertaintyis statisticalandthesecond systematic. The sta-tisticaluncertainty oftheratiodominates,asmanycommon sys-tematicuncertaintiescancel.Thisisconsistentwithandmore pre-cisethanthevaluepreviouslymeasuredbytheE687 Collaboration

(

0

.

53

±

0

.

15

±

0

.

07

)

[16].

Acknowledgements

The BESIII Collaboration thanks the staff of BEPCII and the IHEPcomputingcenterfortheirstrongsupport.Thisworkis sup-ported in part by National Key Basic Research Program ofChina underContractNo.2015CB856700;NationalNaturalScience Foun-dationofChina(NSFC)underContractsNos.11125525,11235011,

11275266,11305180, 11322544,11322544, 11335008, 11425524, 11505010; the Chinese Academy of Sciences (CAS) Large-Scale Scientific Facility Program; the CAS Center for Excellencein Par-ticle Physics (CCEPP); Joint Large-Scale Scientific Facility Funds ofthe NSFC andCAS under Contracts Nos. U1332201,U1532257, U1532258; CAS under Contracts Nos. KJCX2-YW-N29, KJCX2-YW-N45, QYZDJ-SSW-SLH003; 100 Talents Program of CAS; National 1000TalentsProgram ofChina; INPACand Shanghai Key Labora-toryforParticlePhysicsandCosmology;GermanResearch Founda-tionDFG underContracts Nos.Collaborative ResearchCenter CRC 1044,FOR2359;IstitutoNazionalediFisicaNucleare,Italy; Konin-klijke Nederlandse Akademie van Wetenschappen (KNAW) under ContractNo.530-4CDP03;MinistryofDevelopmentofTurkey un-der Contract No. DPT2006K-120470; National Science and Tech-nology fund; The Swedish Research Council; U.S. Department of EnergyunderContractsNos.DE-FG02-05ER41374,DE-SC-0010118, DE-SC-0010504, DE-SC-0012069; University of Groningen (RuG) and the Helmholtzzentrum fuer Schwerionenforschung GmbH (GSI),Darmstadt; WCU Program ofNational ResearchFoundation ofKorea underContractNo.R32-2008-000-10155-0. Thispaperis also supported by Beijing municipal government under Contract Nos.KM201610017009,2015000020124G064,CIT&TCD201704047.

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