JHEP03(2022)090
Published o SISSA by Sp inge
Recei ed:Decembe 30, 2021
Accep ed:Feb ua y 25, 2022
Published:Ma ch 15, 2022
Fi s measu emen o he Λ+
c→pη0decay
BELLE
The BELLE collabo a ion
S. X. Li,14 J. X. Cui,14 C. P. Shen,14 I. Adachi,20,16 H. Aiha a,84 S. Al Said,79,39
D. M. Asne ,3H. A macan,8T. Aushe ,22 R. Ayad,79 V. Babu,9P. Behe a,27
K. Belous,29 M. Bessne ,19 V. Bha dwaj,24 B. Bhuyan,25 T. Bilka,5D. Bod o ,22,45
G. Bon icini,88 J. Bo ah,25 A. Bozek,60 M. B ačko,49,36 P. B anchini,32
T. E. B owde ,19 A. Budano,32 M. Campajola,31,56 D. Če enko ,5M.-C. Chang,13
P. Chang,59 V. Chekelian,50 A. Chen,58 B. G. Cheon,18 K. Chilikin,45 H. E. Cho,18
K. Cho,41 S.-J. Cho,90 S.-K. Choi,7Y. Choi,77 S. Choudhu y,34 D. Cinab o,88
S. Cunli e,9S. Das,48 N. Dash,27 G. De Na do,31,56 G. De Pie o,32 R. Dhamija,26
F. Di Capua,31,56 Z. Doležal,5T. V. Dong,11 D. Epi ano ,4,64 T. Fe be ,9
D. Fe lewicz,51 B. G. Fulsom,66 R. Ga g,67 V. Gau ,87 N. Gabyshe ,4,64 A. Gi i,26
P. Goldenzweig,37 B. Golob,46,36 E. G aziani,32 T. Gu,68 T. Ha a,20,16 K. Hayasaka,62
H. Hayashii,57 W.-S. Hou,59 K. Inami,55 A. Ishikawa,20,16 M. Iwasaki,65 Y. Iwasaki,20
W. W. Jacobs,28 E.-J. Jang,17 S. Jia,14 Y. Jin,84 K. K. Joo,6J. Kahn,37
A. B. Kaliya ,80 K. H. Kang,38 Y. Ka o,55 T. Kawasaki,40 H. Kichimi,20 C. Kiesling,50
C. H. Kim,18 D. Y. Kim,76 Y.-K. Kim,90 K. Kinoshi a,8P. Kodyš,5T. Konno,40
A. Ko obo ,4,64 S. Ko pa ,49,36 E. Ko alenko,4,64 P. K ižan,46,36 R. K oege ,52
P. K oko ny,4,64 M. Kuma ,48 R. Kuma ,69 K. Kuma a,88 Y.-J. Kwon,90 T. Lam,87
M. Lau enza,32,72 S. C. Lee,43 J. Li,43 L. K. Li,8Y. Li,14 L. Li Gioi,50 J. Libby,27
D. Li en se ,88,20 A. Ma ini,9M. Masuda,83,70 T. Ma suda,53 D. Ma ienko,4,64,45
S. K. Mau ya,25 F. Meie ,10 M. Me ola,31,56 K. Miyabayashi,57 R. Mizuk,45,22
R. Mussa,33 M. Nakao,20,16 D. Na wal,25 Z. Na kaniec,60 A. Na ochii,19 L. Nayak,26
N. K. Nisa ,3S. Nishida,20,16 K. Nishimu a,19 K. Ogawa,62 S. Ogawa,81 H. Ono,61,62
P. Oskin,45 P. Pakhlo ,45,54 G. Pakhlo a,22,45 T. Pang,68 S. Pa di,31 S.-H. Pa k,20
S. Pa a,24 T. K. Pedla ,47 R. Pes o nik,36 L. E. Piilonen,87 T. Podobnik,46,36
V. Popo ,22 M. T. P im,2M. Röh ken,9A. Ros omyan,9N. Rou ,27 G. Russo,56
D. Sahoo,34 S. Sandilya,26 A. Sangal,8T. Sanuki,82 V. Sa ino ,68 G. Schnell,1,23
J. Schuele ,19 C. Schwanda,30 A. J. Schwa z,8Y. Seino,62 K. Senyo,89
M. E. Se io ,51 M. Shapkin,29 C. Sha ma,48 V. Shebalin,19 J.-G. Shiu,59
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JHEP03(2022)090
B. Shwa z,4,64 F. Simon,50 E. Solo ie a,45 S. S anič,63 M. S a ič,36 Z. S. S o le ,87
M. Sumihama,15,70 K. Sumisawa,20,16 T. Sumiyoshi,86 W. Su cli e,2
M. Takizawa,74,21,71 U. Tamponi,33 K. Tanida,35 F. Tenchini,9K. T abelsi,44
M. Uchida,85 Y. Unno,18 K. Uno,62 S. Uno,20,16 P. U quijo,51 S. E. Vahsen,19
R. Van Tonde ,2G. Va ne ,19 A. Vinoku o a,4,64 E. Waheed,20 D. Wang,12
E. Wang,68 M.-Z. Wang,59 S. Wa anuki,90 E. Won,42 X. Xu,75 B. D. Yabsley,78
W. Yan,73 H. Ye,9J. H. Yin,42 Y. Yusa,62 Y. Zhai,34 V. Zhilich4,64 and V. Zhuko a45
1Depa men o Physics, Uni e si y o he Basque Coun y UPV/EHU, 48080 Bilbao, Spain
2Uni e si y o Bonn, 53115 Bonn, Ge many
3B ookha en Na ional Labo a o y, Up on, New Yo k 11973, U.S.A.
4Budke Ins i u e o Nuclea Physics SB RAS, No osibi sk 630090, Russian Fede a ion
5Facul y o Ma hema ics and Physics, Cha les Uni e si y, 121 16 P ague, The Czech Republic
6Chonnam Na ional Uni e si y, Gwangju 61186, Sou h Ko ea
7Chung-Ang Uni e si y, Seoul 06974, Sou h Ko ea
8Uni e si y o Cincinna i, Cincinna i, OH 45221, U.S.A.
9Deu sches Elek onen-Synch o on, 22607 Hambu g, Ge many
10Duke Uni e si y, Du ham, NC 27708, U.S.A.
11Ins i u e o Theo e ical and Applied Resea ch (ITAR), Duy Tan Uni e si y,
Hanoi 100000, Vie nam
12Uni e si y o Flo ida, Gaines ille, FL 32611, U.S.A.
13Depa men o Physics, Fu Jen Ca holic Uni e si y, Taipei 24205, Taiwan
14Key Labo a o y o Nuclea Physics and Ion-beam Applica ion (MOE)
and Ins i u e o Mode n Physics, Fudan Uni e si y, Shanghai 200443, PR China
15Gi u Uni e si y, Gi u 501-1193, Japan
16SOKENDAI (The G adua e Uni e si y o Ad anced S udies), Hayama 240-0193, Japan
17Gyeongsang Na ional Uni e si y, Jinju 52828, Sou h Ko ea
18Depa men o Physics and Ins i u e o Na u al Sciences, Hanyang Uni e si y,
Seoul 04763, Sou h Ko ea
19Uni e si y o Hawaii, Honolulu, HI 96822, U.S.A.
20High Ene gy Accele a o Resea ch O ganiza ion (KEK), Tsukuba 305-0801, Japan
21J-PARC B anch, KEK Theo y Cen e , High Ene gy Accele a o Resea ch O ganiza ion (KEK),
Tsukuba 305-0801, Japan
22Na ional Resea ch Uni e si y Highe School o Economics, Moscow 101000, Russian Fede a ion
23IKERBASQUE, Basque Founda ion o Science, 48013 Bilbao, Spain
24Indian Ins i u e o Science Educa ion and Resea ch Mohali, SAS Naga , 140306, India
25Indian Ins i u e o Technology Guwaha i, Assam 781039, India
26Indian Ins i u e o Technology Hyde abad, Telangana 502285, India
27Indian Ins i u e o Technology Mad as, Chennai 600036, India
28Indiana Uni e si y, Blooming on, IN 47408, U.S.A.
29Ins i u e o High Ene gy Physics, P o ino 142281, Russian Fede a ion
30Ins i u e o High Ene gy Physics, Vienna 1050, Aus ia
31INFN — Sezione di Napoli, I-80126 Napoli, I aly
32INFN — Sezione di Roma T e, I-00146 Roma, I aly
33INFN — Sezione di To ino, I-10125 To ino, I aly
JHEP03(2022)090
34Iowa S a e Uni e si y, Ames, Iowa 50011, U.S.A.
35Ad anced Science Resea ch Cen e , Japan A omic Ene gy Agency, Naka 319-1195, Japan
36J. S e an Ins i u e, 1000 Ljubljana, Slo enia
37Ins i u ü Expe imen elle Teilchenphysik, Ka ls uhe Ins i u ü Technologie,
76131 Ka ls uhe, Ge many
38Ka li Ins i u e o he Physics and Ma hema ics o he Uni e se (WPI), Uni e si y o Tokyo,
Kashiwa 277-8583, Japan
39Depa men o Physics, Facul y o Science, King Abdulaziz Uni e si y, Jeddah 21589, Saudi A abia
40Ki asa o Uni e si y, Sagamiha a 252-0373, Japan
41Ko ea Ins i u e o Science and Technology In o ma ion, Daejeon 34141, Sou h Ko ea
42Ko ea Uni e si y, Seoul 02841, Sou h Ko ea
43Kyungpook Na ional Uni e si y, Daegu 41566, Sou h Ko ea
44Uni e si é Pa is-Saclay, CNRS/IN2P3, IJCLab, 91405 O say, F ance
45P.N. Lebede Physical Ins i u e o he Russian Academy o Sciences,
Moscow 119991, Russian Fede a ion
46Facul y o Ma hema ics and Physics, Uni e si y o Ljubljana, 1000 Ljubljana, Slo enia
47Lu he College, Deco ah, IA 52101, U.S.A.
48Mala iya Na ional Ins i u e o Technology Jaipu , Jaipu 302017, India
49Facul y o Chemis y and Chemical Enginee ing, Uni e si y o Ma ibo , 2000 Ma ibo , Slo enia
50Max-Planck-Ins i u ü Physik, 80805 München, Ge many
51School o Physics, Uni e si y o Melbou ne, Vic o ia 3010, Aus alia
52Uni e si y o Mississippi, Uni e si y, MS 38677, U.S.A.
53Uni e si y o Miyazaki, Miyazaki 889-2192, Japan
54Moscow Physical Enginee ing Ins i u e, Moscow 115409, Russian Fede a ion
55G adua e School o Science, Nagoya Uni e si y, Nagoya 464-8602, Japan
56Uni e si à di Napoli Fede ico II, I-80126 Napoli, I aly
57Na a Women’s Uni e si y, Na a 630-8506, Japan
58Na ional Cen al Uni e si y, Chung-li 32054, Taiwan
59Depa men o Physics, Na ional Taiwan Uni e si y, Taipei 10617, Taiwan
60H. Niewodniczanski Ins i u e o Nuclea Physics, K akow 31-342, Poland
61Nippon Den al Uni e si y, Niiga a 951-8580, Japan
62Niiga a Uni e si y, Niiga a 950-2181, Japan
63Uni e si y o No a Go ica, 5000 No a Go ica, Slo enia
64No osibi sk S a e Uni e si y, No osibi sk 630090, Russian Fede a ion
65Osaka Ci y Uni e si y, Osaka 558-8585, Japan
66Paci ic No hwes Na ional Labo a o y, Richland, WA 99352, U.S.A.
67Panjab Uni e si y, Chandiga h 160014, India
68Uni e si y o Pi sbu gh, Pi sbu gh, PA 15260, U.S.A.
69Punjab Ag icul u al Uni e si y, Ludhiana 141004, India
70Resea ch Cen e o Nuclea Physics, Osaka Uni e si y, Osaka 567-0047, Japan
71Meson Science Labo a o y, Clus e o Pionee ing Resea ch, RIKEN, Sai ama 351-0198, Japan
72Dipa imen o di Ma ema ica e Fisica, Uni e si à di Roma T e, I-00146 Roma, I aly
73Depa men o Mode n Physics and S a e Key Labo a o y o Pa icle De ec ion and Elec onics,
Uni e si y o Science and Technology o China, He ei 230026, PR China
JHEP03(2022)090
74Showa Pha maceu ical Uni e si y, Tokyo 194-8543, Japan
75Soochow Uni e si y, Suzhou 215006, China
76Soongsil Uni e si y, Seoul 06978, Sou h Ko ea
77Sungkyunkwan Uni e si y, Suwon 16419, Sou h Ko ea
78School o Physics, Uni e si y o Sydney, New Sou h Wales 2006, Aus alia
79Depa men o Physics, Facul y o Science, Uni e si y o Tabuk, Tabuk 71451, Saudi A abia
80Ta a Ins i u e o Fundamen al Resea ch, Mumbai 400005, India
81Toho Uni e si y, Funabashi 274-8510, Japan
82Depa men o Physics, Tohoku Uni e si y, Sendai 980-8578, Japan
83Ea hquake Resea ch Ins i u e, Uni e si y o Tokyo, Tokyo 113-0032, Japan
84Depa men o Physics, Uni e si y o Tokyo, Tokyo 113-0033, Japan
85Tokyo Ins i u e o Technology, Tokyo 152-8550, Japan
86Tokyo Me opoli an Uni e si y, Tokyo 192-0397, Japan
87Vi ginia Poly echnic Ins i u e and S a e Uni e si y, Blacksbu g, VA 24061, U.S.A.
88Wayne S a e Uni e si y, De oi , MI 48202, U.S.A.
89Yamaga a Uni e si y, Yamaga a 990-8560, Japan
90Yonsei Uni e si y, Seoul 03722, Sou h Ko ea
E-mail: [email p o ec ed]
Abs ac : We p esen he i s measu emen o he b anching ac ion o he singly
Cabibbo-supp essed (SCS) decay Λ+
c→pη0wi h η0→ηπ+π−, using a da a sample co -
esponding o an in eg a ed luminosi y o 981 b−1, collec ed by he Belle de ec o a he
KEKB e+e−asymme ic-ene gy collide . A signi ican Λ+
c→pη0signal is obse ed o he
i s ime wi h a signal signi icance o 5.4σ. The ela i e b anching ac ion wi h espec
o he no maliza ion mode Λ+
c→pK−π+is measu ed o be
B(Λ+
c→pη0)
B(Λ+
c→pK−π+)= (7.54 ±1.32 ±0.73) ×10−3,
whe e he unce ain ies a e s a is ical and sys ema ic, espec i ely. Using he wo ld-a e age
alue o B(Λ+
c→pK−π+) = (6.28 ±0.32) ×10−2, we ob ain
B(Λ+
c→pη0) = (4.73 ±0.82 ±0.46 ±0.24) ×10−4,
whe e he unce ain ies a e s a is ical, sys ema ic, and om B(Λ+
c→pK−π+), espec i ely.
Keywo ds: B anching ac ion, e+-e−Expe imen s, Cha m Physics, Pa icle and Reso-
nance P oduc ion
A Xi eP in : 2112.14276
JHEP03(2022)090
Con en s
1 In oduc ion 1
2 The Belle de ec o and da a sample 1
3 Selec ion c i e ia 2
4 Signal and backg ound es ima ion 4
5 Sys ema ic unce ain ies 6
6 Conclusions 8
1 In oduc ion
Had onic decays o cha med ba yons p o ide an ideal labo a o y o unde s and he in e -
play o weak and s ong in e ac ions in he cha m sys em [1–3]. Decays o cha med ba yons
ecei e sizable non ac o izable con ibu ions om W-exchange diag ams, which a e subjec
o colo and helici y supp ession [4–7]. The e o e, he s udy o non ac o izable con ibu-
ions is c i ical o unde s and he dynamics o cha med ba yon decays. To a oid heo e ical
di icul ies in he ac o iza ion app oach [4], one can use he SU(3)F la o symme y o
ela e he ampli udes among di e en decays [5,8,9]. O he heo e ical app oaches p o-
ide calcula ions based on dynamical models [10–12]. Fo he singly Cabibbo-supp essed
(SCS) decay Λ+
c→pη0, heo e ical p edic ions on i s b anching ac ion unde di e en
assump ions a y by mo e han an o de o magni ude as lis ed in able 1. Cu en ly, his
decay has no ye been obse ed.
In his s udy, based on an e+e−annihila ion da a sample o 981 b−1collec ed by
he Belle expe imen , we measu e he b anching ac ion o he signal mode Λ+
c→pη0
wi h espec o he no maliza ion mode Λ+
c→pK−π+. Th oughou his pape , cha ge-
conjuga e modes a e implici ly included unless s a ed o he wise. The pape is o ganized as
ollows. Sec ion 2in oduces he Belle de ec o and da a sample. Sec ion 3discusses he
e en selec ion c i e ia. The signal and backg ound es ima ions a e p esen ed in sec ion 4.
Sec ions 5and 6desc ibe he sys ema ic unce ain y and conclusion, espec i ely.
2 The Belle de ec o and da a sample
This measu emen is based on a da a sample co esponding o an in eg a ed luminosi y
o 981 b−1, collec ed wi h he Belle de ec o a he KEKB asymme ic-ene gy e+e−col-
lide [14,15]. Abou 70% o he da a we e eco ded a he Υ(4S) esonance, and he es
– 1 –
JHEP03(2022)090
SU(3)Fsymme y [5]SU(3)Fsymme y [13] Cons i uen qua k model [3]
B(Λ+
c→pη0) 0.4−0.6 1.22+1.43
−0.87 0.04 −0.2
Table 1. Compa ison o di e en heo e ical p edic ions o B(Λ+
c→pη0)(in uni s o 10−3).
we e collec ed a o he Υ(nS)(n= 1, 2, 3, o 5) s a es o a cen e -o -mass (CM) ene gies
a ew ens o MeV below he Υ(4S)o he Υ(nS)peaks.
The Belle de ec o is a la ge-solid-angle magne ic spec ome e ha consis s o a sil-
icon e ex de ec o (SVD), a 50-laye cen al d i chambe (CDC), an a ay o ae ogel
h eshold Che enko coun e s (ACC), a ba el-like a angemen o ime-o - ligh scin il-
la ion coun e s (TOF), and an elec omagne ic calo ime e comp ised o CsI(Tl) c ys als
(ECL) loca ed inside a supe conduc ing solenoid coil ha p o ides a 1.5 T magne ic ield.
An i on lux- e u n loca ed ou side o he coil is ins umen ed o de ec K0
Lmesons and
o iden i y muons. The de ec o is desc ibed in de ail elsewhe e [16,17]. The o igin o he
coo dina e sys em is de ined as he posi ion o he nominal in e ac ion poin , and he axis
aligning wi h he di ec ion opposi e he e+beam is de ined as he zaxis. Mon e Ca lo (MC)
simula ed e en s a e used o op imize he selec ion c i e ia, s udy backg ounds, and de e -
mine he signal econs uc ion e iciency. Samples o simula ed signal MC e en s a e gene -
a ed by E Gen [18] and p opaga ed h ough a de ec o simula ion based on gean 3 [19].
The e+e−→c¯ce en s a e simula ed using py hia [20]; he decays Λ+
c→pK−π+and
η0→ηπ+π−a e gene a ed wi h a phase space model. We ake in o accoun he e ec o
inal-s a e adia ion om cha ged pa icles by using he pho os package [22]. Simula ed
samples o Υ(4S)→B+B−/B0¯
B0,Υ(5S)→B(∗)
s¯
B(∗)
s/B(∗)¯
B(∗)(π)/Υ(4S)γ,e+e−→q¯q
(q=u, d, s, c)a √s= 10.52, 10.58, and 10.867 GeV, and Υ(1S, 2S, 3S)decays, no -
malized o he same in eg a ed luminosi y as eal da a, a e used o de elop he selec ion
c i e ia and pe o m he backg ound s udy [23].
3 Selec ion c i e ia
Selec ion c i e ia a e op imized by maximizing a igu e-o -me i /(a
2+√nB)[24], whe e
is he signal e iciency; ais he a ge signal signi icance exp essed in s anda d de ia ions
in a one-sided Gaussian es , selec ed o be 5; nBis he numbe o backg ound e en s
expec ed in a wo-dimensional signal egion o η0and Λ+
csignals, which is de ined as (0.95,
0.965) GeV/c2in M(ηπ+π−)and (2.27, 2.31) GeV/c2in M(pη0).
We econs uc he decays Λ+
c→pη0and Λ+
c→pK−π+, wi h he η0decay econ-
s uc ed in he cascade ηπ+π−,η→γγ. Final-s a e cha ged acks a e iden i ied as p,K,
o πcandida es using in o ma ion om he cha ged-had on iden i ica ion sys ems (ACC,
TOF, CDC) combined in o a likelihood a io, R(h|h0) = L(h)/(L(h)+L(h0)), whe e hand
h0a e π,K, o pas app op ia e [25]. T acks ha ing R(p|π)>0.9and R(p|K)>0.9a e
iden i ied as p o on candida es; cha ged kaon candida es a e equi ed o ha e R(K|p)>0.4
and R(K|π)>0.9; and cha ged pion candida es o ha e R(π|p)>0.4and R(π|K)>0.4.
A likelihood a io o elec on iden i ica ion, R(e), is o med om ACC, CDC, and ECL
in o ma ion [26], and is equi ed o be less han 0.9 o all cha ged acks o supp ess elec-
– 2 –
JHEP03(2022)090
]
2
) [GeV/c
-
π
+
πηM(
0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1
2
E en s/ 1 MeV/c
0
500
1000
1500
2000
2500
Figu e 1. The in a ian mass dis ibu ion o ηπ+π−. The egion be ween wo ed lines is selec ed
as he η0signal egion, and he egions be ween wo blue lines a e he η0sidebands.
ons. The iden i ica ion e iciencies o p,K, and πa e 82%, 70%, and 97%, espec i ely.
The p obabili ies o misiden i ying has h0,P(h→h0), a e es ima ed o be 3% [P(p→π)],
7% [P(p→K)], 10% [P(K→π)], 2% [P(K→p)], 5% [P(π→K)], and 1% [P(π→p)].
Fo each cha ged ack, he dis ance o he closes app oach wi h espec o he in e ac ion
poin along he zaxis and in he ans e se x−yplane is equi ed o be less han 2.0 cm
and 0.1 cm, espec i ely. Each ack mus ha e a leas one SVD hi in bo h he zdi ec ion
and he x−yplane.
Pho on candida es a e selec ed om ECL clus e s no associa ed wi h any cha ged
acks. The pho on ene gy is equi ed o be g ea e han 90 MeV in he ba el egion
(−0.63 <cosθ < 0.85) and g ea e han 120 MeV in he endcap egions (−0.91 <cosθ <
−0.63 o 0.85 <cosθ < 0.98) o he ECL, whe e θis he pola angle ela i e o he posi i e
zaxis. To ejec neu al had ons, he a io o he ene gy deposi ed in he cen al 3×3
a ay o ECL c ys als o he o al ene gy deposi ed in he enclosing 5×5a ay o c ys als
is equi ed o be a leas 0.9 o each pho on candida e.
The ηcandida es a e econs uc ed ia hei decay o wo pho ons. The γγ in a ian
mass is equi ed o sa is y 0.45 < M(γγ)<0.65 GeV/c2, and hen a mass-cons ained i
is pe o med o ηcandida es o imp o e he momen um esolu ion. The co esponding χ2
alue o he mass-cons ained i on η(χ2
η) is equi ed o be less han 10. To u he supp ess
backg ound e en s, we emo e ηcandida es in which ei he o he daugh e pho ons can
be combined wi h o he pho ons in he e en o o m π0→γγ candida es sa is ying
|M(γγ)−mπ0|<12 MeV/c2, whe e mπ0is he nominal π0mass [21]. Wi h his e o, we
ejec 42% o he backg ound, while e aining 83% o he signal.
The η0candida es a e econs uc ed by combining wo opposi e-cha ge π acks wi h
an ηcandida e. The in a ian mass dis ibu ion o ηπ+π− om da a is shown in igu e 1.
Candida es η0a e e ained i 0.950 < M(ηπ+π−)<0.965 GeV/c2, co esponding o an
e iciency o 96%. The η0sidebands a e de ined as 0.915 o 0.930 GeV/c2and 0.980 o
0.995 GeV/c2, which a e he egions be ween wo blue lines in he M(ηπ+π−)dis ibu ion.
Candida es o Λ+
c→pK−π+and Λ+
c→pη0decays a e econs uc ed by combining
p,K−,π+candida es, and p,η0candida es, espec i ely. A e ex i is pe o med wi h
he h ee cha ged acks o supp ess combina o ial backg ound e en s. The esul ing i
– 3 –
JHEP03(2022)090
2
E en s / 0.002 GeV/c
0
50
100
150
200
250
300
350
3
10×
]
2
) [GeV/c
+
π
-
M(pK
2.24 2.26 2.28 2.3 2.32 2.34
Pull
-4
-2
0
2
4
2
E en s / 0.005 GeV/c
0
50
100
150
200
250
]
2
') [GeV/cηM(p
2.15 2.2 2.25 2.3 2.35 2.4
Pull
-2
0
2
Figu e 2. Fi s o he in a ian mass dis ibu ions o he pK−π+(Le ) and pη0combina ions
(Righ ). Black do s wi h e o ba s ep esen he da a; ed solid lines ep esen he o al i ed esul ;
blue dashed lines ep esen he signal shape; magen a do -dashed lines ep esen he backg ound
shape; and he g een his og am is om no malized η0sidebands.
quali y is labeled χ2
x. Fo Λ+
c→pK−π+, he χ2
x is equi ed o be less han 40, while o
Λ+
c→pη0,χ2
x <15 is equi ed. Fo bo h χ2
x equi emen s, he e iciency is la ge han
98%. A scaled momen um o xp>0.53 is equi ed o supp ess backg ound, especially om
B-meson decays, whe e xp=p∗/pE2
cm/4c2−M2c2,Ecm is he CM ene gy, and p∗and M
a e he momen um and in a ian mass, espec i ely, o he Λ+
ccandida es in he CM ame.
A e he p elimina y selec ion, abou 0.8% o he Λ+
c→pK−π+e en s and 13.3% o
he Λ+
c→pη0e en s ha e wo o mo e Λ+
ccandida es. We choose he bes ηcandida e
acco ding o he smalles alue o χ2
η; he a e o e en s ha ing mul iple Λ+
c→pη0candi-
da es wi h his c i e ion is 1.6%. Fo such mul i-candida e e en s, we choose a single Λ+
c
candida e andomly. This bes -candida e selec ion, based on he MC simula ion, iden i ies
he co ec candida e 65% o he ime. The pη0mass dis ibu ion o w ong-combina ion
simula ed signal e en s is ound o be smoo h. We keep all candida es o Λ+
c→pK−π+in
mul iple-candida e e en s as he mul iplici y is negligible.
4 Signal and backg ound es ima ion
Wi h he abo e selec ion c i e ia applied, he in a ian mass dis ibu ions o no maliza ion
and signal modes a e shown in igu e 2. F om a s udy o gene ic MC samples [23], no known
peaking backg ound p ocesses con ibu e o mass dis ibu ions in he Λ+
csignal egion.
To ex ac he numbe o signal e en s, we pe o m an unbinned maximum-likelihood
i o he M(pK−π+)o M(pη0)dis ibu ion. The likelihood unc ion is de ined in e ms
o a signal PDF (FS) and a backg ound PDF (FB) as
L=e−(nS+nB)
N!
N
Y
i
[nSFS(Mi) + nBFB(Mi)] ,(4.1)
whe e Nis he o al numbe o obse ed e en s; nSand nBa e he numbe s o signal e en s
and backg ound e en s, espec i ely; Mis pK−π+o pη0in a ian mass; and ideno es he
– 4 –
JHEP03(2022)090
e en index. The i is pe o med o candida e e en s su i ing he selec ion c i e ia; nS
and nBa e ee pa ame e s in he i .
Fo he Λ+
c→pK−π+channel, we ex ac he Λ+
csignal yields by i ing he
M(pK−π+)dis ibu ion. The signal PDF is a sum o wo Gaussian unc ions wi h a
common mean, and he backg ound PDF is a second-o de polynomial. All pa ame e s
o FSand FBa e loa ed. The i esul is shown in igu e 2(Le ), along wi h he pull
dis ibu ion. The i ed signal yield is Nno m = 1472190 ±5726, whe e he unce ain y is
s a is ical. F om he MC simula ion, he mass esolu ion o Λ+
c→pK−π+is 8 MeV/c2.
Fo he Λ+
c→pη0channel, we i s check he M(pπ+π−η)dis ibu ion om no malized
η0sidebands, as shown in igu e 2(Righ ). The dis ibu ion om he no malized η0side-
bands is smoo hly alling, indica ing a negligible con ibu ion om Λ+
c→pπ+π−ηdecays.
We subsequen ly i he M(pη0)dis ibu ion o ex ac he Λ+
csignal yield. A sum o a
Gaussian unc ion and a C ys al Ball (CB) unc ion [27] is used as he signal PDF, and
a second-o de polynomial as he backg ound PDF. The Gaussian and CB unc ions a e
ixed o ha e a common mean. All o he pa ame e s a e loa ed in he i . The i esul ,
along wi h he pull dis ibu ion, is shown in igu e 2(Righ ). A clea Λ+
csignal is obse ed
in he M(pη0)dis ibu ion. The i ed signal yield is Nsig = 294±52, whe e he unce ain y
is s a is ical. The mass esolu ion o Λ+
c→pη0is 13 MeV/c2 om he MC simula ion,
which is he hal -wid h a hal maximum. The s a is ical signi icance o he Λ+
csignal is
6.3σ, calcula ed om he di e ence o he loga i hmic likelihoods, −2ln(L0/Lmax) = 59.2,
whe e L0and Lmax a e he maximized likelihoods wi hou and wi h a signal componen ,
espec i ely [28]. The signi icance akes in o accoun he di e ence in he numbe o de-
g ees o eedom in he wo i s (∆nd =7). Since he la ges sys ema ic unce ain y is due
o he i , as desc ibed in sec ion 5, al e na i e i s o he M(pη0)spec um unde di e en
i condi ions a e pe o med and he Λ+
csignal signi icance is la ge han 5.4σin all cases.
To measu e he b anching ac ion, we mus di ide hese ex ac ed signal yields by
hei econs uc ion e iciencies. Since Λ+
c→pη0is a wo-body decay and η0→ηπ+π−
is well modeled by he phase space [29], we es ima e he econs uc ion e iciency di ec ly
om he simula ed e en s by he a io nsel/ngen, whe e nsel and ngen a e he numbe s o
ue signal e en s su i ing he selec ion c i e ia and gene a ed e en s, espec i ely. The
signal mode econs uc ion e iciency is de e mined o be sig = (2.22 ±0.02)%. Howe e ,
he econs uc ion e iciency o he decay Λ+
c→pK−π+can a y ac oss he h ee-body
phase space, as isualized in a Dali z plo [30] wi h pola iza ion neglec ed. To ake his
in o accoun , we co ec he econs uc ion e iciency acco ding o he Dali z plo om
da a as ollows. Figu e 3shows he Dali z dis ibu ion o M2(pK−) e sus M2(K−π+)
in he Λ+
c→pK−π+signal egion om da a which is de ined as 2.274 <M(pK−π+)<
2.298 GeV/c2. The numbe o backg ound e en s has been sub ac ed using he no malized
Λ+
csidebands de ined as (2.260, 2.272) GeV/c2and (2.300, 2.312) GeV/c2. The e ec o
he a ia ion o he kinema ic bounda ies wi h M(pK−π+)is neglec ed. We di ide he
Dali z plo o he da a in o 120×120 bins, wi h a bin size o 0.027 GeV2/c4 o M2(pK−)
and 0.016 GeV2/c4 o M2(K−π+). The co ec ed econs uc ion e iciency is de e mined
– 5 –
