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Author(s)Observation of Υ(4S)→ 𝜂𝜂'Υ(1S)

Citation

Physical Review Letters,121(6),p.062001_1-062001_7

Title

Text Version

Guido E., Tanida Kiyoshi, Belle Collaboration, 168 of others
Published Journal Article

URL

https://jopss.jaea.go.jp/search/servlet/search?5064657

DOI

https://doi.org/10.1103/PhysRevLett.121.062001

Published by the American Physical Society under the terms of
the Creative Commons Attribution 4.0 International license.
Right

Further distribution of this work must maintain attribution to
the author(s) and the published article’s title, journal citation,
and DOI. Funded by SCOAP3.

Japan Atomic Energy Agency

PHYSICAL REVIEW LETTERS 121, 062001 (2018)

Observation of ϒð4SÞ → η0 ϒð1SÞ
E. Guido,26 R. Mussa,26 U. Tamponi,26,82 H. Aihara,79 S. Al Said,73,33 D. M. Asner,61 H. Atmacan,69
V. Aulchenko,3,59 T. Aushev,50 R. Ayad,73 V. Babu,74 I. Badhrees,73,32 A. M. Bakich,72 V. Bansal,61 P. Behera,20
M. Berger,70 V. Bhardwaj,17 J. Biswal,28 A. Bondar,3,59 G. Bonvicini,84 A. Bozek,56 M. Bračko,44,28 T. E. Browder,13D. Červenkov,4 V. Chekelian,45 A. Chen,53 B. G. Cheon,12 K. Chilikin,39,49 K. Cho,34 S.-K. Choi,11 Y. Choi,71
S. Choudhury,19 D. Cinabro,84 S. Cunliffe,61 S. Di Carlo,84 Z. Doležal,4 S. Eidelman,3,59 D. Epifanov,3,59 J. E. Fast,61
T. Ferber,6 B. G. Fulsom,61 R. Garg,62 V. Gaur,83 N. Gabyshev,3,59 A. Garmash,3,59 M. Gelb,30 A. Giri,19 P. Goldenzweig,30
J. Haba,14,10 T. Hara,14,10 H. Hayashii,52 M. T. Hedges,13 W.-S. Hou,55 K. Inami,51 G. Inguglia,6 A. Ishikawa,77 R. Itoh,14,10
M. Iwasaki,60 Y. Iwasaki,14 W. W. Jacobs,21 H. B. Jeon,37 S. Jia,2 Y. Jin,79 T. Julius,46 K. H. Kang,37 G. Karyan,6
T. Kawasaki,58 C. Kiesling,45 D. Y. Kim,68 J. B. Kim,35 K. T. Kim,35 S. H. Kim,12 K. Kinoshita,5 P. Kodyš,4 S. Korpar,44,28
D. Kotchetkov,13 P. Križan,40,28 R. Kroeger,47 P. Krokovny,3,59 R. Kulasiri,31 R. Kumar,64 T. Kumita,81 A. Kuzmin,3,59
Y.-J. Kwon,85 I. S. Lee,12 S. C. Lee,37 L. K. Li,22 Y. B. Li,63 L. Li Gioi,45 J. Libby,20 D. Liventsev,83,14 M. Lubej,28 T. Luo,8J. MacNaughton,14 M. Masuda,78 T. Matsuda,48 D. Matvienko,3,59 M. Merola,25 K. Miyabayashi,52 H. Miyata,58
R. Mizuk,39,49,50 G. B. Mohanty,74 H. K. Moon,35 T. Mori,51 E. Nakano,60 M. Nakao,14,10 T. Nanut,28 K. J. Nath,18
M. Nayak,84,14 M. Niiyama,36 S. Nishida,14,10 S. Ogawa,76 H. Ono,57,58 W. Ostrowicz,56 G. Pakhlova,39,50 B. Pal,5 S. Pardi,25
C. W. Park,71 S. Paul,75 T. K. Pedlar,42 R. Pestotnik,28 L. E. Piilonen,83 V. Popov,50 A. Rostomyan,6 G. Russo,25 Y. Sakai,14,10
M. Salehi,43,41 T. Sanuki,77 O. Schneider,38 G. Schnell,1,16 C. Schwanda,23 Y. Seino,58 V. Shebalin,3,59 C. P. Shen,2T.-A. Shibata,80 J.-G. Shiu,55 B. Shwartz,3,59 A. Sokolov,24 M. Starič,28 J. F. Strube,61 M. Sumihama,9 T. Sumiyoshi,81
M. Takizawa,67,15,65 K. Tanida,27 F. Tenchini,46 M. Uchida,80 T. Uglov,39,50 Y. Unno,12 Y. Usov,3,59 C. Van Hulse,1
G. Varner,13 A. Vinokurova,3,59 B. Wang,5 C. H. Wang,54 M. -Z. Wang,55 P. Wang,22 X. L. Wang,8 M. Watanabe,58
Y. Watanabe,29 E. Widmann,70 E. Won,35 H. Ye,6 J. Yelton,7 C. Z. Yuan,22 Y. Yusa,58 S. Zakharov,39,50 Z. P. Zhang,66
V. Zhilich,3,59 V. Zhukova,39,49 and V. Zhulanov3,59(Belle Collaboration)
1

University of the Basque Country UPV/EHU, 48080 Bilbao
2
Beihang University, Beijing 100191
3
Budker Institute of Nuclear Physics SB RAS, Novosibirsk 630090
4Faculty of Mathematics and Physics, Charles University, 121 16 Prague
5
University of Cincinnati, Cincinnati, Ohio 45221
6
Deutsches Elektronen-Synchrotron, 22607 Hamburg
7
University of Florida, Gainesville, Florida 32611
8
Fudan University, Shanghai 200443
9Gifu University, Gifu 501-1193
10
SOKENDAI (The Graduate University for Advanced Studies), Hayama 240-0193
11
Gyeongsang National University, Chinju 660-701
12
Hanyang University, Seoul 133-791
13
University of Hawaii, Honolulu, Hawaii 96822
14High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801
15
J-PARC Branch, KEK Theory Center, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801
16
IKERBASQUE, Basque Foundation for Science, 48013 Bilbao
17
Indian Institute of Science Education and Research Mohali, SAS Nagar, 140306
18
Indian Institute of Technology Guwahati, Assam 781039
19
Indian Institute of Technology Hyderabad, Telangana 502285
20
Indian Institute of Technology Madras, Chennai 600036
21
Indiana University, Bloomington, Indiana 47408
22
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049
23
Institute of High Energy Physics, Vienna 1050
24
Institute for High Energy Physics, Protvino 142281
25
INFN—Sezione di Napoli, 80126 Napoli
26
INFN—Sezione di Torino, 10125 Torino

0031-9007=18=121(6)=062001(7)062001-1

Published by the American Physical Society

PHYSICAL REVIEW LETTERS 121, 062001 (2018)27

Advanced Science Research Center, Japan Atomic Energy Agency, Naka 319-1195
28
J. Stefan Institute, 1000 Ljubljana
29
Kanagawa University, Yokohama 221-8686
30
Institut für Experimentelle Teilchenphysik, Karlsruher Institut für Technologie, 76131 Karlsruhe
31
Kennesaw State University, Kennesaw, Georgia 30144
32
King Abdulaziz City for Science and Technology, Riyadh 11442
33
Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589
34
Korea Institute of Science and Technology Information, Daejeon 305-806
35
Korea University, Seoul 136-713
36
Kyoto University, Kyoto 606-8502
37
Kyungpook National University, Daegu 702-701
38
École Polytechnique Fédérale de Lausanne (EPFL), Lausanne 1015
39
P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow 119991
40
Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana
41
Ludwig Maximilians University, 80539 Munich
42
Luther College, Decorah, Iowa 52101
43
University of Malaya, 50603 Kuala Lumpur
44
University of Maribor, 2000 Maribor
45
Max-Planck-Institut für Physik, 80805 München
46
School of Physics, University of Melbourne, Victoria 3010
47
University of Mississippi, University, Mississippi 38677
48
University of Miyazaki, Miyazaki 889-2192
49
Moscow Physical Engineering Institute, Moscow 115409
50
Moscow Institute of Physics and Technology, Moscow Region 141700
51
Graduate School of Science, Nagoya University, Nagoya 464-8602
52
Nara Women’s University, Nara 630-8506
53
National Central University, Chung-li 32054
54
National United University, Miao Li 36003
55
Department of Physics, National Taiwan University, Taipei 10617
56
H. Niewodniczanski Institute of Nuclear Physics, Krakow 31-342
57
Nippon Dental University, Niigata 951-8580
58
Niigata University, Niigata 950-2181
59
Novosibirsk State University, Novosibirsk 630090
60
Osaka City University, Osaka 558-8585
61
Pacific Northwest National Laboratory, Richland, Washington 99352
62
Panjab University, Chandigarh 160014
63
Peking University, Beijing 100871
64
Punjab Agricultural University, Ludhiana 141004
65
Theoretical Research Division, Nishina Center, RIKEN, Saitama 351-0198
66
University of Science and Technology of China, Hefei 230026
67
Showa Pharmaceutical University, Tokyo 194-8543
68
Soongsil University, Seoul 156-743
69
University of South Carolina, Columbia, South Carolina 29208
70
Stefan Meyer Institute for Subatomic Physics, Vienna 1090
71
Sungkyunkwan University, Suwon 440-746
72
School of Physics, University of Sydney, New South Wales 2006
73
Department of Physics, Faculty of Science, University of Tabuk, Tabuk 71451
74
Tata Institute of Fundamental Research, Mumbai 400005
75
Department of Physics, Technische Universität München, 85748 Garching76
Toho University, Funabashi 274-8510
77
Department of Physics, Tohoku University, Sendai 980-8578
78
Earthquake Research Institute, University of Tokyo, Tokyo 113-0032
79
Department of Physics, University of Tokyo, Tokyo 113-0033
80
Tokyo Institute of Technology, Tokyo 152-8550
81
Tokyo Metropolitan University, Tokyo 192-0397
82
University of Torino, 10124 Torino
83
Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
84
Wayne State University, Detroit, Michigan 48202
85
Yonsei University, Seoul 120-749

062001-2

PHYSICAL REVIEW LETTERS 121, 062001 (2018)(Received 27 March 2018; revised manuscript received 11 July 2018; published 6 August 2018)
We report the first observation of the hadronic transition ϒð4SÞ → η0 ϒð1SÞ, using 496 fb−1 datacollected at the ϒð4SÞ resonance with the Belle detector at the KEKB asymmetric-energy eþ e− collider. We
reconstruct the η0 meson through its decays to ρ0 γ and to π þ π − η, with η → γγ. We measureBðϒð4SÞ → η0 ϒð1SÞÞ ¼ ½3.43  0.88ðstatÞ  0.21ðsystÞ × 10−5 , with a significance of 5.7σ.
DOI: 10.1103/PhysRevLett.121.062001

One of the major challenges in particle physics is thetreatment of quantum chromodynamics (QCD) in the
nonperturbative regime [1]. In particular, heavy quarkonia,
i.e., bound states of one heavy quark and its antiquark,thanks to their intrinsic multiscale behavior, are one of themost promising and clean laboratories in which to explore
these dynamics [2]. Hadronic transitions between bottomonia (i.e., bb̄ bound states) have been, in the past few
years, a fertile field for both experiment and theory. On thebasis of heavy quark spin symmetry, the QCD multipole
expansion (QCDME) model predicts that transitions
involving the η particle should be suppressed relative todipion transitions [3]. Several recent results [4–7] challenge
this long-standing expectation. Following these measurements, it has been argued that the light-quark degrees of
freedom actively intervene in the transitions [8].
Few processes for the ϒð4SÞ meson decaying to the nonBB̄ system have been measured thus far [9]. There havebeen no searches for the kinematically allowed transitionϒð4SÞ → η0 ϒð1SÞ, which is expected to be as strong asϒð4SÞ → ηϒð1SÞ [8], where the relative strength of the η0and η transitions depends on the relative uū þ dd̄ content of
the mesons, and is predicted to range between 20% and60%. In contrast, a significant dominance of the η0transition is predicted by QCDME models. In the charmonium sector, searches for ψð4160Þ → η0 J=ψ andYð4260Þ → η0 J=ψ transitions have been made by CLEO
[10] without the observation of significant signals, while
the observation of eþ e− → η0 J=ψ at center-of-mass energies of 4.226 and 4.258 GeV has been reported by
BESIII [11].
In this Letter, we present the first observation of thetransition ϒð4SÞ → η0 ϒð1SÞ. The ϒð1SÞ meson is reconstructed via its leptonic decay to two muons, which isconsiderably cleaner than the dielectron mode. The η0meson is reconstructed via its decays to ρ0 γ and toπ þ π − η, with the η meson reconstructed as two photons.
We use a sample of ð538  8Þ × 106 ϒð4SÞ mesons,corresponding to an integrated luminosity of 496 fb−1 ,Published by the American Physical Society under the terms of
the Creative Commons Attribution 4.0 International license.
Further distribution of this work must maintain attribution to
the author(s) and the published article’s title, journal citation,
and DOI. Funded by SCOAP3.

collected by the Belle experiment at the KEKB
asymmetric-energy eþ e− collider [12,13]. In addition, adata sample corresponding to 56 fb−1 , collected about
60 MeV below the resonance, is used to estimate thebackground contribution.
The Belle detector (described in detail elsewhere[14,15]) is a large-solid-angle magnetic spectrometer thatconsists of a silicon vertex detector, a 50-layer central drift
chamber (CDC), an array of aerogel threshold Cherenkov
counters (ACC), a barrel-like arrangement of time-of-flightscintillation counters, and an electromagnetic calorimetercomprised of CsI(Tl) crystals (ECL) located inside asuperconducting solenoid coil that provides a 1.5 T magnetic field. An iron flux return located outside of thecoil (KLM) is instrumented to detect K 0L mesons and to
identify muons.
Monte Carlo (MC) simulated events are used for theefficiency determination and the selection optimization;these are generated using EVTGEN [16] and simulated tomodel the detector response using GEANT3 [17]. Thechanging detector performance and accelerator conditions
are taken into account in the simulation. The distributionsof generated dimuon decays incorporate the ϒð1SÞ polarization. The angular distribution in the ϒð4SÞ → η0 ϒð1SÞ
transition is simulated as a vector decaying to a pseudoscalar and a vector. The η0 → π þ π − η and the η → γγ decays
are generated uniformly in phase space, while the η0 →
ρ0 γ → π þ π − γ decay is generated assuming the appropriate
helicity. Final state radiation effects are modeled in thegenerator by PHOTOS [18].
Charged tracks must originate from a cylindrical regionof length 5 cm along the z axis, which is aligned opposite
to the positron beam, and radius 1 cm in the transverse
plane, centered on the eþ e− interaction point, and musthave a transverse momentum (pT ) greater than 0.1 GeV=c.
Charged particles are assigned a likelihood Li , with i ¼ μ,π, K [19], based on the range of the particle extrapolated
from the CDC through the KLM; particles are identified asmuons if the likelihood ratio P μ ¼ Lμ =ðLμ þ Lπ þ LK Þ
exceeds 0.8, corresponding to a muon efficiency of about
91.5% over the polar angle range 20° ≤ θ ≤ 155° and themomentum range 0.7 GeV=c ≤ p ≤ 3.0 GeV=c in thelaboratory frame. Electron identification uses a similar
likelihood ratio P e based on CDC, ACC, and ECL
information [20]. Charged particles that are not identified
as muons and having a likelihood ratio P e < 0.1 are treated062001-3

PHYSICAL REVIEW LETTERS 121, 062001 (2018)broadly distributed for signal events, while backgrounds
are sharply peaked at the values 563.0  0.4, 894.9  0.6,
and 1119.1  1.2 MeV=c2 , for ϒð2SÞ, ϒð3SÞ, andϒð4SÞ → π þ π − ϒð1SÞ, respectively [9], with experimental
resolutions of a few MeV=c2. It has been verified that, withrespect to a cut-based approach, the BDT method enhances
the dipion rejection while retaining a higher signal
efficiency. The reconstructed invariant mass of the η0candidate must lie within 0.93 GeV=c2 < Mðπ þ π − γÞ <0.98 GeV=c2 , which retains 90% of signal events.
The overall selection efficiencies for the signal events inthe 2π1γ and 2π2γ final states are ϵ ¼ ð17.64  0.05Þ% andð5.02  0.03Þ%, respectively, as determined from MC-simulated samples. The selection efficiency for ϒð2S; 3S; 4SÞ →π þ π − ϒð1SÞ events is in the range of 10−6 − 10−4 , making
their contribution negligible. The contributions from theseand other background sources are measured with a data
sample collected below the ϒð4SÞ resonance; a fraction ofless than ∼10−8 of the data remains in the 2π1γ final state,while no events are present in the 2π2γ final state.
The signal events are identified by the variable
ΔMη0 ¼ M½ϒð4SÞ − M½ϒð1SÞ − Mðη0 Þ;

ð1Þ

where M½ϒð1SÞ ¼ Mðμþ μ− Þ in both final states;
for the 2π1γ [2π2γ] final state, M½ϒð4SÞ ¼
Mðμþ μ− π þ π − γÞ½Mðμþ μ− π þ π − γγÞ and Mðη0 Þ¼Mðπ þ π − γÞ[Mðπ þ π − γγÞ]. The expected resolution for the signal
is 7–8 MeV=c2 , depending on the reconstructed η0decay mode. The distribution of ΔMη0 versus Mðη0 Þ[Mðη0 Þ − MðηÞ] for the 2π1γ [2π2γ] candidates is shownin Fig. 1 [Fig. 2] in a broad range of the abscissa in order toillustrate the distribution.
0.4
0.35
-3σ

0.3

Δ Mη’ [GeV/c2]

as pions. Calorimeter clusters not associated with reconstructed charged tracks and with energies greater than
50 MeV are classified as photon candidates. Pairs of
oppositely charged tracks, of which at least one is positively identified as a muon, are selected as dimuon
candidates. Pairs of oppositely charged tracks, both classified as pions, are selected as dipion candidates. Retained
events contain exactly one dimuon candidate and one
dipion candidate. For η0 → ρ0 γ decays, hereinafter labeled
as 2π1γ, only events with at least one photon and with thephoton-dipion invariant mass within 50 MeV=c2 (3σ)of the nominal η0 mass [9] are retained. Similarly, for theη0 → π þ π − η, η → γγ decay chain, hereinafter labeled as
2π2γ, only events with at least two photons having aninvariant mass within 50 MeV=c2 (3σ) of the nominal
η mass [9] and with an invariant-mass differenceMðπ þ π − γγÞ − MðγγÞ within 20 MeV=c2 (3σ) of Mðη0 Þ −MðηÞ are considered. In 2π1γ (2π2γ) final states, 1.2 (1.4)
candidates per event are present on average, where themultiplicity is due to one or more accidental photons.
The ambiguity is resolved by choosing the one whose
reconstructed η0 mass is closest to the nominal value. This
choice has an efficiency of ∼90%pon
ﬃﬃﬃ the MC-simulated
signal samples. The events with j s − M½ϒð1SÞη0 c2 j <150 MeV,where
M½ϒð1SÞη0  ¼ Mðμþ μ− π þ π −pγÞﬃﬃﬃ
þ − þ −
[Mðμ μ π π γγÞ] in the 2π1γ [2π2γ] final state and s
is the center-of-mass (c.m.) eþ e− energy, are retained.
The kinematic bound expressed by the quantity pKB ¼
pﬃﬃﬃ
pðμμÞc:m: − ½s − MðμμÞ2 c4 =ð2c sÞ, where pðμμÞc:m: is thec.m. momentum of the dimuon system, is constrained tonegative values for signal events and is used to reject partof the background contribution due to QED processes[eþ e− → eþ e− ðγÞ and eþ e− → μþ μ− ðγÞ]. Further reductions of QED processes and of cosmic background
events are achieved by requiring the opening angle ofthe charged pion candidates in the c.m. frame to satisfy
j cos θðππÞc:m: j < 0.9.
The 2π1γ final state has contributions from dipiontransitions to the ϒð1SÞ resonance from either ϒð2S; 3SÞ
resonances produced in initial state radiation events or theϒð4SÞ resonance in which a random photon is incorporated
into the η0 candidate. The high production cross section
values [21] and decay rates [9] make these processes
competitive with the signal transition, and particular careis needed to reduce them to negligible levels. A boosted
decision tree (BDT) method, as implemented in the Toolkit
for Multivariate Data Analysis package [22], is trained toseparate the signal events from those due to dipiontransitions. The performance of the classifier is optimized
and tested using MC-simulated samples for both the signal
and dipion transitions. The input variables used to construct
the BDT are the difference between invariant masses
ΔMππ ¼ Mðμþ μ− π þ π − Þ − Mðμþ μ− Þ and the total reconstructed mass of the event Mðμþ μþ π þ π − γÞ. The highest
discrimination is provided by ΔM ππ. This variable is

+3σ

0.25
0.2
0.15
0.1
0.05
0

2π1γ
0.86 0.88 0.9 0.92 0.94 0.96 0.98

1

1.02 1.04

M(η’) [GeV/c2 ]FIG. 1. Distribution of ΔM η0 versus Mðη0 Þ for the selected
events (binned into the boxes) in the 2π1γ final state. The verticaldashed lines show the 3σ selected region. The signal-selection
region of 0.93 GeV=c2 < Mðπ þ π − γÞ < 0.98 GeV=c2 is bounded by the vertical solid lines. The two-dimensional region where
97% of the signal events are expected is bounded by these verticallines and the two red horizontal lines.

062001-4

PHYSICAL REVIEW LETTERS 121, 062001 (2018)F ðxÞ ¼ exp −

0.4
0.35
-3σ

Δ Mη' [GeV/c2]

0.3

+3σ

0.25
0.2
0.15
0.1
0.05
0

0.32 0.34 0.36 0.38 0.4 0.42 0.44 0.46 0.48 0.5

M(η') - M(η) [GeV/c2]

FIG. 2. Distribution of ΔM η0 versus Mðη0 Þ − MðηÞ for theselected events (binned into the boxes) in the 2π2γ final state.
The vertical dashed lines show the 3σ selected region. The twodimensional region where 97% of the signal events are expected
is bounded by these vertical lines and the two red horizontal lines.

The signal and background yields are determined by an
unbinned maximum likelihood fit to the ΔM η0 distribution,
shown in Fig. 3. The signal component is parametrized by aGaussian-like analytical function
18

Events / (10 MeV/c2)

16

2π1γ

14
12
10
8
6
4
2
0

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.35

0.4

ΔMη’ [GeV/c2]

Events / (20 MeV/c2)

6
5

2π2γ

4
3
2
1
0

0

0.05

0.1

0.15

0.2

0.25

0.3

ΔMη’ [GeV/c2]

FIG. 3. Fit to the ΔM η0 distribution for ϒð4SÞ → η0 ϒð1SÞ
candidates reconstructed in the 2π1γ (top) and 2π2γ (bottom)
final states. Data are shown as points, and the solid blue line
shows the best fit to the data, while the dashed red line shows the
background contribution.

ðx − μÞ2;
2σ 2L;R þ αL;R ðx − μÞ2ð2Þwith mean value μ, distinct widths σ L;R , and asymmetrictail parameters αL;R on either side of the peak. The
background is described by a very broad Gaussian (linear)function in the 2π1γ (2π2γ) final state. The signal shapeparameters are fixed to the values determined from the MCsimulated sample. The signal and background yields in the
2π1γ final state are N sig ¼ 22  7 and N bkg ¼ 96  11,
respectively. In the 2π2γ final state, the signal and background yields are N sig ¼ 5.0  2.3 and N bkg ¼ 2.0  1.6,
respectively.
statistical significanceof the signal is determined as
ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ﬃ
pThe
2 log½LðN sig Þ=Lð0Þ, where LðN sig Þ=Lð0Þ is the ratio
between the likelihood values for a fit that includes a signal
component versus a fit with only the background hypothesis. The statistical significance is estimated to be 4.2σ
(4.1σ) in the 2π1γ (2π2γ) final state.
Several sources of systematic uncertainty affect thebranching fraction measurement, including the numberof ϒð4SÞ events N ϒð4SÞ (1.4%) and the values used forthe secondary branching fractions Bsecondary (2.7% for
2π1γ and 2.6% for 2π2γ) [9]. The uncertainties incharged track reconstruction (1.4%) and muon identification efficiency (1.1%) are determined by comparing
data and MC events using independent control samples.
The largest contribution to the systematic uncertainty
comes from the signal extraction procedure (6.8% for
2π1γ and 2.0% for 2π2γ). The uncertainty due to the
choice of signal parameterizations is estimated by changing
the functional forms used; the systematic uncertainty forthe background form is evaluated by using second-orderpolynomial or exponential functions and by varying therange chosen for the fit. An additional uncertainty is related
to the chosen values for the signal shape parameters and isevaluated by repeating the fit while varying each of them by
1σ with respect to their nominal value. In each case, theuncertainty is estimated as the variation in the signal yield
when using an alternate configuration with respect to thatobtained with the nominal one. Not all of the partial widthof η0 → π þ π − γ can be explained by a resonant decay
through a ρ0 [23,24]. The potential systematic bias in
the signal efficiency due to the presence of nonresonant
decays is estimated by comparing the selection efficiencies
between the default resonant sample and a completely
nonresonant one. Half of the difference is conservatively
assigned as a systematic error (−1.9% for 2π1γ). Other
possible sources of systematic uncertainties, due to discrepancies between the data and MC simulations in theefficiency of the applied selection requirements or in thephoton energy calibration, have been found to be relatively
small. The total systematic uncertainty is obtained byadding in quadrature all of the contributions and amounts

062001-5

PHYSICAL REVIEW LETTERS 121, 062001 (2018)
to 7.6% for the 2π1γ final state and 3.5% for the 2π2γ
final state.
The value of the branching fraction B is calculated as
B¼

N sig
:
ϵ × N ϒð4SÞ × Bsecondary

ð3Þ

We measure B ¼ ð3.19  0.96ðstatÞ  0.24ðsystÞÞ × 10−5in the 2π1γ final state and B ¼ ð4.53  2.12ðstatÞ 
0.16ðsystÞÞ × 10−5 in the 2π2γ final state. The measurements
obtained from the two independent subsamples are combinedin a weighted average, where the weight is the inverse of thesquared sum of the statistical and systematic uncertainties oneach yield, considering only the systematic contributions thatare uncorrelated between the two channels. The systematic
uncertainties in common between the two channels are thenadded in quadrature to obtain the total uncertainty. The
measured branching fraction is Bðϒð4SÞ → η0 ϒð1SÞÞ ¼
ð3.43  0.88ðstatÞ  0.21ðsystÞÞ × 10−5 . The statistical significance of the combined measurement is estimated by
performing a simultaneous fit to the two disjoint data sets,using the same parameterizations as before and constraining
the signal normalization so that the ratio of the signal yield
divided by the signal efficiency and the secondary branching
fractions is the same in the two data sets. The statisticalsignificance of the combined measurement is 5.8σ; this is
reduced to 5.7σ when considering yield-related systematic
uncertainties by convolving the likelihood function with aGaussian whose width equals the systematic uncertainty.
This measurement represents the first observation of thehadronic transition ϒð4SÞ → η0 ϒð1SÞ.
We also determine the ratios of branching fractions:Rη0 =h ¼Bðϒð4SÞ → η0 ϒð1SÞÞ
;Bðϒð4SÞ → hϒð1SÞÞ

ð4Þ

where the decay is mediated by a hadronic state h ¼ η orπ þ π − . For Bðϒð4SÞ → hϒð1SÞÞ, we use the values obtainedin Ref. [5], which analyzes the same data sample consideredin this Letter. Several systematic uncertainties cancel, being
common to the numerator and denominator. The resultsfrom the two η0 decay modes are combined in a weighted
average, as for the branching fraction measurement, and areRη0 =η ¼ 0.20  0.06 and Rη0 =πþ π− ¼ 0.42  0.11. The former ratio, in particular, is in agreement with the expectedvalue in the case of an admixture of a state containing lightquarks in addition to the bb̄ pair in the ϒð4SÞ in bottomonium
hadronic transitions; a value of 0.2 is predicted in Eq. (6) inRef. [8], with a reasonable assumption of the ratio of the form
factors, i.e., Fðpη Þ=Fðpη0 Þ ≈ 1.
The past few years have seen a large amount of activity
by both experiment and theory to study more closely theunexpected nature of η transitions between bottomonium
states. Following this path, the described measurement,
being the first observation of an η0 transition, adds anothertile to our effort to understand the puzzle of hadronictransitions between heavy quarkonia.
We thank the KEKB group for excellent operation of theaccelerator; the KEK cryogenics group for efficient solenoid
operations; and the KEK computer group, the NII, and PacificNorthwest National Laboratory (PNNL)/Environmental
Molecular Sciences Laboratory (EMSL) for valuable computing and Science Information NETwork 5 (SINET5) network
support. We acknowledge support from Ministry of
Education, Culture, Sports, Science, and Technology(MEXT), Japan Society for the Promotion of Science(JSPS), and Nagoya’s TLPRC (Japan); Australian ResearchCouncil (ARC) (Australia); FWF (Austria); National Natural
Science Foundation of China (NSFC) and CAS Center for
Excellence in Particle Physics (CCEPP) (China); MSMT
(Czechia); CZF, Deutsche Forschungsgemeinschaft (DFG),
EXC153, and VS (Germany); DST (India); Instituto
Nazionale di Fisica Nucleare (INFN) (Italy); Ministry of
Education (MOE), Ministry of Science, ICT and Future
Planning (MSIP), National Research Foundation (NRF),
RSRI, FLRFAS project, and GSDC of Korea Institute of
Science and Technology Information (KISTI) (Korea);
Ministerstwo Nauki i Szkolnictwa Wyzszego (MNiSW)
and Narodowe Centrum Nauki (NCN) (Poland); MES underContract No. 14.W03.31.0026 (Russia); ARRS (Slovenia);
IKERBASQUE and Ministry of Economy andCompetitiveness (MINECO) (Spain); SNSF (Switzerland);
Ministry of Education (MOE) and Ministry of Science andTechnology, Taiwan (MOST) (Taiwan); and U.S. Departmentof Energy (DOE) and National Science Foundation(NSF) (USA).

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