doi: 10.1103/PhysRevLett.121.232001.
Observation of ϒ(2S)→γη_{b}(1S) Decay
B G Fulsom
1
, T K Pedlar
2
, I Adachi
3
4
, H Aihara
5
, S Al Said
6
7
, D M Asner
8
, H Atmacan
9
, V Aulchenko
10
11
, T Aushev
12
, R Ayad
7
, V Babu
13
, I Badhrees
14
7
, A M Bakich
15
, V Bansal
1
, P Behera
16
, C Beleño
17
, M Berger
18
, V Bhardwaj
19
, B Bhuyan
20
, T Bilka
21
, J Biswal
22
, A Bondar
10
11
, G Bonvicini
23
, A Bozek
24
, M Bračko
22
25
, T E Browder
26
, L Cao
27
, D Červenkov
21
, V Chekelian
28
, A Chen
29
, B G Cheon
30
, K Chilikin
31
, K Cho
32
, S-K Choi
33
, Y Choi
34
, S Choudhury
35
, D Cinabro
23
, S Cunliffe
36
, N Dash
37
, S Di Carlo
38
, J Dingfelder
39
, Z Doležal
21
, T V Dong
3
4
, Z Drásal
21
, S Eidelman
10
31
11
, D Epifanov
10
11
, J E Fast
1
, T Ferber
36
, R Garg
40
, V Gaur
41
, N Gabyshev
10
11
, A Garmash
10
11
, M Gelb
27
, A Giri
35
, P Goldenzweig
27
, E Guido
42
, J Haba
3
4
, K Hayasaka
43
, H Hayashii
44
, S Hirose
45
, W-S Hou
46
, T Iijima
45
47
, K Inami
45
, G Inguglia
36
, A Ishikawa
48
, R Itoh
3
4
, M Iwasaki
49
, Y Iwasaki
4
, W W Jacobs
50
, H B Jeon
51
, S Jia
52
, Y Jin
5
, D Joffe
53
, K K Joo
54
, T Julius
55
, T Kawasaki
43
, H Kichimi
4
, C Kiesling
28
, D Y Kim
56
, H J Kim
51
, J B Kim
57
, K T Kim
57
, S H Kim
30
, K Kinoshita
58
, P Kodyš
21
, S Korpar
22
25
, D Kotchetkov
26
, P Križan
22
59
, R Kroeger
60
, P Krokovny
10
11
, T Kuhr
61
, R Kulasiri
53
, A Kuzmin
10
11
, Y-J Kwon
62
, J S Lange
63
, I S Lee
30
, S C Lee
51
, L K Li
64
, Y B Li
65
, L Li Gioi
28
, J Libby
16
, D Liventsev
4
41
, M Lubej
22
, T Luo
66
, M Masuda
67
, T Matsuda
68
, D Matvienko
10
31
11
, M Merola
69
70
, K Miyabayashi
44
, H Miyata
43
, R Mizuk
31
71
12
, G B Mohanty
13
, H K Moon
57
, T Mori
45
, R Mussa
42
, M Nakao
3
4
, T Nanut
22
, K J Nath
20
, Z Natkaniec
24
, M Nayak
4
23
, M Niiyama
72
, N K Nisar
73
, S Nishida
3
4
, S Ogawa
74
, S Okuno
75
, H Ono
76
43
, P Pakhlov
31
71
, G Pakhlova
31
12
, B Pal
8
, S Pardi
69
, H Park
51
, S Paul
77
, R Pestotnik
22
, L E Piilonen
41
, V Popov
31
12
, E Prencipe
78
, A Rabusov
77
, M Ritter
61
, A Rostomyan
36
, G Russo
69
, Y Sakai
3
4
, M Salehi
61
79
, S Sandilya
58
, L Santelj
4
, T Sanuki
48
, V Savinov
73
, O Schneider
80
, G Schnell
81
82
, C Schwanda
83
, Y Seino
43
, K Senyo
84
, M E Sevior
55
, V Shebalin
10
11
, C P Shen
52
, T-A Shibata
85
, J-G Shiu
46
, B Shwartz
10
11
, F Simon
28
86
, J B Singh
40
, A Sokolov
87
, E Solovieva
31
12
, M Starič
22
, J F Strube
1
, M Sumihama
88
, K Sumisawa
3
4
, T Sumiyoshi
89
, W Sutcliffe
27
, M Takizawa
90
91
92
, U Tamponi
42
, K Tanida
93
, F Tenchini
55
, M Uchida
85
, T Uglov
31
12
, Y Unno
30
, S Uno
3
4
, P Urquijo
55
, S E Vahsen
26
, C Van Hulse
81
, R Van Tonder
27
, G Varner
26
, A Vinokurova
10
11
, V Vorobyev
10
31
11
, A Vossen
94
, B Wang
58
, C H Wang
95
, P Wang
64
, X L Wang
66
, M Watanabe
43
, S Watanuki
48
, E Widmann
18
, E Won
57
, H Ye
36
, J H Yin
64
, C Z Yuan
64
, Z P Zhang
96
, V Zhilich
10
11
, V Zhukova
31
71
, V Zhulanov
10
11
, A Zupanc
22
59
; Belle Collaboration
Affiliations
Affiliations
- 1 Pacific Northwest National Laboratory, Richland, Washington 99352.
- 2 Luther College, Decorah, Iowa 52101.
- 3 SOKENDAI (The Graduate University for Advanced Studies), Hayama 240-0193.
- 4 High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801.
- 5 Department of Physics, University of Tokyo, Tokyo 113-0033.
- 6 Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589.
- 7 Department of Physics, Faculty of Science, University of Tabuk, Tabuk 71451.
- 8 Brookhaven National Laboratory, Upton, New York 11973.
- 9 University of South Carolina, Columbia, South Carolina 29208.
- 10 Budker Institute of Nuclear Physics SB RAS, Novosibirsk 630090.
- 11 Novosibirsk State University, Novosibirsk 630090.
- 12 Moscow Institute of Physics and Technology, Moscow Region 141700.
- 13 Tata Institute of Fundamental Research, Mumbai 400005.
- 14 King Abdulaziz City for Science and Technology, Riyadh 11442.
- 15 School of Physics, University of Sydney, New South Wales 2006.
- 16 Indian Institute of Technology Madras, Chennai 600036.
- 17 II. Physikalisches Institut, Georg-August-Universität Göttingen, 37073 Göttingen.
- 18 Stefan Meyer Institute for Subatomic Physics, Vienna 1090.
- 19 Indian Institute of Science Education and Research Mohali, SAS Nagar, 140306.
- 20 Indian Institute of Technology Guwahati, Assam 781039.
- 21 Faculty of Mathematics and Physics, Charles University, 121 16 Prague.
- 22 J. Stefan Institute, 1000 Ljubljana.
- 23 Wayne State University, Detroit, Michigan 48202.
- 24 H. Niewodniczanski Institute of Nuclear Physics, Krakow 31-342.
- 25 University of Maribor, 2000 Maribor.
- 26 University of Hawaii, Honolulu, Hawaii 96822.
- 27 Institut für Experimentelle Teilchenphysik, Karlsruher Institut für Technologie, 76131 Karlsruhe.
- 28 Max-Planck-Institut für Physik, 80805 München.
- 29 National Central University, Chung-li 32054.
- 30 Hanyang University, Seoul 133-791.
- 31 P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow 119991.
- 32 Korea Institute of Science and Technology Information, Daejeon 305-806.
- 33 Gyeongsang National University, Chinju 660-701.
- 34 Sungkyunkwan University, Suwon 440-746.
- 35 Indian Institute of Technology Hyderabad, Telangana 502285.
- 36 Deutsches Elektronen-Synchrotron, 22607 Hamburg.
- 37 Indian Institute of Technology Bhubaneswar, Satya Nagar 751007.
- 38 LAL, Université Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, Orsay F-91898.
- 39 University of Bonn, 53115 Bonn.
- 40 Panjab University, Chandigarh 160014.
- 41 Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061.
- 42 INFN-Sezione di Torino, 10125 Torino.
- 43 Niigata University, Niigata 950-2181.
- 44 Nara Women's University, Nara 630-8506.
- 45 Graduate School of Science, Nagoya University, Nagoya 464-8602.
- 46 Department of Physics, National Taiwan University, Taipei 10617.
- 47 Kobayashi-Maskawa Institute, Nagoya University, Nagoya 464-8602.
- 48 Department of Physics, Tohoku University, Sendai 980-8578.
- 49 Osaka City University, Osaka 558-8585.
- 50 Indiana University, Bloomington, Indiana 47408.
- 51 Kyungpook National University, Daegu 702-701.
- 52 Beihang University, Beijing 100191.
- 53 Kennesaw State University, Kennesaw, Georgia 30144.
- 54 Chonnam National University, Kwangju 660-701.
- 55 School of Physics, University of Melbourne, Victoria 3010.
- 56 Soongsil University, Seoul 156-743.
- 57 Korea University, Seoul 136-713.
- 58 University of Cincinnati, Cincinnati, Ohio 45221.
- 59 Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana.
- 60 University of Mississippi, University, Mississippi 38677.
- 61 Ludwig Maximilians University, 80539 Munich.
- 62 Yonsei University, Seoul 120-749.
- 63 Justus-Liebig-Universität Gießen, 35392 Gießen.
- 64 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049.
- 65 Peking University, Beijing 100871.
- 66 Key Laboratory of Nuclear Physics and Ion-beam Application (MOE) and Institute of Modern Physics, Fudan University, Shanghai 200443.
- 67 Earthquake Research Institute, University of Tokyo, Tokyo 113-0032.
- 68 University of Miyazaki, Miyazaki 889-2192.
- 69 INFN-Sezione di Napoli, 80126 Napoli.
- 70 Università di Napoli Federico II, 80055 Napoli.
- 71 Moscow Physical Engineering Institute, Moscow 115409.
- 72 Kyoto University, Kyoto 606-8502.
- 73 University of Pittsburgh, Pittsburgh, Pennsylvania 15260.
- 74 Toho University, Funabashi 274-8510.
- 75 Kanagawa University, Yokohama 221-8686.
- 76 Nippon Dental University, Niigata 951-8580.
- 77 Department of Physics, Technische Universität München, 85748 Garching.
- 78 Forschungszentrum Jülich, 52425 Jülich.
- 79 University of Malaya, 50603 Kuala Lumpur.
- 80 École Polytechnique Fédérale de Lausanne (EPFL), Lausanne 1015.
- 81 University of the Basque Country UPV/EHU, 48080 Bilbao.
- 82 IKERBASQUE, Basque Foundation for Science, 48013 Bilbao.
- 83 Institute of High Energy Physics, Vienna 1050.
- 84 Yamagata University, Yamagata 990-8560.
- 85 Tokyo Institute of Technology, Tokyo 152-8550.
- 86 Excellence Cluster Universe, Technische Universität München, 85748 Garching.
- 87 Institute for High Energy Physics, Protvino 142281.
- 88 Gifu University, Gifu 501-1193.
- 89 Tokyo Metropolitan University, Tokyo 192-0397.
- 90 J-PARC Branch, KEK Theory Center, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801.
- 91 Theoretical Research Division, Nishina Center, RIKEN, Saitama 351-0198.
- 92 Showa Pharmaceutical University, Tokyo 194-8543.
- 93 Advanced Science Research Center, Japan Atomic Energy Agency, Naka 319-1195.
- 94 Duke University, Durham, North Carolina 27708.
- 95 National United University, Miao Li 36003.
- 96 University of Science and Technology of China, Hefei 230026.
- PMID: 30576207
- DOI: 10.1103/PhysRevLett.121.232001
Item in Clipboard
Observation of ϒ(2S)→γη_{b}(1S) Decay
B G Fulsom et al.
Phys Rev Lett.
.
Display options
Format
doi: 10.1103/PhysRevLett.121.232001.
Authors
B G Fulsom
1
, T K Pedlar
2
, I Adachi
3
4
, H Aihara
5
, S Al Said
6
7
, D M Asner
8
, H Atmacan
9
, V Aulchenko
10
11
, T Aushev
12
, R Ayad
7
, V Babu
13
, I Badhrees
14
7
, A M Bakich
15
, V Bansal
1
, P Behera
16
, C Beleño
17
, M Berger
18
, V Bhardwaj
19
, B Bhuyan
20
, T Bilka
21
, J Biswal
22
, A Bondar
10
11
, G Bonvicini
23
, A Bozek
24
, M Bračko
22
25
, T E Browder
26
, L Cao
27
, D Červenkov
21
, V Chekelian
28
, A Chen
29
, B G Cheon
30
, K Chilikin
31
, K Cho
32
, S-K Choi
33
, Y Choi
34
, S Choudhury
35
, D Cinabro
23
, S Cunliffe
36
, N Dash
37
, S Di Carlo
38
, J Dingfelder
39
, Z Doležal
21
, T V Dong
3
4
, Z Drásal
21
, S Eidelman
10
31
11
, D Epifanov
10
11
, J E Fast
1
, T Ferber
36
, R Garg
40
, V Gaur
41
, N Gabyshev
10
11
, A Garmash
10
11
, M Gelb
27
, A Giri
35
, P Goldenzweig
27
, E Guido
42
, J Haba
3
4
, K Hayasaka
43
, H Hayashii
44
, S Hirose
45
, W-S Hou
46
, T Iijima
45
47
, K Inami
45
, G Inguglia
36
, A Ishikawa
48
, R Itoh
3
4
, M Iwasaki
49
, Y Iwasaki
4
, W W Jacobs
50
, H B Jeon
51
, S Jia
52
, Y Jin
5
, D Joffe
53
, K K Joo
54
, T Julius
55
, T Kawasaki
43
, H Kichimi
4
, C Kiesling
28
, D Y Kim
56
, H J Kim
51
, J B Kim
57
, K T Kim
57
, S H Kim
30
, K Kinoshita
58
, P Kodyš
21
, S Korpar
22
25
, D Kotchetkov
26
, P Križan
22
59
, R Kroeger
60
, P Krokovny
10
11
, T Kuhr
61
, R Kulasiri
53
, A Kuzmin
10
11
, Y-J Kwon
62
, J S Lange
63
, I S Lee
30
, S C Lee
51
, L K Li
64
, Y B Li
65
, L Li Gioi
28
, J Libby
16
, D Liventsev
4
41
, M Lubej
22
, T Luo
66
, M Masuda
67
, T Matsuda
68
, D Matvienko
10
31
11
, M Merola
69
70
, K Miyabayashi
44
, H Miyata
43
, R Mizuk
31
71
12
, G B Mohanty
13
, H K Moon
57
, T Mori
45
, R Mussa
42
, M Nakao
3
4
, T Nanut
22
, K J Nath
20
, Z Natkaniec
24
, M Nayak
4
23
, M Niiyama
72
, N K Nisar
73
, S Nishida
3
4
, S Ogawa
74
, S Okuno
75
, H Ono
76
43
, P Pakhlov
31
71
, G Pakhlova
31
12
, B Pal
8
, S Pardi
69
, H Park
51
, S Paul
77
, R Pestotnik
22
, L E Piilonen
41
, V Popov
31
12
, E Prencipe
78
, A Rabusov
77
, M Ritter
61
, A Rostomyan
36
, G Russo
69
, Y Sakai
3
4
, M Salehi
61
79
, S Sandilya
58
, L Santelj
4
, T Sanuki
48
, V Savinov
73
, O Schneider
80
, G Schnell
81
82
, C Schwanda
83
, Y Seino
43
, K Senyo
84
, M E Sevior
55
, V Shebalin
10
11
, C P Shen
52
, T-A Shibata
85
, J-G Shiu
46
, B Shwartz
10
11
, F Simon
28
86
, J B Singh
40
, A Sokolov
87
, E Solovieva
31
12
, M Starič
22
, J F Strube
1
, M Sumihama
88
, K Sumisawa
3
4
, T Sumiyoshi
89
, W Sutcliffe
27
, M Takizawa
90
91
92
, U Tamponi
42
, K Tanida
93
, F Tenchini
55
, M Uchida
85
, T Uglov
31
12
, Y Unno
30
, S Uno
3
4
, P Urquijo
55
, S E Vahsen
26
, C Van Hulse
81
, R Van Tonder
27
, G Varner
26
, A Vinokurova
10
11
, V Vorobyev
10
31
11
, A Vossen
94
, B Wang
58
, C H Wang
95
, P Wang
64
, X L Wang
66
, M Watanabe
43
, S Watanuki
48
, E Widmann
18
, E Won
57
, H Ye
36
, J H Yin
64
, C Z Yuan
64
, Z P Zhang
96
, V Zhilich
10
11
, V Zhukova
31
71
, V Zhulanov
10
11
, A Zupanc
22
59
; Belle Collaboration
Affiliations
- 1 Pacific Northwest National Laboratory, Richland, Washington 99352.
- 2 Luther College, Decorah, Iowa 52101.
- 3 SOKENDAI (The Graduate University for Advanced Studies), Hayama 240-0193.
- 4 High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801.
- 5 Department of Physics, University of Tokyo, Tokyo 113-0033.
- 6 Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589.
- 7 Department of Physics, Faculty of Science, University of Tabuk, Tabuk 71451.
- 8 Brookhaven National Laboratory, Upton, New York 11973.
- 9 University of South Carolina, Columbia, South Carolina 29208.
- 10 Budker Institute of Nuclear Physics SB RAS, Novosibirsk 630090.
- 11 Novosibirsk State University, Novosibirsk 630090.
- 12 Moscow Institute of Physics and Technology, Moscow Region 141700.
- 13 Tata Institute of Fundamental Research, Mumbai 400005.
- 14 King Abdulaziz City for Science and Technology, Riyadh 11442.
- 15 School of Physics, University of Sydney, New South Wales 2006.
- 16 Indian Institute of Technology Madras, Chennai 600036.
- 17 II. Physikalisches Institut, Georg-August-Universität Göttingen, 37073 Göttingen.
- 18 Stefan Meyer Institute for Subatomic Physics, Vienna 1090.
- 19 Indian Institute of Science Education and Research Mohali, SAS Nagar, 140306.
- 20 Indian Institute of Technology Guwahati, Assam 781039.
- 21 Faculty of Mathematics and Physics, Charles University, 121 16 Prague.
- 22 J. Stefan Institute, 1000 Ljubljana.
- 23 Wayne State University, Detroit, Michigan 48202.
- 24 H. Niewodniczanski Institute of Nuclear Physics, Krakow 31-342.
- 25 University of Maribor, 2000 Maribor.
- 26 University of Hawaii, Honolulu, Hawaii 96822.
- 27 Institut für Experimentelle Teilchenphysik, Karlsruher Institut für Technologie, 76131 Karlsruhe.
- 28 Max-Planck-Institut für Physik, 80805 München.
- 29 National Central University, Chung-li 32054.
- 30 Hanyang University, Seoul 133-791.
- 31 P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow 119991.
- 32 Korea Institute of Science and Technology Information, Daejeon 305-806.
- 33 Gyeongsang National University, Chinju 660-701.
- 34 Sungkyunkwan University, Suwon 440-746.
- 35 Indian Institute of Technology Hyderabad, Telangana 502285.
- 36 Deutsches Elektronen-Synchrotron, 22607 Hamburg.
- 37 Indian Institute of Technology Bhubaneswar, Satya Nagar 751007.
- 38 LAL, Université Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, Orsay F-91898.
- 39 University of Bonn, 53115 Bonn.
- 40 Panjab University, Chandigarh 160014.
- 41 Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061.
- 42 INFN-Sezione di Torino, 10125 Torino.
- 43 Niigata University, Niigata 950-2181.
- 44 Nara Women's University, Nara 630-8506.
- 45 Graduate School of Science, Nagoya University, Nagoya 464-8602.
- 46 Department of Physics, National Taiwan University, Taipei 10617.
- 47 Kobayashi-Maskawa Institute, Nagoya University, Nagoya 464-8602.
- 48 Department of Physics, Tohoku University, Sendai 980-8578.
- 49 Osaka City University, Osaka 558-8585.
- 50 Indiana University, Bloomington, Indiana 47408.
- 51 Kyungpook National University, Daegu 702-701.
- 52 Beihang University, Beijing 100191.
- 53 Kennesaw State University, Kennesaw, Georgia 30144.
- 54 Chonnam National University, Kwangju 660-701.
- 55 School of Physics, University of Melbourne, Victoria 3010.
- 56 Soongsil University, Seoul 156-743.
- 57 Korea University, Seoul 136-713.
- 58 University of Cincinnati, Cincinnati, Ohio 45221.
- 59 Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana.
- 60 University of Mississippi, University, Mississippi 38677.
- 61 Ludwig Maximilians University, 80539 Munich.
- 62 Yonsei University, Seoul 120-749.
- 63 Justus-Liebig-Universität Gießen, 35392 Gießen.
- 64 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049.
- 65 Peking University, Beijing 100871.
- 66 Key Laboratory of Nuclear Physics and Ion-beam Application (MOE) and Institute of Modern Physics, Fudan University, Shanghai 200443.
- 67 Earthquake Research Institute, University of Tokyo, Tokyo 113-0032.
- 68 University of Miyazaki, Miyazaki 889-2192.
- 69 INFN-Sezione di Napoli, 80126 Napoli.
- 70 Università di Napoli Federico II, 80055 Napoli.
- 71 Moscow Physical Engineering Institute, Moscow 115409.
- 72 Kyoto University, Kyoto 606-8502.
- 73 University of Pittsburgh, Pittsburgh, Pennsylvania 15260.
- 74 Toho University, Funabashi 274-8510.
- 75 Kanagawa University, Yokohama 221-8686.
- 76 Nippon Dental University, Niigata 951-8580.
- 77 Department of Physics, Technische Universität München, 85748 Garching.
- 78 Forschungszentrum Jülich, 52425 Jülich.
- 79 University of Malaya, 50603 Kuala Lumpur.
- 80 École Polytechnique Fédérale de Lausanne (EPFL), Lausanne 1015.
- 81 University of the Basque Country UPV/EHU, 48080 Bilbao.
- 82 IKERBASQUE, Basque Foundation for Science, 48013 Bilbao.
- 83 Institute of High Energy Physics, Vienna 1050.
- 84 Yamagata University, Yamagata 990-8560.
- 85 Tokyo Institute of Technology, Tokyo 152-8550.
- 86 Excellence Cluster Universe, Technische Universität München, 85748 Garching.
- 87 Institute for High Energy Physics, Protvino 142281.
- 88 Gifu University, Gifu 501-1193.
- 89 Tokyo Metropolitan University, Tokyo 192-0397.
- 90 J-PARC Branch, KEK Theory Center, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801.
- 91 Theoretical Research Division, Nishina Center, RIKEN, Saitama 351-0198.
- 92 Showa Pharmaceutical University, Tokyo 194-8543.
- 93 Advanced Science Research Center, Japan Atomic Energy Agency, Naka 319-1195.
- 94 Duke University, Durham, North Carolina 27708.
- 95 National United University, Miao Li 36003.
- 96 University of Science and Technology of China, Hefei 230026.
- PMID: 30576207
- DOI: 10.1103/PhysRevLett.121.232001
Item in Clipboard
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Format
Abstract
We report the observation of ϒ(2S)→γη_{b}(1S) decay based on an analysis of the inclusive photon spectrum of 24.7 fb^{-1} of e^{+}e^{-} collisions at the ϒ(2S) center-of-mass energy collected with the Belle detector at the KEKB asymmetric-energy e^{+}e^{-} collider. We measure a branching fraction of B[ϒ(2S)→γη_{b}(1S)]=(6.1_{-0.7-0.6}^{+0.6+0.9})×10^{-4} and derive an η_{b}(1S) mass of 9394.8_{-3.1-2.7}^{+2.7+4.5} MeV/c^{2}, where the uncertainties are statistical and systematic, respectively. The significance of our measurement is greater than 7 standard deviations, constituting the first observation of this decay mode.
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