Improved Measurement of the Evolution of the Reactor Antineutrino Flux and Spectrum at Daya Bay F. P. An1W. D. Bai1A. B. Balantekin2M. Bishai3S. Blyth4G. F. Cao5J. Cao5J. F. Chang5Y . Chang6

2025-05-08 4 0 399.33KB 8 页 10玖币

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Improved Measurement of the Evolution of the Reactor Antineutrino Flux and Spectrum at Daya

Bay

F. P. An,1W. D. Bai,1A. B. Balantekin,2M. Bishai,3S. Blyth,4G. F. Cao,5J. Cao,5J. F. Chang,5Y. Chang,6

H. S. Chen,5H. Y. Chen,7S. M. Chen,7Y. Chen,8, 1 Y. X. Chen,9J. Cheng,9J. Cheng,9Y.-C. Cheng,4Z. K. Cheng,1

J. J. Cherwinka,2M. C. Chu,10 J. P. Cummings,11 O. Dalager,12 F. S. Deng,13 Y. Y. Ding,5M. V. Diwan,3T. Dohnal,14

D. Dolzhikov,15 J. Dove,16 K. V. Dugas,12 H. Y. Duyang,17 D. A. Dwyer,18 J. P. Gallo,19 M. Gonchar,15 G. H. Gong,7

H. Gong,7W. Q. Gu,3J. Y. Guo,1L. Guo,7X. H. Guo,20 Y. H. Guo,21 Z. Guo,7R. W. Hackenburg,3Y. Han,1S. Hans,3, ∗

M. He,5K. M. Heeger,22 Y. K. Heng,5Y. K. Hor,1Y. B. Hsiung,4B. Z. Hu,4J. R. Hu,5T. Hu,5Z. J. Hu,1H. X. Huang,23

J. H. Huang,5X. T. Huang,17 Y. B. Huang,24 P. Huber,25 D. E. Jaffe,3K. L. Jen,26 X. L. Ji,5X. P. Ji,3R. A. Johnson,27

D. Jones,28 L. Kang,29 S. H. Kettell,3S. Kohn,30 M. Kramer,18, 30 T. J. Langford,22 J. Lee,18 J. H. C. Lee,31 R. T. Lei,29

R. Leitner,14 J. K. C. Leung,31 F. Li,5H. L. Li,5J. J. Li,7Q. J. Li,5R. H. Li,5S. Li,29 S. C. Li,25 W. D. Li,5X. N. Li,5

X. Q. Li,32 Y. F. Li,5Z. B. Li,1H. Liang,13 C. J. Lin,18 G. L. Lin,26 S. Lin,29 J. J. Ling,1J. M. Link,25 L. Littenberg,3

B. R. Littlejohn,19 J. C. Liu,5J. L. Liu,33 J. X. Liu,5C. Lu,34 H. Q. Lu,5K. B. Luk,30, 18, 35 B. Z. Ma,17 X. B. Ma,9X. Y. Ma,5

Y. Q. Ma,5R. C. Mandujano,12 C. Marshall,18, †K. T. McDonald,34 R. D. McKeown,36, 37 Y. Meng,33 J. Napolitano,28

D. Naumov,15 E. Naumova,15 T. M. T. Nguyen,26 J. P. Ochoa-Ricoux,12 A. Olshevskiy,15 J. Park,25 S. Patton,18 J. C. Peng,16

C. S. J. Pun,31 F. Z. Qi,5M. Qi,38 X. Qian,3N. Raper,1J. Ren,23 C. Morales Reveco,12 R. Rosero,3B. Roskovec,14

X. C. Ruan,23 B. Russell,18 H. Steiner,30, 18 J. L. Sun,39 T. Tmej,14 K. Treskov,15 W.-H. Tse,10 C. E. Tull,18 Y. C. Tung,4

B. Viren,3V. Vorobel,14 C. H. Wang,6J. Wang,1M. Wang,17 N. Y. Wang,20 R. G. Wang,5W. Wang,1, 37 X. Wang,40

Y. Wang,38 Y. F. Wang,5Z. Wang,5Z. Wang,7Z. M. Wang,5H. Y. Wei,3, ‡L. H. Wei,5L. J. Wen,5K. Whisnant,41

C. G. White,19 H. L. H. Wong,30, 18 E. Worcester,3D. R. Wu,5Q. Wu,17 W. J. Wu,5D. M. Xia,42 Z. Q. Xie,5Z. Z. Xing,5

H. K. Xu,5J. L. Xu,5T. Xu,7T. Xue,7C. G. Yang,5L. Yang,29 Y. Z. Yang,7H. F. Yao,5M. Ye,5M. Yeh,3B. L. Young,41

H. Z. Yu,1Z. Y. Yu,5B. B. Yue,1V. Zavadskyi,3, 15 S. Zeng,5Y. Zeng,1L. Zhan,5C. Zhang,3F. Y. Zhang,33 H. H. Zhang,1

J. L. Zhang,38 J. W. Zhang,5Q. M. Zhang,21 S. Q. Zhang,1X. T. Zhang,5Y. M. Zhang,1Y. X. Zhang,39 Y. Y. Zhang,33

Z. J. Zhang,29 Z. P. Zhang,13 Z. Y. Zhang,5J. Zhao,5R. Z. Zhao,5L. Zhou,5H. L. Zhuang,5and J. H. Zou5

(The Daya Bay Collaboration)

1Sun Yat-Sen (Zhongshan) University, Guangzhou

2University of Wisconsin, Madison, Wisconsin 53706

3Brookhaven National Laboratory, Upton, New York 11973

4Department of Physics, National Taiwan University, Taipei

5Institute of High Energy Physics, Beijing

6National United University, Miao-Li

7Department of Engineering Physics, Tsinghua University, Beijing

8Shenzhen University, Shenzhen

9North China Electric Power University, Beijing

10Chinese University of Hong Kong, Hong Kong

11Siena College, Loudonville, New York 12211

12Department of Physics and Astronomy, University of California, Irvine, California 92697

13University of Science and Technology of China, Hefei

14Charles University, Faculty of Mathematics and Physics, Prague

15Joint Institute for Nuclear Research, Dubna, Moscow Region

16Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801

17Shandong University, Jinan

18Lawrence Berkeley National Laboratory, Berkeley, California 94720

19Department of Physics, Illinois Institute of Technology, Chicago, Illinois 60616

20Beijing Normal University, Beijing

21Department of Nuclear Science and Technology, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an

22Wright Laboratory and Department of Physics, Yale University, New Haven, Connecticut 06520

23China Institute of Atomic Energy, Beijing

24Guangxi University, No.100 Daxue East Road, Nanning

25Center for Neutrino Physics, Virginia Tech, Blacksburg, Virginia 24061

26Institute of Physics, National Chiao-Tung University, Hsinchu

27Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221

28Department of Physics, College of Science and Technology, Temple University, Philadelphia, Pennsylvania 19122

29Dongguan University of Technology, Dongguan

30Department of Physics, University of California, Berkeley, California 94720

31Department of Physics, The University of Hong Kong, Pokfulam, Hong Kong

arXiv:2210.01068v2 [hep-ex] 23 May 2023

32School of Physics, Nankai University, Tianjin

33Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai Laboratory for Particle Physics and Cosmology, Shanghai

34Joseph Henry Laboratories, Princeton University, Princeton, New Jersey 08544

35The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong

36California Institute of Technology, Pasadena, California 91125

37College of William and Mary, Williamsburg, Virginia 23187

38Nanjing University, Nanjing

39China General Nuclear Power Group, Shenzhen

40College of Electronic Science and Engineering, National University of Defense Technology, Changsha

41Iowa State University, Ames, Iowa 50011

42Chongqing University, Chongqing

(Dated: October 4, 2022)

Reactor neutrino experiments play a crucial role in advancing our knowledge of neutrinos. A precise

measurement of reactor electron antineutrino ﬂux and spectrum evolution can be key inputs in improving the

knowledge of neutrino mass and mixing as well as reactor nuclear physics and searching for physics beyond the

standard model. In this work, the evolution of the ﬂux and spectrum as a function of the reactor isotopic content

is reported in terms of the inverse-beta-decay yield at Daya Bay with 1958 days of data and improved systematic

uncertainties. These measurements are compared with two signature model predictions: the Huber-Mueller

model based on the conversion method and the SM2018 model based on the summation method. The measured

average ﬂux and spectrum, as well as their evolution with the 239Pu isotopic fraction, are inconsistent with the

predictions of the Huber-Mueller model. In contrast, the SM2018 model is shown to agree with the average

ﬂux and its evolution but fails to describe the energy spectrum. Altering the predicted IBD spectrum from

239Pu ﬁssion does not improve the agreement with the measurement for either model. The models can be

brought into better agreement with the measurements if either the predicted spectrum due to 235Uﬁssion is

changed or the predicted 235U,238U,239Pu, and 241Pu spectra are changed in equal measure.

PACS numbers: 14.60.Pq, 29.40.Mc, 28.50.Hw, 13.15.+g

Keywords: reactor antineutrino anomaly, sterile neutrino, 5 MeV bump, Huber-Mueller Model, Daya Bay

The detection of reactor electron antineutrinos with the

inverse-beta-decay (IBD) process plays a crucial role in

advancing our knowledge of neutrinos including the discovery

of neutrinos [1], establishment of large mixing angle solution

of neutrino oscillation [2], and the discovery of non-zero

mixing angle θ13 [3]. Looking forward, the JUNO experiment

requires an accurate knowledge of the reactor neutrino

spectrum to determine the neutrino mass ordering [4].

For commercial reactors, uranium isotopes are introduced

at beginning of a fueling cycle and plutonium isotopes are

gradually generated. Four ﬁssion isotopes 235U,238U,239Pu,

and 241Pu account for over the 99.7% of the antineutrino

ﬂux with energy above the IBD detection threshold [5].

A reactor antineutrino prediction, the Huber-Mueller (HM)

model [6, 7], is determined by converting cumulative beta

spectra to antineutrino spectra for 235U,239Pu, and 241Pu and

by summing all involved beta decay branches in databases for

238U. The average of reactor neutrino ﬂux measurements is

only 95%-96% of the HM prediction, known as the reactor

antineutrino anomaly (RAA) [8–11]. Another anomaly is

about the spectrum. The measured neutrino spectrum is

poorly described by the HM model, e.g. a notable “bump”

around 5 MeV [12–14].

Together with other experimental anomalies at

short-baseline [15–17], the RAA has motivated a new

generation of short-baseline reactor neutrino experiments

to search for a sterile neutrino [18–24]. The effect of

weak magnetism [25], neutron capture [26], ﬁssion-neutron

energy [27] and database inaccuracies [28] on the prediction

has been postulated. In particular, approximately 30% of the

antineutrino ﬂux comes from forbidden decays which can

imply an uncertainty as large as the total ﬂux deﬁcit and the

bump [29–33].

Another prediction approach is the summation method,

which adds up all related decay branches from databases

for all four isotopes. One such example, the SM2018

calculation [34], with the latest experimental inputs, predicted

a uniformly lower ﬂux from 235Uthan the HM model.

Kopeikin et al. [35] reported the measured ratio between

cumulative βspectra from 235Uand 239Pu that is also

systematically lower than the HM prediction. Both SM2018

and Kopeikin imply a much smaller discrepancy with neutrino

ﬂux measurement than HM.

The most recent results from Daya Bay on the total ﬂux

in terms of IBD yield, i.e., the number of antineutrinos

per ﬁssion multiplied by the IBD cross section [9] and

evolution of the spectrum as a function of reactor burnup

used a 1230-day data sample [36]. These results showed

that the 235Uyield is about 8% less than the HM prediction

while the 239Pu yield is consistent with the model. The

latest total and energy differential yields from 235Uand

239Pu with a 1958-day data sample are reported in Ref. [37].

Evolution studies have been performed for the NEOS [38] and

RENO [39] experiments.

In this Letter, using the 1958-day data sample taken

from December 2011 to August 2017 with the Daya Bay

experiment [40], we report the direct measurement of the

total and energy differential IBD yields, σand σe, and

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ImprovedMeasurementoftheEvolutionoftheReactorAntineutrinoFluxandSpectrumatDayaBayF.P.An,1W.D.Bai,1A.B.Balantekin,2M.Bishai,3S.Blyth,4G.F.Cao,5J.Cao,5J.F.Chang,5Y.Chang,6H.S.Chen,5H.Y.Chen,7S.M.Chen,7Y.Chen,8,1Y.X.Chen,9J.Cheng,9J.Cheng,9Y.-C.Cheng,4Z.K.Cheng,1J.J.Cherwinka,2M.C.Chu,10J.P.Cummings,11...

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Improved Measurement of the Evolution of the Reactor Antineutrino Flux and Spectrum at Daya Bay F. P. An1W. D. Bai1A. B. Balantekin2M. Bishai3S. Blyth4G. F. Cao5J. Cao5J. F. Chang5Y . Chang6

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