Also at ]Coimbra Polytechnic - ISEC, 3030-199 Coimbra, PortugalAlso at
]Physikalisches Institut, Universität Heidelberg, Heidelberg, GermanyAlso at
]INFN, Sez. di Ferrara and Dip. di Fisica e Scienze della Terra, Università di
Ferrara, via G. Saragat 1, Edificio C, I-44122 Ferrara (FE), Italy

XENON Collaboration

# Search for New Physics in Electronic Recoil Data from XENONnT

E. Aprile  Physics Department, Columbia University, New York, NY 10027, USA
K. Abe  Kamioka Observatory, Institute for Cosmic Ray Research, and Kavli
Institute for the Physics and Mathematics of the Universe (WPI), University of
Tokyo, Higashi-Mozumi, Kamioka, Hida, Gifu 506-1205, Japan  F. Agostini
Department of Physics and Astronomy, University of Bologna and INFN-Bologna,
40126 Bologna, Italy  S. Ahmed Maouloud  LPNHE, Sorbonne Université,
CNRS/IN2P3, 75005 Paris, France  L. Althueser  Institut für Kernphysik,
Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany  B. Andrieu
LPNHE, Sorbonne Université, CNRS/IN2P3, 75005 Paris, France  E. Angelino
INAF-Astrophysical Observatory of Torino, Department of Physics, University of
Torino and INFN-Torino, 10125 Torino, Italy  J. R. Angevaare  Nikhef and the
University of Amsterdam, Science Park, 1098XG Amsterdam, Netherlands  V. C.
Antochi  Oskar Klein Centre, Department of Physics, Stockholm University,
AlbaNova, Stockholm SE-10691, Sweden  D. Antón Martin  Department of Physics &
Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL
60637, USA  F. Arneodo  New York University Abu Dhabi - Center for Astro,
Particle and Planetary Physics, Abu Dhabi, United Arab Emirates  L. Baudis
Physik-Institut, University of Zürich, 8057 Zürich, Switzerland  A. L. Baxter
Department of Physics and Astronomy, Purdue University, West Lafayette, IN
47907, USA  L. Bellagamba  Department of Physics and Astronomy, University of
Bologna and INFN-Bologna, 40126 Bologna, Italy  R. Biondi  INFN-Laboratori
Nazionali del Gran Sasso and Gran Sasso Science Institute, 67100 L’Aquila,
Italy  A. Bismark  Physik-Institut, University of Zürich, 8057 Zürich,
Switzerland  A. Brown  Physikalisches Institut, Universität Freiburg, 79104
Freiburg, Germany  S. Bruenner  Nikhef and the University of Amsterdam,
Science Park, 1098XG Amsterdam, Netherlands  G. Bruno  SUBATECH, IMT
Atlantique, CNRS/IN2P3, Nantes Université, Nantes 44307, France  R. Budnik
Department of Particle Physics and Astrophysics, Weizmann Institute of
Science, Rehovot 7610001, Israel  T. K. Bui  Kamioka Observatory, Institute
for Cosmic Ray Research, and Kavli Institute for the Physics and Mathematics
of the Universe (WPI), University of Tokyo, Higashi-Mozumi, Kamioka, Hida,
Gifu 506-1205, Japan  C. Cai  Department of Physics & Center for High Energy
Physics, Tsinghua University, Beijing 100084, China  C. Capelli  Physik-
Institut, University of Zürich, 8057 Zürich, Switzerland  J. M. R. Cardoso
LIBPhys, Department of Physics, University of Coimbra, 3004-516 Coimbra,
Portugal  D. Cichon  Max-Planck-Institut für Kernphysik, 69117 Heidelberg,
Germany  M. Clark  Department of Physics and Astronomy, Purdue University,
West Lafayette, IN 47907, USA  A. P. Colijn  Nikhef and the University of
Amsterdam, Science Park, 1098XG Amsterdam, Netherlands  J. Conrad  Oskar Klein
Centre, Department of Physics, Stockholm University, AlbaNova, Stockholm
SE-10691, Sweden  J. J. Cuenca-García  Physik-Institut, University of Zürich,
8057 Zürich, Switzerland  Institute for Astroparticle Physics, Karlsruhe
Institute of Technology, 76021 Karlsruhe, Germany  J. P. Cussonneau  Deceased
SUBATECH, IMT Atlantique, CNRS/IN2P3, Nantes Université, Nantes 44307, France
V. D’Andrea  Department of Physics and Chemistry, University of L’Aquila,
67100 L’Aquila, Italy  INFN-Laboratori Nazionali del Gran Sasso and Gran Sasso
Science Institute, 67100 L’Aquila, Italy  Physikalisches Institut, Universität
Freiburg, 79104 Freiburg, Germany  M. P. Decowski  Nikhef and the University
of Amsterdam, Science Park, 1098XG Amsterdam, Netherlands  P. Di Gangi
Department of Physics and Astronomy, University of Bologna and INFN-Bologna,
40126 Bologna, Italy  S. Di Pede  Nikhef and the University of Amsterdam,
Science Park, 1098XG Amsterdam, Netherlands  A. Di Giovanni  New York
University Abu Dhabi - Center for Astro, Particle and Planetary Physics, Abu
Dhabi, United Arab Emirates  R. Di Stefano  Department of Physics “Ettore
Pancini”, University of Napoli and INFN-Napoli, 80126 Napoli, Italy  S. Diglio
SUBATECH, IMT Atlantique, CNRS/IN2P3, Nantes Université, Nantes 44307, France
K. Eitel  Institute for Astroparticle Physics, Karlsruhe Institute of
Technology, 76021 Karlsruhe, Germany  A. Elykov  Physikalisches Institut,
Universität Freiburg, 79104 Freiburg, Germany  S. Farrell  Department of
Physics and Astronomy, Rice University, Houston, TX 77005, USA  A. D. Ferella
Department of Physics and Chemistry, University of L’Aquila, 67100 L’Aquila,
Italy  INFN-Laboratori Nazionali del Gran Sasso and Gran Sasso Science
Institute, 67100 L’Aquila, Italy  C. Ferrari  INFN-Laboratori Nazionali del
Gran Sasso and Gran Sasso Science Institute, 67100 L’Aquila, Italy  H. Fischer
Physikalisches Institut, Universität Freiburg, 79104 Freiburg, Germany  W.
Fulgione  INAF-Astrophysical Observatory of Torino, Department of Physics,
University of Torino and INFN-Torino, 10125 Torino, Italy  INFN-Laboratori
Nazionali del Gran Sasso and Gran Sasso Science Institute, 67100 L’Aquila,
Italy  P. Gaemers  Nikhef and the University of Amsterdam, Science Park,
1098XG Amsterdam, Netherlands  R. Gaior  LPNHE, Sorbonne Université,
CNRS/IN2P3, 75005 Paris, France  A. Gallo Rosso  Oskar Klein Centre,
Department of Physics, Stockholm University, AlbaNova, Stockholm SE-10691,
Sweden  M. Galloway  Physik-Institut, University of Zürich, 8057 Zürich,
Switzerland  F. Gao  Department of Physics & Center for High Energy Physics,
Tsinghua University, Beijing 100084, China  R. Gardner  Department of Physics
& Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL
60637, USA  R. Glade-Beucke  Physikalisches Institut, Universität Freiburg,
79104 Freiburg, Germany  L. Grandi  Department of Physics & Kavli Institute
for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA  J.
Grigat  Physikalisches Institut, Universität Freiburg, 79104 Freiburg, Germany
M. Guida  Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany  R.
Hammann  Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany  A.
Higuera  Department of Physics and Astronomy, Rice University, Houston, TX
77005, USA  C. Hils  Institut für Physik & Exzellenzcluster PRISMA+, Johannes
Gutenberg-Universität Mainz, 55099 Mainz, Germany  L. Hoetzsch  Max-Planck-
Institut für Kernphysik, 69117 Heidelberg, Germany  J. Howlett  Physics
Department, Columbia University, New York, NY 10027, USA  M. Iacovacci
Department of Physics “Ettore Pancini”, University of Napoli and INFN-Napoli,
80126 Napoli, Italy  Y. Itow  Kobayashi-Maskawa Institute for the Origin of
Particles and the Universe, and Institute for Space-Earth Environmental
Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602,
Japan  J. Jakob  Institut für Kernphysik, Westfälische Wilhelms-Universität
Münster, 48149 Münster, Germany  F. Joerg  Max-Planck-Institut für Kernphysik,
69117 Heidelberg, Germany  A. Joy  Oskar Klein Centre, Department of Physics,
Stockholm University, AlbaNova, Stockholm SE-10691, Sweden  N. Kato  Kamioka
Observatory, Institute for Cosmic Ray Research, and Kavli Institute for the
Physics and Mathematics of the Universe (WPI), University of Tokyo, Higashi-
Mozumi, Kamioka, Hida, Gifu 506-1205, Japan  M. Kara  Institute for
Astroparticle Physics, Karlsruhe Institute of Technology, 76021 Karlsruhe,
Germany  P. Kavrigin  Department of Particle Physics and Astrophysics,
Weizmann Institute of Science, Rehovot 7610001, Israel  S. Kazama
<EMAIL_ADDRESS>Kobayashi-Maskawa Institute for the Origin of
Particles and the Universe, and Institute for Space-Earth Environmental
Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602,
Japan  M. Kobayashi  Kobayashi-Maskawa Institute for the Origin of Particles
and the Universe, and Institute for Space-Earth Environmental Research, Nagoya
University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan  G. Koltman
Department of Particle Physics and Astrophysics, Weizmann Institute of
Science, Rehovot 7610001, Israel  A. Kopec  Department of Physics, University
of California San Diego, La Jolla, CA 92093, USA  F. Kuger  Physikalisches
Institut, Universität Freiburg, 79104 Freiburg, Germany  H. Landsman
Department of Particle Physics and Astrophysics, Weizmann Institute of
Science, Rehovot 7610001, Israel  R. F. Lang  Department of Physics and
Astronomy, Purdue University, West Lafayette, IN 47907, USA  L. Levinson
Department of Particle Physics and Astrophysics, Weizmann Institute of
Science, Rehovot 7610001, Israel  I. Li  Department of Physics and Astronomy,
Rice University, Houston, TX 77005, USA  S. Li  Department of Physics and
Astronomy, Purdue University, West Lafayette, IN 47907, USA  S. Liang
Department of Physics and Astronomy, Rice University, Houston, TX 77005, USA
S. Lindemann  Physikalisches Institut, Universität Freiburg, 79104 Freiburg,
Germany  M. Lindner  Max-Planck-Institut für Kernphysik, 69117 Heidelberg,
Germany  K. Liu  Department of Physics & Center for High Energy Physics,
Tsinghua University, Beijing 100084, China  J. Loizeau  SUBATECH, IMT
Atlantique, CNRS/IN2P3, Nantes Université, Nantes 44307, France  F. Lombardi
Institut für Physik & Exzellenzcluster PRISMA+, Johannes Gutenberg-Universität
Mainz, 55099 Mainz, Germany  J. Long  Department of Physics & Kavli Institute
for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA  J. A.
M. Lopes  [  LIBPhys, Department of Physics, University of Coimbra, 3004-516
Coimbra, Portugal  Y. Ma  Department of Physics, University of California San
Diego, La Jolla, CA 92093, USA  C. Macolino  Department of Physics and
Chemistry, University of L’Aquila, 67100 L’Aquila, Italy  INFN-Laboratori
Nazionali del Gran Sasso and Gran Sasso Science Institute, 67100 L’Aquila,
Italy  J. Mahlstedt  Oskar Klein Centre, Department of Physics, Stockholm
University, AlbaNova, Stockholm SE-10691, Sweden  A. Mancuso  Department of
Physics and Astronomy, University of Bologna and INFN-Bologna, 40126 Bologna,
Italy  L. Manenti  New York University Abu Dhabi - Center for Astro, Particle
and Planetary Physics, Abu Dhabi, United Arab Emirates  F. Marignetti
Department of Physics “Ettore Pancini”, University of Napoli and INFN-Napoli,
80126 Napoli, Italy  T. Marrodán Undagoitia  Max-Planck-Institut für
Kernphysik, 69117 Heidelberg, Germany  K. Martens  Kamioka Observatory,
Institute for Cosmic Ray Research, and Kavli Institute for the Physics and
Mathematics of the Universe (WPI), University of Tokyo, Higashi-Mozumi,
Kamioka, Hida, Gifu 506-1205, Japan  J. Masbou  SUBATECH, IMT Atlantique,
CNRS/IN2P3, Nantes Université, Nantes 44307, France  D. Masson  Physikalisches
Institut, Universität Freiburg, 79104 Freiburg, Germany  E. Masson  LPNHE,
Sorbonne Université, CNRS/IN2P3, 75005 Paris, France  S. Mastroianni
Department of Physics “Ettore Pancini”, University of Napoli and INFN-Napoli,
80126 Napoli, Italy  M. Messina  INFN-Laboratori Nazionali del Gran Sasso and
Gran Sasso Science Institute, 67100 L’Aquila, Italy  K. Miuchi  Department of
Physics, Kobe University, Kobe, Hyogo 657-8501, Japan  K. Mizukoshi
Department of Physics, Kobe University, Kobe, Hyogo 657-8501, Japan  A.
Molinario  INAF-Astrophysical Observatory of Torino, Department of Physics,
University of Torino and INFN-Torino, 10125 Torino, Italy  S. Moriyama
Kamioka Observatory, Institute for Cosmic Ray Research, and Kavli Institute
for the Physics and Mathematics of the Universe (WPI), University of Tokyo,
Higashi-Mozumi, Kamioka, Hida, Gifu 506-1205, Japan  K. Morå  Physics
Department, Columbia University, New York, NY 10027, USA  Y. Mosbacher
Department of Particle Physics and Astrophysics, Weizmann Institute of
Science, Rehovot 7610001, Israel  M. Murra  Physics Department, Columbia
University, New York, NY 10027, USA  J. Müller  Physikalisches Institut,
Universität Freiburg, 79104 Freiburg, Germany  K. Ni  Department of Physics,
University of California San Diego, La Jolla, CA 92093, USA  U. Oberlack
Institut für Physik & Exzellenzcluster PRISMA+, Johannes Gutenberg-Universität
Mainz, 55099 Mainz, Germany  B. Paetsch  Department of Particle Physics and
Astrophysics, Weizmann Institute of Science, Rehovot 7610001, Israel  J.
Palacio  Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany  P.
Paschos  Department of Physics & Kavli Institute for Cosmological Physics,
University of Chicago, Chicago, IL 60637, USA  R. Peres  Physik-Institut,
University of Zürich, 8057 Zürich, Switzerland  C. Peters  Department of
Physics and Astronomy, Rice University, Houston, TX 77005, USA  J. Pienaar
Department of Physics & Kavli Institute for Cosmological Physics, University
of Chicago, Chicago, IL 60637, USA  M. Pierre  SUBATECH, IMT Atlantique,
CNRS/IN2P3, Nantes Université, Nantes 44307, France  V. Pizzella  Max-Planck-
Institut für Kernphysik, 69117 Heidelberg, Germany  G. Plante  Physics
Department, Columbia University, New York, NY 10027, USA  J. Qi  Department of
Physics, University of California San Diego, La Jolla, CA 92093, USA  J. Qin
Department of Physics and Astronomy, Purdue University, West Lafayette, IN
47907, USA  D. Ramírez García  Physik-Institut, University of Zürich, 8057
Zürich, Switzerland  S. Reichard  Institute for Astroparticle Physics,
Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany  A. Rocchetti
Physikalisches Institut, Universität Freiburg, 79104 Freiburg, Germany  N.
Rupp  Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany  L.
Sanchez  Department of Physics and Astronomy, Rice University, Houston, TX
77005, USA  J. M. F. dos Santos  LIBPhys, Department of Physics, University of
Coimbra, 3004-516 Coimbra, Portugal  I. Sarnoff  New York University Abu Dhabi
- Center for Astro, Particle and Planetary Physics, Abu Dhabi, United Arab
Emirates  G. Sartorelli  Department of Physics and Astronomy, University of
Bologna and INFN-Bologna, 40126 Bologna, Italy  J. Schreiner  Max-Planck-
Institut für Kernphysik, 69117 Heidelberg, Germany  D. Schulte  Institut für
Kernphysik, Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany
P. Schulte  Institut für Kernphysik, Westfälische Wilhelms-Universität
Münster, 48149 Münster, Germany  H. Schulze Eißing  Institut für Kernphysik,
Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany  M. Schumann
Physikalisches Institut, Universität Freiburg, 79104 Freiburg, Germany  L.
Scotto Lavina  LPNHE, Sorbonne Université, CNRS/IN2P3, 75005 Paris, France  M.
Selvi  Department of Physics and Astronomy, University of Bologna and INFN-
Bologna, 40126 Bologna, Italy  F. Semeria  Department of Physics and
Astronomy, University of Bologna and INFN-Bologna, 40126 Bologna, Italy  P.
Shagin  Institut für Physik & Exzellenzcluster PRISMA+, Johannes Gutenberg-
Universität Mainz, 55099 Mainz, Germany  S. Shi  Physics Department, Columbia
University, New York, NY 10027, USA  E. Shockley<EMAIL_ADDRESS>Department of Physics, University of California San Diego, La Jolla, CA 92093,
USA  M. Silva  LIBPhys, Department of Physics, University of Coimbra, 3004-516
Coimbra, Portugal  H. Simgen  Max-Planck-Institut für Kernphysik, 69117
Heidelberg, Germany  J. Stephen  Department of Physics & Kavli Institute for
Cosmological Physics, University of Chicago, Chicago, IL 60637, USA  A. Takeda
Kamioka Observatory, Institute for Cosmic Ray Research, and Kavli Institute
for the Physics and Mathematics of the Universe (WPI), University of Tokyo,
Higashi-Mozumi, Kamioka, Hida, Gifu 506-1205, Japan  P.-L. Tan  Oskar Klein
Centre, Department of Physics, Stockholm University, AlbaNova, Stockholm
SE-10691, Sweden  A. Terliuk  [  Max-Planck-Institut für Kernphysik, 69117
Heidelberg, Germany  D. Thers  SUBATECH, IMT Atlantique, CNRS/IN2P3, Nantes
Université, Nantes 44307, France  F. Toschi  Physikalisches Institut,
Universität Freiburg, 79104 Freiburg, Germany  G. Trinchero  INAF-
Astrophysical Observatory of Torino, Department of Physics, University of
Torino and INFN-Torino, 10125 Torino, Italy  C. Tunnell  Department of Physics
and Astronomy, Rice University, Houston, TX 77005, USA  F. Tönnies
Physikalisches Institut, Universität Freiburg, 79104 Freiburg, Germany  K.
Valerius  Institute for Astroparticle Physics, Karlsruhe Institute of
Technology, 76021 Karlsruhe, Germany  G. Volta  Physik-Institut, University of
Zürich, 8057 Zürich, Switzerland  Y. Wei  Department of Physics, University of
California San Diego, La Jolla, CA 92093, USA  C. Weinheimer  Institut für
Kernphysik, Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany
M. Weiss  Department of Particle Physics and Astrophysics, Weizmann Institute
of Science, Rehovot 7610001, Israel  D. Wenz  Institut für Physik &
Exzellenzcluster PRISMA+, Johannes Gutenberg-Universität Mainz, 55099 Mainz,
Germany  C. Wittweg  Physik-Institut, University of Zürich, 8057 Zürich,
Switzerland  T. Wolf  Max-Planck-Institut für Kernphysik, 69117 Heidelberg,
Germany  D. Xu  Department of Physics & Center for High Energy Physics,
Tsinghua University, Beijing 100084, China  Z. Xu  Physics Department,
Columbia University, New York, NY 10027, USA  M. Yamashita  Kamioka
Observatory, Institute for Cosmic Ray Research, and Kavli Institute for the
Physics and Mathematics of the Universe (WPI), University of Tokyo, Higashi-
Mozumi, Kamioka, Hida, Gifu 506-1205, Japan  L. Yang  Department of Physics,
University of California San Diego, La Jolla, CA 92093, USA  J. Ye
<EMAIL_ADDRESS>Physics Department, Columbia University, New York,
NY 10027, USA  L. Yuan  Department of Physics & Kavli Institute for
Cosmological Physics, University of Chicago, Chicago, IL 60637, USA  G.
Zavattini  [  Department of Physics and Astronomy, University of Bologna and
INFN-Bologna, 40126 Bologna, Italy  M. Zhong  Department of Physics,
University of California San Diego, La Jolla, CA 92093, USA  T. Zhu  Physics
Department, Columbia University, New York, NY 10027, USA<EMAIL_ADDRESS>
###### Abstract

We report on a blind analysis of low-energy electronic-recoil data from the
first science run of the XENONnT dark matter experiment. Novel subsystems and
the increased 5.9 tonne liquid xenon target reduced the background in the (1,
30) keV search region to $(16.1\pm 1.3)$
events/(tonne$\times$year$\times$keV), the lowest ever achieved in a dark
matter detector and $\sim$5 times lower than in XENON1T. With an exposure of
1.16 tonne-years, we observe no excess above background and set stringent new
limits on solar axions, an enhanced neutrino magnetic moment, and bosonic dark
matter.

In 2020 we reported an unexpected excess of electronic recoil (ER) events
below $\sim$7 keV in the XENON1T dark matter (DM) experiment [1]. The excess
was compatible with decays from trace amounts of tritium, the presence of
which we were unable to confirm nor exclude at the time. The result was also
interpreted as physics beyond the Standard Model (BSM) such as from solar
axions, bosonic DM with a mass of $\sim$2.3 keV/c2, solar neutrinos with
enhanced magnetic moment, and many other models [2].

This Letter presents the first results from a blind analysis of science data
from XENONnT, aimed at investigating the nature of the XENON1T excess. Due to
its larger active target mass and lower radioactive background, XENONnT is an
order of magnitude more sensitive to rare events than its predecessor [3].

The XENONnT experiment [4, 5, 6], located at the INFN Laboratori Nazionali del
Gran Sasso (LNGS) in Italy, was designed as a fast upgrade of XENON1T [6] and
shares many of the systems and infrastructure. For instance, the XENON1T
systems for cooling, gas storage, purification, and Kr-removal [7] are all
reused for XENONnT. At the core of the experiment is a new dual-phase xenon
time projection chamber (TPC), enclosed in a double-walled stainless-steel
(SS) cryostat filled with 8.5 tonnes of liquid xenon (LXe).

The cryostat is suspended at the center of the XENON1T water Cherenkov muon
veto [8]. A neutron veto was added, surrounding the cryostat, to detect gammas
produced from neutron capture on Gd, to be added at a later stage. It consists
of an octagonal enclosure with an average diameter of 4 m and a height of 3 m
composed of reflective panels and 120 8-inch Hamamatsu R5912 photomultiplier
tubes (PMTs). For the first science run, referred to as SR0, the neutron veto
was operated with pure water, with an estimated neutron tagging efficiency of
$\sim$68%.

The low-energy ER background in XENON1T was dominated by 214Pb, a
$\upbeta$-emitter originating from 222Rn. For XENONnT, in addition to an
extensive material radioassay campaign [9], a new high-flow radon removal
system was developed to further reduce this background [10]. The system can
operate in two independent modes: the gaseous xenon (GXe)-mode, where radon is
extracted from warm sections of the detector system (e.g. from around PMT high
voltage and signal cables) with a GXe flow of 20 SLPM before it enters the
LXe, and the LXe-mode, where in addition to the GXe extraction the entire LXe
mass is exchanged every 5.5 days, which matches the mean lifetime of 222Rn.
While commissioning this new system we found that, at the time, operating it
in LXe-mode resulted in a drastic drop of xenon purity. For this reason, the
system was used only in GXe-mode for SR0, resulting in a 222Rn level of 1.7
$\mathrm{\mu}$Bq/kg.

The cylindrical TPC, 1.33 m in diameter and 1.49 m tall, encloses an active
mass of 5.9 tonnes of LXe, viewed by two arrays of 3-inch Hamamatsu R11410-21
PMTs [11]. The TPC walls are made of PTFE panels. The top (bottom) array
contains 253 (241) PMTs, arranged in a hexagonal pattern to maximize the light
collection efficiency (LCE).

Five electrodes made of parallel SS wires set the electric fields in the TPC.
The anode and gate electrodes, featuring 5 mm pitch 216
$\mathrm{\mu}$m-diameter wires, are positioned 3.0 mm above and 5.0 mm below
the gas-liquid interface, respectively. Four (two) additional 304
$\mathrm{\mu}$m-diameter wires were installed perpendicular to the anode
(gate) wires to minimize sagging. The cathode electrode features 304
$\mathrm{\mu}$m-diameter wires arranged with a 7.5 mm pitch. The two remaining
electrodes, the top and bottom screens, are positioned 28 mm above the anode
and 55 mm below the cathode, respectively, and protect the PMTs from high
electric fields.

The cathode and gate electrodes, together with a field cage, define and shape
the electric field in the drift region. The field cage consists of two sets of
alternating concentric copper rings, connected by two redundant resistive
chains. The bottom-most ring is connected to the cathode with a resistor,
while the top is independently biased (VFSR). This allows for the tuning of
the electric field during operations, strongly improving its homogeneity.

The filling of the TPC was completed in the fall of 2020 and all components
were successfully operated for several weeks until November 2020, when a
short-circuit between the cathode and bottom screen limited the cathode
voltage to $-$2.75 kV. Therefore, during SR0 the electrodes were kept at
Vanode = +4.9 kV, Vgate = +0.3 kV, Vcathode = $-$2.75 kV and VFSR = +0.65 kV,
corresponding to a drift field of 23 V/cm and an electron extraction field of
2.9 kV/cm in the liquid. With these fields, the response of the TPC was
sufficient to acquire science data and exploit the low background level of
XENONnT.

Energy depositions above $\sim$1 keV in LXe can produce detectable prompt
scintillation photons (S1) at 175 nm [12] and ionization electrons which are
drifted by the external field and extracted into the GXe above the liquid,
where they produce secondary scintillation photons (S2) via
electroluminescence at the same wavelength. The time difference between the S1
and S2 signals is proportional to the interaction depth $z$.

Both S1 and S2 signals are attenuated due to absorption by impurities
dissolved in xenon; thus, high xenon purity is critical for the detector
performance. During operation, the xenon is purified continuously using a new
cryogenic purification system, based on the work detailed in [13]. The LXe is
purified through a dedicated adsorbent at a rate of 2 liters per minute,
corresponding to 8.3 tonnes/day. With liquid purification we achieved an
electron lifetime, the drift time after which the number of electrons are
attenuated to 1/e, of $>$10 ms, more than an order of magnitude improvement
compared to XENON1T. In addition, the GXe volume of the cryostat is
continuously purified with a high-temperature getter upstream of the gas inlet
to the radon removal system.

Since tritium was a potential explanation for the XENON1T excess, a number of
measures were taken to minimize the possibility of introducing it in the form
of HT and HTO [1]. The TPC was outgassed for a period of $\sim$3 months before
filling the cryostat with GXe. The entire xenon inventory was processed
through the Kr-removal system during its transfer into the gas storage system,
thereby considerably reducing any initial HT content. In preparation to
filling the cryostat with GXe and eventually LXe, the xenon was transferred to
the liquid storage system via high temperature getters, which include hydrogen
removal units. Prior to cool down and filling, the cryostat and TPC were also
treated by continuously circulating GXe for $\sim$3 weeks. Every time GXe or
LXe was filled into the cryostat it was always purified via the getters.
Following these measures, the hydrogen removal units were regenerated before
the start of SR0.

The SR0 dataset was collected from July 6, 2021 to November 10, 2021 with a
total livetime of 97.1 days. During this period, the detector’s temperature,
pressure, and liquid level above the gate electrode remained stable at
($176.8\pm 0.4$) K , ($1.890\pm 0.004$) bar, and ($5.02\pm 0.20$) mm,
respectively. The PMT gains were set at $\sim$2$\times$106 and were stable
within $3\,\%$. The electron extraction efficiency and the mean single
electron gain were measured to be $\sim$53 % and $\sim$31 photoelectrons (PE)
per extracted electron, respectively. These values were affected by temporary
ramp-downs of the anode, caused by localized, high-rate bursts of electrons.
The effect stabilized within 3 days and is corrected for in the analysis.

The PMT signals are amplified by a factor of 10, digitized at a sampling rate
of 100 MHz, and reconstructed with the open-source software straxen (v1.7.1)
[14, 15]. Data is collected in ‘triggerless’ mode: each PMT hit that crosses
its threshold, typically 2.06 mV, is written to disk without additional
trigger requirements and considered during reconstruction [16]. The mean
single-PE acceptance was 91 %.

17 out of 494 PMTs were either turned off or excluded from analysis due to
internal vacuum degradation, light emission, or noise. PMT hits within a 700
ns time-window are grouped into clusters, and these clusters are successively
subdivided into smaller ones to yield S1 or S2 peaks. Classification of S1s
and S2s is performed based on their hit-pattern and pulse shape. Peaks with
hits from three or more PMTs occurring within 100 ns (3-fold coincidence) are
classified as S1s. S2-peaks are defined as non-S1 peaks with at least four
contributing PMTs. An event time-window is defined by an S2 peak above 100 PE
($\sim$3 extracted electrons) and the preceding maximum drift-time of 2.2 ms
where an S1 may have occurred, extended on both sides by 0.25 ms. If another
S2 above 100 PE occurs within the duration of the event, the two time-windows
are combined.

The interaction position in the horizontal plane ($r$, $\theta$) of an event
is reconstructed using the S2 hit pattern of the top PMT array. Three
independent machine learning algorithms (multilayer perceptron, convolutional
neural network, and graph constrained network [17]) are used. The uncertainty
of a reconstructed position at the edge of the detector is estimated to be
$\sim$1 cm for an S2 of 1000 PE, validated with events on the inner surface of
the PTFE panel.

Injections of 83mKr (internal conversion; transitions of 32.1 keV and 9.4 keV)
were performed every two weeks to monitor the stability and calibrate the
energy reconstruction and position dependencies in the detector response. The
homogeneity of this internal calibration source is used to correct for the
effect of the non-uniform drift field, which results in the reconstructed
position of events being biased to smaller radii. The drift field as a
function of $r$ and $z$ was evaluated by matching the boundary of the 83mKr
distribution to that of a simulation accounting for potential charge
accumulation on the PTFE surface using COMSOL [18]. This simulation-driven
field map was validated using the measured ratio of the two 83mKr S1 signals
[19], and is in agreement with simulation to the level of 3% in the fiducial
volume used in the analysis.

The S1 signal size is normalized to the mean response in the central part of
the detector to account for spatial dependence of the LCE, determined with
83mKr data. When evaluating this correction, the effect of the non-uniform
drift field on the 83mKr light yields, as measured in [20], is factored out.
This ensures that the correction map only includes geometric effects and is
applicable to events in a wide energy range.

The S2 signal size is corrected for the time-dependent electron lifetime,
measured with 83mKr and 222Rn $\alpha$-decays. The impact of the drift field
on the charge yield of the respective source [20] is considered when
determining the electron lifetime. After correction, the measurements with
both sources agree, and are consistent with those from a purity monitor
installed in the LXe purification system.

Two additional corrections are applied to S2s: first, the spatially-dependent
S2 response due to, e.g., electrode sagging is determined using 83mKr and used
to correct the S2 size based on the reconstructed horizontal position at
extraction. No electric field effects were considered for this correction due
to the small electric field radial dependence. Second, a time-dependent
correction for variations in the single electron gain and extraction
efficiency is included to account for transient effects following ramping up
of the anode voltage.

In this analysis, we consider ER interactions only. The S1 and S2 response to
low energy ERs was calibrated with two radioactive sources. First, 220Rn,
whose $\upbeta$-emitting daughter 212Pb produces a roughly uniform ER spectrum
at low energies [21], was injected into the TPC in June 2021. Second, 37Ar,
which primarily produces low-energy depositions of 2.82 keV via (K-shell)
electron capture [22], was injected in December 2021 – after the conclusion of
SR0. Being a mono-energetic and a low-energy source, 37Ar is primarily used to
calibrate the response near the energy threshold (and the location of the
XENON1T excess).

Using the 220Rn calibration as a reference, the ER signal region in S1-S2
space was blinded below 20 keV in the SR0 dataset while the analysis was
ongoing. The primary science goal of XENONnT is to search for Weakly
Interacting Massive Particles (WIMPs), which are expected to produce nuclear
recoils (NRs). The NR response was calibrated with an 241AmBe neutron source
and used to define the region of interest (ROI) for WIMPs, which is still
blinded in this analysis.

Like in [1], this analysis is performed in the space of reconstructed energy
using a combined energy scale defined by
$E=W\left(c\mathrm{S1}/g1+c\mathrm{S2}/g2\right)$, where $E$ is the
reconstructed energy, the mean energy to produce a quantum, $W$, is fixed to
$13.7~{}\rm eV/quanta$ [23], $c$ denotes the corrected S1 and S2 variables,
and $g1$ ($g2$) is the gain constant that defines the average S1 (S2) signal
in PE observed per unit photon (electron) produced in the interaction. Using
mono-energetic calibration sources 37Ar, 83mKr, 131mXe, and 129mXe the gain
constants are found to be $g1=(0.151\pm 0.001)$ PE/photon and $g2=(16.5\pm
0.6)$ PE/electron. These values are extracted after accounting for an energy-
dependent ‘reconstruction bias’, a non-linearity in the S1 and S2
reconstruction due to, e.g., photon pile-up and/or PMT afterpulsing and
determined via simulation. This effect, 1 % at 41.5 keV (83mKr) and 2 % at
236.1 keV (129mXe), is modeled empirically in reconstructed energy space and
included when constructing the background models. Informed primarily by the
37Ar calibration, the energy resolution is modeled using a skew-Gaussian
smearing function, rather than a pure Gaussian as done in [1].

The ROI for this analysis is (1, 140) keV in reconstructed energy, with event-
selection criteria similar to [1, 24]. All events must have a valid S1-S2
pair. Signals are expected to induce single scatters; thus, multiple-scatter
events are removed. An S2 threshold of 500 PE is applied. We remove events far
below and above the ER region in cS1-cS2 space to avoid background events
originating from 222Rn daughters on the TPC surface [25] and events in GXe,
respectively. TPC events coincident within 300 ns of neutron veto events are
removed.

The efficiencies and uncertainties of the event selections are estimated
following the procedure in [1, 24]. The detection efficiency is dominated by
the 3-fold coincidence requirement for valid S1s and was determined using both
a data-driven method of sampling PMT hits from S1s using 37Ar and 83mKr
calibration data to mimic low-energy S1s as well as from waveform simulation
[24]. The two methods agree within 1 $\%$, and the waveform simulation method
is taken as the nominal one (see Fig. 1). The S2 reconstruction efficiency is
determined via simulation to be $\sim$100 $\%$ for the energies considered
here. The combined efficiency of detection and event selection with
uncertainties are also shown in Fig. 1. The discontinuity of the combined
efficiency at 10 keV is due to the blinded WIMP search region.

Figure 1: Efficiency and the solar axion signal. The purple curve represents
the detection efficiency dominated by the 3-fold S1 coincidence requirement.
The black curve is the total efficiency, which is a combination of the
detection and event-selection efficiencies. The bands indicate the 1 $\sigma$
uncertainty. The discontinuity at 10 keV is caused by the still-blinded WIMP
search region. The black dashed line shows the 1 keV energy threshold of this
search. The red solid and dashed lines represent the solar axion signal model
before and after accounting for energy smearing and efficiency loss, with
axion couplings of $g_{\mathrm{ae}}=3.1\times 10^{-12}$,
$g_{\mathrm{a\gamma}}=9.1\times 10^{-11}\,\mathrm{GeV}^{-1}$, and
$g_{\mathrm{an}}^{\mathrm{eff}}=8.3\times 10^{-7}$, respectively.

Fig. 2 shows the reconstructed energy spectra of calibration data at low
energy, along with unbinned maximum likelihood fits with the response model
used in this analysis. The fit to 220Rn suggests that the energy threshold,
selection efficiency, and energy reconstruction are well-understood. The fit
to 37Ar, which assumes a fixed mean of 2.8 keV and a fixed resolution,
validates the skew-Gaussian smearing model and anchors the energy
reconstruction of peaks down to 2.8 keV.

Figure 2: Calibration data and models at low energy. Both 220Rn and 37Ar data
are fit using unbinned maximum likelihoods. The 220Rn data fit is performed in
the energy interval (1, 140) keV, with the low-energy region showing the
efficiency near the energy threshold. The 37Ar data validates the energy
reconstruction and skew-Gaussian smearing model.

A fiducial mass of $(4.37\pm 0.14)$ tonne is used for this analysis, yielding
a total exposure of 1.16 tonne-years. The fiducial volume was optimized based
on the spatial distribution of the gamma-ray induced material background, as
well as instrumental backgrounds near the detector wall and liquid-gas
interface. The uncertainty of the fiducial mass is dominated by the estimation
of the charge-insensitive volume of the TPC, a region near the edge of the
detector where extraction of ionization electrons is impossible due to the
inhomogeneity in the drift field caused by the limited drift field strength.

We consider three categories of potential BSM signals in this search: a) solar
axions, b) solar neutrinos with an enhanced magnetic moment, and c) bosonic
DM, which primarily includes Axion-like Particles (ALPs) and dark photon DM.
The solar axion signal model is the one of [1] updated to include the inverse
Primakoff effect [26, 27, 28]. The model thus has six components governed by
three couplings, i.e. axion couplings to electrons ($g_{\mathrm{ae}}$),
photons ($g_{\mathrm{a\gamma}}$), and nucleons
($g_{\mathrm{an}}^{\mathrm{eff}}$). An example of the solar axion signal is
shown in Fig. 1.

In order to further explore the possibility of tritium as an explanation for
the XENON1T excess, we operated XENONnT in a different mode for 14.3 days
bypassing the getter purifying the GXe volume of the cryostat after the SR0
data was collected. Being considerably more volatile than xenon, any outgassed
hydrogen (and therefore HT) is more effectively removed by this purification
scheme. Furthermore, the shorter purification time constant ($\sim$2 h),
compared to that of liquid purification ($\sim$1 day), also makes the removal
more effective. Therefore, the equilibrium concentration of HT in LXe is
expected to be enhanced by bypassing this getter. The enhancement factor is
conservatively estimated to be at least 10, but could be as large as 100. This
‘tritium-enhanced’ dataset, when unblinded, showed no evidence for a tritium-
like excess. Based on this null result, combined with the aforementioned
reduction measures, tritium is not included in the background model.

37Ar was suggested as another potential source of the XENON1T excess [29].
This hypothesis was ruled out due to the strong constraints set on trace
amounts of 37Ar both from cosmogenic activation and from a potential air leak
[1]. Similar considerations apply to XENONnT. As mentioned earlier, the entire
xenon inventory was cryogenically distilled underground by the Kr-removal
system, which is also extremely effective in reducing 37Ar to a negligible
level [30]. The variation of natKr concentration in xenon over the SR0 period,
measured with rare-gas mass spectrometry [31], sets a constraint on any air
leak that leads to a negligible level of 37Ar. For this reason, 37Ar is also
not included in the background model.

Table 1: The background model $B_{0}$ with fit constraint and best-fit number of events for each component in (1, 140) keV. Component | Constraint | Fit
---|---|---
214Pb | (584, 1273) |  980 | $\pm$ |  120
85Kr |  90 | $\pm$ |  59 |  91 | $\pm$ |  58
Materials |  266 | $\pm$ |  51 |  267 | $\pm$ |  51
136Xe |  1537 | $\pm$ |  56 |  1523 | $\pm$ |  54
Solar neutrino |  297 | $\pm$ |  30 |  298 | $\pm$ |  29
124Xe | - |  256 | $\pm$ |  28
AC |  0.70 | $\pm$ |  0.04 |  0.71 | $\pm$ |  0.03
133Xe | - |  163 | $\pm$ |  63
83mKr | - |  80 | $\pm$ |  16

We consider nine components in the background model $B_{0}$, as listed in Tab.
1. The dominant background at low energies is still the $\upbeta$-decay of
214Pb, the activity of which is bound between ($0.777\pm
0.006_{\mathrm{stat}}\pm 0.032_{\mathrm{sys}}$) $\mathrm{\mu}$Bq/kg and
($1.691\pm 0.006_{\mathrm{stat}}\pm 0.072_{\mathrm{sys}}$)
$\mathrm{\mu}$Bq/kg. These bounds are determined by the rates of 218Po and
214Po $\alpha$-decays, parent and daughter of 214Pb in the decay chain,
respectively. We used the 214Pb spectrum from [32] that is calculated as a
forbidden transition.

The natKr concentration was measured to be ($56\pm 36$) ppq. The abundance of
85Kr in natKr is taken as $2\times 10^{-11}$ based on seasonal measurements of
85Kr activity in the LNGS air of 1.4 $\mathrm{\mu}\mathrm{Bq}/\mathrm{m^{3}}$,
which is consistent with [33]. Those measurements are then propagated to
constrain the 85Kr rate with an uncertainty of $\sim$66 %, dominated by the
natKr concentration measurement.

Gamma-ray backgrounds from materials are found to have a flat spectrum below
140 keV in the fiducial volume from Geant4 simulation [34, 35]. We expect the
rate to be ($2.1\pm 0.4$) events/(tonne$\times$year$\times$keV) (abbreviated
as events/(t$\cdot$y$\cdot$keV) for the rest of the paper), where the
uncertainty originates from the simulation and from the measurement
uncertainties of material radioassay [9].

Having successfully reduced other sources, the two-neutrino double-beta
($2\upnu\upbeta\upbeta$) decay of 136Xe becomes an important background in
this analysis, overtaking 214Pb as the dominant component above 40 keV. This
background is constrained by an in-situ measurement of the xenon isotopic
abundance with a residual gas analyzer (RGA) and the half-life from [36]. We
also allow for a small shape change to account for the uncertainty on the
theoretical calculation of this spectrum, specifically whether this isotope is
better described by the higher state dominance [37] or single state dominance
[38] model of $2\upnu\upbeta\upbeta$ decay.

The double-electron capture (2$\upnu$ECEC) decay rate of 124Xe is left
unconstrained in $B_{0}$. The energy spectrum adopts the updated model of
[39], which takes into account the contributions from higher atomic shells
compared to [1] and uses fixed branching ratios. The reconstruction of the
dominant KK-capture peak at 64.3 keV was also used as validation of the energy
reconstruction.

The spectrum of electron scattering from solar neutrinos is computed as in
[1]. We assign a 10 % solar neutrino flux uncertainty based on the Borexino
measurement [40]. 133Xe was produced by neutron activation from the 241AmBe
calibration several months before the SR0 science data taking and a tiny
fraction survived to the start of SR0. Given that it does not impact the low-
energy region and this rate is small, the background is allowed to vary freely
in the fit. Trace amounts of 83mKr leftover from calibrations are also present
in the SR0 data, the rate of which is also left unconstrained.

The last background component, accidental coincidences (ACs), is the only non-
ER background in $B_{0}$. Uncorrelated S1s and S2s can randomly pair and form
fake events, and a small fraction survives all event selections [24]. AC
events overlap with the ER band in cS1-cS2 space and produce a spectrum that
increases towards low energies. Its rate in the ER region is predicted to be
$(0.61\pm 0.03)$ events/(t$\cdot$y) using a data-driven method.

Figure 3: Science data (black dots) in cS1-cS2 space, overlaid on 220Rn data
(2D histogram). The WIMP search region (orange) is still blinded and not used
in this search. Regions (gray shaded) far away from the ER band are excluded
to avoid anomalous backgrounds. Iso-energy lines are represented by the gray
dashed lines.

After all analysis components had converged and a good agreement between the
background model and data above 20 keV was found (p-value $\sim$ 0.2), the
region above the $-2\,\sigma$ quantile of ER events in S2 was unblinded. The
NR region below ER $-2\,\sigma$ remains blinded while the WIMP analysis
continues, as shown in Fig. 3.

Figure 4: Fit to SR0 data using the background model $B_{0}$. The fit result
of $B_{0}$ is the red line. The subdominant AC background is not shown. Figure
5: Data and best-fit $B_{0}$ model below 30 keV. No significant excess above
the background was found. The bump at $\sim$10 keV is from the LL-shell of
124Xe 2$\upnu$ECEC [39], while the discontinuity at 10 keV is caused by the
blinded WIMP search region, see Fig. 1 and 3. A finer binning than in Fig. 4
is used to show the event rate change near the threshold.

We performed a fit in reconstructed energy space using an unbinned maximum
likelihood similar to that in [1]. The efficiency at low energies is allowed
to vary within its uncertainty band. The best-fit of $B_{0}$ is illustrated in
Fig. 4 and Fig. 5, and the results are listed in Tab. 1. The SR0 dataset
agrees well with $B_{0}$, and no excess above the background is found. The
efficiency-corrected average ER background rate within (1, 30) keV is measured
to be ($16.1\pm 1.3_{\mathrm{stat}}$) events/(t$\cdot$y$\cdot$keV), a factor
of $\sim$5 lower than the rate in XENON1T [1]. This is the lowest background
rate ever achieved at these energies among dark matter direct detection
experiments. The spectral shape in Fig. 4 is, for the first time, mostly
determined by two second-order weak processes: the $2\upnu\upbeta\upbeta$ of
136Xe and 2$\upnu$ECEC of 124Xe.

The best-fit activity of 214Pb is ($1.4\pm 0.2_{\mathrm{stat}}$)
$\mathrm{\mu}$Bq/kg assuming the branching ratio to the ground state is 12.7 %
[41]. The best-fit rate of 124Xe 2$\upnu$ECEC translates to a half-life of
$T_{1/2}^{\mathrm{2\upnu ECEC}}=(1.15\pm 0.13_{\mathrm{stat}}\pm
0.14_{\mathrm{sys}})\times 10^{22}\,\mathrm{yr}$, where the 12 % systematic
uncertainty is from selection efficiency (8 %), exposure (3 %), 124Xe
abundance (6 %), and capture fraction (6 %). This result is consistent with
the half-life reported in [39].

Figure 6: 90% C.L. upper limit on different new physics models. Constraints on
the axion-electron $g_{\mathrm{ae}}$ and axion-photon $g_{\mathrm{a\upgamma}}$
couplings from a search for solar axions are shown in (a). Constraints on
solar neutrinos with an enhanced magnetic moment (b), ALP DM (c), and dark
photon DM (d) are shown together with the 1 $\sigma$ (green) and 2 $\sigma$
(yellow) sensitivity bands estimated with the background-only fit. Constraints
between (39, 44) keV/c2 are excluded in (c) and (d) due to the unconstrained
83mKr background. Selected limits from other experiments [42, 43, 44, 45, 46,
47, 48, 49, 50, 51, 52, 53, 54, 55] and astrophysical observations [56, 57,
58, 59, 60, 61] are also shown.

Fig. 6 shows the 90 $\%$ confidence level (C.L.) upper limit on solar axions,
bosonic DM models, and solar neutrinos with an enhanced magnetic moment
together with sensitivity bands estimated from the background-only fit. The
solar axion limit is the upper bound of a 3D confidence volume evaluated in
the space of $g_{\mathrm{ae}}$, $g_{\mathrm{a\gamma}}$, and
$g_{\mathrm{an}}^{\mathrm{eff}}$, and projected onto the
$g_{\mathrm{a\gamma}}$ vs. $g_{\mathrm{ae}}$ space. The 90 % C.L. upper limit
on the 14.4 keV peak from the solar axion 57Fe component is 20
events/(t$\cdot$y). Since the 83mKr rate is left unconstrained, we only place
upper limits for bosonic DM with a mass between (1, 39) and (44, 140) keV/c2.
The maximum local significance of this search is around 1.8 $\sigma$. The
excess observed in the XENON1T experiment [1], when modeled as a 2.3 keV mono-
energetic peak, is excluded with a statistical significance of
$\sim$4$\,\sigma$. The 90 $\%$ C.L. upper limit on solar neutrinos with an
enhanced magnetic moment is $\mu_{\upnu}<6.3\times
10^{-12}\,\mu_{\mathrm{B}}$.

We also searched for a tritium component on top of the background model
$B_{0}$. The best-fit rate of tritium is 0 and the upper limit (90 % C.L.) is
14 events/(t$\cdot$y), corresponding to a concentration of $5.4\times
10^{-26}$ mol/mol of tritium in xenon. If the excess observed in XENON1T was
from trace amounts of tritium, the disappearance of the excess in XENONnT may
be resulted from the aforementioned rigorous tritium prevention measures.

In summary, we performed a search for new physics in the electronic recoil
data from XENONnT using an exposure of 1.16 tonne-years. The average ER
background rate of ($16.1\pm 1.3_{\mathrm{stat}}$)
events/(t$\cdot$y$\cdot$keV) in the (1, 30) keV energy region is the lowest
ever achieved in a DM search experiment. The blind analysis shows no excess
above the background, excluding our previous BSM interpretations of the
XENON1T excess. Upper limits on solar axions, bosonic DM, and solar neutrinos
with an enhanced magnetic moment are set, excluding new parameter spaces. A
measured half-life of 124Xe 2$\upnu$ECEC is also reported, consistent with the
final XENON1T measurement [39].

XENONnT is continuing to take data at LNGS. Since the conclusion of SR0,
upgrades to the radon removal system were made to allow for its operation in
the originally foreseen combined LXe and GXe mode, resulting in a further
reduction in the 222Rn background to a level $<1\,\mathrm{\mu}\mathrm{Bq/kg}$.
This extremely low background level coupled with a large target mass will
allow XENONnT to continue to probe intriguing physics channels such as DM,
solar axions, and solar neutrinos well into the future.

We thank Dr. Jenni Kotila at University of Jyväskylä for providing the single
state dominance spectrum of 136Xe $2\upnu\upbeta\upbeta$ decay and suggesting
the treatment for its spectral uncertainty. We also thank Dr. Roland
Purtschert at Climate and Environmental Physics, University of Bern, for
measurements of the 37Ar concentration at LNGS. We gratefully acknowledge
support from the National Science Foundation, Swiss National Science
Foundation, German Ministry for Education and Research, Max Planck
Gesellschaft, Deutsche Forschungsgemeinschaft, Helmholtz Association, Dutch
Research Council (NWO), Weizmann Institute of Science, Israeli Science
Foundation, Binational Science Foundation, Fundação para a Ciência e a
Tecnologia, Région des Pays de la Loire, Knut and Alice Wallenberg Foundation,
Kavli Foundation, JSPS Kakenhi and JST FOREST Program in Japan, Tsinghua
University Initiative Scientific Research Program and Istituto Nazionale di
Fisica Nucleare. This project has received funding/support from the European
Union’s Horizon 2020 research and innovation programme under the Marie
Skłodowska-Curie grant agreement No 860881-HIDDeN. Data processing is
performed using infrastructures from the Open Science Grid, the European Grid
Initiative and the Dutch national e-infrastructure with the support of SURF
Cooperative. We are grateful to Laboratori Nazionali del Gran Sasso for
hosting and supporting the XENON project.

## References

  * Aprile  _et al._ [2020a] E. Aprile _et al._ (XENON), Excess electronic recoil events in XENON1T, Phys. Rev. D 102, 072004 (2020a), arXiv:2006.09721 [hep-ex] .
  * Leane  _et al._ [2022] R. K. Leane _et al._ , Snowmass2021 Cosmic Frontier White Paper: Puzzling Excesses in Dark Matter Searches and How to Resolve Them, (2022), arXiv:2203.06859 [hep-ph] .
  * Aprile  _et al._ [2020b] E. Aprile _et al._ (XENON), Projected WIMP sensitivity of the XENONnT dark matter experiment, JCAP 2020 (11), 031, arXiv:2007.08796 [physics.ins-det] .
  * Aprile  _et al._ [2011] E. Aprile _et al._ (XENON), Design and Performance of the XENON10 Dark Matter Experiment, Astropart. Phys. 34, 679 (2011), arXiv:1001.2834 [astro-ph.IM] .
  * Aprile  _et al._ [2012] E. Aprile _et al._ (XENON), The XENON100 Dark Matter Experiment, Astropart. Phys. 35, 573 (2012), arXiv:1107.2155 [astro-ph.IM] .
  * Aprile  _et al._ [2017a] E. Aprile _et al._ (XENON), The XENON1T Dark Matter Experiment, Eur. Phys. J. C 77, 881 (2017a), arXiv:1708.07051 [astro-ph.IM] .
  * Aprile  _et al._ [2017b] E. Aprile _et al._ (XENON), Removing krypton from xenon by cryogenic distillation to the ppq level, Eur. Phys. J. C 77, 275 (2017b).
  * Aprile  _et al._ [2014] E. Aprile _et al._ (XENON), Conceptual design and simulation of a water Cherenkov muon veto for the XENON1T experiment, JINST 9, P11006, arXiv:1406.2374 [astro-ph.IM] .
  * Aprile  _et al._ [2021] E. Aprile _et al._ (XENON), Material radiopurity control in the XENONnT experiment, (2021), arXiv:2112.05629 [physics.ins-det] .
  * Murra  _et al._ [2022] M. Murra, D. Schulte, C. Huhmann, and C. Weinheimer, Design, construction and commissioning of a high-flow radon removal system for XENONnT, (2022), arXiv:2205.11492 [physics.ins-det] .
  * Antochi  _et al._ [2021] A. Antochi _et al._ , Improved quality tests of R11410-21 photomultiplier tubes for the XENONnT experiment, JINST 16, P08033, arXiv:2104.15051 [physics.ins-det] .
  * Fujii  _et al._ [2015] K. Fujii _et al._ , High-accuracy measurement of the emission spectrum of liquid xenon in the vacuum ultraviolet region, Nuclear Instruments and Methods in Physics Research Section A 795, 293 (2015).
  * Plante  _et al._ [2022] G. Plante, E. Aprile, J. Howlett, and Y. Zhang, Liquid-phase purification for multi-tonne xenon detectors, (2022), arXiv:2205.07336 [physics.ins-det] .
  * [14] J. Aalbers _et al._ , The strax data processor v1.2.3, https://doi.org/10.5281/zenodo.6620274.
  * [15] E. Aprile _et al._ (XENON), The straxen data processor v1.7.1, https://doi.org/10.5281/zenodo.6553131.
  * Aprile  _et al._ [2019a] E. Aprile _et al._ (XENON), The XENON1T Data Acquisition System, JINST 14 (07), P07016, arXiv:1906.00819 [physics.ins-det] .
  * Liang  _et al._ [2022] S. Liang _et al._ , Domain-Informed Neural Networks for Interaction Localization Within Astroparticle Experiments, Front. Artif. Intell. 5, 832909 (2022), arXiv:2112.07995 [hep-ex] .
  * [18] COMSOL AB, Stockholm, Sweden, COMSOL Multiphysics.
  * Akerib  _et al._ [2017a] D. Akerib _et al._ (LUX), 83mKr calibration of the 2013 LUX dark matter search, Phys. Rev. D 96, 112009 (2017a), arXiv:1708.02566 [physics.ins-det] .
  * Jörg  _et al._ [2022] F. Jörg, D. Cichon, G. Eurin, L. Hötzsch, T. Marrodán Undagoitia, and N. Rupp, Characterization of alpha and beta interactions in liquid xenon, Eur. Phys. J. C 82, 361 (2022), arXiv:2109.13735 [physics.ins-det] .
  * Aprile  _et al._ [2017c] E. Aprile _et al._ (XENON), Results from a Calibration of XENON100 Using a Source of Dissolved Radon-220, Phys. Rev. D 95, 072008 (2017c), arXiv:1611.03585 [physics.ins-det] .
  * Boulton  _et al._ [2017] E. Boulton _et al._ , Calibration of a two-phase xenon time projection chamber with a 37Ar source, JINST 12, P08004, arXiv:1705.08958 [physics.ins-det] .
  * Dahl [2009] C. E. Dahl, The physics of background discrimination in liquid xenon, and first results from XENON10 in the hunt for WIMP dark matter, PhD thesis, Princeton University (2009).
  * Aprile  _et al._ [2019b] E. Aprile _et al._ (XENON), XENON1T Dark Matter Data Analysis: Signal Reconstruction, Calibration and Event Selection, Phys. Rev. D 100, 052014 (2019b), arXiv:1906.04717 [physics.ins-det] .
  * Aprile  _et al._ [2019c] E. Aprile _et al._ (XENON), Xenon1t dark matter data analysis: Signal and background models and statistical inference, Phys. Rev. D 99, 112009 (2019c).
  * Gao  _et al._ [2020] C. Gao, J. Liu, L.-T. Wang, X.-P. Wang, W. Xue, and Y.-M. Zhong, Reexamining the Solar Axion Explanation for the XENON1T Excess, Phys. Rev. Lett. 125, 131806 (2020), arXiv:2006.14598 [hep-ph] .
  * Dent  _et al._ [2020] J. B. Dent, B. Dutta, J. L. Newstead, and A. Thompson, Inverse Primakoff Scattering as a Probe of Solar Axions at Liquid Xenon Direct Detection Experiments, Phys. Rev. Lett. 125, 131805 (2020), arXiv:2006.15118 [hep-ph] .
  * Abe  _et al._ [2021] T. Abe, K. Hamaguchi, and N. Nagata, Atomic form factors and inverse Primakoff scattering of axion, Physics Letters B 815, 136174 (2021), arXiv:2012.02508 [hep-ph] .
  * Szydagis  _et al._ [2021] M. Szydagis _et al._ , Investigating the xenon1t low-energy electronic recoil excess using nest, Phys. Rev. D 103, 012002 (2021), arXiv:2007.00528 [hep-ex] .
  * Aprile  _et al._ [2022a] E. Aprile _et al._ (XENON), Application and modeling of an online distillation method to reduce krypton and argon in XENON1T, PTEP 2022, 053H01 (2022a), arXiv:2112.12231 [physics.ins-det] .
  * Lindemann and Simgen [2014] S. Lindemann and H. Simgen, Krypton assay in xenon at the ppq level using a gas chromatographic system and mass spectrometer, Eur. Phys. J. C 74, 2746 (2014), arXiv:1308.4806 [physics.ins-det] .
  * Haselschwardt  _et al._ [2020] S. J. Haselschwardt, J. Kostensalo, X. Mougeot, and J. Suhonen, Improved calculations of beta decay backgrounds to new physics in liquid xenon detectors, Phys. Rev. C 102, 065501 (2020), arXiv:2007.13686 [hep-ex] .
  * Bollhöfer  _et al._ [2019] A. Bollhöfer, C. Schlosser, S. Schmid, M. Konrad, R. Purtschert, and R. Krais, Half a century of Krypton-85 activity concentration measured in air over Central Europe: Trends and relevance for dating young groundwater, Journal of Environmental Radioactivity 205-206, 7 (2019).
  * Agostinelli  _et al._ [2003] S. Agostinelli _et al._ (GEANT4), GEANT4–a simulation toolkit, Nucl. Instrum. Meth. A 506, 250 (2003).
  * Allison  _et al._ [2016] J. Allison _et al._ , Recent developments in Geant4, Nucl. Instrum. Meth. A 835, 186 (2016).
  * Albert  _et al._ [2014] J. B. Albert _et al._ (EXO-200), Improved measurement of the $2\nu\beta\beta$ half-life of 136Xe with the EXO-200 detector, Phys. Rev. C 89, 015502 (2014), arXiv:1306.6106 [nucl-ex] .
  * Kotila and Iachello [2012] J. Kotila and F. Iachello, Phase space factors for double-$\beta$ decay, Phys. Rev. C 85, 034316 (2012), arXiv:1209.5722 [nucl-th] .
  * [38] J. Kotila, private communication.
  * Aprile  _et al._ [2022b] E. Aprile _et al._ (XENON), Double-Weak Decays of 124Xe and 136Xe in the XENON1T and XENONnT Experiments, (2022b), arXiv:2205.04158 [hep-ex] .
  * Agostini  _et al._ [2018] M. Agostini _et al._ (BOREXINO), Comprehensive measurement of $pp$-chain solar neutrinos, Nature 562, 505 (2018).
  * Zhu and McCutchan [2021] S. Zhu and E. A. McCutchan, Nuclear Data Sheets for A=214, Nucl. Data Sheets 175, 1 (2021).
  * Barth  _et al._ [2013] K. Barth _et al._ (CAST), CAST constraints on the axion-electron coupling, JCAP 2013 (05), 010.
  * Agostini  _et al._ [2017] M. Agostini _et al._ (BOREXINO), Limiting neutrino magnetic moments with borexino phase-ii solar neutrino data, Phys. Rev. D 96, 091103 (2017).
  * Beda  _et al._ [2013] A. G. Beda _et al._ , Gemma experiment: The results of neutrino magnetic moment search, Physics of Particles and Nuclei Letters 10, 139–143 (2013).
  * Wang  _et al._ [2020] Y. Wang _et al._ (CDEX), Improved limits on solar axions and bosonic dark matter from the CDEX-1B experiment using the profile likelihood ratio method, Phys. Rev. D 101, 052003 (2020), arXiv:1911.03085 [hep-ex] .
  * Zhou  _et al._ [2021] X. Zhou _et al._ (PandaX), A search for solar axions and anomalous neutrino magnetic moment with the complete PandaX-II data, Chin. Phys. Lett. 38, 109902 (2021), arXiv:2008.06485 [hep-ex] .
  * Armengaud  _et al._ [2018] E. Armengaud _et al._ (EDELWEISS), Searches for electron interactions induced by new physics in the EDELWEISS-III Germanium bolometers, Phys. Rev. D 98, 082004 (2018), arXiv:1808.02340 [hep-ex] .
  * Agostini  _et al._ [2020] M. Agostini _et al._ (GERDA), First Search for Bosonic Superweakly Interacting Massive Particles with Masses up to 1 MeV/$c^{2}$ with GERDA, Phys. Rev. Lett. 125, 011801 (2020), arXiv:2005.14184 [hep-ex] .
  * Akerib  _et al._ [2017b] D. S. Akerib _et al._ (LUX), First Searches for Axions and Axionlike Particles with the LUX Experiment, Phys. Rev. Lett. 118, 261301 (2017b), arXiv:1704.02297 [astro-ph.CO] .
  * Abgrall  _et al._ [2017] N. Abgrall _et al._ (Majorana), New limits on Bosonic Dark Matter, Solar Axions, Pauli Exclusion Principle Violation, and Electron Decay from the Majorana Demonstrator, Phys. Rev. Lett. 118, 161801 (2017), arXiv:1612.00886 [nucl-ex] .
  * Fu  _et al._ [2017] C. Fu _et al._ (PandaX), Limits on Axion Couplings from the First 80 Days of Data of the PandaX-II Experiment, Phys. Rev. Lett. 119, 181806 (2017), arXiv:1707.07921 [hep-ex] .
  * Aralis  _et al._ [2020] T. Aralis _et al._ (SuperCDMS), Constraints on dark photons and axionlike particles from the SuperCDMS Soudan experiment, Phys. Rev. D 101, 052008 (2020), [Erratum: Phys.Rev.D 103, 039901 (2021)], arXiv:1911.11905 [hep-ex] .
  * Aprile  _et al._ [2017d] E. Aprile _et al._ (XENON), Search for Bosonic Super-WIMP Interactions with the XENON100 Experiment, Phys. Rev. D 96, 122002 (2017d), arXiv:1709.02222 [astro-ph.CO] .
  * Aprile  _et al._ [2019d] E. Aprile _et al._ (XENON), Light Dark Matter Search with Ionization Signals in XENON1T, Phys. Rev. Lett. 123, 251801 (2019d), arXiv:1907.11485 [hep-ex] .
  * Abe  _et al._ [2018] K. Abe _et al._ (XMASS), Search for dark matter in the form of hidden photons and axion-like particles in the XMASS detector, Phys. Lett. B 787, 153 (2018), arXiv:1807.08516 [astro-ph.CO] .
  * Ayala  _et al._ [2014] A. Ayala, I. Domínguez, M. Giannotti, A. Mirizzi, and O. Straniero, Revisiting the bound on axion-photon coupling from globular clusters, Phys. Rev. Lett. 113, 191302 (2014).
  * Viaux  _et al._ [2013] N. Viaux, M. Catelan, P. B. Stetson, G. G. Raffelt, J. Redondo, A. A. R. Valcarce, and A. Weiss, Neutrino and axion bounds from the globular cluster m5 (ngc 5904), Phys. Rev. Lett. 111, 231301 (2013).
  * Bertolami  _et al._ [2014] M. M. Bertolami, B. Melendez, L. Althaus, and J. Isern, Revisiting the axion bounds from the galactic white dwarf luminosity function, JCAP 2014 (10), 069.
  * Córsico  _et al._ [2014] A. Córsico, L. Althaus, M. M. Bertolami, S. Kepler, and E. García-Berro, Constraining the neutrino magnetic dipole moment from white dwarf pulsations, JCAP 2014 (08), 054.
  * Ferreira  _et al._ [2022] R. Z. Ferreira, M. C. D. Marsh, and E. Müller, Do Direct Detection Experiments Constrain Axionlike Particles Coupled to Electrons?, Phys. Rev. Lett. 128, 221302 (2022), arXiv:2202.08858 [hep-ph] .
  * An  _et al._ [2015] H. An, M. Pospelov, J. Pradler, and A. Ritz, Direct Detection Constraints on Dark Photon Dark Matter, Phys. Lett. B 747, 331 (2015), arXiv:1412.8378 [hep-ph] .