Physics Letters B 774 (2017) 656–661
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Physics Letters B
www.elsevier.com/locate/physletb
Low-energy electronic recoil in xenon detectors by solar neutrinos
Jiunn-Wei Chen
a,b,c
, Hsin-Chang Chi
d
, C.-P. Liu
d,∗
, Chih-Pan Wu
a,∗
a
Department of Physics and Center for Theoretical Sciences, National Taiwan University, Taipei 10617, Taiwan
b
Leung Center for Cosmology and Particle Astrophysics, National Taiwan University, Taipei 10617, Taiwan
c
Center for Theoretical Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
d
Department of Physics, National Dong Hwa University, Shoufeng, Hualien 97401, Taiwan
a r t i c l e i n f o a b s t r a c t
Article history:
Received
20 October 2016
Received
in revised form 13 October 2017
Accepted
13 October 2017
Available
online 18 October 2017
Editor: W.
Haxton
Low-energy electronic recoil caused by solar neutrinos in multi-ton xenon detectors is an important
subject not only because it is a source of the irreducible background for direct searches of weakly-
interacting
massive particles (WIMPs), but also because it provides a viable way to measure the solar
pp and
7
Be neutrinos at the precision level of current standard solar model predictions. In this work
we perform ab initio many-body calculations for the structure, photoionization, and neutrino-ionization
of xenon. It is found that the atomic binding effect yields a sizable suppression to the neutrino-electron
scattering cross section at low recoil energies. Compared with the previous calculation based on the
free electron picture, our calculated event rate of electronic recoil in the same detector configuration is
reduced by about 23%. We present in this paper the electronic recoil rate spectrum in the energy window
of 100 eV to 30 keV with the standard per ton per year normalization for xenon detectors, and discuss
its implication for low energy solar neutrino detection as the signal and WIMP search as a source of
background.
© 2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Direct searches of weakly-interacting massive particles (WIMPs),
one of the favored dark matter (DM) candidates, have been actively
pursued in experimental nuclear and particle physics. Although no
concrete evidence of WIMPs has been obtained so far, a large por-
tion
of the parameter space in terms of WIMP mass and their
cross section to normal matter has been ruled out. For example,
recent results by the PandaX-II [1] and LUX [2] experiments, both
employing xenon detectors, set their best upper limits on the spin-
independent
WIMP-nucleon cross section: 2.5 × 10
−46
cm
2
for a
40 GeV/c
2
WIMP and 2.2 × 10
−46
cm
2
for a 50 GeV/c
2
WIMP,
respectively.
Using
xenon as detector material has several advantages: It is
relatively cheap to obtain, easy to scale up, and has enhanced cross
sections when scattered coherently. Next-generation WIMP search
proposals with xenon detectors include XENON1T [3], LZ [4], and
DARWIN [5]. These multi-ton scale detectors aim at improving the
*
Corresponding authors.
E-mail
addresses: jwc@phys.ntu.edu.tw (J.-W. Chen),
hsinchang@mail.ndhu.edu.tw (H.-C. Chi), cpliu@mail.ndhu.edu.tw (C.-P. Liu),
d01222003@ntu.edu.tw (C.-P. Wu).
current sensitivity in WIMP-nucleon cross section by one order of
magnitude with a ton-year exposure (a modest goal) to three or-
ders
of magnitude with 200 ton-year exposure (an ambitious goal).
To
reach high sensitivity in those experiments, proper back-
ground
subtraction is crucial. Direct WIMP searches use nuclear
recoil (NR) as the signal of WIMP-nucleus collision. However, nu-
clear
recoil due to coherent neutrino-nucleus scattering could fake
the signal. This kind of background is hard to shield and forms an
irreducible background called “neutrino floor”, which limits the ul-
timate
sensitivity the experiments can achieve [6].
Neutrino
electron scattering is another type of neutrino back-
ground
which is in principle reducible, but in practice hard to be
removed completely in experiments. It has been demonstrated re-
cently
by the LUX experiment that its xenon detector can reject
electron recoil (ER) at 99.6% with a NR acceptance of 50% [7].
As a recent study [8] shows, at a conventional level of 99.5% ER
rejection and 50% NR acceptance with a mulit-ton liquid xenon
detector, the ER caused by solar neutrinos is the limiting back-
ground
to measure the WIMP-nucleon cross section lower than
2 ×10
−48
cm
2
with a WIMP mass of 40 GeV/c
2
. While forthcoming
and future experiments looking forward to improving the ER/NR
discriminating power even further, the tiny remaining ER signals
can still be important.
https://doi.org/10.1016/j.physletb.2017.10.029
0370-2693/
© 2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.