July 13, 2018
University of California -
Riverside
Scientists have imposed
conditions on how dark matter may interact with ordinary matter. In the search
for direct detection of dark matter, the experimental focus has been on WIMPs,
or weakly interacting massive particles, the hypothetical particles thought to
make up dark matter. But the research team invokes a different theory to
challenge the WIMP paradigm: the self-interacting dark matter model, or SIDM.
An international team of
scientists that includes University of California, Riverside, physicist Hai-Bo
Yu has imposed conditions on how dark matter may interact with ordinary matter
-- constraints that can help identify the elusive dark matter particle and
detect it on Earth.
Dark matter -- nonluminous
material in space -- is understood to constitute 85 percent of the matter in
the universe. Unlike normal matter, it does not absorb, reflect, or emit light,
making it difficult to detect.
Physicists are certain dark
matter exists, having inferred this existence from the gravitational effect
dark matter has on visible matter. What they are less certain of is how dark
matter interacts with ordinary matter -- or even if it does.
In the search for direct
detection of dark matter, the experimental focus has been on WIMPs, or weakly
interacting massive particles, the hypothetical particles thought to make up
dark matter.
But Yu's international
research team invokes a different theory to challenge the WIMP paradigm: the
self-interacting dark matter model, or SIDM, a well-motivated framework that
can explain the full range of diversity observed in the galactic rotation
curves. First proposed in 2000 by a pair of eminent astrophysicists, SIDM has
regained popularity in both the particle physics and the astrophysics
communities since around 2009, aided, in part, by work Yu and his collaborators
did.
Yu, a theorist in the
Department of Physics and Astronomy at UCR, and Yong Yang, an experimentalist
at Shanghai Jiaotong University in China, co-led the team analyzing and
interpreting the latest data collected in 2016 and 2017 at PandaX-II, a
xenon-based dark matter direct detection experiment in China (PandaX refers to
Particle and Astrophysical Xenon Detector; PandaX-II refers to the experiment).
Should a dark matter particle collide with PandaX-II's liquefied xenon, the result
would be two simultaneous signals: one of photons and the other of electrons.
Yu explained that PandaX-II
assumes dark matter "talks to" normal matter -- that is, interacts
with protons and neutrons -- by means other than gravitational interaction
(just gravitational interaction is not enough). The researchers then search for
a signal that identifies this interaction. In addition, the PandaX-II
collaboration assumes the "mediator particle," mediating interactions
between dark matter and normal matter, has far less mass than the mediator
particle in the WIMP paradigm.
"The WIMP paradigm
assumes this mediator particle is very heavy -- 100 to 1000 times the mass of a
proton -- or about the mass of the dark matter particle," Yu said.
"This paradigm has dominated the field for more than 30 years. In
astrophysical observations, we don't, however, see all its predictions. The
SIDM model, on the other hand, assumes the mediator particle is about 0.001
times the mass of the dark matter particle, inferred from astrophysical
observations from dwarf galaxies to galaxy clusters. The presence of such a
light mediator could lead to smoking-gun signatures of SIDM in dark matter
direct detection, as we suggested in an earlier theory paper. Now, we believe
PandaX-II, one of the world's most sensitive direct detection experiments, is
poised to validate the SIDM model when a dark matter particle is
detected."
The international team of
researchers reports July 12 in Physical Review Letters the strongest
limit on the interaction strength between dark matter and visible matter with a
light mediator. The journal has selected the research paper as a highlight, a
significant honor.
"This is a particle
physics constraint on a theory that has been used to understand astrophysical
properties of dark matter," said Flip Tanedo, a dark matter expert at UCR,
who was not involved in the research. "The study highlights the
complementary ways in which very different experiments are needed to search for
dark matter. It also shows why theoretical physics plays a critical role to
translate between these different kinds of searches. The study by Hai-Bo Yu and
his colleagues interprets new experimental data in terms of a framework that
makes it easy to connect to other types of experiments, especially
astrophysical observations, and a much broader range of theories."
PandaX-II is located at the
China Jinping Underground Laboratory, Sichuan Province, where pandas are
abundant. The laboratory is the deepest underground laboratory in the world.
PandaX-II had generated the largest dataset for dark matter detection when the
analysis was performed. One of only three xenon-based dark matter direct
detection experiments in the world, PandaX-II is one of the frontier facilities
to search for extremely rare events where scientists hope to observe a dark
matter particle interacting with ordinary matter and thus better understand the
fundamental particle properties of dark matter.
Particle physicists' attempts
to understand dark matter have yet to yield definitive evidence for dark matter
in the lab.
"The discovery of a dark
matter particle interacting with ordinary matter is one of the holy grails of
modern physics and represents the best hope to understand the fundamental,
particle properties of dark matter," Tanedo said.
For the past decade, Yu, a
world expert on SIDM, has led an effort to bridge particle physics and
cosmology by looking for ways to understand dark matter's particle properties
from astrophysical data. He and his collaborators have discovered a class of
dark matter theories with a new dark force that may explain unexpected features
seen in the systems across a wide range, from dwarf galaxies to galaxy
clusters. More importantly, this new SIDM framework serves as a crutch for
particle physicists to convert astronomical data into particle physics
parameters of dark matter models. In this way, the SIDM framework is a
translator for two different scientific communities to understand each other's
results.
Now with the PandaX-II
experimental collaboration, Yu has shown how self-interacting dark matter
theories may be distinguished at the PandaX-II experiment.
"Prior to this line of
work, these types of laboratory-based dark matter experiments primarily focused
on dark matter candidates that did not have self-interactions," Tanedo
said. "This work has shown how dark forces affect the laboratory signals
of dark matter."
Yu noted that this is the
first direct detection result for SIDM reported by an experimental
collaboration.
"With more data, we will
continue to probe the dark matter interactions with a light mediator and the
self-interacting nature of dark matter," he said.
Story Source:
Materials provided
by University of California
- Riverside. Original written by Iqbal Pittalwala. Note: Content may
be edited for style and length.
Journal Reference:
Xiangxiang Ren, Li Zhao,
Abdusalam Abdukerim, Xun Chen, Yunhua Chen, Xiangyi Cui, Deqing Fang, Changbo
Fu, Karl Giboni, Franco Giuliani, Linhui Gu, Xuyuan Guo, Ke Han, Changda He, Di
Huang, Shengming He, Xingtao Huang, Zhou Huang, Xiangdong Ji, Yonglin Ju, Yao
Li, Heng Lin, Huaxuan Liu, Jianglai Liu, Yugang Ma, Yajun Mao, Kaixiang Ni,
Jinhua Ning, Andi Tan, Hongwei Wang, Meng Wang, Qiuhong Wang, Siguang Wang,
Xiuli Wang, Shiyong Wu, Jingkai Xia, Mengjiao Xiao, Pengwei Xie, Binbin Yan,
Jijun Yang, Yong Yang, Hai-Bo Yu, Jianfeng Yue, Tao Zhang, Jifang Zhou, Ning
Zhou, Qibin Zheng, Xiaopeng Zhou. Constraining Dark Matter Models with a
Light Mediator at the PandaX-II Experiment. Physical Review Letters, 2018;
121 (2) DOI: 10.1103/PhysRevLett.121.021304
