季向东
上海交通大学
PARTICLE COSMOLOGY
Outline
 Particle Cosmology
 WIMPs Miracle
 PandaX
What is particle cosmology?
 Can particle physicists say something about
cosmology? Or vice versa, can cosmologists
say something about particle physics?
Particle Physics = Standard Model
+ a bunch of untested ideas
(planck scale, GUT, SUSY,
extra dimension, LR symmetry,
techni-color, little Higgs…)
Importance issues
 What is dark matter? Why there is 23% dark
matter?
 Why there is 4% baryonic matter?
 Can particle physicists say something about
cosmological constant?
 Other questions:
 What is the role of neutrinos?
 Particle physics from cosmology? The early
universe is an important lab for particle physics
 …
The Cooking recipe
 Assuming Standard Cosmology + (nearly)
thermal equilibrium…
 The third magic assumption!
 Throw in some particles, everything that we
know, quarks+leptons+…., forming the energymomentum density
 Solve the Einstein equation.
Particle and interactions
gravity


weak


strong

e/m


Quarks
Lepton+
(W, H+)
Neutrinos 

(Z, H0)
Photons


Gluons


Gravitons

Symmetries: spacetime + gauge symmetries +
other
There are unknowns
 Heavy particles that live short.
 In the GUT model, there are GUT scales multiplets
which play important role in the very early universe
 Heavy right-handed neutrinos…
 Particles that are weakly interacting and live
long





Axions
Sterile neutrinos
Gravitino
WIMPs
...
Particles that live long
 Electron: lightest charged particle (charge
conservation)
 Proton: baryon number symmetry breaking
interaction is small.
 Neutrinos: lightest fermion
 Photon and graviton: cannot decay due to
energy-momentum conservation
Particles that drop out of
thermal equilibrium
 Standard Boltzmann equation approach
 Depend on the rate of annihilation and
recombination
 Can trace out particle’s cosmic history fairly
accurately

Baryon Asymmetry and
Leptogenesis
 Understanding baryon-antibaryon
asymmetry in the present universe, why
matter dominates over antimatter?
 Heavy right-handed neutrino required by
seasaw might play a very important role
 Constraint on the CP violation parameter in
the lepton sector
What is dark matter?
 Axion
 Sterile neutrinos
 Gravitinos
 WIMPs
 …
 Non-particle possibilities
Gravitino dark matter?
 Because of its weak gravity coupling,
gravitino decouples from the rest of the world
very early, left with a huge quantity (it must
be very light to avoid over-closure).
 It can be diluted through inflation
 Gravitino will be regenerated through
reheating process.
 If gravitino decays, its life time will be around
M2pl/M3, which could affect BBN.
Gravitino dark matter?
 If gravitino is the lightest supersymmetric
particle, it lives long
 However, decay (to gravitino) of the next-tothe lightest supersymmetric particles can
affect BBN
 This problem can be solve through small Rparity breaking decays, which lead to a
gravitino with life time much longer than that
of the universe.
WIMPs
 WIMPs are particles that have mass on the
order of electroweak symmetry breaking
scale and has only weak interactions
 WIMPs has long life-time due to certain
symmetry
Z2 symmetry, U(1) symmetry
R-parity
Dark Matter Relics in the
Universe
 T ≫ M, WIMPs in thermal
equilibrium
 T < M, number density
becomes Boltzmann
suppressed
 T ~ M/20, Hubble expansion
dominates over annihilations
freeze-out occurs
 Precise temperature at
which freeze-out occurs, and
the density which results,
depends on the WIMP’s
annihilation cross section
WIMP Miracle
 To understand the percent of DM energy in the
universe today, we need the DM particles having
annihilation cross sections on the order of
~ 10-40 cm2
 Therefore, apart from the gravitational
interactions, the DM particles probably have
weak interactions as well!
 If DM particles do have weak interactions, they
may have something to do with the electroweak
symmetry breaking!
 WIMP Miracle might be the most exciting
thing the cosmology teaches us about
particle physics!
Searching for WIMPs
 Collider search
 Indirect search
 Direct search
 Review by 倪凯旋（上海交通大学）
PandaX Experiment
 An experiment that designed to look for
WIMPs through direct search
 PandaX collaboration (30+ people)
 上海交通大学
 中科院上海应用物理研究所
 山东大学
 北京大学
 We are considering other institution to join from China
and US
PandaX experiment
 Formed in 2009
 Shanghai Jiaotong University
 Shanghai Institue of Applied Physics, CAS
 Shangdong University
 Peking University
 (University of Maryland)
The technology
 PandaX: Particle AND Astrophysical Xenon TPC
can be used for both dark matter search and
double beta decay search
 Following the preceding experiments: Zeplin,
XENON, and LUX, build a state-of-art large size
xenon dual phase TPC detector.
 Two features:
1. It emphasizes light collection efficiency so as to
enhance the sensitivity to low-mass WIMPs
2. It can accommodate a ton-scale experiment
PandaX 实验是怎么工作的？
 在一个大的、高纯度的容器装入2吨的氙、并冷
却到零下100度。
 在液氙的上部和下部装上大量的光电管，可以
探测到单个光子
 在液氙的中心加上10万伏的高压，单个电子可
以其中漂移80cm 以上
 液氙必须不断更新提纯，去除从容器壁渗进的
杂质。
 探测容器必须放在高真空和由聚乙烯和铅做成
的上百吨的屏蔽体中。
 整个实验必须放在至少2公里深的地下。
Layout of jinping lab
PandaX 实验进展
 PandaX 首个100公斤级探测器设计已经完成，
正在建造中，预计6月可以完成。
 已购买了所有的光电管和500公斤氙气
 十万伏高测试正在进行中
 屏蔽体设计已经完成，正在采购材料， 准备
安装
 电子学信号采集系统正在建设中
Sensitivity plot
PandaX funding
 科技部973
 自然科学基金委
 上海交通大学
 山东大学
 北京大学
summary
 Great things happen now at the intersection
of particle physics and cosmology
 A lot of challenge
 Cosmological constant
 Dark matter
 Grand unification? Extra dimension?