季向东 (Xiangdong Ji) Shanghai JiaoTong University /University of Maryland. Dark Matter & Its Direct Detection. Four Lectures. Lecture 1: Astrophysical Evidences for Dark Matter (gravity) Lecture 2: Dark Matter Candidates and WIMPs Lecture 3: Collider and Indirect Search for WIMPs
季向东 (Xiangdong Ji) Shanghai JiaoTong University /University of Maryland Dark Matter & Its Direct Detection
Four Lectures • Lecture 1: Astrophysical Evidences for Dark Matter (gravity) • Lecture 2: Dark Matter Candidates and WIMPs • Lecture 3: Collider and Indirect Search for WIMPs • Lecture 4: Direct Detection of WIMPs
Ordinary Matter • The world around us is made of ordinary matter! • Ordinary matter is made of atoms and molecules (19th century chemistry) • Atoms are made of atomic nuclei and electrons (beginning of 20th century) • Atomic nuclei are made of protons and neutrons (1930’s) • Protons and neutrons are made of quarks and gluons (1970’s) Atomic spectroscopy indicates the sun, the milky way, and all stars in the sky are made of ordinary matter!
Origin of the DM Concept • Virial theorem：In the stationary gravitational system, the potential energy is twice the kinetic energy! • In 1933，Prof. Zwicky at Caltech studied the kinetic energy of the Coma cluster, he found that the kinetic energy is far bigger than the potential energy created by luminous mass. He proposed the concept of “dark matter” • According to his calcualtion, the mass of the dark matter must be as much as 300 times of the ordinary matter.
Astrophysical Evidences • Galaxy Structure • Rotational curve， • Gravitational lensing • Clusters of Galaxy • Gravitational lensing • Velocity distribution • Hot gas (X-ray) • Cosmic Microwave Background • Large Scale Structure of the Universe.
Sun’s Rotation Speed Around Milky Way • In the milky way, all stars rotates around the center of the galaxy • According to Newton’s gravitational theory，the rotation speed of the sun depends on the mass distribution and the distance to the center • According to this formula, the Rotation speed of the sun Shall be around 170km/s, however The actual speed is about 220 -250km/s. r v(r)
Rotation Curve（Ford,Robin,70’) • In a galaxy，stars rotation speed is a function of distace to the center. The result is the so-called galaxy rotation curve. 95%质量来自暗物质！
Dark Matter content in a galaxy • This implies the existence of a dark halo, with mass density ρ(r) ∝ 1/r2, i.e., M(r) ∝ r; • At some point ρ will have to fall off faster (in order to keep the total mass of the galaxy finite), but we do not know at what radius this will happen. • This leads to a lower bound on the DM mass density, ΩDM>∼0.1, where ΩX ≡ ρX/ρcrit, ρcrit being the critical mass density to be described later (i.e., Ωtot = 1)
Local Dark Matter Density • The DM density in the “neighborhood” of our solar system was first estimated as early as 1922 by J.H. Jeans, who analyzed the motion of nearby stars transverse to the galactic plane. He concluded that in our galactic neighborhood, the average density of DM must be roughly equal to that of luminous matter (stars, gas, dust). • Remarkably enough, the most recent estimates, based on a detailed model of our galaxy, find quite similar results ρlocal DM = 0.3 GeV/cm3; This value is known to within a factor of two or so.
Gravitational Lensing (引力透镜) • When light-ray passes through a gravitational field，its direction will be bent. From the magnitude of the bending, we can calculate the distribution of the gravitational field, hence the dark matter.
Gravitational Lensing • Strong Lensing (Tyson et al.) Dark Matter can extend as far as 200kpc and beyond!
DM content from clusters of galaxies • The observation of clusters of galaxies tends to give somewhat larger values, ΩDM0.2 to 0.3. • These observations include measurements of • the peculiar velocities of galaxies in the cluster, which are a measure of their potential energy if the cluster is virialized; • measurements of the X-ray temperature of hot gas in the cluster, which again correlates with the gravitational potential felt by the gas; and—most directly— • studies of (weak) gravitational lensing of background galaxies on the cluster.
Standard BigBag Cosmology • According to the standard theory of cosmology, the universe started 13 billion years ago with a big bang, expands and cools ever since. • At about 300,000 years, the atomic nuclei and electrons combine to form neutral atoms, the light can propagates now freely. • The first light is propagating for nearly 13by And became the fossil of the universe. Cosmic microwave background radiation (CMB) (Dicke, Gamow, 1946)
Critical mass/energy density • Hubble expansion parameter • Critical mass density
Cosmic Microwave Background（CMB） • In 1965，Penzias& Wilson (Bell Lab) found the CMB for the first time, measured the temperature around 3K, received the 1978 Nobel Prize in physics. • 1990,J. Mather through COBE satellite，found the CMB is a perfect black-body radiation. Moreover, the temperature is almost the same in all directions.
Fluctuation of CMB • In 1992，G. Smoot found, again through COBE data, that CMB temp has fluctuations at the level of 10-5. • The fluctuation can be explained using inflationary models, however, there must be 23% of dark matter！ CMB Fluctuation
2006年诺贝尔奖 • Mather 和 Smoot
Large Scale Structure of the universe • 我们今天的宇宙是非常不均匀的。这个不均匀是通过宇宙早期的涨落和引力的不稳定演化而来。 背景辐射的涨落
Dark Matter Content • The currently most accurate determination of ΩDM comes from global fits of cosmological parameters to a variety of observations: the anisotropy of CMB and of the spatial distribution of galaxies, one finds a density of cold, non–baryonic matter Ωnbmh2 = 0.106 ± 0.008 where h is the Hubble constant in units of 100 km/(s·Mpc). • Some part of the baryonic matter density, Ωbh2 = 0.022 ± 0.001 may well contribute to (baryonic) DM, e.g., MACHOs or cold molecular gas clouds
MOND • In 1983, Milgrom proposed a modified Newtonian dynamics in which F=ma is modified to F=maµ, which µ is 1 for large acceleration, becomes a/a0 when a is small. • To explain the rotational curve, one can choose
Problems with MOND • Cannot fit into a framework consistent with GR. • Hard to describe the expansion history, therefore the CMB fluctuation and galaxy distribution. • Hard to explain the bullet cluster. • No MOND can explain all gravitational anomalies without introducing DM.