Dr. Nicolò Masi Bologna University and INFN – November 13th 2015 Knowledge and Prejudices Hard to question that: • Non-gravitational interactions between DM and Standard Model particles are highly suppressed: the lack of observations disfavors DM that is electrically charged or interacts by the strong nuclear force. • It was called “dark” by F. Zwicky in 1933, long before the underground experiments and the direct detection studies that are testing also its interactions with fermions. • DM must clump gravitationally to form galaxies. This requires DM to be “cold”, that is non- relativistic at the time of structure formation. • The theoretically best-motivated candidates for a DM particle are a weakly interacting massive particle (WIMP) or an axion or axion-like particle (ALP). • WIMPS could solve the Hierarchy problem whereas an axion solves the Strong CP problem of the Standard Model. Typically WIMPs are considered to be thermal relics left over from the Early Universe; ALPs are usually not thermal. • We will focuss on the WIMP case, because we have less and more controversial evidences about axion-like particles properties. And because WIMPs are the most suitable candidates for direct and indirect searches. Nicolò Masi, 13/11/2015, DM State of Art • The abundance today is inversely proportional to the WIMP self-annihilation cross section. The fractional abundance, relative to the critical density, and the thermal averaged annihilation cross section are We believed that: 휎푣 ≈ 3 ∙ 10−26푐푚3푠−1 is a typical value expected for a particle with mass near the weak scale [O(100 GeV)] and a weak gauge coupling: the fact that the observed abundance of DM points to new physics at the weak scale, independently of particle physics motivations, is the so-called WIMP miracle (a Naturalness Criterion). But • Any combination of 푚퐷푀, 푔푋 can be taken. • The only real bound is that the DM must be relatively “light” as a consequence of the existence of a maximum annihilation cross section for a particle of a given mass: the so-called Unitarity Bound. • Griest and Kamionkowski applied this bound to infer an upper limit on the dark matter particle mass: using PLANCK constraints, this bound is something like: mDM ≲ 120 TeV (for a scalar). It slightly varies according to the spin-statistics of the candidate. A recent study shows a mDM ≲ 139 TeV constraint for a Dirac fermion. • Run I LHC researches excluded DM candidates up to 600 GeV-1 TeV scale. Nicolò Masi, 13/11/2015, DM State of Art Prejudices Some well spread paradigms, or “theoretical prejudices”: 1. DM has a zero electromagnetic cross section (some theories try to overcome the charge issue introducing a milli-charge or electric/magnetic dipole moments, conserving the main dark behavior of the particle). But, at the same time, it is capable to collide with our visible world particles. 2. DM particles are thermal relics which produce the EW scale Miracle. A not too heavy SUSY particle was the perfect candidate. 3. DM is made of one particle and resembles usual Standard Model “simplest” particles, stable and not affected by Standard Model-like anomalies and violations: some asymmetric theories try to overcome this simplification. 4. DM is much more a particle (a Majorana neutralino from SUSY inheritance) than a force, i.e. a real boson. 5. DM forms “simple” spherical halos around galaxies, without important substructures. This is a dangerous assumption when we have to deal with simulations of dark matter cosmic rays (CR) fluxes and with recoil physics in direct detection experiment (a Dark Disk can change everything!). Eur. Phys. J. Plus (2015) 130: 69 Nicolò Masi, 13/11/2015, DM State of Art Galactic Evidences Local Galactic Evidences 1. Rotation Curves From the Kepler’s law, for r much larger than the luminous radius, you should have v ∝ r-1/2: instead it is flat or rises slightly. Milky Way 2015 Mgrav : Mvis = 8÷6 : 1 Nicolò Masi, 13/11/2015, DM State of Art 2. Velocity dispersions of dwarf spheroidal galaxies Not only spiral galaxies evidences Nicolò Masi, 13/11/2015, DM State of Art 3. Milky Way warp and dark galaxies Milky Way like a long play record …But Magellanic Clouds don’t explain the entire Warp Factor problem. • With the Sloan Digital Sky Survey a lot of low-brightness satellites galaxies have been discovered, but not enough to account the whole dynamical Milky Way warp: there should be tens of almost dark galaxies. • This is known as “the dwarf galaxy problem”: in the Universe there are 10 to 100 times fewer small galaxies than predicted by dark matter theory of galaxy formation and spiral galaxies warp. The solution: VIRGOHI21 VIRGOHI21 is an extended region of neutral hydrogen (HI) in the Virgo Cluster discovered in 2005. Analysis of its internal motion indicates that it may contain a large amount of DM, as much as a small galaxy, but no stars: the first Dark Galaxy. Nicolò Masi, 13/11/2015, DM State of Art Cosmic Evidences Cosmic Evidences 1. Dynamics of galaxy cluster Virial theorem U = 2K 2 K = i mi vi U ~ GM2/R • The Bullet cluster (1E 0657-56) consists of two colliding clusters of galaxies. At a statistical significance of 8σ, it was found that the spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law. It provides "evidence against some of the more popular versions of Modified Newtonian Dynamics (MOND)" . • In other words: The lensing is strongest in two separated regions near the visible galaxies. This provides support for the idea that most of the mass in the cluster pair is in the form of collisionless dark matter. Nicolò Masi, 13/11/2015, DM State of Art 2. X-ray cluster The mass of a cluster can be determined via the profile of X–ray emission that traces the distribution of hot emitting gas in rich clusters Hydrostatic equilibrium: Beta model: Estimated Temperature: The disparity between the temperature of the previous formula and the corresponding observed temperature, T ≈ 10 keV, when Mr is identified with the baryonic mass, suggests that Mgrav/Mvis ~ 15 Nicolò Masi, 13/11/2015, DM State of Art 3. Lensing Weak Lensing: the distortions are small and can only be detected by analyzing large numbers of sources to find coherent distortions of only a few percent. COSMOS 3D DM distribution from 3.5 to 6.5 billion years ago. Strong Gravitational Lensing: there are easily visible distortions such as the formation of Einstein’s rings, arcs and multiple images. 4퐺푀 Deflection Angle 휃 = 푟푐2 Nicolò Masi, 13/11/2015, DM State of Art 4. DM Skeleton between galaxies Galaxy clusters occur at the intersection of large-scale structure filaments. The thread-like structure of this “cosmic web” has been traced by galaxy redshift surveys for decades. The COSMIC DM WEB Contour lines outline an invisible dark matter filament connecting the galaxy clusters Abell 222 (bottom) and Abell 223 (top) in the night sky. Nicolò Masi, 13/11/2015, DM State of Art Dwarf Galaxy Planes (2014): the discovery of symmetric structures in the Local Group Great spiral galaxies Spiral galaxies have their form on the main nodes satellites confined in a plane, of the Cosmic DM Web orthogonal to their own luminous one, which seems to follow the ingoing filaments Milky Way Dwarf Galaxies Nicolò Masi, 13/11/2015, DM State of Art Cosmological Evidences Cosmological Scale Evidences: 1. PLANCK Acoustic Peacks Universe Curvature Baryon density Matter density Nicolò Masi, 13/11/2015, DM State of Art Planck Pie Chart Perfect Λ퐶퐷푀 scenario Nicolò Masi, 13/11/2015, DM State of Art 2. Baryonic Oscillation, Structure Formation and Evolution • Baryon Acoustic Oscillations (BAO) refers to regular, periodic fluctuations in the density of the visible baryonic matter of the universe, caused by acoustic primordial waves. • In the same way that SNe Ia provide a “standard candle" for astronomical observations, BAO matter clustering provides a “standard rules" for length scale in cosmology. The scale of BAO depends on 휴풎: one can quantify the amount of dark matter on very large scales: Ω푏 Ω푚 = 0.155 ± 0.006. Furthermore, galaxies could not have formed from primordial density fluctuations in a purely baryonic medium. Nicolò Masi, 13/11/2015, DM State of Art Not a particle? Modified Newtonian Dynamics (MOND) is a MOND hypothesis that proposes a modification of Newton’s law of gravity/second law to explain the galaxy rotation term Milgrom’s idea: the acceleration due to gravitational force depends upon the function μ(a/a0), which approaches 1 for Newtonian cases and a/a0 for small arguments. −10 2 (a0 ≈ 10 m/s ) The velocity of stars on a circular orbit far from 4 the center is a constant and does not depend 푣 = 퐺푁푀푎0 on the distance r → the rotation curve is flat The AQUAL and TeVes Lagrangian generalizations were developed by Milgrom and Bekenstein. Studies of the aforementioned Bullet Cluster (August 2006-2008) provide evidence against some of the more popular versions of MOND. In addition, an important 2011 study observing the gravity-induced redshift of galactic clusters found results that strongly supported General Relativity: MOND can fit the determined redshifts slightly worse than does General Relativity with dark halos. Nicolò Masi, 13/11/2015, DM State of Art With Modified Gravity (MOG) several theories MOG which introduce new cosmological fields are addressed, starting from a tensor generalization of Einstein Gravitational Theory. • MOG has evolved as a result of investigations of Nonsymmetric Gravity Theory (NGT) and, most recently, it has taken the form of Scalar-Tensor-Vector Gravity (STVG), that extends Einstein-Hilbert action using new scalar, tensor, and vector fields and new constants above the Newtonian one.