Dark Matter Candidates Astroparticlephysics, UZH, Spring 2012 Marc Schumann [email protected] What will we learn today? ● What kind of Dark Matter do we „need“? ● Baryonic Dark Matter? - Why not? - Primordial Nucleosynthesis ● Particle Dark Matter: - Axions - WIMPs: thermal production – the „WIMP miracle“ SUSY and the neutralino (Extra Dimensions: Kaluza-Klein particles) - sterile neutrinos This lecture is to learn about the models that predict Dark Matter candidates → lots of theoretic ideas CDM Model The Standard Model of Cosmology („Concordance Model“) Describes the Universe since the Big Bang with a few parameters only (6) Uses Friedmann equation to describe evolution of Universe since Inflation Agrees with the most important cosmological observations: ● CMB Fluctuation ● Large Scale Structures ● Accelerated Expansion (SN observations) ● Distribution of H, D, He, Li Ingredients: Cosmological Constant CDM Cold Dark Matter Cold vs. Hot ● Hot: particle moving with relativistic speed at the time when galaxies could just start to form ● Cold: moving non-relativistically at that time ● Important implication for structure formation ● Hot Dark Matter cannot cluster on galaxy scales until it has cooled down to non-relativistic speeds and so gives rise to a considerably different primordial fluctuation spectrum We are looking for Cold Dark Matter: Invisible Cold (v < 10-8 c) Collisionless Stable Do we have to invent something new? Baryonic Matter in the Universe Centaurus A Remember: Baryonic Matter might also be „dark“ in the optical... BUT we are looking for something without e/m interaction Why not Baryonic Matter? ● too little: < 0.05 b ● Big Bang Nucleosynthesis < 0.05 b fixes quite precisely (+CMB) b (1940s: Gamov, Alpher, Herman) - abundances of light elements depend on number of baryons - D production is most sensitive ● not collisionless ● not found in microlensing searches ● Black Holes? → No Baryonic Candidates main class: MACHOs – massive compact halo objects ● Brown Dwarfs: H/He spheres with m < 0.08 M⊙ (too light, H-burning will never start) ● Jupiters: similar but with m < 0.001 M⊙ ● Black Holes with m ~ 100 M⊙ could be remnants of an early generation of stars which were massive enough so that not many heavy elements were dispersed when they exploded as supernovae Less popular: fractal or specially placed clouds of molecular hydrogen EROS, MACHO, OGLE Microlensing with OGLE ● Polish project started 1992 ● telescope located in Chile ● main targets: GMC and galactic bulge ● some MACHOs and 14 extrasolar planets found so far Primordial Black Holes Fraction of the Universe's mass which could be in form of a Carr et al, PRD 81, 104019 (2010) primordial black hole BUT ● some of the dark matter must be baryonic! ● We expect b~0.05 (nucleosynthesis, CMB) but what we see (stars, gas, dust) only accounting for lum~0.01 ● It seems that there are way too many MACHOs to explain the discrepancy Why not Neutrinos? Neutrinos are a part of the SM ● collisionless ● massive ( -oscillations) ● produced in the early Universe: decouple at kT ~ 3 MeV n ~ 115 cm-3 ● compare with critical density 3 crit = 5.1 GeV/m 3 = 5100 eV/cm → neutrinos can make up the entire energy content of the Universe if much too large! Large Scale Structures BUT: neutrinos move too far and too fast (decoupling at kT=3 MeV) From direct e 0.63 eV mass limit; oscillations; WMAP data ⇒ hot Dark Matter The smallest scale with „clumpy“ structure sets a lower limit on the particle mass: low mass → high speed (if created thermally) → travels large distances → scale on which density perturbations are washed out Probing small scale structures at z~3: mDM2 keV Back to Particle Physics? the Standard Model provides an excellent description of all experimental observations... H BUT it is incomplete... The Standard Model > 18 free parameters No grand unification No gravity Why P and CP violation? Why three particle generations? Strong CP problem Hierarchy Problem (m ≪ m ) H P l ⇒ Not the fundamental theory Popular extensions: Supersymmetry (SUSY) → WIMP H Extra Dimensions → LKP Peccei-Quinn Theory → Axion ... and many, many more Non baryonic DM: new particles or „old“ particles with non-standard properties stolen from Gianfranco Bertone (Some) Dark Matter Candidates ● Axion n o ● i t WIMPs c e - Neutralino s s s - (LKP) o r c ● sterile neutrinos mass DM Production Two production mechanisms: Thermal Production Non thermal production In thermal equilibrium with Production in a the Universe („freeze out“) Phase Transition → WIMPs → Axions Candidates for non-baryonic DM must be ● stable on cosmological time scales (otherwise they would have been decayed by now) ● must interact very weakly (otherwise would not be considered as Dark Matter) ● must have the right relic density (=amount of DM) Note: There is a 3rd production mechanism at very large T, soon after or soon before inflation. These particles are usually superheavy, e.g. Wimpzillas The Axion in a Nutshell The strong CP-Problem: CP violating term + BUT: no neutron EDM found (< 3x10-2 6 e cm) ­ → no CP violation in QCD ( < 10-1 0 ) → Question: Why is so small? Naturalness Problem Peccei, Quinn (1977): Add new global symmetry spont. broken U(1) → make a dynamical variable Weinberg, Wilzcek (1978): Theory contains a new particle: Axion DM candidate: cosmological E density cold Dark Matter V ~ 10 -- 1 7 c non-thermal production a Effective Axion Potential very high E spontaneous symmetry QCD epoch: vacuum breaking; the axion field (instanton) effects tilt relaxes somewhere in the potential, explicitly the potential breaking the symmetry axion gets mass CP symmetry restored A Pooltable Analogy <10 – 9 We live on a pool-table which CP seems to be a perfect is perfectly flat (such that we symmetry in strong interactions can play pool properly...) stolen from P. Sikivie, arXiv:hep-ph/9506229 A Pooltable Analogy <10 – 9 At some point we jump off the It is strange that CP is conserved table an realize that it is standing in strong interactions while it is on a non-flat room floor violated in weak interactions → why is the table so remarkably flat? → Why is so small (or zero)? stolen from P. Sikivie, arXiv:hep-ph/9506229 A Pooltable Analogy <10 – 9 The easiest way to make The Peccei-Quinn mechanism every pool table perfectly makes a dynamic field. flat is to build it on a post Non-perturbative QDC effects than can pivot on an axle, then pull to zero. countered by a weight. → then gravity does the adjustment stolen from P. Sikivie, arXiv:hep-ph/9506229 A Pooltable Analogy <10 – 9 L One can try to test this The axion is the quantum of hypothesis by inducing oscillation of the parameter oscillations in the pool table. in QCD. The oscillation frequency Its properties depend in the axion depends on the lever arm L decay constant f ∝ ma– 1 stolen from P. Sikivie, arXiv:hep-ph/9506229 A Pooltable Analogy * L Assume the pool table was Depending on how the QCD brought from outer space effects appear at kT~1 GeV there (no gravity) and the initial angle are initial coherent axion field was –* oscillations. If f is large, these might constitute an axion relic Depending on how gravity started energy density. to act (when the spaceship landed) → dark matter candidate there might be relic oscillations which „vacuum misalingnment mechanism“ depend on the initial misalignment angle * non thermal DM production Axion Searches / Limits Current Axion Limits (... from 2010) Generalized Formalism for Dark Matter Candidates ● most „new physics“ models need to have a mechanism to make the lightest new particle stable → Dark Matter Candidate ● this is usually achieved by introducing a multiplikative discrete D-symmetry (D=Dark) with D=+1 standard model sector D=−1 new particle sector ● D is a multiplikative quantum number → particles in the D=−1 sector can only be pair-annihilated or -produced → the lightest particle with D=−1 is stable ● if the particle is electrically neutral → Dark Matter Candidate WIMPs ● Weakly Interacting Massive Particles ● Some of the best motivated candiates from „new“ physics ● WIMPs interact only via gravity and weak interactions ● WIMPs are somewhat similar to neutrinos, but far more massive (>GeV) and slower ● sub-GeV WIMPs could be Light Dark Matter ● Why weak scale masses/interactions? The Planck Scale ● Mpl2 = ℏc/G ≈ 101 9 GeV → Planck mass ● At this scale, the strength of Expansion and the Temperature gravity becomes similar to the other forces of the early Universe → „natural“ scale for gravity interactions (radiation dominated): ● Compton wavelength is about the size of a Schwarzschild radius of a black hole → QFT breaks down ● Any photon energetic enough to precisely measure a Planck-sized object could actually create a particle of that dimension, but it would be massive enough to immediately become a black hole → Quantum gravity is needed (here string theory comes into play) ● Early universe (right after the Big Bang) is governed by Planck scale dynamics Thermal WIMP Production „The WIMP Miracle“ ● suppose WIMP candidates can be created/annihilated in pairs ● assume that the 's are in thermal eq. with all light particles ● number density n follows the Boltzmann equation: ● when T < m, pair creation needs from tail of v-distribution → in equilibrium, number density falls exponentially Thermal WIMP Production II When the annihilation rate nannv〉 < expansion rate H, the probability for to find a partner for annihilation becomes small expanding Universe: „freeze out“ WIMPs fall out of equilibrium,