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DARK MATTER ON DEPARTMENT SCALE Daniele Fantin (M. Merrifield, A. Green) (M. Merrifield, A. Green)...
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Transcript of DARK MATTER ON DEPARTMENT SCALE Daniele Fantin (M. Merrifield, A. Green) (M. Merrifield, A. Green)...
DARK MATTER ON DEPARTMENT SCALE
Daniele Fantin (M. Merrifield, A. Green)University of Nottingham
Bologna, 2 April 2009
Summary
Dark Matter Candidates
Dark Matter Detection
State of art of N-body simulations
Alternative approaches (My research)
Daniele Fantin, University of Nottingham
Dark Matter Evidence
Daniele Fantin, University of Nottingham
Universe Energy Budget
Potential dark matter candidate
Optically dark: does not couple strongly with photons
Electrically neutral
Non-relativistic
Collisionless: in order to form extended halos
Stable: doesn’t decay on timescale shorter than age of the Universe
Daniele Fantin, University of Nottingham
Cold Dark Matter – Candidates
Non-baryonic matter
WIMPs (Weakly Interactive Massive Particles) (Lee & Weinberg, 77; Gunn et al, 78)
Neutralino (lightest supersymmetric particle )
Axions (Peccei & Quinn, 77; Weinberg, 78)
Daniele Fantin, University of Nottingham
Why WIMPs?Any stable WIMP in thermal equilibrium in the early Universewill have the right density at the present day to be the DM
Daniele Fantin, University of Nottingham
χ + χ ↔ U + Ū
1 32
1. High T: Efficient Creation/destruction
2. Destruction
3. Creation in overdense regions
Particle Colliders (LHC)
How can we detect WIMPs?
.
Generic signal: missing energy/momentumWon’t demonstrate the existence of DM
Daniele Fantin, University of Nottingham
Via products of annihilations (e.g. γ-rays, positrons, anti-protons,neutrinos) in high density regions.
Indirectly
Daniele Fantin, University of Nottingham
γ-rays: FERMI, HESS, MAGIC, VERITAS
Anti-matter: PAMELA,ATIC Neutrinos: IceCube, ANTARES
Dependence of signals on local dark matter distribution
Daniele Fantin, University of Nottingham
Via elastic scattering on detector nuclei in the lab.
χ+N χ+N
Solutions: go underground (mines)
Detect recoil energy via ionisation, scintillation and/or heat.
Problem: radioactivity
Directly
Daniele Fantin, University of Nottingham
ZEPLIN III
CDMSII DAMA
Daniele Fantin, University of Nottingham
The event rates in direct detection experiments depend on the DM density (and in some cases velocity) distribution.
Why is the DM Distribution important?
WIMP cross-section on proton
dR/dE ≈ σp ρχ ∫ f(v)/v dvvmin
∞
Differential event rate (per kg/day/KeV):
minimum WIMP speed whichcan cause a recoil of energy E.
WIMP speed distributionin rest frame of detector
local WIMP density
Daniele Fantin, University of Nottingham
To confirm the existence of DM we need to detect it .
WIMPs are a good dark matter candidate
They can be detected:
directly via elastic scattering in the lab
signals depend on ultra-local dark matter density and speed distribution
indirectly via annihilation products
signals depend on Milky Way density distribution in high density regions
Intro-Summary
Daniele Fantin, University of Nottingham
Event rate depends on dark matter distribution
The standard halo model:assumes Milky Way halo as isotropic, isothermal spherevelocity distribution is then Maxwellian
BUT “observed” and simulated halos are triaxial, anisotropic and contain substructure.
Halo Modelling
Moore et al, 04
Daniele Fantin, University of Nottingham
Actual predictions about f(v) based on very simple assumptions:
Maxwellian (Freese et al. 1988)
Multivariate Gaussian (Evans et al. 2000, Helmi et al. 2002)
Probably NOT valid in reality!!
Dark Matter Distribution
Daniele Fantin, University of Nottingham
How good is the assumption of a Maxwellian speed distribution?
Depends on the ultra-local (sub-mpc) WIMP distribution.
Which depends on how well-mixed the tidal debris from disrupted sub-halos is.
i) Ultra-local WIMP distribution is smooth, consisting of large number of streams. [Helmi, White & Springel,02; Vogelsberger et al,08]
ii) Ultra-local WIMP distribution consists of a finite number of streams [Stiff & Widrow,01 ; Moore et al., 04; Fantin, Merrifield & Green,08]
Daniele Fantin, University of Nottingham
First cosmological simulationable to resolve the building bricks of massive MW-like DM halo (10¹²M⊙) at z=0
Presence in the Phase S ofunderdense elongated streams formed by material removed from accreted subhalos
Substructures: Via Lactea - GHALO
Diemand et al, 08
800 kpc
40 kpc
Daniele Fantin, University of Nottingham
4 generations of substructures
Smooth emission from the main halo
Smooth emission from the subhalos
Substructures: AQUARIUS
Springel et al, 08
Daniele Fantin, University of Nottingham
Name Authors CodeParticle
mass(M⊙)
Softening(pc)
Number ofhalos
simulated
Aquarius Springel et al.
GADGET3(TreePM)
1.7 x 10³ 21 6
GHALO Stadel et al. PKDGRAV2 1.0 x 10³ 61 5
Via Lactea2 Diemand etal.
PKDGRAV2 4.0 x 10³ 40 1
Recent simulations of Milky Way-like halos
Daniele Fantin, University of Nottingham
Substructures
DistributionAbundanceMass ProfileAnnihilation signal
Phase-space distribution of DM
DM Halos: Open Questions
Daniele Fantin, University of Nottingham
The smallest halos resolved in sim are close to 10⁵ M⊙, while the mass of the smallest WIMP microhalos 10⁻⁶ M⊙ (Green et al., 2004)
Resolution in simulations ~ 100 pc, while the expected scales probed by direct detection experiments are 0.1-1 mpc
NEW APPROACHES REQUIRED!!!
Sim-Summary/Conclusions
Daniele Fantin, University of Nottingham
Method based on reverse integration process
Collision head-on between unbound system of particles and a galaxy
No numerical integration
First model of the ultra-fine structure of the DM halo in the solar neighbourhood
My Research
Daniele Fantin, University of Nottingham
Realistic gravity
Results in a single timestep
Orbits history can be calculated analytically
No numerical integration: quick
Exploration of parameter space with very high resolution
Positive Aspects
Daniele Fantin, University of Nottingham
No detailed quantitative/realistic comparison with Milky Way
Isochrone potential
Negative Aspects
Daniele Fantin, University of Nottingham
i. Set the IC for the satellite and the time in the past t0
at which it was at that locationii. Set the present-day PS coordinates of the detector
locationiii. Evolve analytically these coords. backwards in timeiv. Assume for the merging halo a Gaussian distributionv. Evaluate the initial PS density due to the satellite for
this PS locationvi. Repeat for a grid of v to map out the full velocity
distribution within the detector
Main Steps
Daniele Fantin, University of Nottingham
Evolution at t = 1.23 Gyr
Look at the difference in resolution!!!
Results
Daniele Fantin, University of Nottingham
fx
vx
fx
vx
fx
vx
t =13.6 Gyr (MW’s age):
Single Merger: Distribution Function
Halo perturbed Whole series of
peaks
Daniele Fantin, University of Nottingham
Distribution Function Evolution
Daniele Fantin, University of Nottingham
Fantin et al, 08
cosθ
v
Diagnostic for Directional Detectors
Cos θ = vy/v
Smooth background+
Evident features
Daniele Fantin, University of Nottingham
Fantin et al, 08
Many mergers at each timestep
Add to each merger a signal
Analysis of the final signal
Multiple Merger
cosθ
vDaniele Fantin, University of Nottingham
Merger Tree
• Merger tree of a MW-like halo
• Add f(v) at different z
• Look for the final velocity/angle distribution
Daniele Fantin, University of Nottingham
Multiple Merger: Angle Distribution
Daniele Fantin, University of NottinghamM(M⊙)
Time(Gyr)
10⁶
10⁸
10¹⁰
13.6 1.36 7.35 136
Merger Tree
Daniele Fantin, University of Nottingham
132
1. Relations between velocity components
2. Presence of shell structure
3. Presence of escape velocity
Fantin et al, in prep.
• Event rate in direct detection of DM depends on the Ultra-local spatial(velocity) distribution
• In this case N-body simulation cannot help us• New approaches are necessary to investigate • Presence of “features” in the velocity
distribution coming from merging events• Work is still ongoing
Final Summary
Daniele Fantin, University of Nottingham
Daniele Fantin, University of Nottingham
Any Question?