Transcript Halo-Shape and Relic-Density Exclusions of Sommerfeld

Dark Matter:
halo-shape and relic density constraints on dark
matter theories with Sommerfeld enhancement
Recent observations by PAMELA, etc. have been
interpreted as evidence of cold dark matter self
annihilation. If these observations and their speculative
interpretation are correct, then the dark matter self
interaction cross section would need enhancement. In
this journal club talk I will discuss the weakly interacting
massive particle (WIMP) 'miracle' that results in the dark
matter relic density that we see today, I will review
recent observations, and I will discuss how proposed
interaction enhancement mechanisms are inconsistent
with data. Finally, I will discuss signals that dark matter
may have on observable halo-shapes.
Dark Matter:
halo-shape and relic density constraints on dark
matter theories with Sommerfeld enhancement
Journal Club. A. B. Fry 4/22/2010
Halo-Shape and Relic-Density Exclusions
of Sommerfeld-Enhanced Dark Matter
Explanations of Cosmic Ray Excesses
Feng, J., Kaplinghat, M., & Yu, H.
(2010) Physical Review Letters, 104 (15)
DOI: 10.1103/PhysRevLett.104.151301
A Theory of Dark Matter
Nima Arkani-Hamed, Douglas P. Finkbeiner,
Tracy R. Slatyer, Neal Weiner
Outline
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Motivation
Relic wimps
Observations
Constraints
The dark sector
○ Sommerfeld
enhancement
○ halo shape
• Conclusions
The Question
If the particles that make up most of the
mass of the universe are not baryons what
are they?
The Question
If the particles that make up most of the
mass of the universe are not baryons what
are they?
• They are dark (doesn’t play well with
radiation)
• They are electrically neutral
• They are highly non-relativistic (cold)
• Dissipationless
The Answer
If the particles that make up most of the
mass of the universe are not baryons what
are they?
Can we even understand the information
data provides us without tying it to any
specific models for DM’s nature?
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Weakly Interacting Massive Particles
• Massive particles in the Universe decay to
lower energetically favored states,
however massive particles may survive to
the present if they carry some sort of
conserved additive or multiplicative
number.
Weakly Interacting Massive Particles
1
2
3
1. Assume DM
particle in
equilibrium
χχ ↔ff
2. Universe cools
χχ → ff
3. Freeze out
//
χχ ↔ff
Weakly Interacting Massive Particles
• The WIMP miracle is that parameter space
perfectly allows for a CDM WIMP particle
that is independently predicted in
particle physics and it has the right density
to be dark matter.
Weakly Interacting Massive Particles
• In the rest of this talk I will be assuming
some sort of supersymmetric dark matter
particle.
Neutralinos & Supersymmetry
• Supersymmetry is a theoretical scheme
that pairs every known particle with a
heavier, undiscovered superpartner.
• The lightest superpartner, expected to be
a few hundred times as massive as a
proton (~1 Tev), is the neutralino which
could be a WIMP
• According to supersymmetry WIMPs act
as their own antimatter particles!
Neutralinos & Supersymmetry
• Neutralino WIMPs in a galactic halo will
collide and annihilate each other to
produce high energy gamma ray photons
or other ordinary particles like positrons
and electrons (consistent because the
supersymmetric quantum number is
multiplicative)
Observations
Observations
• PAMELA: anomalous abundance of
positrons in cosmic rays above 10 GeV.
• ATIC: Excess in total flux of electrons and
positrons in cosmic rays
• WMAP: excess of microwave emission
from galactic center “WMAP haze”
• The time variation of a signal seen by the
DAMA-LIBRA collaboration
Observations
• FERMI: despite initial speculation, the 10
month data release seems to be
consistent with a single power law
distribution of gama-rays as predicted by
the cosmic background of AGN.
Observations
• Pulsars can explain PAMELA…
Constraints
• Relic density and ΩDM (Arkani-Hamed
2009 et al.)
• Late decays would modify nucelosynthesis
particularity the 7Li/H ratio (Feng lecture
slides 2007).
• CMB observations constrain dark matter
annihilation energy injection rate during
recombination (Zavala 2010 et al.).
Constraints
• The Pamela and Atic signals require a
cross section that is of order 100 times
greater than what would be expected from
thermal relic WIMPs
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Require a low cross section to hadrons.
Require a high cross section into leptons.
The Dark Sector
The Dark Sector
• From a theoretical viewpoint a new
interaction for the dark sector arises
naturally in a variety of theories beyond
the standard model, and is thus well
motivated from a theoretical point of view
Sommerfeld Enhancements
• A plethora of papers propose Sommerfeld
enhancement, S, to the DM cross section
• Sommerfeld enhancement increases the
cross section of particles at low velocities
similarly to classical gravitational
enchantment.
• The Sommerfeld enhancement is the
quantum counterpart to the classical
gravitational phenomena.
Sommerfeld Enhancements
• Consider classical gravitational
enhancement: for a test particle at velocity
ν approaching a massive particle of radius
R and mass M it can be shown that the
cross section is increased to
Where σ0=πR2 and νesc2=2GM/R
Sommerfeld Enhancements
• If you work through the entire derivation
the result is that the enhancement leads to
a cross section that scales at low energies
as S(σν) ∝ 1/ν.
• It turns out that the velocity of dark matter
particles is a factor of 10- 3 smaller now
compared to at.
• It is a miracle of dark matter that
Sommerfeld enhancement therefore
provides an elegant mechanism for
Halo Shapes
• Dark matter has an observable effect haloshapes.
• Self-interactions can be strong enough to
create first order changes in the energies
of dark matter particles which will
isotropize the velocity dispersion and
creative spherical halos (which are not
seen)
Halo Shapes
• In Feng et al. 2010 they look at the halo of
NGC 720 and consider halo-shape
constraints from various DM models
Halo Shapes
• Strong self-interacting DM would cause the
formation of constant density cores.
• Strong self-interacting DM would make
subhalos (which have comparable or higher
densities than that of the halo and also
generally have lower velocity dispersions
than the approximately thermal bulk)
especially important.
• Strong self-interacting DM would isotropize
the velocity dispersion and creative spherical
halos
Sommerfeld
enhancement
factor
Mass of WIMP
M and S regions
which explain
PAMELA and
Fermi data
Sommerfeld
enhancement
factor
Mass of WIMP
Sommerfeld
enhancement
factor
Halo shape
observations set
upper limits on the
effective mass of
the force carrier
which mediates
interactions in halo
Mass of WIMP
Sommerfeld
enhancement
factor
Halo shape
observations set
upper limits on the
effective mass of
the force carrier
which mediates
interactions in halo
Mass of WIMP
Sommerfeld
enhancement
factor
Upper
limit from
ΩDM
Mass of WIMP
Conclusions
• A satisfactory solution to the dark matter
problem must not only have dark matter
annihilating at the correct rate, but it must
also produce the right density and
structure on all cosmological scales which
is consistent with observations…
Conclusions
We weren’t able to get the cross section high
enough to explain ATIC or PAMELA results
without invoking Sommerfeld enhancement
and even then the enhancement was
inconsistent with relic DM density and further it
Crash and burn?
changed the halo shapes.
Conclusions (for theorists)
• WIMPs are motivated from fundamental
particle physics!
• A simple modification of a standard
candidate such as a neutralino in the
supersymmetric standard model is
insufficient.
• We need a simple theory with a small
number of parameters to more
quantitatively confront future data.
Conclusions (for observers)
• Hopes for detection of dwarf galaxies through
dark matter annihilation
• Although the cross section is not Sommerfeld
enhanced during freeze-out, it keeps pace
with the expansion over the cosmic history.
This may have significant implications for a
variety of early-universe phenomena as well
as the cosmic gamma-ray background.
• Keep analyzing Fermi data.
• Look for local astrophysical sources of
cosmic rays!
Questions?
Image Credits
• NGC 720: X-ray: NASA/CXC/UCI/D.Buote et al.,
Optical: DSS U.K.Schmidt Image/STScI
• The Thinker: Rodin
• DM observations square: WMAP, SDSS, Chandra,
Hubble
• Dark matter decay interactions: NASA
• Pulsar: NASA
• Observation interactions: Colanfanseco 2010
• Dark hallway: http://www.xcravn.com/
• Slide background: galaxy cluster CL0025+1654 by
J.-P. Kneib (Observatoire MidiPyrenees, Caltech) et al., ESA, NASA
Particles
Hadrons is a particle made
of quarks held together by
the strong force. Hadrons
are categorized into two
families: baryons (made of
three quarks), and mesons
(made of one quark and
one antiquark). Protons and
neutrons are hadrons. All
hadrons except protons are
unstable and undergo
particle decay.
Particles
Fermions include quarks -which
are the constituents of protons
and neutrons, i.e. nuclear
matter- and leptons -the
electron and its neutrino, and
two pairs of heavier copies of
the former. Bosons are the
messengers of the forces
experienced by fermions; they
include the photon, the W and Z
bosons, and the gluons.
Photons carry electromagnetic
forces, W and Z carry the weak
forces, and the 8-strong family
of gluons keeps quarks bound
together.
The enhancement is proportional to the coupling of
dark matter to the force carrier, and if the coupling
is large enough to give the needed enhancement,
then it is too large to be consistent with the relic
density, so the desired reconciliation cannot be
achieved. Moreover, for very light force carriers,
self-interactions of the dark matter in models of
this type lead to spherical galactic halos of dark
matter, inconsistent with observations of elliptical
halos. Unless a way around these problems can
be found, this approach to explaining the positron
excess seems ruled out. – Stanley Brown
http://physics.aps.org/synopsisfor/10.1103/PhysRevLett.104.151301