1. Lisa Randall Harvard University @lirarandall
What is dark Matter?
Lisa Randall
Harvard University
@lirarandall

2. What do we know about dark matter?
It has gravitational interactions—of matter!
Gravitational lensing
Rotation curves in galaxies
Detailed measurements of energy abundances—total and normal matter
It has no other discernible interactions
It’s not dark—it’s effectively transparent!
Hopes to see it based on it being a little opaque…

3. So…What Is It? Clearly we don’t yet know
For a long time WIMP “miracle” has been the reigning paradigm
Now in position to test it fairly well
So far no sign…
We need to consider all possibilities
Does it interact as we might hope?

4. Dark Matter
For a long time WIMP “miracle” has been the reigning paradigm
Now in position to test it fairly well
So far not looking promising
Though some hints…
Also particular models challenging to make fit
SUSY heavy?
Worth asking if we really understand dark matter
Does it interact as we might hope?

5. WIMP “Miracle”

6. How to search? Searches based on somewhat optimistic assumptions
Namely dark matter does interact with our matter at some level
In principle could be purely gravity coupling
We see that already!
But does it have other interactions?
Talk today: reasons to think it might
And alternatives to standard WIMP paradigm
Asymmetric dark matter/Xogenesis
Partially interacting dark matter

7. Lampposts for Dark Matter?
Existence of dark matter not necessarily so mysterious
Why should everything be like our matter?
What is mysterious is that energy stored in dark matter and ordinary matter so similar
But how to find what it is?
Look under the lamppost
Find theoretical, experimental clues
What are the right lampposts?

8. Experimental Lampposts: Direct Detection
Look for low probability dark matter interactions with large detectors
Look for small nuclear recoil
Good way to look for a well-motivated class of candidates (WIMPs)
We haven’t seen it yet
Waiting for more sensitive searches

9. Experimental Lampposts: LHC
LHC: Look for evidence of a stable particle with weak scale mass
Remarkably, has the right energy density to constitute dark matter
Such a particle likely in ANY weak scale model that supplements Higgs theory
WIMP not necessarily supersymmetric!
Any stable weak scale particle can be a candidate
We haven’t yet seen beyond Higgs
Waiting for higher energies, more intensity
Don’t yet know if this lamppost in the right region

10. Experimental Lampposts: Indirect Searches
Rather than directly interact with nuclei,
Dark matter particle hits another dark matter particle and annihilates
Hope is
This happens often enough
Annihilation produces Standard Model particles
The kind we can detect
Not dark!
Focus on any signal that is distinguishable from astrophysical background

11. Indirect Searches
Focus on what is less likely to occur in ordinary astrophysical settings
Antiparticles
AMS: antideuteron
Gamma ray signals
Particularly gamma ray lines
Continuum from stars
Lines direct consequence of annihilation
Observations have seen a lot!
Need to determine
What really is not background
What models of dark matter can produce such signals

12. Indirect Detection Exciting thing about Indirect Detection
Is the many signals
Has precipitated better understanding of astrophysics
And of range of dark matter models
I’ll talk about one motivated by Fermi satellite “signal” soon
Again, general lessons whether or not signal survives
Important to understand range of reasonable models
Part of art is deciding what is reasonable
Interesting
Has consequences

13. Status of WIMP searches?
Direct dark matter detection
Look for feeble interactions
LHC searches
Look for missing energy indicating noninteracting weak-mass particle produced
Neither has seen anything yet
But both are increasing sensitivity
Dark matter “vats” becoming bigger
LHC exploring higher energy and higher intensity (sensitive to lower probability)
Both are important

14. Lampposts: theoretical
Most-researched candidates are WIMPS
WIMPS have weak-scale mass
That is mass such that it can be produced at LHC
Doesn’t have to have anything to do with supersymmetry
Any weak mass stable particle could yield correct abundance
And if connected to weak scale have reason to believe will be produced
WIMP—coincidence that weak mass particle has measured abundance?

15. But another theoretical lamppost?
Similarity of amount of energy in dark matter and ordinary matter
Maybe matter and dark matter are produced in similar ways?
Excess “matter” over “antimatter”

16. Xogenesis/Asymmetric Dark Matter Models
Key observation:
rX~6rB
Why should dark matter and ordinary matter energy densities be at all comparable?
Could just be independently generated—baryogenesis somehow and weak miracle
On other hand, maybe clue their origin is in fact related

17. Three Papers Xogenesis w/ Matthew Buckley
Asymmetric Dark Matter
Dark matter produced first
Weak scale dark matter still natural
Emergent Dark Matter and Baryon/Lepton Numbers
w/Yanou Cui, Brian Shuve
Natural models explaining ADM
Alternative explanation: WIMPy Baryogenesis w/Yanou Cui, Brian Shuve
WIMP annihilation trigger for baryogenesis
WIMPs annihilate and lepton/baryon number created together

18. All Sorts of Miracles Possible
Asymmetric Dark Matter
Make B, Transfer B to X, nB~nX, light DM
Zurek,Luty,Kaplan
Hylogenesis
Make B, X together nB~nX, light DM
Morrissey, Tulin, Hall, March-Russell
Darkogenesis, Dark Genesis
Make X, Transfer X to B nB~nX, light DM
Shelton, Zurek
Xogenesis
Make X, Transfer X to B, nB<nX, weak scale DM
Buckley, LR
Xogenesis’
Make X, B, nB<nX, weak scale DM
Cui, Kahawala,LR, Shuve
Light Dark Matter
Weak Scale Dark Matter

19. Xogenesis: New Class of Models
Asymmetric Dark Matter
Create dark matter first
Then transfer asymmetry from dark matter to matter
Can be weak scale
Can be light
Baryon number and Dark matter number could be connected in early universe
Produce both at the same time

20. Asymmetric Dark Matter
Explain connection dark matter and ordinary matter energy densities
Dark matter energy similar in spirit to that of baryons
Asymmetry in dark matter density; not thermal
Need interactions between baryons and dark matter to explain the similar relative size
Chemical potentials related
Number densities are too nB~nX;
Nonrelativistic solution allows more general possibilities
Xogenesis
And DM created first

21. Can nature do better? ADM compelling
But origin of operators that mix two sectors?
Higher-dimensional operators can violate both L (or B) and DM numbers
Don’t necessarily expect L, X conservation in early universe

22. Light Dark Matter: “Relativistic Solution”
Chemical equilibrium between B or L and X
Net asymmetry
Ratio chemical potential~ratio number density~ratio energy density

23. Weak Scale (or Heavy) Dark Matter “Nonrelativistic Solution”
More generally
Number density suppressed for m>>T

24. Nonrelativistic Solution
Need to solve
Number density of X less than B
Chemical, but no longer thermal equilibrium
Allows for different masses

25. Naturalness allows hierarchy of order 10
Right ratio of densities found for wide range of m/T
Usually need m/T~10, which is quite reasonable
Expect comparable densities over the whole range

26. Emergent Lepton and Dark matter Numbers
w/Cui, Shuve
Are B and X conserved?
Maybe not in early universe
Transfer and then restore
Two Higgses, Modulus decay, higher-dimensional operators

27. Another Option WIMPy baryogenesis
Creates dark matter density inversely proportional to annihilation cross section
As conventional
Baryon density proportional to dark matter density AT WASHOUT FREEZEOUT

28. Detection Prospects

29. Other interesting possibilities?
Lots of attention devoted to dark matter
Both theory and detection
Sometimes signals are unexpected
They might be wrong
They might lead to interesting unexplored options
Surprisingly, unexplored option:
Interacting dark matter
But rather than assume all dark matter
Assume it’s only a fraction (maybe like baryons?)
w/fan,katz,reece

30. Since we don’t know what dark matter is
Should keep an open mind
Especially in light of abundance of astronomical data
Today talk about Self-interacting dark matter
But rather than assume all dark matter
Assume it’s only a fraction
maybe like baryons?
Nonminimal assumption
But one with significant consequences
Will be tested
In any case leads to rethinking of implications of almost all dark matter, astronomical, cosmological measurements

31. On the surface surprising…
Dark matter thought to be non-interacting
Or at best very weakly interacting
First piece of evidence is spherical halo
No means of cooling down
Second piece of evidence is some nonsphericity in core
Interactions would make it more uniform
Third piece of evidence is Bullet Cluster
Gas left behind on merger but dark matter passes right through
Finally: lack of detection
That of course just refers to interactions with ordinary matter
Doesn’t tell about self-interactions

32. This changes everything!
Almost all constraints on interacting dark matter assume it is the dominant component
If it’s only a fraction, we’ll see most bounds generally don’t apply
structure
Galaxy or cluster interactions
But if a fraction, you’d expect even smaller signals!
However, not necessarily true…

33. Turns out none of these two serious anyway
But for us even less so
Clearly Bullet Cluster okay if only a fraction –most dark matter would pass through
Shapes tricker—but even if the fraction very strongly interacting, can smooth out only that fraction at first
Maybe? This is something that could be interesting to better understand

34. Strongest Bound (for us)
Just comes from matter accounting
Gravitational potential measured
Both in and out of plane of galaxy
Measured from star velocities
Baryonic matter independently constrained
Ordinary dark matter constrained
Extrapolate halo
Total constraint on matter
Constrains any new (nonhalo) component

35. Strongest bound (when paper written): Oort Limit

36. Implication If dark matter interacts, either
Reasonably strong constraints
Actually not all that strong…
Or it’s not all the dark matter!
At most about baryonic energy density or fraction thereof

37. Why would we care? Implications of a subdominant component
Can be relevant for signals if it is denser
Can be relevant for structure (to be done…)
Depends on “shape”
Baryons matter because formed in a dense disk
Perhaps same for component of dark matter
Perhaps dark disk inside galactic plane
However, to generate a disk, cooling required
Baryons cool because they radiate
They thereby lower kinetic energy and velocity
Get confined to small vertical region
Disk because angular momentum conserved

38. Could interacting dark matter also cool into a disk?
Requires a means of dissipating energy
Assume interacting component has the requisite interaction
Simplest option perhaps independent gauge symmetry
“Dark light”
Could be U(1) or a nonabelian group
U(1) has fewer DOF: good for “neutrino constraint”
Nonabelian permits formation of stable dark atoms
Also good for U(1) mixing constraint
Check when enough cooling can occur to form a disk
Most interesting possibility

39. Simple PIDM Model U(1)D, αD
Two matter fields: a heavy fermion X and a light fermion C
For “coolant” as we will see
qX=1, qC=-1
(In principle, X and C could also be scalars)
Also interesting will be nonabelian generalization SU(N)D
X fundamental, C antifundamental
Assume confinement scale below relevant cooling temps

40. Relic Density X
Won’t violate Oort Limit with big enough alpha—reasonable values

41. Density of C? Thermal abundance of C will however be too small
Will expect both thermal and nonthermal contributions to X
Nonthermal to C

42. Thermal and Nonthermal
In principle other processes to produce C
Still would annihilate away
Unless bound with X
Possible in nonabelian scenarios
We make simpler assumption of nonthermal component
Interesting that thermal component of X naturally survives as well

43. Cooling: Three cooling processes can be relevant Bremsstrahlung
Compton scattering off dark photons
Recombination cooling (only relevant when an additional light species, which we will need)
Heating processes (since that’s when cooling stops!)
For normal matter, photoionization
Gravitational heating (small)
Compton heating can be relevant for us
At very least will be recombination, which stops brehmstrahllung and Compton
We make assumption that cooling stops when recombination can occur

44. Cooling for reasonable parameters

45. When does it stop? Presumably when dense enough no longer ionized
Cooling very suppressed at that point
(?)
This would be nice to simulate
How thickened will disk become?

46. Cooling temp determines disk height
And therefore density of new component

47. Disk Height In reality, gravitational heating can occur
Reasonable to assume disk height between
mP/mX---1 times baryonic disk height
Can be very narrow disk
For 100 GeV particle, can get boost factor of 10,000!

48. Consequence Can have dark atoms Dark disk
Could be much denser and possibly titled with respect to plane of our galaxy
Very significant implications
Even though subdominant component
Velocity distributions in or near galactic plane constrain fraction to be comparable or less to that of baryons
But because it is in disk and dense signals can be rich

49. Implication? Photons from plane of galaxy!
Not only center but unassociated sources throughout plane would be expected
Seems rather specific to this type of model
Component of dark matter sitting in small disk in plane of galaxy
Furthermore will affect structure formatoin
Work not yet in progress….

50. I:Interacting Dark Matter and Indirect Signals
An enormous boost factor is needed to account for any indirect signal so far
Eg Of order 1000 for reasonable parameters for Fermi signal
Too high to assume clumping
But what if dark matter actually had structure?
Like baryons for example!
Consider interacting dark matter
Dissipative dark matter in particular
Idea is to have more collapsed component of dark matter
Even if only a fraction of dark matter, will be most important for signals

51. Distinctive Shape to Signal

52. IIIMany Other Consequences Things to think about
New species (Planck can detect)
Possibly small scale structure
Atomic physicbs
Numerical simulations (structure, alignment)
Velocity distributions, lensing (look for structure)
Large scale structure
Acoustic Oscillations
galaxy-galaxy correlation funcionts
Indirect detection
Direct detection (at very low threshold)
Aside: anthropic limits

53. Conclusions Whether or not 130 GeV signal survives,
Very interesting new possibility for dark matter
That one might expect to see signals from
Since in some sense only minor modification (just a fraction of dark matter)
hard to know whether or not it’s likely
But presumably would affect structure
Just like baryons do
Research area
Rich arena: lots of questions to answer

54. Also Interesting to explore slightly more complex dark matter sectors
Even if not dominant component, new species can have significant observable signals to distinguish it
I know what everyone wants to know is when we will see dark matter
Answer could be sooner--or later--than we think!

55. Extra slides

56. Motivation: Gamma Ray Signal
Neal Weiner’s talk

57. Motivation: Fermi Lines
Finkbeiner, Sug

58. Motivation: Fermi 130 GeV line
Suppose you want to explain Fermi signal with dark matter
If you also assume relic thermal abundance want annihilation into something to be about an order of magnitude bigger
However can’t annihilate into charged particles since the signal would already rule it out
One option is to annihilate to photons through a loop of charged particle that is kinematically inaccessible

59. Can Dark Matter Explain Signal?
Clearly an enormous boost factor is needed
Of order for reasonable parameters
Too high to assume clumping
But what if dark matter actually had structure?
Like baryons for example!
So we consider interacting dark matter
Dissipative dark matter in particular
Idea is to have more collapsed component of dark matter
Even if only a fraction of dark matter, will be most important for signals

60. Density enhancement
Consider possibility that due to interactions, portion of dark matter (like baryons) collapses into a disk
Involves
Dark force (we take U(1)D)
Additional light particles in dark sector
Necessary for cooling in time
Even if new component a fraction of dark matter, if it collapsed to baryonic disk (eg) enhancement factors ~100—10,000

61. Disk Height Reasonable to assume disk height between
mP/mX---1 times baryonic disk height
Can be very narrow disk
For 100 GeV particle, can get boost factor of 10,000!

62. Implication? Photons from plane of galaxy!
Not only center but unassociated sources throughout plane would be expected
Seems rather specific to this type of model
Component of dark matter sitting in small disk in plane of galaxy
Furthermore will affect structure formation

63. Many Other Consequences
New species (Planck can detect)
Possibly small scale structure
Velocity distributions, lensing (look for structure)
Acoustic Oscillations
Indirect detection
Direct detection (at very low threshold)
Many ways to search and constrain

64. Status Whether or not 130 GeV signal survives,
Very interesting new possibility for dark matter
That one might expect to see signals from
Since in some sense only minor modification (just a fraction of dark matter)
hard to know whether or not it’s likely
But presumably would affect structure
Just like baryons do
Research area
Rich arena: lots of questions to answer

65. Summary Clearly dark matter experiments telling us something
If we find evidence soon could be great vindication of WIMP scenario
If we don’t we’ll still want to know what it means
Perhaps we have been too focused on conventional WIMPs?
Other coincidences worth exploring and explaining
ADM
Xogenesis (weak scale)
Emergent
WIMP annihilation connected to leptogenesis
Usually tradeoff between genericness of model and parameter space
Admittedly much more challenging for experiment
But nature ultimately decides…