What is dark Matter? - PowerPoint Presentation

Slide1

What is dark Matter?

Lisa Randall

Harvard University

@

lirarandall

Slide2

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…Slide3

So…What Is It?Clearly we don’t yet knowFor 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?Slide4

Dark MatterFor a long time WIMP “miracle” has been the reigning paradigmNow 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?Slide5

WIMP “Miracle”Slide6

How to search?Searches based on somewhat optimistic assumptionsNamely 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 Slide7

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?Slide8

Experimental Lampposts: Direct DetectionLook 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 searchesSlide9

Experimental Lampposts: LHCLHC: 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 regionSlide10

Experimental Lampposts: Indirect SearchesIndirect SearchesRather 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 backgroundSlide11

Indirect SearchesFocus 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 signalsSlide12

Indirect DetectionExciting 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 consequencesSlide13

Status of WIMP searches?Direct dark matter detectionLook 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 importantSlide14

Lampposts: theoreticalMost-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?Slide15

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”Slide16

Xogenesis/Asymmetric Dark Matter ModelsKey observation:

r

X

~6

r

B

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 relatedSlide17

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 togetherSlide18

All Sorts of Miracles PossibleAsymmetric Dark MatterMake B, Transfer B to X,

n

B

~n

X

, light DM

Zurek,Luty,Kaplan

Hylogenesis

Make B, X together

n

B

~n

X

, light DM

Morrissey,

Tulin

, Hall, March-Russell

Darkogenesis

, Dark Genesis

Make X, Transfer X to B

n

B

~n

X

, light DM

Shelton,

Zurek

Xogenesis

Make X, Transfer X to B,

n

B

<

n

X

, weak scale DM

Buckley, LR

Xogenesis

’

Make X, B,

nB<nX, weak scale DMCui, Kahawala,LR, Shuve

Weak Scale Dark Matter

Light Dark MatterSlide19

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 timeSlide20

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 firstSlide21

Can nature do better?ADM compellingBut 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 universeSlide22

Light Dark Matter: “Relativistic Solution”Chemical equilibrium between B or L and X

Net asymmetry

Ratio chemical

potential~ratio

number

density~ratio

energy densitySlide23

Weak Scale (or Heavy) Dark Matter“Nonrelativistic Solution”

More generally

Number density suppressed for m>>TSlide24

Nonrelativistic SolutionNeed to solve

Number density of X less than B

Chemical, but no longer thermal equilibrium

Allows for different massesSlide25

Naturalness allows hierarchy of order 10Right 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 rangeSlide26

Emergent Lepton and Dark matter NumbersAre B and X conserved? Maybe not in early universe

Transfer and then restore

Two

Higgses

, Modulus decay, higher-dimensional operators

w/Cui,

ShuveSlide27

Another OptionWIMPy baryogenesis

Creates dark matter density inversely proportional to annihilation cross section

As conventional

Baryon density proportional to dark matter density AT WASHOUT FREEZEOUTSlide28

Detection ProspectsSlide29

Other interesting possibilities?Lots of attention devoted to dark matterBoth 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,reeceSlide30

Since we don’t know what dark matter isShould keep an open mindEspecially 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 measurementsSlide31

On the surface surprising…Dark matter thought to be non-interactingOr 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-interactionsSlide32

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…Slide33

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 understandSlide34

Strongest Bound (for us)Just comes from matter accountingGravitational potential measuredBoth 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

Slide35

Strongest bound (when paper written):Oort LimitSlide36

ImplicationIf dark matter interacts, eitherReasonably strong constraintsActually not all that strong…

Or it’s not all the dark matter!

At most about baryonic energy density or fraction thereofSlide37

Why would we care?Implications of a subdominant componentCan 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

componen

t 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 conservedSlide38

Could interacting dark matter also cool into a disk?Requires a means of dissipating energyAssume 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 possibilitySlide39

Simple PIDM ModelU(1)D,

α

D

Two matter fields: a heavy

fermion

X and a light

fermion

C

For “coolant” as we will see

q

X

=1,

q

C

=-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 tempsSlide40

Relic Density X

Won’t violate

Oort

Limit with big enough alpha—reasonable valuesSlide41

Density of C?Thermal abundance of C will however be too smallWill expect both thermal and

nonthermal

contributions to X

Nonthermal

to CSlide42

Thermal and NonthermalIn principle other processes to produce CStill 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 wellSlide43

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 occurSlide44

Cooling for reasonable parametersSlide45

When does it stop?Presumably when dense enough no longer ionizedCooling very suppressed at that point (?)

This would be nice to simulate

How thickened will disk become?Slide46

Cooling temp determines disk height And therefore density of new componentSlide47

Disk HeightIn reality, gravitational heating can occurReasonable to assume disk height betweenm

P

/

m

X

---1 times baryonic disk height

Can be very narrow disk

For 100

GeV

particle, can get boost factor of 10,000!Slide48

ConsequenceCan have dark atomsDark 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 richSlide49

Implication?Photons from plane of galaxy!Not only center but unassociated sources throughout plane would be expectedSeems 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….Slide50

I:Interacting Dark Matter and Indirect SignalsAn enormous boost factor is needed to account for any indirect signal so farEg

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 signalsSlide51

Distinctive Shape to SignalSlide52

IIIMany Other ConsequencesThings 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

limitsSlide53

ConclusionsWhether 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 answerSlide54

AlsoInteresting to explore slightly more complex dark matter sectorsEven 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!Slide55

Extra slidesSlide56

Motivation: Gamma Ray Signal

Neal Weiner’s talkSlide57

Motivation: Fermi LinesFinkbeiner

,

SugSlide58

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

inaccessibleSlide59

Can Dark Matter Explain Signal?Clearly an enormous boost factor is needed

Of order 100-1000 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 signalsSlide60

Density enhancementConsider 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—10,000Slide61

Disk HeightReasonable to assume disk height between

m

P

/

m

X

---1 times baryonic disk height

Can be very narrow disk

For 100

GeV

particle, can get boost factor of 10,000!Slide62

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 formationSlide63

Many Other ConsequencesNew 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 constrainSlide64

StatusWhether 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 answerSlide65

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…