lisa randall - university of california, los angeles · pdf filefor a long time wimp...
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Lisa Randall
w/Matthew Buckley,
Yanou Cui,
Brian Shuve
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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?
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WIMP dark matter: Coincidence of energy carried by stable
weak scale particle and observed dark matter density
Another key observation: X~5 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 related
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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
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ADM explains connection dark
matter and ordinary matter energy
densities
Matter and dark matter densities both
explained by asymmetries
Asymmetries connected: interaction
terms violating both B and X
Original models yield light DM
Xogenesis Create dark matter first
Transfer asymmetry from DM to matter
Can be weak scale
Nonrelativistic solution allows more
general possibilities
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ADM establishes chemical
equilibrium between B or L and X
Net asymmetry prop
to chemical potential
Ratio chemical potentials~ratio
number densities~ratio energy
densities
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More generally….
Number density suppressed for m>>T
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In general, need to solve
Number density of X can be less than
that of B
Allows for different masses
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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
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Sakharov Conditions:
Create X or B
X and B violating operators to
transfer asymmetry
Need symmetric component to be
eliminated
This might be best bet for detection.
Also possible to have B and X violation
and both created together
Then need to explain why they can have
comparable densities
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•XB-Violation
•XL Violation
•Higher-Dim Op Suppressed by Weak Scale
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Indirect signals unlikely
Mix with photon: see others’ work
Measure direct detection signal?
Symmetric component must annihilate
Model-dependent signal from doublet in
UV completion
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Vector: annihilate to dark (eg W’s)
Generally can have entire DM sector
Bad: invisible
Detection not guaranteed
Relies on model-dependent annihilation
generally
Not ADM operators themselves
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Chief nonobvious assumption: Symmetry-transferring, breaking operators
Sphaelerons might be most obvious-just need doublets—but hard to meet detection bounds
Existing ADM models employ higher dimensional operator involving weak scale suppression Least compelling aspect
Operators don’t shout out at you
UV completion requires additional structure
Certainly possible natural
Just not obvious
Detectability not guaranteed Symmetric component can annihilate into dark sector
Heavy dark matter gives weaker signal
On the other hand: natural, and explains weird near-coincidence of matter densities
And can still be connected to weak scale where we expect new matter
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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
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Are B and X conserved?
Do we need to invoke new operators
Keep in mind we need these operators
only in early universe
Suppose normalizable mass mixing or mass
mixing generated by Planck-suppressed ops
No odd ops, no odd scales
B and X accidental symmetries!
w/Cui, Shuve
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XL mixing in early universe to
transfer asymmetry--but must turn off
B, L, and X conserved at late times
Option I: Dynamical mechanism:
Φ is a scalar field
ΦXL turns on mass when vev turns on
Key is VEV that switches
Turns on, then off
Opposite to usual transition, but
readily happens in early universe eg
Multifield models-akin to hybrid inflation
Moduli fields, flat directions that roll to
zero
Option 2 Early universe kinetic terms,
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Three Models
Order from more specific and less
specific parameters to less specific
model and more specific parameters
2 Higgs
Moduli-driven
Thermal Background
Get mass mixing: interplay of
neutrino-like oscillation and thermal
interaction
and new way to accommodate heavier
mX ...
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Rapid shutoff of Φ triggered by
interaction with another scalar:
As with hybrid inflation
Two Stage Phase Transition in Two
Higgs Models
X dark matter, Yukawa mixing term, Z2 symmetry
Φ VEV first
H VEV later on
With temperature corrections:
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Two step phase transition
At high temperature, all VEVs vanish
At critical temperature turns on h
vanishing
induces mass XL mass mixing
At second critical temperature, new
minimum develops
h turns on
tunnels to zero
This is true vacuum today
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Direct Detection: Loop-suppressed
Can also look directly for second doublet
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Now VEV doesn’t carry SU(2)
Dark matter neutral
So lepton-number carrying field neutral:
Χ
Simplifying assumption: Dirac
leptogenesis to get lepton number into
singlet
now modulus: induces mass
mixing as before
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Need mixing at leptogenesis time to determine transfer
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Φ rolls to its minimum
En route leptogenesis occurs
Lepton asymmetry shared with DM
Amount of transfer depends on value
of Φ field at leptogenesis time
Can get lower dark matter number
density relative to lepton number
when field has rolled to lower values
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•Nothing assumed in this model except fields
•These operators should occur
•Global symmetries expected to be violated at
Planck scale)
•Furthermore early universe is hot
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Two of Sakharov conditions (B and
CP violation) assumed in Lagrangian
Third dynamical—
Need to be out of equilibrium
eg out of equilibrium decay
Another possibility: WIMP freezeout
is source of out of equilibrium
behavior
WIMP annihilation directly (or
indirectly) leads to lepton [baryon]
production
DM/B near concidence explained
without ADM
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Dark matter obtained as usual in
WIMP paradigm with order unity
couplings and weak scale dark matter
Baryon asymmetry created by
annihilations themselves
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DM annihilates; B number created
Reverse can also occur
So washout plays a critical role
Essentially baryon number created
when washout decouples
Baryon density proportional to dark
matter density at washout freezeout
Implies washout decoupling must happen
before dark matter annihilation
decoupling
Guarantees number density of dark
matter less than that of baryons
Aside: weak scale baryogenesis
Bonus: no issue with reheat temperature
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Need washout to freeze out first
Implies annihilation channel should
involve field heavier than DM
Guarantees kinematic suppression
Also: final state field mass must
(obviously) be less than twice dark
matter mass
Baryon number shouldn’t be washed
out by late decays, etc of exotic field
Need exotic field to decay into light
neutral object that carries baryon
number but doesn’t disturb visible
baryon density
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WIMPy baryogenesis
Creates dark matter density inversely
proportional to annihilation cross
section
As conventional
Baryon density proportional to dark
matter density AT WASHOUT
FREEZEOUT
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For
decay
Compare to leptogenesis via S decay
Question is whether S decays in or out of
equilibrium
If we assume S is weak scale, requires tiny
coupling .000001
Same sorts of interference: multiple S states, etc
But here baryon production from off-shell S via
Annihilating dark matter
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Similar to leptogenesis
Differences
No constraint for completion before
sphaeleron temp
Can decay with true B violation or
preserving U(1) as with lepton case
Decay
either
Or
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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 or baryogenesis
Usually tradeoff between generiticity of model and parameter space
Admittedly much more challenging for experiment
But nature ultimately decides…
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