Novel Probes of Dark Matter
Cora Dvorkin
Harvard University“Cosmic Controversies” Conference
Chicago, October, 20190

Outline
What is the particle nature of Dark Matter?
Ø Cosmological observables as a probe of dark matterinteractions at large/intermediate scales.
Ø Strong gravitational lensing as a model-independent
probe of dark matter at small scales.
Ø Conclusions and future directions.
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Looking for Dark Matteroff the beaten track
Where do Dark Matter interactions matter?
Some well known avenues:
Excess high energy cosmic/gamma rays;
Missing energy at colliders;
Nucleon recoil deep underground;…
Important to look for new avenues
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Going Beyond the WIMP Paradigm
Ø The WIMP parameter space is being actively explored.
Ø A wealth of knowledge is and will soon be available
from cosmological surveys, which will reveal new
information about the dark sector.
We should look beyond the WIMP scenario, exploitingthese data sets as much as we can!
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Going beyond the WIMP scenario

Probing Dark Matter Interactions through the CMB
and the large-scale structure of the Universe.
Case study: Dark Matter – baryon strong interactions
C. Dvorkin, K. Blum and M. Kamionkowski, PRD (2014)

Boehm et al. (2014), Bucklet et al. (2014), Ali-Haimoud et al. (2015),…,Muñoz et al. (2015),…,Aloni et al.(2016), Poulin et al. (2017),Gluscevic
et al.(2018), Slatyer et al. (2018), Chang, Essig, and McDermott, etc,etc…

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Probing sub-GeV Dark Matter-Baryon
Scattering with Cosmological Observables

Linda Xu

Andrew Chael

CMB temperatureand polarization +Lyman-alpha forestPlanck 2015 +Sloan Digital Sky
Survey (SDSS)

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Dark Matter scatteringcross section [cm2]Velocity-Independent Scattering

Linda Xu, C. Dvorkin and Andrew Chael, PRD (2018)

Ø Limits will get much (order of magnitude) better with CMB-S4: main sciencedriver for the Dark Matter science case in the CMB-S4 Decadal Survey Report.

Cosmological observables provide an extremely complementary probe
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to that of direct detection and other indirect searches!

Going beyond the freeze-out mechanism

Freeze-out

Freeze-in

WIMPs

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New channel for Freeze-in

C. Dvorkin, T. Lin, K. Schutz, PRD (2019)8

MillichargeProbing Dark Matter scatteringwith current/future cosmological probes

Stellaremission
boundsDark matter mass
C. Dvorkin, T. Lin, K. Schutz, in prep.

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Probing Dark Matter fluctuations
at smaller scales

Probing Dark Matter substructure
at small scales
via strong gravitational lensing

10Strong Gravitational Lensing

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Looking for Dark Subhalos
Idea: subhalos can locally perturb lensed images, so by looking at
the residual between the images predicted by modeling the lens asa smooth mass and what is actually observed we can infer thepresence of subhalos.
The advantage relative to other methods for detecting dark matteris that we do not need to assume a coupling between the Standard
Model and Dark Matter (in contrast to direct/indirect detection andcolliders): model-independent method.
Another main advantage is that by focusing on the lowest mass
subhalos present in galaxies (largely devoid of stars) we canminimize baryonic feedback.
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The numerous population of low mass subhalos (< 107 M¤) may bestatistically detected by their collective perturbations on images.
Hezaveh, Dalal, et al. (2016)13

A different approach to substructure lensing:statistical detection of dark subhalos

Key Question:
What can we learn about low-masssubhalos from measuring thesubstructure convergence power
spectrum?

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A General Formalism
Ana DiazRivero• We developed a general formalism to study the N-point function of theconvergence field from first principles, which can be easily applied to subhalo
populations with different properties.
• We model the convergence field as a fluctuation field superimposed on thesmoothly varying background density profile of the host:, where((Surface mass density))qi are a set of parameters that represent the intrinsic properties of a subhalo.
A. Diaz Rivero, F. Cyr-Racine, and C. Dvorkin, PRD (2017)15

A General Formalism
Change of language: instead of talking about lensing perturbations in terms ofindividual subhalos, look at the correlation function of the projected density field.
• Start from first principles to derive the lens plane-averaged convergence correlation
function.
;•1-subhalo term: arises from ensemble-averaging over the spatial distribution ofa single subhalo.

•2-subhalo term: arises from averaging over pairs of distinct subhalos.

A. Diaz Rivero, F. Cyr-Racine, and C. Dvorkin, PRD (2017)16

Substructure Power Spectrum:
Truncated NFW Subhalo Population
The Power Spectrum can be described mainly by three quantities:

A. Diaz Rivero, F. Cyr-Racine, and C. Dvorkin, PRD (2017)17

Substructure Power Spectrum:
Truncated Cored Profile
Key probe of the inner subhalo density profile: asymptotic slope.

A. Diaz Rivero, F. Cyr-Racine, and C. Dvorkin, PRD (2017)18

The Convergence Power Spectrum:
Insights from Simulations
Simulations from Vogelsberger et al.
- Effect on substructure due to a cutoff in the initialpower spectrum + Dark Matter self-interactions.

CDM

Self Interacting Dark Matter (SIDM)+ cutoff in initial power spectrum

A. Diaz Rivero, C. Dvorkin, F. Cyr-Racine, et al., PRD (2018)19

Redshift and Scale-Dependence
Comparing the amplitude and slope of the power spectrum on scales
0.1 kpc-1<k<10 kpc-1 from lenses at different redshifts can help us distinguishbetween CDM and other DM scenarios.
The effective mass is reduced
between z = 0.5 and z = 0 due tohigher susceptibility to tidal stripping.
SIDM + cutoff

A. Diaz Rivero, C. Dvorkin, F. Cyr-Racine, et al., PRD (2018)20

Redshift and Scale-Dependence
Comparing the amplitude and slope of the power spectrum on scales
0.1 kpc-1<k<10 kpc-1 from lenses at different redshifts can help us distinguishbetween CDM and other DM scenarios.
Change in the slope at k ≳ 2 kpc-1 due
to tidal stripping transferring powerfrom larger to smaller scales.
SIDM + cutoff

A. Diaz Rivero, C. Dvorkin, F. Cyr-Racine, et al., PRD (2018)
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The Road Ahead
In the next decade, we will discover tens of thousands of lensed galaxies.
This vast increase in sample sizes (in coordination with other facilities,e.g. HST, ALMA) will provide much stronger statistical constraints ondark matter models than what is currently possible.
“Probing the Fundamental Nature of Dark Matterwith the Large Synoptic Survey Telescope”, 2019arXiv:1902.01055

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Convolutional Neural Networks (CNNs)
to find Dark Matter Substructure
Classification: binary output - is an image likely to contain substructure or not?
Can help identify promising images for further (expensive) traditional analysis.
Our simulation pipeline for the training and validations sets:

A. Diaz Rivero and C. Dvorkin (2019)23

Conclusions and the Road Ahead• The CMB and the large-scale structure of the universe (Lyman-alpha forest,
future 21 cm observations, galaxy surveys, weak lensing measurements, etc)
encode a wealth of information about the interactions of the dark sector.
• Important clues about Dark Matter physics lie on small scales: detection ofdark subhalos via strong gravitational lensing has great potential for revealing
the particle nature of dark matter.

This coming decade will bring a wealth of new and complementary
cosmological data.
We should continue to look off the beaten track,and hopefully we will shed light on the dark sector soon.
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