Indirect Dark Matter Searches: a Review Eric Charles SLAC National Lab. 13 eme Recontres de Vietnam: Exploring the Dark Universe 24 July 2017, Quy Nhon, Vietnam
Outline 2 I. Review / Context: indirect detection of dark matter II. Indirect dark matter searches A. Astrophysical dark matter signals B. Astrophysical backgrounds C. Summary of current search status D. Focus on Galactic center GeV excess & dwarf spheroidal galaxies III. Summary and prospects
3 I. REVIEW / CONTEXT: INDIRECT DETECTION OF DARK MATTER
Evidence for / Salient Features of Dark Matter Comprises majority of mass in Galaxies Missing mass on Galaxy Cluster scale Zwicky (1937) Almost collisionless Bullet Cluster Clowe+(2006) Large halos around Galaxies Rotation Curves Rubin+(1980) Non-Baryonic Big-Bang Nucleosynthesis, CMB Acoustic Oscillations WMAP(2010), Planck(2015) 4
Evidence for / Salient Features of Dark Matter All of the evidence for dark Large halos around Galaxies Rotation Curves matter is astrophysical! Rubin+(1980) Comprises majority of mass in Galaxies Missing mass on Galaxy Cluster scale Zwicky (1937) Almost collisionless Bullet Cluster Clowe+(2006) Non-Baryonic Big-Bang Nucleosynthesis, CMB Acoustic Oscillations WMAP(2010), Planck(2015) 5
Cosmological Constraints on Dark Matter 6 Λ-CDM Concordance Fits Adapted from Kowalski+ (2008) DM Candidates by Mass & Cross Section Park+ (2007) No Standard Model particle matches the known properties of dark matter Many candidate particles have been proposed: In this talk I will focus on WIMPs Current experiments are also sensitive to signals from axion-like particles, primordial black holes and gravitinos & other exotica
Indirect Detection of WIMPs 7 This Talk & Thursday PM Session Fermi-LAT, IACTs, HAWC Tuesday AM Session: Brunner & Scott Afternoon Sessions: Cirelli & others AMS-02, DAMPE, CALET Also, consider cosmological effects of annihilation/ decay products on environment: e.g., Shirasaki + (2016PhRvD..94f3522S), Slatyer + (2017PhRvD..95b3010S).
Gamma-Ray Telescopes 8 Imaging Atmospheric Cerenkov Telescopes Fermi Large Area Telescope High Altitude Water Cerenkov
9 II. INDIRECT DARK MATTER SEARCHES
DM Structures are Present on Many Scales Zoom Sequence of DM Structure on 100 Mpc Scales We can probe DM by looking for signal contributions from halos: On cosmological scales (left) In the Milky Way virial radius (~300 kpc = ~1 MLY, right) (Visible size of MW = ~ 20 kpc) Milky Way-like Halo and Several Sub-Halos Left: Boylan-Kolchin+ 2009MNRAS.398.1150B Right: Springel+ 2008MNRAS.391.1685S 10
Dark Matter as Seen From Earth 11 Satellites Galactic Center Milky Way Halo Galaxy Clusters Isotropic contributions Dark Matter simulation: Pieri+ 2011PhRvD..83b3518P
The Key Formula for WIMP Searches 12 Particle Physics Astrophysics (J-Factor) dn /de [MeV 1 ] 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 10 e + e µ + µ + W + W gg dn/de for 200 GeV DM b b t t s s c c 10 2 10 3 10 4 10 5 Energy [GeV] 200 GeV J [GeV 2 cm 5 sr 1 ]] 10 24 10 23 10 22 10 21 10 20 J-factor for the Galactic Center NFW gnfw 1.2 gnfw 1.4 Burkert Isothermal Einasto 10 20 30 40 50 60 70 80 90 Angular Sep. [deg] Note: J-factor includes distance, i.e., J-factor would decrease by four if a point-like source were twice as far away Note: the key factor of 1/m χ 2 is b/c we express the J-factor as a function of mass density (which we can measure), not number density
WIMP Dark Matter as a Thermal Relic 13 DM Density v. Temperature Small cross-section: freeze out too early, too many WIMPs Freeze out Feng (2010) Large cross-section: freeze out too late, too few WIMPs The calculation of the thermal-averaged cross-section <σv> needed to obtain the relic density is robust and gives <σv> ~ 3 10-26 cm 3 s -1 At that cross-section we are probing the entire class of particle models that would generate GeV-to-TeV dark matter
Searches for Dark Matter with γ rays 14??? γ-ray Sky Galactic Point Sources Isotropic
Astrophysical Backgrounds (GeV) Diffuse Backgrounds: Cosmic-ray interactions with interstellar matter and radiation fields Source Backgrounds: Pulsars Blazars and Active Galactic Nuclei Supernova Remnants Galaxies (starburst galaxies) Unresolved Sources 15
Improvement of Source Catalogs as a Function of Instrument Sensitivity (S) 16 Flux falls as 1 / d 2 Volume grows as r 3 The number of cataloged sources increases faster than sqrt(t) For a population of objects with the same intrinsic brightness uniformly distributed in space: N ( Flux > S ) scales as S -1.5 The total unresolved flux decreases linearly with S
Unresolved Sources 17 These two images contain roughly the same number of γ rays, but in the top left they are truly randomly distributed, in the bottom right they are produced by individual sources that we can not resolve.
Dark Matter Search Strategies 18 Satellites Low background and good source id, but low statistics Galactic Center Good statistics, but source confusion/diffuse background Milky Way Halo Large statistics, but diffuse background Spectral Lines Little or no astrophysical uncertainties, good source id, but low sensitivity because of expected small branching ratio Galaxy Clusters Low background, but low statistics Isotropic contributions Large statistics, but astrophysics, Galactic diffuse background Dark Matter simulation: Pieri+ 2011PhRvD..83b3518P
Search Strategies (against the γ-ray Sky) Galactic Center Satellites Low background and good Good statistics, but source source id, but low statistics confusion/diffuse background Spectral Lines Milky Way Halo Large statistics, but diffuse background Isotropic contributions Little or no astrophysical uncertainties, good source id, but low sensitivity because of expected small branching ratio 19 Galaxy Clusters Low background, but low statistics Large statistics, but astrophysics, Galactic diffuse background LAT 7 Year Sky > 1 GeV
Summary of Results from Indirect-Detection DM Searches as of Fall 2015 χχ -> bb channel 20 LAT limits from other targets shown in gray
A Sample of Published Results from Indirect DM Searches with LAT Data 21 h vi [cm 3 s 1 ] 10 22 10 23 10 24 10 25 10 26 10 27 Representative Results for Different Search Targets for the b-quark Channel b b MW Halo: Ackermann+ (2013) MW Center: Gomez-Vargas+ (2013) dsphs: Ackermann+ (2015) Unid. Sat.: Bertoni+ (2015) Virgo: Ackermann+ (2015) Isotropic: Ajello+ (2015) X-Correl.: Cuoco+ (2015) APS: Gomez-Vargas+ (2013) Daylan+ (2014) Gordon & Macias (2013) Thermal Relic Cross Section (Steigman+ 2012) Calore+ (2014) Abazajian+ (2014) 10 1 10 2 10 3 10 4 m [GeV]
Summary of Dark Matter Searches with Fermi 22 Comparison of measured upper limits on annihilation of dark matter in dwarf satellite galaxies with annihilation interpretation of the GC excess
The Milky Way Galactic Halo Recent papers: Gordon & Macias 2013PhRvD..88h3521G, Abazajian+ 2014PhRvD..90b3526A Calore+ 2015PhRvD..91f3003C, Daylan+ 2016PDU...12...1D, Ajello+ [LAT Clb] 2016ApJ...819...44A Talks by Moulin, Weniger, Gonthier The goal is to look for DM in the inner Galaxy Because of the large astrophysical foregrounds, we must first understand the γray emission from the Galaxy and from known source classes 23
Galactic Center GeV Excess 24 Ackermann et al. (2017) The presence for an γ-ray excess with respect to the modeled diffuse emission at the Galactic center at a few GeV is well established However, the characteristics (and the interpretation) of the excess depend on the modeling of the astrophysical fore/background
Galactic Center Excess as Unresolved Sources 25 In 2016, two groups analyzed the Galactic center excess statistically and found that it appears to consist of numerous unresolved point sources, not the sort of smooth halo we would expect from a dark matter signal.
Tip of the γ-ray Pulsar iceberg Positions of γ-ray Pulsars Abdo+ 2013ApJS..208...17A 26 Positions of γ-ray Pulsars Calore+ 2016ApJ...827..143C Measuring the γ-ray pulsar luminosity function is difficult for several reasons including: sample incompleteness, differences in beaming between radio and gamma emission, large variations in detection efficiency across the sky, difficult of blind searches for γ-ray pulsations, increased radio pulse dispersion towards the Galactic center In short, rough agreement that there are ~1500-5500 Galactic disk γ-ray pulsars (with Lγ > 1033 erg / s) and that Luminosity function is reasonable well modelled by a power-law with index ~1.2 to 1.3
Pulsar Candidates in the Galactic Bulge 27 Positions of γ-ray pulsar candidates in the 2FIG source list Ajello+ 2017 (Fermi-LAT) arxiv:1705.00009 Identified Pulsars 3FGL Sources New Sources
Confirmation of Bulge Population of Pulsar-Like Sources 28 Ajello+ 2017 (Fermi-LAT) arxiv:1705.00009 Pulsar-like sources as a function of distance from GC Preliminary Recent analysis confirms findings of Lee et al. and Bartels et al., and finds strong independent evidence that the unresolved sources are indeed pulsars. Most important open question: are these young pulsar (τ ~ few Myr) or recycled pulsars in binary systems (τ ~ few Gyr)
Known Satellites of the Milky Way Segue 1 Keck Observatory 29 Recent papers: Ackermann+ [LAT Clb] 2015PhRvL.115w1301A, Drlica-Wagner+ [LAT + DES Clbs] 2015ApJ...809L...4D Albert+ [LAT Clb] 2017ApJ...834..110A Geringer-Sameth+ 2015PhRvL.115h1101G Talks by Geringer-Sameth, Robles Look for γ-ray emission from Dwarf spheroidal galaxies (dsphs), which are the most dark matter dominated objects known This is a moderate-signal, low-background search strategy
Dwarf Spheroidal Satellites of the Milky Way 30 The Milky Way is surrounded by small satellite galaxies Close to Earth (25 kpc to 250 kpc) Optical Luminosities range from 10 3 to 10 7 L Astrophysically inactive Most dark matter dominated objects known 30 kpc
Rapidly Growing Number of Known Dwarf Galaxies 31 DES Year 2 Data: Drlica-Wagner+, 2015ApJ...813..109D DECam DES Year 1 Data: Bechtol+: 2015ApJ...807...50B Koposov+: 2015ApJ...805..130K SDSS Advent of deep, digital survey era in optical astronomy has led to the discovery of numerous new Milky Way-satellite dwarf galaxies. LSST & other surveys will continue to find new dwarf galaxies after the Fermi mission.
Example γ-ray Count Maps for a dsph Galaxy 32 Albert+ (LAT & DES) 2017ApJ...834..110A NFW scale radius PSF containment (68%,98%)
Trials Effect: Most dsphs Lie Inside Expectation Bands 33 Significance v. DM mass for dsphs targets for the ττ and bb annihilation channels Albert+ (LAT & DES) 2017ApJ...834..110A Here we have highlighted the 4 dsphs that fall outside the 95% containment band for any DM particle mass Note: these are NOT the 4 dsphs with the highest J-factors
No Correlation Between J-factor and dsph Signal 34 Flux and <σv> upper limits v. J-factor for dsphs targets Albert+ (LAT & DES) 2017ApJ...834..110A Here we plotted the Flux (left) and <σv> (right) upper limits versus J- factor to emphasize that we are not seeing excesses from many of the highest J-factor dsphs
Joint Fitting Results from dsphs 35 Upper limits from joint analysis of all dsphs Albert+ (LAT & DES) 2017ApJ...834..110A Stacked analysis of dsphs is well within the expectation band for null hypothesis and is in mild tension with DM interpretations of GC excess
III. SUMMARY 36
Looking Forward 37 Large Synoptic Survey Telescope (LSST) Cerenkov Telescope Array (CTA) Existing experiments (Fermi, IACTs, HAWC) will continue to push sensitivity at thermal relic cross section to higher masses (see talk by Moulin). New optical survey (e.g., LSST) telescopes are coming online that will find more dark matter targets and better quantify the dark matter in existing targets. New ground-based high-energy γ-ray telescope array (CTA) that will search for γ rays from dark matter annihilation to much higher energies than Fermi (see talk by Knödlseder).
Dark Matter Sensitivity, circa 2025 38 Comparison of Projected Limits with Direct-Detection and Collider Limits Conversion of direct detection and collider limits following EFT methodology of Bauer+ 2015PDU...7...16B Charles+ [Fermi-LAT Clb] 2016PhR...636...1C