Dark Matter: Observations, Theories, Experiments Szydagis 02.25.2015 1 / 17 Figure credit: X-ray: NASA/CXC/CfA/M. Markevitch et al.; Lensing Map: NASA/STScI;

Dark Matter: Observations, Theories, Experiments Szydagis 02.25.2015 1 / 17 Figure credit: X-ray: NASA/CXC/CfA/M. Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U. Arizona/ D. Clowe et al.; Optical image: NASA/STScI; Magellan/U. Arizona/D. Clowe et al.; Right: NASA / ESA / M. Bradac et al. An invisible universe out there!

A Fun Activity Three sets of groups based on where you are siting Window: theory of modified gravity (use Rubin) Near me: dark matter. Debate with window group Center of room: Design your own dark matter detection experiment (direct or indirect/observational) geared towards discovery and/or telling the difference between dark matter and modified gravity models From articles, previous knowledge, or fresh ideas Ambiguous location? Choose what you want to do 5 minutes for discussion. 5-10 for public reporting 2 / 17

Historical Perspective In 1933, the Swiss astronomer Fritz Zwicky discovered there was insufficient luminous matter in the stars of the Coma cluster Based on looking at kinetic energies of galaxies Too high: cluster should fly apart, unbound Famous for coining todays term dark matter In 1970s, American astronomer Vera Rubin found new, intragalactic piece of the puzzle Today: wealth of observational evidence yet *conclusive* detection remains elusive Image credit: zwicky-stiftung.ch Image credit: Princeton University 3 / 17

Rotation Curves Galaxies rotation curves consistently exhibit unexpected behavior, consistent with missing mass at the ~90% level ~0.3 GeV/cm 3 in the Milky Way Intragalactic rotational speeds gravitationally consistent with there being more matter than is visible (shining) in the stars 4 / 17 http://w3.iihe.ac.be/icecube/3_Activities/1_WIMPs%20Analysis /, ~200 km/s for dark matter near sun, earth

Gravitational Lensing Gravitational distortion of light by matter enables a calculation of the mass of the matter doing the distorting (we saw on Feb. 9) Predicted by Einstein (in GR) and first hint observed during a solar eclipse in 1919, but not as galactic-scale lens until 1979 Our gravitational lensing studies (especially of weak) concur with the rotation curves: extra mass! 5 / 17 Credit: Karen Teramura, University of Hawai'i Institute for Astronomy Image: NASA/ESA

The Bullet Cluster Considered nail in coffin for modified gravity ideas Hot x-ray emitting gas in red superimposed in false color on optical image Greatest mass, probable dark matter, in blue (mapped using lensing) Galaxies collided but dark matter evaded collision 6 / 17 Composite Credit: X-ray: NASA/CXC/CfA/M. Markevitch et al.;M. Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U. Arizona/D. Clowe et al.D. Clowe et al. Optical: NASA/STScI; Magellan/U. Arizona/D. Clowe et al. > 1 Gpc away!

Cosmic Microwave Background Echo, snapshot from ~400,000 years after Big Bang, when photons and the plasma decoupled CMB favors model where 27% of energy content of universe is in matter, but in non-baryonic particles The best fit for explaining the angular power spectrum of the temperature anisotropy Nucleosynthesis agrees! 7 / 17 This pie chart and the intense calculations that accompany it have come to be known by concordance as the Standard Cosmological Model (or, CDM) Images: ESA / Planck Collaboration | 2013

Large-Scale Structure Observations (baryon acoustic oscillations, galaxy clusters) and simulations agree well when the presence of *cold* (non-relativistic) dark matter assumed We need dark matter interacting weakly (as in rarely not necessarily via the weak force), gravitationally (only?) 8 / 17 Michael L. Norman, arXiv:1005.1 100 Difficult to achieve with hot dark matter, like neutrinos. In the essay by Vera Rubin, she said we dont understand the bubble + void structure of clusters and superclusters, but that was late 80s!!

Particle Candidates The WIMP, or, the W eakly I nteracting M assive P article = the vanilla candidate But, no Standard Model particle has all needed traits Cold/slow (must be heavy) Cant be a baryon (p, n, etc.) Interacts very weakly Stable, or very long-lived Natural in SUSY or Kaluza- Klein (higher dimensions) 9 / 17 Fermilab fermions and bosons sfermions and bosinos Dirk Laureyssens, 2002 / 2003 EXTRA SPATIAL DIMENSIONS? (analo- gous to anti- matter)

Alternative Notions MACHOs: Ma ssive C ompact H alo O bjects (joke with WIMP) Black holes, neutron stars, brown dwarfs (protostars that failed to ignite, mega-Jupiter-sized), rogue planets, anything non-luminous Dead concept: would result in disagreement with our latest CMB measurements. Perhaps part of dark matter, but cant be majority Axion (named after detergent), scalar boson like heavy photon Hypothetical new particle that explains why the strong nuclear force does not appear to break CP (charge-parity) symmetry, when it should (strong CP problem, QCD: Quantum Chromodynamics) Still in the running! Though as with WIMPs no hard evidence yet Looking for them by trying to shine light through walls, looking at sun, using high magnetic fields (axioelectric effect), RF cavities 10 / 17

The WIMP Miracle As the universe expands, different types (masses) of particles thermally freeze out at different times Temperatures falling, with universe cooling off Dilution: drop in densities Freeze-out occurs when particles can no longer find each other (too far) or other types and interact 11 / 17 Dan Hooper, FNAL With a weak-force cross- section (probability of scattering) a ~100 GeV mass (100x proton) particle would give us ALMOST EXACTLY the dark matter energy density observed today, as a thermal relic Beautiful picture, but sadly kind of dead (WIMP-less miracle?) because havent found anything

Detection Strategies Indirect detection of dark matter (DM) self- annihilation into Standard Model (SM) particles (gammas and neutrinos) Direct detection, my focus today, since this is my work Production of dark matter particles from the high- energy collisions of a particle accelerator (LHC) 12 / 17 Diagram: Mike Woods Collider experiments, like CMS, ATLAS (UAlbany works on) Fermi-LAT, IceCube

Direct Detection Method Most searches are geared toward finding WIMPs, in a model-independent fashion Something going bump in the night, above background In most models, WIMPs scatter elastically off nucleons (billiard-ball-like) Experiments deployed deep underground, because depth reduces (overwhelming-rate) cosmic-ray backgrounds 13 / 17 Mike Attisha, CDMS collaboration 2 main interaction types: nuclear and electron recoil Low-energy (exponentially favored) nuclear recoils (NR) are expected from WIMPs, and electron recoils (ER) constitute primary background to avoid misidentifying

Detector Response Atoms can be excited and scintillate and/or be fully ionized by NR/ER Recoils can also cause lattice vibrations, or boil superheated liquids Most robust searches combine two methods Given rare interaction, figure of merit = target mass X exposure time 14 / 17 charge from ionization (electrons liberated) phonons (excitations within crystalline structures) bubbles, boiling heat (atomic motion) light from de-excitation (scintillation)

How Works A two-phase Xe detector Example of a time- projection chamber (TPC) 122 photomultiplier tubes (PMTs) convert photons into photo-electrons (phe) via photoelectric effect S1 (primary) and S2 (secondary) scintillation light, latter from charge Ratio tells you ER or NR Sum gives you energy 15 / 17

Competitors (Lots) XENON, PandaX: same as LUX, the leader (Europe, China) Spherical version (no electric field and all-liquid): XMASS DarkSide, ArDM, DEAP/CLEAN: Similar concept except instead of xenon with argon (not as dense, but also scintillates, noble, and has advantage of S1 pulse-shape discrimination) CDMS (one of first), CoGeNT: germanium crystals, reading ionization (CDMS phonons too). Costly, difficult to scale up, but low in background and good for low-mass WIMPs PICO, SIMPLE: superheated fluids. Blind to ER backgrounds, but energy=? on event-by-event level. Best for SD-proton 16 / 17

Conclusion & Homework 17 / 17 LUX (2014), 85 live-days exclusion limit curve uncertainty band (in expected result) XENON100 (2012), 225 live-days XENON100 (2011), 100 live-days ZEPLIN-III CDMS-II Ge Edelweiss-II Some claims of discovery exist (for light WIMPs) NOT discovering something is oftentimes equally as valuable as discovering something (think Michelson-Morley and the aether) Old HW: Make sure you do written assignment #5 by next time (Friday) And you should have finished all of your dark matter reading by today !! New HW: The 10 Greatest Discoveries of Modern Astronomy http://wildammo.com/2010/10/22/t he-10-most-amazing-discoveries-of- modern-astronomy/ (dark energy tops the list) http://wildammo.com/2010/10/22/t he-10-most-amazing-discoveries-of- modern-astronomy/ A Faster Walk on the Dark Side http://cosmictimes.gsfc.nas a.gov/teachers/guide/2006 /guide/faster_walk.html http://cosmictimes.gsfc.nas a.gov/teachers/guide/2006 /guide/faster_walk.html HubbleSite Hubble Breakthroughs Cosmology Science http://hubblesite.org/hubb le_discoveries/breakthroug hs/cosmology http://hubblesite.org/hubb le_discoveries/breakthroug hs/cosmology Including sublink http://hubblesite.org/h ubble_discoveries/dark _energy/ (Find Out: Discovering Dark Energy) http://hubblesite.org/h ubble_discoveries/dark _energy/ Dark Questions Remain over Dark Energy (ABC) http://www.abc.net.au/sc ience/articles/2009/12/0 9/2765371.htm http://www.abc.net.au/sc ience/articles/2009/12/0 9/2765371.htm

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Dark Matter: Observations, Theories, Experiments Szydagis 02.25.2015 1 / 17 Figure credit: X-ray: NASA/CXC/CfA/M. Markevitch et al.; Lensing Map: NASA/STScI;