Dark Matters Mavourneen Wilcox Physics 711 Presentation

Overview Definition Current Understanding Some Methods of Detection Final Remark

Definition of Dark Matter Matter that can be observed by its gravitational effects, but does not emit light. Luminous Matter Dark Matter

Strong Gravitational Lensing HST image of background blue galaxies lensed by orange galaxies in a cluster “Einstein’s rings” can be formed for the correct alignment

Rotation Curves of Galaxies What we expect: What we observe: Mass of a galaxy grows with radius. Requires dark matter halo 

Hot or Cold Dark Matter Comes in two forms: Hot Dark Matter (HDM) -A form of non-baryonic dark matter with individual particle masses not more than eV/c 2 (e.g., neutrinos)  relativistic velocities Cold Dark Matter (CDM)  a form of non-baryonic dark matter with typical mass around 1 GeV/c2 (e.g., WIMPs) -able to form smaller structures like galaxies -more massive and slower

Baryons vs. Non-Baryons CDM may be made of two types of matter Baryons -Strongly interacting fermions -Ordinary matter such as MACHO’s, white, red or brown dwarfs, black holes, neutron stars, gas and dust. Non-Baryons -Formed during the Big Bang -Suitable candidate not directly observed yet are WIMPs or other Supersymmetric particles and axions.

MACHOs Brown Dwarfs Exist in the halo of galaxies Attempts to explain Cold Dark Matter without new particles MAssive Compact Halo Objects

Undiscovered non-baryonic particle Interacts only through the weak and gravitational forces High mass corresponds to a lower kinetic energy, making the particle “cold” Weakly Interacting Massive Particles WIMPs

General Consensus No WIMPs have been directly observed Groups studying MACHOs have not found enough objects to account for the missing mass problem Cold Dark Matter probably is a mixture of both baryonic and non-baryonic matter We still do not know for sure

Detecting Wimps Several groups are currently running experiments to find WIMPs Cryogenic Dark Matter Search (CDMS) * Cryogenically cooled GE and SI crystals DAMA Experiment * Scintillation detectors Zeplin * Liquid Xenon EDELWEISS * GE Cryogenic All detect the collisions between a WIMP and a target nuclei

Collision between a WIMP and target nuclei  wimp will strike a target nucleus in the detector material head on and cause an elastic recoil. The recoil energy depends on the mass and velocity of The WIMP together with the Mass of the target nucleus. The energy can be measured in several ways, depending on the Detector. A scintillation photon may be emitted, an electric charge may be liberated or a phonon may cause a slight rise in temperature in the cryogenic material.

Direct Search Principle

DAMA NaI PMT OFHC low radioactivity Copper low radioactivity Lead polyethylene Glove box for calibration sources N2N2 Experiment running since 1996 Located at Gran Sasso Lab, Italy, at a depth of 1400 meters 9  9.7 kg low-activity NaI (sodium- iodide) scintillator crystals, each viewed by 2 PMTs –Known technology –Low cost –Large mass 107,000 kg-days exposure through July 2002 Goal is to detect the Annual modulation in wimp signal.

Signal of WIMPs Earth motion through the galactic halo produce asymmetries distinctive of WIMPs. Earth orbital motion around the Sun (15 km/s) Annual modulation of the WIMP interaction rate.

DAMA Results Figure: Model independent residual rate for single hit events, in the (2–4), (2–5)and (2–6) keV energy intervals as a function of the time elapsed since January 1-st of he first year of data taking. The experimental points present the errors as vertical bars and the associated time bin width as horizontal bars. The superimposed curves represent the cosinusoidal functions behaviors expected for a WIMP signal with a period equal to 1 year and phase at 2nd June Detected an Annual modulation in wimp signal of 6.3 

CDMS II: Overview Located at the Soudan mine in sunny Minnesota CDMS II is 2341 feet below the surface

CDMS II Aim to measure nuclear recoil energy in Ge or Si semiconductor crystals following χ nucleon elastic scattering. Measure, or place upper limits on, WIMP-nucleus cross-sections. Both recoiling nuclei and electrons (due to photon background) produce phonons and electron-hole pairs (ionization) within the crystal. Two parameters characterize particle events. Ionization/phonon ratio (ionization yield) differs for electron and nuclear recoil. Allows for event-by-event discrimination.

Sources of Background Detectors must effectively discriminate between Nuclear Recoils (Neutrons, WIMPs) Electron Recoils (gammas, betas) Use Ge and Si based detectors with two-fold interaction signature: - Ionization signal - Athermal phonon signal

CDMS II Setup Neutrons interact with matter in a similar way to WIMPs. No discrimination between Neutron and WIMP events. Soudan Mine: Cosmic ray muons induce neutron production in matter. ~800m rock overburden reduces muon flux by 5 x Shielding: 0.5cm of Cu, 22.5cm of Pb, 50cm of polyethylene provide shielding from neutron background of rock. Cu is low background and provides shielding from Pb background. Active Muon Veto: A scintillator identifies ~99% of all muon induced neutron events.

Data analysis Fig 10. shows cuts (black lines) for neutron and gamma calibration. Blue dots = nuclear recoils. Red dots = electron recoil. Gives a low rate of misidentified surface events. Fig 11. shows plot of calibration data. Solid curves are the ±2σ electron- recoil band. Dashed curves are ±2σ nuclear-recoil band. Efficiency of combined cuts ~0.4 in 20 keV – 100 keV range.

Data Analysis 2 Fig 12. shows a data plot for an exposure of 19.4 kg day following timing and ionization cuts. Black vertical line is the 10 keV analysis threshold. Note missing nuclear recoil events. No candidate WIMP events for total exposure. Triangles and plus marks are Surface electrons Null result used to set upper limit on WIMP-nucleus rate/cross- section.

CDMS II Limit What’s Next? Currently operating 2 towers Adding 3 new towers over the next 6 months (4kg Ge, 1.4kg Si) Factor of 4 below best previous limits set by EDELWEISS New analysis methods and increased exposure promises 20x improvement over current limit Set an upper limit on the WIMP- nucleon cross-section of 4X cm 2 at the 90% C.L. at a WIMP mass of 60Gev/c 2 for coherent scalar interactions and a standard wimp halo.

The XENON Project A 1 tonne Liquid Xenon experiment for a sensitive Dark Matter Search Elena Aprile Columbia University

The XENON Experiment : Design Overview The XENON design is modular. An array of 10 independent 3D position sensitive LXeTPC (Time projection Chamber) modules, each with a 100 kg active Xe mass, is used to make the 1- tonne scale experiment. The TPC fiducial LXe volume is self- shielded by a few cm thick layer of additional LXe. The active scintillator shield is very effective for charged and neutral background rejection. One common vessel of ~ 60 cm diameter and 60 cm height is used to house the TPC teflon and copper rings structure filled with the 100 kg Xe target and the ~50 kg Xe for shielding.

The XENON TPC: Principle of Operation 30 cm drift gap to maximize active target  long electron lifetime in LXe demonstrated 5 kV/cm drift field to detect small charge from nuclear recoils  internal HV multiplier (Cockroft Walton type) Electrons extraction into gas phase to detect charge via proportional scintillation (~1000 UV  /e/cm)  demonstrated Internal CsI photocathode with QE~31% (Aprile et al. NIMA 338,1994) to enhance direct light signal and thus lower threshold  demonstrated PMTs readout inside the TPC for direct and secondary light  need PMTs with low activity from U/Th/K

The XENON TPC Signals: Nuclear Recoil Discrimination Redundant information from charge (secondary light) signal (S2) and primary scintillation light (S1) signal from PMTs and CsI photocathode Background ( ,e,  produce electron recoils with S2/S1 >>0 WIMPs (and neutrons) produce nuclear recoils with S2/S1~1 3D event localization for effective background rejection via fiducial volume cuts

XENON Summery The XENON experiment is proposed as an array of ten independent, self shielded, 3D position sensitive LXeTPCs each with 100 kg active mass. The detector design, largely based on established technology and >10 yrs experience with LXe detectors development at Columbia, maximizes the fiducial volume and the signal information useful to distinguish the rare WIMP events from the large background. With a total mass of 1-tonne, a nuclear recoil discrimination > 99.5% and a threshold of 10 keV, the projected sensitivity for XENON is  events/kg/day in 3 yrs operation, covering most SUSY predictions.

Evolution of Direct Searches

LSST (The Large Synoptic Survey Telescope) At least 8.4 meter telescope About 10 square degree field of view with high angular resolution Resolve all background galaxies and find redshifts Goal is 3D maps of universe back to half its current age

Other Features A camera with over 3 billion pixels LSST's camera will produce 20 million megabytes of data every night. A single ten-second exposure will detect sources at 24th magnitude LSST will detect and classify 840 million persistent sources. Over time, LSST will survey 30,000 square degrees. By adding together the first five years of data, the all-sky map will reach 27th magnitude, and its database will contain over three billion sources, not counting transient events.

LSST will map the Dark Matter Universe From the warping of the visible matter, dark matter clumps can be seen, mapped and charted over their development. To see the dark matter, one "inverts" the cosmic mirage it produces. The analysis of the distorted images produces a unique map of the space-time warp caused by the unseen dark matter in the foreground cluster. HST image of the cluster of galaxies, CL Analysis of all the distorted images in the HST picture above.

LSST will map the Dark Matter Universe Here the a image of the distribution of mass from the previous image is shown as a two-dimensional surface, where the height of the orange surface represents the amount of mass at that point in the image. This mass distribution shows that most of the dark matter is not clinging to the galaxies in the cluster (the narrow, high peaks), but instead is smoothly distributed. Kochanski, Dell’antonio,Tyson

Pan-STARRS Panoramic Survey Telescope & Rapid Response System Developed at the University of Hawaii's Institute for Astronomy The Pan-STARRS design is weighted towards detecting potentially hazardous objects in our Solar System like earth bound asteroids and comets but will be ideal for making three dimensional maps of the distribution of dark matter in clusters of galaxies. PS1 - the Prototype Telescope

Pan-STARRS Four comparatively small telescopes, each with a 3 degree field A single observation with the broad- band filter will reach a 5 σ depth of 24th magnitude. Determine positions on an individual image to within 0.07 arcseconds, based on an image size of 0.6 arcseconds FWHM, and a signal-to- noise ratio of 5. 30,000 square degrees can be observed from Hawaii. PanStarrs looks at about 7 square degrees in each 30 seconds exposure, so in an eight-hour night it can map about 6,000 square degrees. Therefore it takes about a week to survey the whole sky once, using one filter. 4, 1.8-m concave primary mirror that follow the Richey-Chretien design

Final Thought “The discovery of cold dark-matter particles would be one of the most important in the history of physics. It would clarify many questions concerning the birth, evolution and final destiny of our universe. A definitive confirmed discovery would certainly merit a Nobel prize and a distinguished place in history for those who provided the intellectual leadership” Frank T Avignone

References UKDMC: Microlensing planet search: Neutrinoless double β-decay: WIMP direct detection: CDMS Collaboration: Pan-STARRS WEBSITE Perkins, D. H. Introduction to High Energy Physics, 4th Ed CUP. Roos, M. Introduction to Cosmology, 3rd Ed Wiley Liddle, A.R. An Introduction to Modern Cosmology. 2nd Ed Wiley. D.S. Akerib et al. "First Results from the Cryogenic Dark Matter Search in the Soudan Underground Lab". astro-ph/ astro-ph/ T. Saab, Search for Weakly Interacting Massive Particles with the Cryogenic Dark Matter Search Experiment, PhD Dissertation, Stanford University Aug

Web Resources Jonathan Dursi’s Dark Matter Tutorials & Java applets MACHO project National Center for Supercomputing Applications Pete Newbury’s Gravitational Lens movies