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Space Telescope Science Institute
2011 May Symposium Talks

The Search for Dark Matter with the XENON100 Experiment

Elena Aprile (Columbia University)

The XENON100 experiment is designed to search for interactions of dark matter Weakly Interacting Massive Particles (WIMPs) in a liquid xenon time projection chamber. Featuring a large target mass and an extremely low background, XENON100 is the most sensitive dark matter direct detection experiment in operation today. Located deep underground at the Gran Sasso National Laboratory, in Italy, XENON100 has recently reported results on the elastic scattering of dark matter WIMPs with nucleons, based on 100 days of data acquired in 2010. The recent findings, the status of the experiment and the work towards the next generation XENON1T experiment will be presented.

DAMA/LIBRA Results and Perspectives

Pierluigi Belli (INFN - Roma Tor Vergata)

The DAMA/LIBRA set-up (about 250 kg highly radiopure NaI(Tl)) is running at the Gran Sasso National Laboratory of the INFN. The results obtained by exploiting the model independent annual modulation signature for the presence of Dark Matter (DM) particles in the galactic halo during the first six annual cycles (exposure of 0.87 ton x yr) will be discussed. The cumulative exposure with those previously released by the former DAMA/NaI is 1.17 ton yr, corresponding to 13 annual cycles. The confidence level for the observed effect is 8.9 sigma and the data satisfy all the many peculiarities of this DM model independent signature. No systematics or side processes able to account for the measured modulation amplitude and to simultaneously satisfy all the many requirements of the signature have been found. Further data have been taken during an annual cycle before the realization of the new upgrade of the set-up, which occurred at end of 2010. Presently, DAMA/LIBRA is in data taking in the new configuration. Results, implications and perspectives will be summarized.

Puzzles and Progress for Lambda Cold Dark Matter in the Local Group

James Bullock (Univesity of California, Irvine)

Over the past 5-10 years, surveys of the local group have discovered many new dwarf galaxies and shown that the outer halos of the Milky Way and M31 are filled with streams of stars, the residue of hierarchical assembly. Qualitatively, these results are good news for cold dark matter models, but some puzzles have emerged. One particularly surprising result is that the majority of the most massive subhalos in LCDM simulations are too dense to host any of the known satellite galaxies. The discrepancy sets in at a mass scale of ~10^10 solar masses or a maximum circular velocity scale of ~50 km/s. If standard LCDM predictions are correct, then galaxy formation must become effectively stochastic in halos smaller than ~50 km/s. Another possibility is that the microphysical properties of dark matter differ from those assumed in vanilla CDM simulations. Self-interaction provides an intriguing possibility by preserving the hierachical power spectrum while potentially lowering central densities in halos.

An Update on COUPP and CoGeNT

Juan Collar (University of Chicago)

COUPP employs bubble chambers to look for medium to heavy mass Weakly Interacting Massive Particles (WIMPs). This technique offers the best intrinsic rejection against minimum ionizing backgrounds available to any dark matter detector. Very recently, an additional acoustic rejection of alpha-induced bubbles has been demonstrated. As a result of this, we expect COUPP to become one of the most sensitive dark matter detectors for both spin-dependent and -independent WIMP-nucleus couplings during 2011, following the installation of two chambers at SNOlab. CoGeNT uses P-type Point Contact (PPC) germanium detectors and is designed to look for light WIMP candidates below ~10 GeV in rest mass. We'll discuss a new approach to reject surface events in PPCs, and some "interesting" irreducible backgrounds observed during their deep underground operation, as well as future plans.

Rotation Curves and Dark Matter Halos of Nearby Late-Type Galaxies

Erwin de Blok (University of Cape Town)

The distribution of dark matter in galaxies has been the topic of some debate over the last couple of years. Recent, new observations of the gas kinematics of nearby galaxies as well as high-resolution simulations have enabled us to better constrain this distribution. In this observationally-themed talk I give an overview of some results based on rotation curves derived from the THINGS and HERACLES surveys. These indicate that galaxies without strong bulges prefer dark matter profiles dominated by a central constant-density core. Results using Einasto halos show a similar result. I also discuss results from a number of numerical simulations of dwarf galaxies that have been treated in exactly the same way as the THINGS observations, and discuss their inferred dark matter distributions.

Results from the Large Hadron Collider

Albert de Roeck (CERN)

In March 2010 the Large Hadron Collider (LHC) delivered the first proton-proton collisions at a center of mass energy of 7 TeV. The experiments at the LHC, in particular ALICE, ATLAS, CMS and LHCb recorded the data and had a first glimpse of the high energy interactions in this new energy regime at colliders. By the end of 2010 about 40 pb-1 of data as been collected, and in this presentation we will review the results from this first physics run. In particular we will discuss the results of the initial searches for the Higgs Boson and the searches for new physics beyond the Standard Model. The 2010 data already allows the searches to enter into a new regime, and to obtain the best collider limits. Special attention will be given to those new physics scenarios which are traditionally also considered to be tests for Dark Matter at the LHC, such as supersymmetry and models for Extra Dimension. The LHC has just resumed operation, and will surpass the data sample collected in 2010 very soon, hence we are looking forward to results of these new data.

Constraints on Dark Matter from X-ray Observations

Megan Donahue (Michigan State University)

We can estimate the total mass of clusters of galaxies in at least four ways: galaxy velocity dispersions, gravitational lensing, X-ray observations, and Sunyaev-Zeldovich observations. These methods measure different aspects of the matter, and have different systematics and model dependencies. Clusters of galaxies provide an important independent check on our understanding of gravity, as the cluster correlation function and cluster mass function evolution are determined by gravity and the geometry of the universe, while the observed ratio of gas mass to total mass is determined by geometry and the universal baryon fraction. I will review various determinations of dark matter parameters, particularly Omega_Matter, from X-ray observations of the intracluster gas in clusters of galaxies.

WIMP Annihilation Signals in the CMB

Doug Finkbeiner (Harvard University)

Sommerfeld-enchanced WIMP models can produce sufficient annihilation power around z=600 to change the recombination history of the Universe. Current and upcoming CMB experiments are sensitive to interesting regions of parameter space. I will present a new set of parameters intended to provide an optimal measure of WIMP annihilation power, and describe how both observations and theoretical models can be projected to this convenient, low-dimension parameter space.

Magnificent Magnification: Extracting the other half of the weak lensing signal

Eric Huff (UC, Berkeley/LBNL)

Weak gravitational lensing has attracted a great deal of interest in recent years as a powerful measurement technique capable of probing the invisible components of the concordance cosmology. To date, most of the information available from weak lensing studies has come from analyses of the shearing of galaxy shapes. In this presentation, we describe a method for measuring weak lensing magnification by galaxies. Our method makes use of the existence of tight photometric scaling relations (analogous to the Fundamental Plane) between photometric galaxy properties that are perturbed by gravitational lensing, such as apparent size, and other photometric properties, such as mean surface brightness and light profile concentration, that are not. We present a first detection of magnification by massive galaxy halos at significance approaching that achievable with conventional shear-based measurements. We identify the major sources of systematic error inherent in magnification measurements based on galaxy scaling relations and demonstrate techniques to suppress these systematics below the statistical errors of the measurement. Further work holds the potential to dramatically increase the magnification signal beyond that presented here. Applying this technique to current and future surveys will yield a significant boost in the cosmological information available to lensing experiments and a much-needed device for the suppression of systematic errors.

Weak Lensing Tests of Gravity and Dark Matter

Bhuvnesh Jain (University of Pennsylvania)

Weak lensing probes of dark matter and gravity come from measurements of galaxy-galaxy lensing and mass maps of galaxy clusters. I will summarize current measurements and upcoming lensing surveys. A new arena for lensing and other probes is tests of gravity on astrophysical scales. I will describe how this changes our way of thinking about cosmological tests, which has been developed in the dark matter/dark energy framework.

Gamma-Ray Results from Fermi

Robert Johnson (University of California, Santa Cruz)

The Fermi Gamma-ray Space Telescope is a mission in low-Earth orbit to observe gamma rays from the cosmos in the broad energy range from 20 MeV to >300 GeV, with supporting observations of gamma-ray bursts from 8 keV to 30 MeV. The telescope far surpasses previous generations in its ability to detect and localize faint gamma-ray sources, as well as its ability to see 20% of the sky at any instant and scan the entire sky on a timescale of a few hours. With its launch on 11 June 2008, Fermi opened a new and exciting window on a wide variety of exotic astrophysical objects and is enabling new research on such topics as the origin and circulation of cosmic rays and searches for hypothetical new phenomena such as annihilation of dark matter. This talk presents the latest results on dark matter searches by the Fermi-LAT collaboration, based on observations of gamma rays by the Fermi Large Area Telescope. Included are gamma-ray line searches, and searches for dark matter annihilation at cosmological distances, in galaxy clusters, in local dwarf galaxies, in dark satellites, and in the Galactic halo.

Observational constraints on the nature of Dark Matter and Dark Energy

Ofer Lahav (University College London)

The talk will cover 3 topics: (i) New results from the HST CLASH survey on the nature of Dark Matter in galaxy clusters (ii) Constraints on neutrino mass from Cosmology (iii) The prospects to constrain Dark Matter and Dark Energy from mulit-probes in the next generation of galaxy surveys

Using Galaxy-Galaxy Lensing to Constrain the Connection Between Galaxies and Dark Matter

Alexie Leauthaud (Lawrence Berkeley National Laboratory)

I will present a new theoretical framework that combines measurements of galaxy-galaxy lensing, galaxy clustering, and the galaxy stellar mass function in a self-consistent manner. While considerable effort has been invested in exploring each of these dark matter probes individually, attempts to combine them are still in their infancy despite the potential of such combinations to elucidate the galaxy-dark matter connection, to constrain cosmological parameters, and to test the nature of gravity. I will then present an application of this model to the COSMOS data and will show how the stellar-to-halo mass relation evolves from z=1 to z=0. Finally, I will discuss the potential of future weak large surveys for galaxy-galaxy lensing measurements and I will show some early work from a new 170 deg^2 weak lensing survey ("CS82") we have recently completed in the Stripe82 equatorial region.

Revealing the Invisible: Strong Gravitational Lensing as a Direct Probe of the Particle Nature of Dark Matter

Leonidas Moustakas (Jet Propulsion Laboratory, California Institute of Technology)

Under all viable dark matter particle candidates, we predict there to be sub-structure within all galaxies. These sub-structures will follow spatial and mass-function distributions that depend on the matter power spectrum and the effects of non-linear evolution within galaxies, and will depend on the specific properties of each dark matter particle candidate. These structures need not host gas, or stars, but they will always have some gravitational touchmarks that may be inferred or observed through a variety of techniques. The most comprehensive probe is through strong gravitational lensing. A tailored combination of observations of certain classes of gravitational lenses, especially including high precision time delays between images of multiply imaged active galactic nuclei, can lead to otherwise unattainable conclusions about the viability or even quantitative constraints on the properties of the dark matter particle or particles. I discuss these constraints, compare them against other approaches to studying dark matter, and give both the current state of the art and the prospects for the near future, including through Explorer-scale missions.

Insights into Dark Matter from Galaxy Clusters

Priya Natarajan (Yale University)

Exploiting strong and weak gravitational lensing has proved to be very profitable to understand the detailed distribution of dark matter in clusters of galaxies. I will present the results to date of the comparison of these lensing derived high resolution mass profiles for clusters with cosmological simulations.

Big Bang Nucleosynthesis and the Baryonic Content of the Universe

Keith Olive (University of Minnesota)

An overview of the standard model of big bang nucleosynthesis (BBN) in the post-WMAP era is presented. With the value of the baryon-to-photon ratio determine to relatively high precision by WMAP, standard BBN no longer has any free parameters. In this context, the theoretical prediction for the abundances of D, He4, and Li7 is discussed and compared to the observational determination of the light nuclides. While concordance for D is excellent, and acceptable within the uncertainties for He4, the prediction for Li7 exceeds the observational determination by a factor of about four. Possible solutions to this problem are also discussed.

Dark Matter from Baryon Acoustic Oscillations

Beth Reid (Lawrence Berkeley National Laboratory)

The consistency between fluctuations measured in the Cosmic Microwave Background and in the matter density field at low redshift (including the Baryon Acoustic Oscillations) provides a stringent test of the standard cosmological model: dark matter and baryons are only coupled gravitationally, and dark matter dominates the matter budget of the universe, providing the scaffolding on which baryonic structures grow. I will discuss two other important "dark matter" constraints that we get for free from galaxy redshift surveys: the contribution to the matter budget today from massive neutrinos, and the amplitude of peculiar velocity fluctuations via redshift space distortions.

Alternative Theories to Dark Matter

Constantinos Skordis (University of Nottingham)

The General Theory of Relativity (GR) is an astounding accomplishment: together with quantum field theory, it is now widely considered to be one of the two pillars of modern physics. The great success of GR, however, has not stopped alternatives being proposed, some by Einstein himself. The limits of GR have again come into focus with the emergence of the ‘dark universe’ scenario. For many years there has existed evidence that, if gravity is governed by GR, there should be a substantial amount of dark matter in galaxies and clusters. More recently, dark energy has also been found to be required in order to explain the apparent accelerating expansion of the Universe. Indeed, if GR is correct, it now seems that around 96% of the Universe should be in the form of energy densities that do not interact electromagnetically. Such an odd composition, favoured at such high confidence, has led some to speculate on the possibility that GR may not, in fact, be the correct theory of gravity to describe the Universe on the largest scales. The dark universe may be just another signal that we need to go beyond Einstein’s theory. In this talk, I will discuss the difficulties associated with constructing a new theory of gravity. I will present a compendium of theories of gravity and the extend to which they address the theoretical and observational requirements.

Strong Lensing and Dark Matter

Tommaso Treu (University of Californa, Santa Barbara)

According to the standard cosmological model, cold dark matter dominates the dynamics of structure. However, it is unclear whether the model can reproduce the properties of galaxies and clusters at smaller scales. Strong lensing is an extremely precise probe of the distribution of mass at these scales, and therefore can provide us with unique insights into the properties and nature of dark matter. I will present recent work aimed at answering the following specific questions: Are the density profiles of dark matter halos dark universal? Is there dark matter substructure? I will also show that the our improved understanding of the distribution of mass on galactic scales makes strong gravitational time delays a competitive tool for precision cosmography.

Looking Backward, Darkly

Virgina Trimble (University of California)

Both the concept and the name dark matter are older than you probably think. The talk will begin in the middle, with a data summary that could have been assembled in 1939, but in fact was not until 1974. I will then attempt to identify some of the very oldest DM candidates (a few of which are still viable for at least part of it, though not the Counter-Earth of Philolaus) and, finally, jump forward to a few milestones (Santa Barbara 1961, for instance) en route to dark matter becoming part of our current cosmological aradigm. The work of Vera Rubin, who was originally scheduled for this talk, was, of course, of great importance in this final stage. I am not replacing her; no one could, but feel very honored to be filling in.

Cosmic Microwave Background Constraints and Dark Matter

Licia Verde (ICREA & Institut de Ciencies del Cosmos (ICC-UB-IEEC))

Cosmic Microwave Background (CMB) observations have been fundamental in defining the "standard cosmological model". This model has been so far very succesful in describing, with only few parameters, observations of the Universe from redshift sim 1100 until the present day. Contrary to the present day, at the time when the Cosmic Microwave Background light was emitted, the Universe was matter-dominated. The standard cosmological model indicates that most of the matter in the Universe is dark, and most of it is non-baryonic. Precision measurements of the CMB radiation offer extremely tight constraints on the matter content of the Universe and some insights on dark matter properties. I will review current constraints and consider possible future improvements.

Cores versus Cusps in the Dwarf Spheroidals

Matthew Walker (Harvard-Smithsonian Center for Astrophysics)

The Milky Way's dwarf spheroidal (dSph) satellites have the smallest baryonic masses and largest mass-to-light ratios of any observed galaxies, and therefore provide perhaps the most straightforward available comparison to cold dark matter halos produced in cosmological simulations. I will present a new method for measuring the inner slopes (i.e., `core' vs 'cusp') of dSph dark matter density profiles and briefly discuss implications for CDM-based cosmological models.

Inferences on Dark Matter from Microlensing

Joachim Wambsganss (Universitat Heidelberg)

Gravitational microlensing - the deflection of light by compact objects of roughly stellar mass - comes in two flavors: Quasar microlensing deals with the action of compact objects in the (halo of the) lensing galaxy on a multiply imaged background quasar, Local Group microlensing investigates the lensing effect of foreground objects on stars in the Galactic Bulge, in the SMC, the LMC or in M31. Both scenarios were proposed more than 25 years ago as potential probes for compact dark matter. I will summarize what gravitational microlensing has revealed since on compact dark matter objects. More recently, microlensing has been applied to derive qualitative and even quantitative information on a smooth dark matter component in lensing galaxies by investigating the so-called flux-ratio anomaly in close pairs of (macro-)lensed quasar images. I will review as well what we have learned about smoothly distributed dark matter from microlensing.

Dark Matter and Galaxy Properties

Risa Wechsler (Stanford University)

Dark matter provides the seeds of structure upon which galaxies form. I will review the basics of the formation of dark matter halos, informed by new results based on high-resolution cosmological simulations and gravitationally-consistent merger trees. Recent studies indicate that galaxy properties, including their stellar masses and star formation rates, are strongly correlated with the properties of their dark matter halo hosts. A model based on LCDM halos and this tight connection is in excellent agreement with a wide range of data, including the statistics of Magellanic Cloud mass satellites around Milky Way mass hosts. In the context of a given cosmological model, I will show how the galaxy-halo relation can be tightly constrained at low redshift, and how the galaxy-halo connection can be used to infer the full star formation histories of galaxies. For the faintest dwarfs, there is still considerable degeneracy between the galaxy-halo connection and the properties of dark matter.

Towards Anomaly-Free Dark Matter

Neal Weiner (New York University)

A wide range of results (DAMA, CoGeNT and CRESST, most notably) have found event rates in suggestive of WIMPs. The null results from other experiments challenge these results. I will explore what the existing theoretical options are to explain these results, and what experimental results going forward can conclusively answer whether we have seen dark matter or not.

The Structure of the Dark Matter Distribution on Laboratory Scales

Simon White (MPA Garching)

At the time of recombination, 400,000 years after the Big Bang, the structure of the dark matter distribution was extremely simple and can be inferred directly from observations of structure in the cosmic microwave background. At this time dark matter particles had small thermal velocities and their distribution deviated from uniformity only through a gaussian field of small density fluctuations with associated motions. Later evolution was driven purely by gravity and so obeyed the collisionless Boltzmann equation. This has immediate consequences for the present distribution of dark matter, even in extremely nonlinear regions such as the part of the Galaxy where the Sun resides. I will show how this structure can be followed in full generality by integrating the Geodesic Deviation Equation in tandem with the equations of motion in a high-resolution N-body simulation, enhancing its effective resolution by more than 10 orders of magnitude. I will discuss how the predicted distribution at the Sun's position impacts the expectations for laboratory experiments seeking to detect the dark matter directly, in particular, the possibility of extremely narrow line signals that may be visible in axion detectors.