physics

physicsfaculty

CWRU Physics Faculty

Daniel S. Akerib
Professor of Physics
B.A., University of Chicago (1984)
Ph.D., Princeton University (1991)
Experimental Particle Astrophysics, Dark Matter, Low-temperature Detectors, Particle Physics
Picture of Daniel S. Akerib
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Numerous observations and theoretical models indicate that most of the Universe does not glow in the form of stars or other familiar objects, but rather that it is dark. This dark matter is inferred by its apparent gravitational effects on a wide range of distance scales, from galaxies to super-clusters. Recent data on cosmological scales using supernovae and anisotropy of the cosmic background radiation also indicate a similar amount of total mass not accounted for in luminous objects. Deepening the mystery is the constraint from Big Bang Nucleosynthesis that baryons alone cannot account for all the mass required by these observations. So, not only is most of the matter in the universe "missing" but it is not even made of ordinary particles.

Fortunately there are not just questions, but also some clues. For example, the rate of structure formation requires that the dark matter be "cold", that is, non-relativistic; hot dark matter would prevent gravitational instabilities from growing quickly enough compared to what is observed. One hypothesis for the dark matter suggests a deep connection with particle physics, leading to a generic class of particles called WIMPs, or Weakly Interacting Massive Particles. If WIMPs (and their anti-particle) exist in nature, they would have naturally been produced by thermal collisions in the hot plasma of the early universe. Because they are weakly interacting and the universe was rapidly expanding, WIMPs and anti-WIMPs would eventually fail to find each other and annihilate. Given the right combination of mass and cross section, WIMPs would have "frozen out" when they were non-relativistic and enough would be leftover to make up the dark matter. The deep connection with particle physics comes about because extensions to the Standard Model, e.g., Supersymmetry, lead to hypothetical particles with just the right range of properties.

The goal of my research is to try and detect WIMPs, directly, through their elactic scattering from atomic nuclei in a terrestrial detector. If they were in fact produced in the early universe, an overdense region of WIMPs from that epoch would have had sufficient gravitational binding to overcome the expansion. This WIMP cloud would have formed the initial gravitational core of the Milky Way, upon which the ordinary matter we are made of would have coalesced. The existence of this WIMP "halo" in which the familiar disk of the Milky Way is embedded is the hypothesis we are testing.

Our combined understanding of the structure and density of the Milky Way together with early universe constraints on the properties of WIMPs allows us to make a more refined quantitative hypothesis. That is, these various clues tell us where to look! However, the experimental challenge is formidable. Because WIMPs are slow and weakly interacting, they lead to small energy transfers and very low rates. In a typical particle detector, the rate from ambient radioactivity and cosmic rays is about a billion times higher than from WIMPs so great care must be taken as we looks for this "needle in a haystack."

At present, the two world-leading techniqes in the search are ultra-cold solid-state germanium detectors and 2-phase liquid xenon detectors. Both of these techniques simultaneously measure the energy deposited by a WIMP-nucleus recoil in two different ways, through either ionization and phonons in the germanium and ionization and scinitillation in liquid xenon. In each case the ratio of the measurements allows nuclear recoils to be distinguished from electron recoils, which is useful because almost all of the backgrounds lead to electron recoils.

Since first coming to Case in the mid-1990's, my group has been a member of the Cyrogenic Dark Matter Search, or CDMS [1] [2]. I helped get the first generation CDMS-I experiment underway as a postdoc at UC Berkeley in the early 90's and recently our group completed our work on the CDMS-II experiment. These experiments used the ultra-cold germanium detector technique and have been on the vanguard of WIMP searching over much of the last decade. The CDMS-I experiment took place in a shallow site at Stanford University and CDMS-II at the Soudan Underground Lab 2000' below the surface in Northern Minnesota.

In 2008, our group joined with Case faculty member Tom Shutt and his group to complete construction of the Large Underground Xenon (LUX) experiment. Our combined group, as part of the LUX collaboration, will be commissioning this detector in a surface facility at the Homestake Mine's Sanford Lab in western South Dakota. In 2010 we will be moving it underground into the newly refurbisehd Davis cavern, the site of the famous pioneering solar neutrino detector. This 350-kg detector, using the 2-phase technique, will be well-positioned to make a major stride forward in the search for WIMPs. In addition to pursuing this project, the LUX collaboration has joined with the European ZEPLIN-III collaboration to form the LZ collaboration which is planning to carry out the larger ton-scale followup experiments in the Sanford Lab, and eventually at DUSEL, the Deep Underground Science and Engineering Lab, that is being developed for the Homestake site by the National Science Foundation.

The Case LUX group carries out a broad range of activities, from detector construction and testing, to numerical simulations and long-range R&D. Please contact if you are interested in exploring possible research opportunities.