The Dark Matter Problem remains one of the major unsolved mysteries of contemporary astrophysics and cosmology. Many decades ago astronomers concluded that the amount of visible matter present in galaxies was not nearly large enough to account for the gravitational pull exerted on stars at the outer edges of the galaxies. This unexpectedly strong gravitational pull caused the observed orbital rotation speeds of such stars to be much higher than would have been expected. Further, this extra "dark" mass apparently had a distinctly different and more diffuse spatial distribution than that of the normal matter of a given galaxy.
It is now clear from the angular structure of the cosmic microwave background radiation as measured by the Planck satellite that about 84% of the matter in the universe is "dark". This dark matter is assumed to account for the extra mass of galaxies. It clumps around galaxies in a rough sphere, and it does not form stars (or people). The question is, what is dark matter made of, if it is not made of normal neutrons, protons, and electrons? Theoretical particle physics has conjured up a large number of possible dark matter candidate particles that span a wide range of masses and interaction strengths. Perhaps the most prominent among these candidates are WIMPs, which stands for weakly interacting massive particles. These are suggested by the predictions of super-symmetry and other could-be particle theories.
Twice in the past eight years I have written an AV column about WIMP searches. Specifically, my columns in the December-2009 and May-2014 issues of Analog described the then-current status of several large underground experiments (DAMA/LIBRA, CRESS-II, CoGeNT, LUX) that were being mounted to search for WIMPs, as well as the space-based observatory experiments (GLAST/Fermi, PAMELA, HESS, AMS-II) that were looking for evidence of WIMP annihilation and the ongoing WIMP searches at the LHC facility in Geneva that were trying to produce WIMPs in energetic collisions. The conclusion of those columns was that some promising leads had been developed, but there was not yet any compelling evidence for the existence of WIMPs.
Today that situation has not really changed, but now new results have come in from two extremely sensitive WIMP search experiments. In particular, the LUX Dark Matter Collaboration in the US and the PandaX-II Collaboration in China have both presented new results, each from about 33,000 kilogram-days of data collection (the product of the detector mass and exposure time). Neither data set shows any evidence of WIMP detection.
The LUX experiment is a primarily US-based collaboration funded by the US National Science Foundation and Department of Energy. It was located 4,850 feet underground in the Sanford Research Facility, formerly know as the Homestake Gold Mine, in Lead, South Dakota, not far from Mt. Rushmore. LUX collected the data used in the final analysis for a total of 33,500 kilogram-days of exposure. The LUX detector is time-projection chamber in the form of a vertical cylinder filled with 250.9 kilograms of ultra-pure liquid xenon, with xenon gas above the top liquid surface. If a WIMP collides with a xenon nucleus in the liquid, the nuclear recoil produces ionization and a flash of light. The light flash is detected by two banks of 61 photo-multiplier tubes at the upper and lower ends of the cylinder. The electrons from the ionization are in a strong electric field that causes them to drift upward in the liquid to its surface and then to be accelerated further upward in the gas, producing a second light flash. The positions of the two detected light flashes and the time-distance between them allow a three-dimensional reconstruction of the WIMP collision event.
The PandaX-1I Collaboration is a Chinese effort. The PandaX-1I detector is located 7,874 feet underground at the China Jen-Ping underground laboratory, and collected the data used in the final analysis for a total of 33,000 kilogram-days of exposure. PandaX-II is similar in construction and operation to LUX, but it uses 500 kilograms of liquid xenon and so has twice the sensitivity.
The basic idea behind these experiments is that electrically neutral and weakly interacting WIMPs should stream through the detectors at about 0.1% of the speed of light as the Earth orbits the Sun and the Sun orbits the galaxy. If WIMPs interact with normal matter through the weak interaction (i.e., like a neutrino) they have a probability of interaction that is proportional to the square of the atomic mass number A, the number of neutrons and protons in the nucleus of the atom with which they interact. Xenon, element number 54 in the periodic table, is a heavy inert gas with nine stable isotopes ranging in mass number from A=125 to 136, and as a gas or liquid it can transport electrons without absorbing them. This makes it ideal as a medium for WIMP searches. The incoming WIMP strikes a xenon nucleus and causes it to ionize and recoil. The recoil makes a light flash for the first signal and the ionization produces electrons that make the second signal. This detection scheme is predicted to be sensitive to WIMPS with masses ranging from about 5 GeV (roughly five proton masses) to 1,000 GeV, but it is most sensitive to WIMPs having a mass of around 40 GeV.
The bottom-line results of these experiments, based on the non-detection of WIMPS, are presented as an upper limit that forms a roughly "U" shaped plot of mass vs. interaction strength, with the lowest value of interaction strength around 40 GeV. There, the maximum possible interaction strength is about 0.1 zeptobarns (or 10-50 square meters). To put this interaction strength in perspective, it is 1,000 times lower than the very small interaction strength with which a neutrino from a nuclear reactor interacts with matter. The implication is that if WIMPs exist at all, they are not just weakly interacting, but super-weakly interacting.
Undaunted by these negative results, the PandaX-II experiment will continue to collect data for several more years. The LUX detector system has been removed from its underground location and disassembled. A new larger detector is under construction and will replace LUX at the same site in South Dakota. The LUX group has joined forces with the British ZEPLIN Collaboration, and in 2020 the combined group plans to begin operating the new LUX/ZEPLIN dark matter detector, which will operate with 7,000 kilograms of xenon (as compared to 25.3 kilograms for LUX). This new detector will be more than 28 times more sensitive to interactions with WIMPs.
From space-based gamma ray detectors there is more bad news for WIMPs and the effort to detect and identify gamma rays that were produced by WIMP-antiWIMP annihilation. The underlying assumption behind such searches is that the hot dense plasma of the early universe may have created WIMPs and their antimatter twins in roughly equal amounts, and that these two WIMP populations may now be finding each other and annihilating, particularly in locations like the center of our galaxy. The Fermi satellite's Large Area Telescope (Fermi-LAT) experiment observed gamma rays that were coming from the direction of the galactic center. This has been suggested as evidence for possible WIMP annihilation. However, a recent analysis and simulation of the Fermi-LAT data indicates that all of the observed gamma rays can be accounted for as produced by pulsars near the galactic center, leaving no need to invoke WIMP annihilation to explain any of the gamma ray data.
As the negative WIMP detection results pile up as ever more sensitive experiments are seeing nothing, there is a growing consensus within the physics community that perhaps WIMPs do not exist at all, that perhaps we are looking in the wrong place. To put it another way, the experimental window for possible WIMP detection seems to be closing fast.
What are the alternatives? Perhaps the leading dark matter alternative to the WIMP is the axion. The axion is a particle with zero charge and spin and negative parity (i.e., its quantum wave function changes sign on mirror reflection). The axion was originally suggested as a consequence of the Pecci-Quinn mechanism that explains the absence of large strong-force symmetry violations in charge-conjugation and parity (CP). The hypothetical "axial-vector" field that enforces these CP symmetries produces the axion particle. Therefore the axion is the best-justified candidate particle for explaining dark matter. Particle theorist Ann Nelson has observed that, "There are viable theories, and there are natural and elegant theories. However, all viable, natural, and elegant theories contain dark-matter axions." Unfortunately, these theoretical underpinnings of the axion do not suggest a value for the axion mass. However, cosmological considerations indicate that if the axion is the source of the cold dark matter in the universe, it should have a mass between 2 and 100 micro-electron-volts (where a micro-electron volt is 1.78×10-42 kilograms). Axions near the top of this range could also provide an explanation for the unexplained rapid cooling of neutron stars.
One leading experiment aimed at detecting the axion is the Axion Dark Matter Experiment (ADMX), which was started in 1995 at the Department of Energy's Lawrence Livermore National Laboratory and is presently operating at my own laboratory, the Center for Experimental Nuclear Physics and Astrophysics (CENPA) at the University of Washington in Seattle. ADMX uses a detection scheme first suggested by Pierre Sikivie, in which axions interact with a strong magnetic field and are converted to microwave photons. These photons should deliver power to a resonant cavity that is tuned to their characteristic frequency. The ADMX detector is a super-cooled tunable microwave cavity operating in an 8.0 tesla magnetic field. In previous runs, it has excluded axions with masses between 2 and 4 micro-electron-volts with axion coupling strengths greater than about 10-15 GeV-1. The detector is now being rebuilt to improve mass range and coupling strength sensitivity.
Generation-2 ADMX has recently added a helium-3 dilution refrigerator for operation in the milli-Kelvin temperature range. Between now and 2019 it will be searching for axions in the mass range between 2 to 40 micro-electron-volts with axion coupling strengths that are a factor of three lower than the previous lower limits. This search is probably the best hope for finding non-WIMP dark matter in the form of axions.
But what if dark matter is something else altogether? My own SF novel Twistor was based on such a hypothesis. Dark matter may be some form of matter clumping around galaxies that has absolutely no non-gravitational interactions with normal matter. It may have unknown quantum numbers. It may be made of particles that reside "elsewhere" in some compactified dimension. Alternative theoretical explanations of dark matter cover a wide range of hypotheses, but few of these can easily be tested experimentally. In any case, we should know in a few years if the axion solves the dark matter problem, or if we need to look elsewhere.
The LUX Dark Matter Detector:
"Results from a Search for Dark Matter in the Complete LUX Exposure", Phys. Rev. Letters 118, 021303 (2017); https://arxiv.org/pdf/1608.07648.pdf .
The PandaX-II Dark Matter Detector:
"Dark Matter Results from the first 98.7 days of Data from the PandaX-II Experiment", Phys. Rev. Letters 117, 121303 (2016); see also https://arxiv.org/pdf/1612.01223.pdf .
The ADMX Axon Detector:
John Cramer's new book: a non-fiction work describing his Transactional Interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available for purchase online as a printed or eBook at: http://www.springer.com/gp/book/9783319246406 .
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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