The standard model of cosmology tells us that some 26.8% of the mass in the universe is in the form of dark matter, while only about 4.9% is the normal "baryonic" matter that forms all the atoms, stars, planets, and us.  The dark matter clumps around galaxies, promoting their formation, and shows itself by making stars orbit faster around the galactic center and by its gravitational-lensing effects on starlight.  The nature of dark matter is one of the leading unsolved mysteries of contemporary physics, a secret that Nature has not yet revealed to us.

Physicists have developed several ways of searching for dark matter and trying to understand its properties: (1) by looking for direct production of previously unknown  particle species at accelerators like the LHC; (2) by attempting to detect dark matter particles when they scatter from normal matter as the Earth and the Solar System move through the galactic dark-matter halo at about 0.1 % of the velocity of light; and (3) by searching for the products of annihilation as the matter and antimatter forms of dark matter find each other, collide, and annihilate.  My December-2009 Alternate View column (AV-150) provided a snapshot of the state of dark matter searches at the time.  Today there is still no conclusive solution to the dark matter mystery, but I want to review the current evidence, considering the states of the three lines of investigation.

Direct production of dark matter candidate particles has so far produced no results.  The LHC has been in operation since 2009, and one of the missions of the major detectors there is to search for new weakly interacting massive particle species (WIMPs) that might constitute dark matter.  Of particular interest are particles associated with the extension of the standard model called supersymmetry (SUSY), which predicts that every known fermion particle (having half-integer spin like electrons, neutrinos, and quarks) has a yet-to-be-discovered SUSY twin with integer spin.  Similarly, every known boson particle (having integer spin like photons, gluons, and W and Z bosons) should have a SUSY twin with half-integer spin.   Thus, SUSY predicts that quarks should have "squark"  twins, leptons should have "slepton" twins, a photon should have a "photino" twin, and so on.  None of these particles have yet been observed in collisions at the LHC, meaning that (1) they are have more mass than can be produced in LHC p+p collisions, (2) interactions with them are suppressed for some unknown reason, or (3) they do not exist at all.   In any case, no new dark-matter candidate particles have turned up at the LHC or other accelerators.

Attempts to detect the scattering of dark matter particles streaming by the Earth has proved a bit more promising.   The one element of the search is idea is that the Earth should move faster through the galactic dark matter halo when its orbital velocity around the Sun adds to the orbital velocity of the Solar System around the galactic center than when the two velocities subtract, making collisions with dark matter particles more likely in June than in December.  My December-2009 column described the DAMA/LIBRA results, observation of June/December seasonal variation in the just-above-threshold counting rate of well shielded sodium iodide detectors placed in the Grand Sasso tunnel east of Rome .  Since that time, another Grand Sasso experiment, CRESST-II, has reported observation of similar apparent seasonal variations.  The germanium-detector-based CoGeNT experiment, located in an underground facility in the USA , has also provided some support for the observation of seasonal variations.  On the other hand, the Large Underground Xenon (LUX) experiment, using 368 kilograms of cryogenically-cooled liquid xenon and located in the newly recomissioned Sanford underground facility in Lead, South Dakota, has recently reported that they see no evidence of dark matter collisions.  The observing and non-observing experiments are now attempting to reconcile their differences.  If the Grand Sasso dark-matter scattering observations are to be taken seriously, they imply a significant scattering probability (a cross section of about 10^-41 cm²) for dark matter interacting with normal matter.

This brings us to the third approach, the search for evidence of WIMP + anti-WIMP annihilation products.  Since we do not know what kind of particles are annihilating or what forces and interactions may be involved, the details of such annihilation events must be modeled based on theoretical assumptions.  As the "weak" part of WIMP implies, it is assumed that WIMP particles, like neutrinos, are electrically neutral and are not affected by the strong or electromagnetic forces.  They must be affected by gravity and possibly by the weak force, and they are presumed to come in matter and antimatter forms so that pairs can annihilate.  The products of the annihilation depend on the masses of the WIMPs, but the ultimate result of the annihilation and possible subsequent decays should be to create fairly energetic photons (i.e., gamma rays), electrons, positrons, and neutrinos.  The likelihood of WIMP annihilation depends of their concentration, and they should be most concentrated in deep gravity wells like the center of our galaxy and perhaps the center of the Sun.

There are some observations that are consistent with WIMP annihilation.  First, there is evidence from the Fermi Gamma Ray Space Telescope of energetic gamma rays coming from the center of our galaxy and also from the Virgo Cluster.  The measured energy distribution can be fitted with models based on WIMP annihilation (see below). Second, a byproduct of the measurement of the cosmic microwave background radiation by the WMAP and Planck satellite experiments was the observation of a radio "haze" and a filamentary radio-wave structure near the galactic center.  These radio waves can be explained as synchrotron radiation produced by electrons and positrons from WIMP annihilation.   Analysis by Hooper, Weiner, and Xue (HWX) indicates that the annihilation rate implied by the gamma and radio radiation matches the thermal inverse annihilation rate (the rate of dark matter pair production by heat) required to produce the observed quantity of dark matter in the early universe.

Another signal of dark matter annihilation that has received increasing attention is the spectrum of positrons present in cosmic rays.  The PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) experiment is collaboration of institutions from Russia , Italy , Germany and Sweden , attached to a Russian satellite in a polar elliptical orbit and designed to measure the antiproton and positron components of cosmic rays.  The ongoing experiment has produced  a surprising result:  There was a steep rise in the positron-to-electron ratio with increasing energy, up to 90 GeV, but there was no corresponding rise in the antiproton-to-proton ratio.  Last April-2013, the Alpha Magnetic Spectrometer (AMS-II), a permanent-magnet precision tracking device mounted on the International Space Station, has confirmed this result and produced a high-quality spectrum of the cosmic ray positrons from 0.5 GeV up to about 300 GeV.  There are two competing explanations for the observed  positron excess: WIMP annihilation products or positron production in pulsars.  More modeling and more data on whether the positrons come from particular directions (e.g., pulsars) will be needed to determine which of these explanations is more consistent with the observed positron excess.

Thus, there is a collection of observations pointing in the direction of dark matter annihilation, but the question is, can they be put together in a consistent picture?   To do this, one must provide a description of the WIMP particle: its mass, its interactions involving the four known forces (strong, electromagnetic, weak, and gravitational), its annihilation products, and possibly its interactions involving new unknown forces.  This is tricky, because one must not only explain the existing observations (for instance, the cosmic ray positron rise with energy), but also explain the non-observations (for instance, the lack of any increase in antiprotons with energy or any WIMP candidates at the LHC).  One problem seems to be that PAMELA and the Alpha spectrometer observe too many positrons to accommodate many such models.

The HWX group has proposed a model that seems to fit all the criteria.  It hypothesizes that dark matter and antimatter particles having a mass of around 10 GeV are affected by a new 5th force that is ignored by baryons and leptons.  This new force has a mediating particle (the equivalent of the gluon for the strong force and the photon for the electromagnetic force) that has a mass of around 1 GeV.  The dark matter annihilations produce pairs of these intermediate  particles, which in turn decay to mesons and leptons.  This two-step scenario explains the lack of an antiproton enhancement in cosmic rays and, with a suitable choice of interaction strengths, can fit the gamma ray spectrum from the galactic center, the presumed synchrotron radiation from the WMAP and Planck data, and the scattering that could produce the seasonal variations observed by the Grand  Sasso experiments.

This, of course, is not the end of the story.  The new LUX results place the Grand  Sasso seasonal variations in some doubt.  The cosmic ray positron rise could be coming from pulsars.  Some other astrophysical phenomena could be producing the gamma rays and the synchrotron radiation haze coming from the galactic center.  Several new dark matter searches are currently being mounted, and the ongoing experiments are being improved and are gathering more data.  Watch this column in times to come for further results.

John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.

SF Novels by John Cramer: Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106. His new novel, Fermi's Question may be coming soon.

Alternate View Columns Online: Electronic reprints of 212 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: http://www.npl.washington.edu/av .

References:

Grand Sasso Seasonal Variatons:

"New results from DAMA/LIBRA", R. Bernabei, P. Belli, F. Cappella et al., Eur. Phys. J. C67, 39-49 (2010); arXiv:1002.1028 [astro-ph.GA].

"Results from 730 kg days of the CRESST-II Dark Matter Search", G. Angloher, M. Bauer, I. Bavykina, A. Bento, C. Bucci, C. Ciemniak, G. Deuter, F. von Feilitzsch et al.; arXiv:1109.0702 [astro-ph.CO].

Alpha Spectrometer positrons:

"First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5-350 GeV", M. Aguilar, et al., The AMS Collaboration, Physical Review Letters 110, 141102 (2013).

Theoretical Model:

"Dark Forces and Light Dark Matter",  Dan Hooper, Neal Weiner, and Wei Xue, Phys. Rev. D86, 056009 (2012); arXiv: 1206.2929 [hep-ph].

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