By the time you finish this sentence you will have been bombarded by about 1011 solar neutrinos, each with about 10 times more energy than your standard medical x-ray. Doubtless, you’ve never noticed.
Neutrinos are not dark matter, but tiny, electrically neutral cousins of the electron have some relevant similarities. So maybe discovering neutrino is a case study for finding dark matter? First, one must develop a theory of interaction, i.e., what physical processes do neutrinos participate in? Are there any direct, secondary processes? Next, we can exploit these interactions to develop an experiment, a detector precisely tuned to that specific interaction.
That is the story of the neutrino’s discovery. Neutrinos were one of the last predictions of the era of quantum mechanics. They couldn’t be detected, as their energy was within the errors of existing experimental methods and negligible compared to other subatomic particles. However, contemporary theories left a gap in how some nuclear reactions could conserve fundamental properties of energy, momentum, and angular momentum without some other funny, unknown particle. Neutrinos were first predicted by Wolfgang Pauli and Enrico Fermi in the early 1930s for this reason, but later detected in 1956 by the Cowan–Reines neutrino experiment.
Is this the case with dark matter? Yes and no. Sometimes the scientific method is predictable, as was demonstrated by neutrinos. Other times, unexpected results pop up.
A series of major experiments of the past decade have been focused on a puzzling astronomical signal—and it is these experiments that have had drawn the most media coverage. In 2008, scientists published data from the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) spacecraft that detected this signal. The ratio of positrons, or anti-electrons, to electrons coincided with the possibility that dark matter particles were somehow generating that ratio. Since then, the Fermi satellite and the Alpha Magnetic Spectrometer-2 (AMS-2) on the space station have verified the PAMELA data. The latter generated a media flurry about dark matter in early 2013.
(Understanding why the PAMELA results were noteworthy requires some extra explanation. For almost all particles there is a corresponding antiparticle; like looking into a mirror and seeing everything flipped. The anti-particle of an electron, the positron, has the same mass, same shape, but the electric charge of it is flipped – it has the same charge as a proton! And as science fiction knows very well, when matter and antimatter meet, they annihilate, a totally efficient reaction where all energy is converted and the matter/antimatter pair cease to exist. This annihilation works in reverse: you can never creation of a particle requires the creation of a related antiparticle. This conservation law of (anti)particle number implies that electrons are made with postirons as a result of a previous matter/antimatter annihilation. But despite always forming a pair, space is a big place with magnetic fields that affect positron and electron in the opposite way and different sources of electrons based on the existing but inexplicable paradox of the supremacy of matter. Galactic transport models, software that calculates how particles move around the universe, permit an observed difference. However, they do not predict the observed excess in positrons in the energy range 10-1000 GeV (for context, that is a subatomic particle with an energy just less than that of a flying mosquito. For that reason the authors of this study and other scientists believe that this excess of positrons to electrons might be an indicator for dark matter. Dark matter may annihilate with its antimatter equivalent—and the results of that interaction create electron/positron pairs.)
Oddly, the major theories of dark matter, including the WIMP and axion described in the last post, were not clear candidates. The paper describing the excess and its promise for identifying dark matter leaves the theoretical interactions causing the ratio open for future work. Nevertheless it is undeniably promising. The energy range is convenient—that fits with WIMP theories. However, there are many other requirements based on existing astrophysical data and theory for which no WIMP theories have yet been proven to satisfy all. Several physicists insist this and have forwarded a theory of dynamical dark matter that effectively creates a world of particles parallel to the canonical Standard Model of physics. This parallel ensemble of dark matter particles produces some decays resulting in the production of positrons, within our Standard Model, as detected by the above experiments. Alternatively, the very models we are basing our notion of “correct” positron/electron ratio might be misinformed. There has been debate as to whether or not pulsars, rapidly rotating neutron stars generate this positron excess and not dark matter. There is currently no clear answer, despite notable fanfare.
So while the unexpected PAMELA, et al accidental results have challenged the way we view dark matter—other experiments have followed a more traditional approach. These experiments also contrast because they are striving for direct detection, wherein a physical interaction with dark matter occurs rather than detecting antimatter, a possible secondary effect of dark matter interactions.
Detecting something that doesn’t want to interact with anything is a challenge. The more sensitive a detector is the more false positives it will have. Thus, it’s vitally important to isolate an instrument from any source of noise. It is for that reason that the most notable WIMP direct detection instruments are located deep in mineshafts. Xenon 100 and Cryogenic Dark Matter Search (CDMS) have provided the best limits on how massive a WIMP can be. Xenon 100 was designed to detect the collision between WIMPs and the nucleus of cool, liquid Xenon. The energetic reaction would smack the nucleus away from its electrons and the electrons would form a current that could then be detected. It detected several events, but these were within the expected background limit. CDMS operated like a super-cooled refrigerator—just above absolute zero. Any nuclear collision would raise the temperature indicating the existence of a WIMP. A few events were detected, but within error. The negative results from all these direct WIMP detection experiments give us agreed upon limits.
Axions may also be out there waiting to be snared. The most notable experiment may be the Axion Dark Matter Experiment (ADMX). Axions being fundamentally different particles and participating in a different interaction don’t have the same detection restrictions of the WIMP experiments. Direct detection of an axion involves looking for an axion to spontaneous transform into photons; based on theory, it is expected that these photons would take the form of radio waves. Because the signal is expected to be so weak, the ADMX detector is the most sensitive antennas in the world. To date no events directly attributable to axions have been detected.
But fear not! As reported recently, “Dark Matter Mystery Could Be Solved in 10 Years.” Of course, the article then makes no mention of why 10 years. But this is why I write these posts. The news media, even those who appear to have authority over mainstream science journalism, have failed to relay the facts of specific discoveries and their context. News media also fail to explain the intricacies of the scientific process as a whole. Physics, or science, is an attempt to rationalize a reality we suspect is rational. It seems rational as far as we’ve calculated. But despite whatever theory we develop to lasso this unruly universe and brand every single subatomic particle with the name of a lab or famous scientist, nature seems to buck and romp, provoking and beckoning us further. Sometimes we’re lucky and get it right on the first try, exemplified by the discovery of the predicted Higgs boson. More often, we’re thrown off to try again, as in trying to identify dark matter. Perhaps nature remains untamed most of the time, as may be the case with our understanding of gravity.
This reality—the reality of scientific discovery—rarely makes it across the news. A romantic fiction does, however. And as inspiring as it may be, it is a fiction. Such a fiction is the ever-so-frequently “discovered,” “seen,” or “found” dark matter. But, yes. Dark matter exists. It was discovered a long time ago—but no, we don’t know what it is, but we do know a little bit about what it can’t be. Hopefully someday soon we’ll know what it is, but no promises.