Searching for the Dark Sector in the High Desert
There is a long corrugated steel building, its facade a sun-faded pink, on a protruding fingerlike mesa of the Jemez mountains in the high…
This article was originally published in July 2022 issue of The Minimum Wager
There is a long corrugated steel building, its facade a sun-faded pink, on a protruding fingerlike mesa of the Jemez mountains in the high desert of northern New Mexico. Situated among the juniper and pinion pine, this building contains the Clinton P. Anderson accelerator facility at Los Alamos National Labs. The building houses over half a mile of vacuum tube studded with bronzed electromagnets. It accelerates protons to nearly 90% the speed of light before smashing them into a spindle of tungsten. The energy of the collision creates waves of exotic particles — gamma rays, neutrons, mesons and neutrinos. The conditions within these highly energetic showers of particles could create a portal into a ghostly world of undiscovered particles known as the dark sector. 23 meters away from the tungsten target there is a squat metal cylinder, connected to an intricate mass of black cables and silver cryogenic devices — vessels, tubes, wires, computers and gauges. This is the Coherent Captain Mills detector searching for evidence of the dark sector which promises to explain the nature of dark matter — the elusive substance which makes up over 80% of our universe.
Particle physics detectors are among the most complicated and sensitive instruments ever built. They peer into the smallest things. Theirs is a world on a scale wholly alien from our own. One cannot help but see them as impossibly complex living things. The Coherent Captain Mills detector (CCM) has at its heart a 10-ton cryostat — a vacuum insulated aluminum cylinder not unlike an enormous thermos bottle. It is 6 feet in diameter and 10 feet high. The vessel is filled within the liquid argon, cooled down to below a bone-chilling -300 F. The walls, floor and ceiling of this vessel are lined with photomultiplier tubes, light sensitive vacuum tubes that act as the detector’s eyes. 200 eyes the size of dinner plates stare unblinkingly into the dark liquid argon interior. Each eye is sensitive enough to detect to a single photon. What would appear to any human as total and utter darkness is filled with light to CCM.
This missing 80% is one of the major unsolved problems in physics. Of it nothing can be said with certainty. While some physicists still dispute its very existence, most believe this missing 80% to be some sort of real substance. It interacts gravitationally but not electromagnetically. While we call it dark matter, it is completely transparent to light and should be called something like ‘transparent matter.’ We do not know if dark matter consists of something already known to physics or of something yet to be discovered. We do not know if dark matter is a single particle or many different particles. It is not known how, if at all , dark matter interacts with ordinary matter apart from gravitationally.
However, we do know a great deal about dark matter in other respects. We know dark matter is all around us. We have mapped its location precisely throughout the observable universe. We have precisely measured its density. Over the course of your lifetime approximately a milligram of dark matter will pass through your body. Dark matter abounds even on earth, but direct observation of it has eluded physicists for nearly six decades. Scientists have one by one eliminated many proposed theories of dark matter.
One particularly tantalizing possibility which has survived scrutiny so far posits that dark matter is not a single particle but a whole constellation of particles and forces which interact strongly with each-other but weakly with ordinary matter. These so-called dark sector models imagine a ghostly copy of our world occupying the same physical space but almost completely apart from it. These so-called dark sector models elegantly explain a whole series of discrepancies between theory and empirical results.
How did our conception of dark matter come about? It was first postulated by the giant of 19th century physics, Lord Kelvin. By examining the velocity of stars within our galaxy, Kelvin estimated the galaxy’s total mass to be approximately 10 billion solar masses. He simultaneously estimated that the galaxy contained approximately 100 million stars, which accounted for only 10% of the total mass of the galaxy. Kelvin concluded from this discrepancy that “many of our stars, perhaps the great majority of them, may be dark bodies”. Henri Poincare, in a response to Kelvin, coined the term “dark matter” or matière obscure.
Although many of Kelvin’s estimates were subsequently proven incorrect, the idea that the majority of matter may be non-luminous was later put forward again by the Swiss physicist Fritz Zwicky. In a 1933 paper, Zwicky examined Edwin Hubble’s observation of the Coma galaxy cluster and concluded that it was rotating too quickly to gravitationally hold itself together. He argued that the majority of the cluster’s mass must consist of non-luminous matter. Zwicky’s results were provisional and not widely accepted.
Vera Rubin in her 1974 observations of the Andromeda galaxy finally brought widespread acceptance to the theory of dark matter. Vera Rubin and her collaborator, Ken Ford, observed the rotation of the Andromeda Galaxy (M31). In our solar system, the speed of rotation decreases as the radius increases. Observations of the distribution of luminous matter within a galaxy suggest that the structure of a galaxy resembles the structure of our solar system with the majority of its mass being concentrated at the center. Just as planets further from the sun move more slowly in their orbits, Rubin and Ford expected that stars further from Andromeda’s visual center would also orbit more slowly. The data they collected painted a jarringly different picture: stars in the Andromeda galaxy rotated at the same speed regardless of their distance from the center. In order to account for this result, Rubin and Ford showed that the mass of the galaxy must be evenly distributed, a sort of glue holding the galaxy together as it spins. This matter is non-luminous and makes up most of the mass of the galaxy. Further observations of more galaxies confirmed this result and led to the shocking discovery that the matter of the stars, planets, and even our own bodies accounts for only a sliver of the material universe.
Many independent observations in the years since Rubin’s discovery point to the same bizarre conclusion. Like any massive object, dark matter warps space-time in its vicinity, bending light which passes through it in a phenomenon called gravitational lensing. This gravitational lensing has allowed the Hubble telescope to observe the presence of dark matter and to map its distribution. In the Bullet Cluster, a cluster of galaxies formed from the remnants of two colliding galaxy clusters, lensing studies show that ordinary baryonic matter was slowed and stretched as the gasses in each galaxy collided with each other, but the ghostly dark matter in each galaxy sailed through the collisions without interacting with ordinary matter or other dark matter. From these observations of the Bullet Cluster, astronomers conclude that if dark matter interacts with itself or with ordinary matter, it does so only minimally.
The dark sector models under study at the Los Alamos National Lab have two components. First, there must be one or more particles in the dark sector which do not interact with ordinary matter at all. Second, there must be a portal. A portal is a particle which interacts both with the dark sector and with ordinary matter. One of the simplest models, the vector portal model, involves two particles: the dark matter itself, and another particle called a dark photon. A special type of symmetry known as gauge symmetry beautifully underpins the fundamental interactions between particles. Each force carrying particle of ordinary matter, known as a gauge boson, is associated with an underlying gauge symmetry which specifies many of its properties and interactions. The symmetry group of an ordinary photon is called Unitary group of order I, or U(I) gauge group. The dark photon shares the same U(I) symmetry with ordinary photons in the Standard Model. Thus, the photon and the dark photon have similar physical properties and can interact in a process called kinetic mixing. This interaction gives the vector portal model its portal: dark matter interacts with dark photons which then mix with standard model photons.
While the unmistakable fingerprint of dark matter is clear in the astrophysical evidence cited above, the true test is direct detection, i.e. to build a detector here on earth which can detect the particles of dark matter themselves. One strategy has been to try to catch particles of dark matter afloat in the galactic halo as the earth moves through it. Such experiments are very sensitive and often located underground at the bottom of decommissioned mines or buried under mountains. But this strategy suffers from two major disadvantages: 1) a dark matter particle could pass through the detector at any time, so physicists must constantly collect data, which in turn increases the chances of stray cosmic rays entering the detector and being mistaken for dark matter, and 2) the earth is moving through the galactic halo relatively slowly at a mere 700 km/sec. This means that dark matter interactions would be at low energies and thus harder to detect.
CCM is searching for evidence of vector portal dark matter using a novel approach. Instead of trying to detect dark matter which already exists in the galactic halo, one can create dark matter at a particle accelerator. This relativistic dark matter will be traveling near the speed of light, and thus will have more energy — making it easier to detect. Additionally, the dark matter will be guaranteed to appear in the detector nanoseconds after the proton beam collides with its target, greatly reducing the chances of there being any stray cosmic rays in the detector. This eliminates the need for deep sub-terranean shielding.
As subatomic particles travel through the volume of the detector, they ricochet off argon nuclei releasing miniscule flashes of light. This light then travels and is absorbed by the photo-multiplier tubes (PMT) around the detector. Each photo-multiplier tube acts as a sort of inverse lightbulb. In a lightbulb, a current passes through the filament to generate light. In a photo-multiplier tube, light hitting the tube generates a current. Here is how it works. Photons hit the polished metal electrode known as a photo-cathode. These photons deposit their energy in a handful of electrons in the metal, knocking them out of the material and generating a tiny undetectable current. These few electrons are then multiplied through a series of electromagnets known as dynodes. Each electron slams into a dynode releasing another cascade of electrons. This avalanche of electrons is eventually amplified to a detectable pulse which can be read out and digitized by a computer. The amount of current in the pulse is proportional to the amount of light incident to the PMT. By collecting pulses from all 200 PMTs in the Los Alamos detector, one can reconstruct how much light was released by the collision. And by measuring the differences in time between PMTs one can triangulate the actual position of the initial event.
CCM is searching for several dark matter candidates, including vector portal dark matter. If vector portal dark matter exists, the process for detecting it would be as follows. When the linear accelerator collides its beam of protons with the tungsten target it releases showers of exotic particles including a particle known as a neutral pion. Pions are a type of meson, consisting of one up quark and one anti up-quark, or one down quark and one anti-down quark. Their mass is approximately 1/10 the mass of a neutron and they decay very very quickly, in one-quintillionth of a second (1 /1,000,000,000,000,000,000 seconds) . Pions can decay into two high energy photons. Very occasionally, as these photons fly away at the speed of light from the target one of the photons will kinetically mix, turning into a dark photon. This dark photon has a chance of decaying into one particle of dark matter and one anti-particle of dark matter. The dark matter particles then continue to fly at nearly the speed of light through tons of concrete and steel around the target and eventually into the volume of the CCM detector. As the dark matter travels through the liquid argon interior of the detector it can scatter off one of the argon nuclei in a process which is mediated by yet another dark photon. Through this scattering the dark matter imparts some energy to the argon nucleus which is then released in the form of ordinary light. This light is what the CCM would detect.
The interactions involved are almost impossibly weak, and the production of a single dark matter particle is very unlikely. This is why the CCM must search for dark matter over the course of five years. In that time CCM will witness sextillions of protons collide with the tungsten target. Each proton carries with it the chance of dark matter being created and detected. If the theory is correct, out of these sextillions of chances CCM expects to detect only hundreds of individual dark matter particles. These are very small odds.