As a member of the collaboration I could learn of many new developments in detail, but I cannot discuss them here as they are work in progress by my colleagues. What I can do here, however, is to describe the observatory as we would like to build it, and a few other things that have been decided and are now public.
One of these - a very important one, in fact - is the decision to build the observatory in Pampa La Bola, a flat stretch of land at high altitude (4770 meters above sea level) on the mountains due East of the city of San Pedro de Atacama, a very well-known location in northern Chile (I wrote about this site and pasted there several pictures, in a previous post here). The site is part of an area dubbed "Parque Astronomico", which includes several other scientific instruments and experiments and has been agreed by the Chilean government to be exclusively devoted to science. The most famous of the existing experiments there is ALMA.
ALMA is an array of radiotelescopes that does radio waves interferometry, with dishes that can be moved around by huge dedicated trucks, such that they can provide extremely high resolution (when the dishes are moved very far from one another) or wide field of view observations (when they are placed close together). But let me go back to SWGO...
In order to detect very-high-energy gamma rays, which is an excellent means of furthering our understanding of our galaxy as well as of a host of sources in the cosmos, SWGO intends to deploy Cherenkov detectors at high altitude. Why high altitude, and why Cherenkov detectors? Let me answer these obvious questions below.
Two basic ingredients of a ground-based array of cosmic ray detectors
1) First of all - what is the phenomenon we wish to observe? What we go after is called an extended atmospheric shower (EAS). This originates when the high-energy gamma ray (which I will sometimes address here as a photon - that is what a gamma radiation quantum is) incides on the upper atmosphere, hitting hard an atom of Oxygen or Nitrogen. This results in the gamma ray turning into an electron-positron pair (the process called "pair production").
The produced electron and positron share the initial energy of the incoming gamma ray as well as its direction, and soon meet with other atoms, where they undergo the process called "bremsstrahlung", emitting a new photon. The newly produced photon can then again do a pair production, so the process multiplies into a large shower of electrons, positrons, and photons. This shower continues to expand as the particles travel into the atmosphere, but it peters out when the energy of each particle becomes small and they get progressively absorbed in air. In order to be sensitive to the energy and direction of the primary gamma ray, an experiment needs to detect these shower particles before they extinguish, so placing the observatory at high altitude is a requirement of high importance.
2) Cherenkov light is emitted by charged particles that traverse a medium with a speed that is higher than the speed of light in the same material. This is possible because light slows down in dense media: the speed of light in a material is indeed equal to c/n, where c=299790 km/second is the speed light has in vacuum, and n is the refraction index of the material. Since water has a refractive index n=1.33 (at 20 degrees), any electrically-charged particle can emit Cherenkov light if they have a speed beta (in c units) larger than 1/n. The light is emitted in a cone around the direction of the particle, much like the sonic boom waves of a plane traveling faster than the speed of sound. With photomultiplier tubes in the water, it is easy to detect the light and infer information on the number of particles crossing the volume, and their energy.
The simplest arrangement of a Cherenkov detector is a tank filled with water, where the body of water is observed by light-sensitive detectors. However, one can imagine a different scheme whereby bags filled with water (bladders) are set below the surface of a lake. The lake option removes the need of a physical sturdy tank containing the water, but implies other logistic requirements.
3) Ok, I lied. There are three ingredients to consider. We need to not only detect and measure the gamma ray showers: we have a background to sort out. High-energy protons and light nuclei also hit our atmosphere - in fact, they do so at much higher rates than high-energy photons. How do we distinguish these from our signal? It turns out that many of the processes that take place when a proton hits our atmosphere are similar to those of gamma rays, but there is a difference.
Contrarily to what happens in a photon shower, in a proton shower many charged pions and kaons get produced (while we do not expect almost any of these in gamma-originated showers). Pions and kaons are short-lived particles, and they decay to muons while flying down; muons can insted reach the ground before decaying. So if we can distinguish muons from other particles reaching the ground, we know that the shower is probably not due to a gamma ray, but to a proton or light nucleus - so we can discard it from consideration.
Distinguishing muons from electrons and positrons hitting our detector tanks is possible because muons traverse large amounts of material without being stopped. Instead, electrons and positrons can only travel limited thickness of water. By placing two photomultiplier tubes in a tall tank filled with 3 or more meters of water, one looking up and one looking down, it can then be possible to distinguish the Cherenkov light created by particles of low penetration power (electrons and positrons) from the light generated by muons, which illuminate all the way to the bottom of the tank!
Above: a view of what SWGO could look like, and a Cherenkov tank. In the center you can also see (in red) a sketch of what a gamma-ray shower looks like.
Where to build SWGO
The decision to build SWGO on the mountains of Chile was taken by the collaboration after a careful consideration of several alternatives: in particular, a couple of sites in Peru, and one site in Argentina. In Peru there was a possibility to build the observatory in a lake at high altitude, while the Argentinian site was similar to the Chilean one in terms of the foreseen kind of array, but it was at slightly lower altitude and had some logistic issues.
Costs of construction, materials, water, as well as logistic factors and risks connected with the different options were carefully assessed before reaching a decision, which was hard to make because scientists from different countries and institutions had spent several years concentrating their studies on only one of the options - it is never easy to have to tell your colleagues that all the research work they have done does not produce a tangible result.
In particular, building an array of detectors in a lake involves solving a number of complex problems connected with the deployment, the maintenance, and the weather effects (waves are hard to cope with, e.g., and in a high-altitude lake they can easily reach 1m of height).
But the option to build an array of detector tanks in Chile also involves issues. In particular, the most troublesome one is the need to carry the water necessary to fill the detectors from far away - Pampa La Bola and the whole Parque Astronomico are situated at very high altitude in a desertic region, with no water sources nearby.
If you take the nominal number of detector units that SWGO originally considered to build (of the order of 6000), and the volume of water they need to contain (about 50 tons each, as the water needs to be deep enough to allow discrimination of the muons from the electrons, positrons and photons that are more characteristic of gamma-ray-originated showers), this boils down to 300,000 metric tons of water. It is a lot of water to carry up from 2000 to 4800m of altitude through a winding road with trucks! Unfortunately, there seem to be no viable alternative - the cost of transportation is very high, but it is fundamentally connected with the gain in potential energy that the water gets by being moved up!
The future of SWGO
Now a difficult phase opens, when the institutions that participate in the SWGO Collaboration have to convince their funding agencies of the scientific value of building a gamma-ray detector in the southern hemisphere.
The scientific case is indeed extremely strong - the LHAASO observatory built in China for the same purpose has been producing a host of groundbreaking results, and it cannot access sources located in the southern hemisphere, including the galactic center. Hence a lot of new scientific discoveries await to be made, as SWGO can be sensitive to gamma rays of energy that no other instrument can measure. If you ask me, the few tens of million $ required for the array are money very well spent! However, the funding agencies might argue against the specific choices and plan that the collaboration has converged on.
So, if you love to know more about our universe, its birth, the monsters it contains, and the possible sources of dark matter and other discoveries that await to be made, please join me in rooting for SWGO to be built soon! As Dante Alighieri wrote some 700 years ago, "Fatte non foste a viver come bruti, ma per seguir virtute e canoscenza." He did mean that humans should invest in knowledge!
Indeed, it is my belief that if an instrument like SWGO can be built, it is a moral imperative of humanity to build it. If you prefer to spend money in new weapons (the yearly US budget ranges in the 10,000 times the cost of the full SWGO array), or if you think a 0.01% of the wealth of the richest men on the planet cannot be taken away from them through higher taxes for the benefit of human knowledge, we have probably evolved to be different kinds of beings - one of us is Homo sapiens sapiens, and the other is Homo sapiens stupidus. I let you decide which is which.
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