Cosmic ray

A cosmic ray is a high-energy particle originating from deep space, meaning outside the Solar System. Cosmic ray energies vary massively, from experimentally accessible ranges ($10^{9}$ eV) to practically unreachable extremes ($>10^{18}$ eV). This makes them wonderful tools to experiment with high-energy physics that is otherwise impossible to achieve with technology, at the cost of having zero control over what we're working with.

Cosmic ray radiation is conventionally divided in two kinds depending on where in the atmosphere they are:

1. Primary cosmic rays are particles that impact the outer layer of the atmosphere from space. 90% of these are protons and the remaining 10% is helium-3 and helium-4. The cross section of these rays is rather high and as such they tend to interact with the atoms in the atmosphere quite frequently. Their scattering and subsequent decay induce a varied cascade of additional particles deeper in the atmosphere, which we know as...
2. Secondary cosmic rays are particles generated by the scattering and decay of primary rays. These penetrate deeper in the atmosphere and are themselves divided in two types. Soft rays make up about 30% of the radiation and are electrons and photons that readily interact with the atmospheric atoms. Hard rays make up the remaining 70% and are made up of a large selection of other, more exotic particles, like muons and pions. Muons are the only ones that make it to the surface of the Earth. Hard secondary cosmic rays are therefore the most scientifically interesting.

Origin#

Cosmic rays originate from outer space. Due to the great number of things in space, there are numerous places and events where a cosmic ray might originate. Galactic rays permeate the interstellar medium and mostly come from supernovae within our Galaxy. Extragalactic rays, of much energies, come from beyond our Galaxy and are generated by exceptionally high-energy events: black hole formations, neutron star mergers, black hole mergers, Quasar jets and other causes of Gamma Ray Bursts.

Measurements#

Cosmic rays have been measured with many different detectors throughout the last century. A historical device (1930s) is the cloud chamber, a sealed chamber filled with vapor (typically a mixture of alcohol and water) and equipped with a rapid piston. When the piston retracts, the volume expands adiabatically, lowering the temperature and creating a supersaturated vapor. Charged cosmic rays passing through the chamber ionize the gas, causing droplets to condense alongside its trajectory. These droplets form visible tracks, allowing observation of the particle's path.

Discoveries#

Several discoveries can be traced back to experiments done on cosmic rays. The positron, muon, pion and lambda particles where all discovered through observation of cosmic rays.

The muon#

The study of cosmic rays was, at the time, motivated in large part by the Yukawa hypothesis and the search for the pion. Yukawa had claimed a mass range of 10 to 200 MeV for the pion and cloud chamber detection of cosmic rays often returned particles fit this range just right. The mass was measured to be about 105 MeV and the mean lifetime was around $2.2\times 10^{-6}\text{ s}$; the particle was named muon (symbol: $\mu$). The question then was: is the muon really the pion we're looking?

The muon came in both positively- and negatively-charged varieties, like the pion was expected (no evidence of a neutral muon, however). To discriminate between the two, Japanese physicists Tomonaga and Araki predicted major differences in interactions within matter for the two particles. Negative pions ($\pi^{-}$) do not decay, rather being progressively slowed down, stopped and ultimately captured into orbits by atoms in the medium in a process known as "k-capture" (another name for electron capture). Positive pions ($\pi^{+}$) would instead decay when stopped. The next step was to conduct more observation on muons and see if their behavior in matter matched these expectations: if it did, realistically muons really were pions and the difference in names was entirely unwarranted.

The experiment that signed the deal was the Conversi-Pancini-Piccioni experiment, conducted in Rome in 1945 by the namesake Italian physicists. Actually I lied: the experiment was conducted twice, specifically with marginally different equipment in order to rule out unexpected behavior. The apparatus went as follows.

A magnetic field was generated using magnetized iron to curve the particles, which passed through an absorber. The particles then hit a 1 cm lead slab and continued downwards. Particle detectors were placed in $A$, $B$, $C$ and $D$ locations. Direction of arrival is measured through the order in which detectors fire. $A$ and $B$ are expected to fire essentially simultaneously, but the presence of the absorber makes $C$ and especially $D$ fire late. More importantly, the particle needs to even get to $C$ or $D$: the point of the absorber is to, well, absorb the incoming particle. The main discriminant is seeing $C$ fire at all. If the particles are absorbed, the only way for it to fire is for the absorbed particle to decay on absorption, as assumed for $\pi^{+}$. If not, as expected for $\pi^{-}$, $C$ wouldn't fire.

As mentioned before, the experiment was ran twice: once with an iron ($Z=26$) absorber and once with a carbon ($Z=6$) absorber. The results were interesting:

1. With iron, only positive particles make $C$ fire. This is just what was expected for $\pi=\mu$.
2. With carbon, negative particles also make $C$ fire, meaning they also decay, which means that $\mu$ cannot be $\pi$.

How's this possible? In iron, the disappearance of the negative particles is explained by the aforementioned "k-capture", which goes like $\mu^{-}+p\to n+X$, where $X$ is some unknown particle. This is basically electron capture, but with a muon, hence the name reuse. The Probability of this occurring goes like $\propto Z^{4}$, so it makes sense that we'd see it iron ($26^{4}$) and not in carbon ($6^{4}$). In other terms, heavy atoms absorb muons, light ones don't. This can't work for pions, which had not been expected to have such behavior, so the $\mu=\pi$ hypothesis had to be discarded.

Physicists then found themselves in a weird spot: they started looking for a pion that worked with strong interaction and ended up with a muon that worked with weak interaction and a mass of 105 MeV, the same range as the other one. Needless to say, the experiment raised as many questions as it answered.