Cosmic rays are energetic particles, mostly atomic nuclei, raining down upon the Earth from the depths of the cosmos. Understanding their detailed nature and origins remains a primary goal in modern-day astroparticle physics.
When cosmic rays enter the Earth’s atmosphere they produce a shower of billions of particles. These particles travel nearly at the speed of light and a large part of them will reach the ground and can be detected by the water-Cherenkov stations of the Pierre Auger Observatory. The most energetic cosmic rays are able to produce particle showers which have a footprint at the ground of a few km2. Among the produced particles there are muons, elementary particles similar to electrons but with a much greater mass. The paper shows a novel technique to estimate where in the atmosphere the muons that we measure at Earth are produced. As the muon signal is a measure for the nature of the primary cosmic ray, this technique may help solve one of the most persisting questions surrounding ultra-high energy cosmic rays: What are they?
Identifying the muons using the surface detector of the Pierre Auger Observatory is not an easy task. However, each type of particle (muon, electron, photon) produces a signal with a characteristic amplitude and time structure. Muonic signals are spiky and have a narrow time distribution (tens to hundreds of nanoseconds) while signals produced by electrons and photons are small, smoother-looking and characterised by a wide time distribution (microseconds). This is especially true for stations far from the impact point of the shower at the ground.
By applying a “low-pass” filter to the signal repeatedly, it is possible to gradually separate the low-frequency smooth electromagnetic signal from the high-frequency component which is primarily due to muons. The technique is effective over a large range of arrival directions (i.e. for zenith angles between 45° and 65°), and for energies greater than 1.5 x 1019 eV. Once the muon signal is estimated station by station, together with its time structure, the atmospheric depth at which muons had been produced is obtained by applying a model of their arrival time at the ground.
The geometry used to reconstruct the muon production point is depicted in the figure on the left. The model is based on the fact that muons are produced close to the shower axis and that they travel to the ground following straight lines. For each muon sampled at the ground, its atmospheric production depth is estimated: the set of these forms the Muon Production Depth (MPD) distribution, as shown in the figure on the right.
Left: Schematic view of the geometry used to obtained the muon production point. Muons are produced at the position z along the shower axis and, after traveling a distance l, they reach the ground and may hit a station of the surface detector. Right: The reconstructed MPD distribution for a imulated shower induced by a proton with θ=48° and E = 6.3 x 1019 eV.
The maximum of the distribution, which is called Xμmax , is the point at which the maximum number of muons is produced, which is a function of the mass of the cosmic ray. Heavy primaries induce showers which reach maximum production higher in the atmosphere compared to light primaries.This method can thus be exploited to study the mass composition of the most energetic cosmic rays detected by the Pierre Auger Observatory.
In addition, Xμmax depends sensitively on the properties of the hadronic particle interactions taking place in the atmosphere. Its measure is a nearly optimal tool to test hadronic interaction models at energies well above those attainable with accelerators such as the Large Hadron Collider (LHC) at CERN.
This novel technique is described in detail in the paper below. Auger results on the mass composition of the highest energy cosmic rays will follow in future publications.
Related paper: Measurement of the Muon Production Depths at the Pierre Auger Observatory,
Laura Collica for the Pierre Auger Collaboration: Eur. Phys. J. Plus (2016) 131: 301,