November 11, 2019 - by Santina Russo
The universe is estimated to be around 13,8 billion years old, but still today, scientists can learn a lot about its beginnings from the cosmic radiation measured on Earth and in the orbit around our planet. Especially high-energy cosmic ray particles can reveal information about the universe’s physical laws, since they originate from the most energetic processes in the universe; these include the remnants of supernova explosions, which generate all matter. That’s why a number of experiments on the ground as well as in the Earth’s orbit are measuring the composition, direction and especially the energy of cosmic particles. About 89 percent of these particles are protons, while some 9 percent are helium nuclei, and the rest are heavier atomic nuclei and electrons.
However, it is difficult to accurately measure the fractions of highest-energy particles that carry information about our own galaxy and partially even from beyond it — meaning particles with an energy in the tera electron Volt (TeV=1012 eV) magnitude and higher. Firstly, they are much rarer than lower-energy particles, since the flux of cosmic rays decreases with increasing energy. Secondly, direct detection of those particles in space, such as on detectors at the international space station (ISS) or on satellites, has been limited due to constraints in weight and size of the detectors.
In addition, scientists have to rely on detector simulations to understand the data. Such simulations calculate the decomposition pattern and spectra of pure particle samples colliding with the detector, thereby allowing differentiation between types of particles — like electrons, protons or gamma rays — and making sense of the observed spectra in the first place. “If you do not have these simulations, you are basically blind”, explains Andrii Tykhonov, an astrophysicist at the University of Geneva, Switzerland. Plus, the more precise the simulations, the more precise the measurements.
However, the software packages for simulating detectors available to date could not handle particles above 10 TeV with reliable accuracy. This is partly because until now, devices collecting data on high-energy particles have almost exclusively been ground-based telescopes, which only measure the secondary radiation emitted by the particles upon entering the Earth’s atmosphere. That’s why Tykhonov and his colleagues from the research group of Xin Wu at the University of Geneva developed new software that enables scientists to directly and precisely measure high-energy cosmic ray protons at and above 100 TeV.
Looking for high-energy particles in space
The promising new software will be used to interpret data collected by the Dark Matter Particle Explorer (DAMPE) satellite. Since its launch in December 2015, DAMPE has been orbiting around Earth measuring cosmic radiation. It carries the thickest and most finely-segmented calorimeter ever used in space. This means that it is able to measure high-energy particles with a much better energy resolution and energy reach than any other existing space experiment. Its main focus is to look for proof for the existence of dark matter by accurately measuring the spectrum of cosmic ray electrons. Beyond that, it is also well-suited for directly measuring high-energy protons, which can reveal information about supernova explosions — such as the shock waves the particles lived through during the explosions, the composition of interstellar medium, or what interstellar processes took place in their surroundings.
Smartly combining two approaches
But: “Protons are much harder to measure than electrons”, says Tykhonov. Upon colliding with the detector, protons decompose into an avalanche of interactions, creating subatomic particles — mostly pions — of variable energy. In the process, contrary to the detection of electrons, only a fraction (30-40 percent) of the particle’s original energy is deposed in the detector and therefore measurable. “All the more important are accurate simulations of these hadronic interactions”, says Tykhonov.
To create a software capable of handling high-energy proton simulations, he and his colleagues combined the algorithms of two existing software packages, CRMC and Geant4. Specifically, they implemented so-called event generators from the CRMC package — used in particular in the popular software CORSIKA for interpreting ground experiments — into the widely-used Geant4 detector simulation tool kit. “The event generators used in CRMC are able to handle high-energy particle interactions,” Tykhonov explains, “while Geant4 offers a straightforward machinery for implementing precise detector geometry."
He and his colleagues first needed to understand how these software packages work, recounts the astrophysicist, which was not trivial. They proceeded to accommodate the physics models for their needs using the codes of 4 different event generators. Those were then connected to Geant4 via a new dedicated software interface. Depending on the interaction energy, the new interface either chooses an internal model of Geant4 or one of the new CRMC event generators to sample the particle’s scattering. For feasibility tests and the extensive validation of the software, the astrophysicists made extensive use of the “Piz Daint” supercomputer. In addition, they performed a preliminary simulation of ultra-high energy proton data from DAMPE in the range of 100 TeV to 1 peta electron Volt 1 (PeV=1015 eV).
Available to the public
For now, the interface is available to a limited part of the astrophysics community as an alpha version. Next, Tykhonov and his colleagues plan to further investigate their interface with data from DAMPE in cooperation with theory experts from both Geant4 and CRMC. In particular, they intend to study hadronic simulations up to PeV energies using simplified detector geometries. After this, a beta version of the software is planned to be released to the community.