Investigating Cosmic Ray Cascades at the Large Hadron Collider

Researchers at the LHC are investigating airshowers — cascades of particles created by collisions between cosmic rays and particles in the upper atmosphere.

Work conducted at CERN’s Large Hadron Collider (LHC) is divided into several different experiments. The smallest of these is LHC-forward (LHCf), and its mission is to investigate what happens when high-energy cosmic rays bombard Earth’s upper atmosphere at near-light speed.

When these cosmic rays —mostly comprised of protons originating from both the Sun and outside the solar system —collide with atomic nuclei in Earth’s upper atmosphere, the event creates a cascade of particles called an airshower which then rains down onto the surface of the planet. This cascade of secondary particles is similar in many ways to debris produced by particle collisions at the LHC.

One of the LHCf experiment’s two detectors, LHCf Arm2, seen here during installation into a particle absorber that surrounds the LHC’s beam pipe. (Image: Lorenzo Bonechi)

When the next run of the LHC begins in 2021, the LHCf will continue to investigate the interactions in the upper-atmosphere that trigger airshowers. The aim of this is to determine which models of cosmic ray and atomic nuclei interaction best fit observations. These models are used in computer simulations that recreate the properties of cosmic rays, so finding the models which best describe such events is a vital step in understanding cosmic rays in general.

“The idea behind the LHCf experiment is to help increase our learning about the nature of high-energy cosmic rays, by measuring and interpreting the properties of the secondary particles released when these cosmic rays collide with the Earth’s atmosphere,” says Lorenzo Bonechi, who leads a team for the LHCf collaboration in Florence, Italy, discussing the experiment in 2016.

The LHCf experiment comprises two detectors, located 280 metres apart on either side of the ALTAS collision point. These detectors measure the secondary particles which fly forward at small angles from proton-proton collisions. This is in contrast to the other LHC experiments — such as CMS, ALICE and LCHb — which can measure particles emitted at almost any angle.

It would seem that one of these aforementioned experiments, with their increase measurement versatility, would be a better choice for making such observations, but LCHf’s focus on forward-travelling particles confers a particular advantage.

Such particles carry away a large portion of the energy generated in such events, as well as barely changing their trajectories. This makes them ideal for understanding the development of showers of particles produced when high-energy cosmic rays strike the atmosphere. The amount of particle ‘debris’ thrown forward by these collisions and the energy they carry can be compared to existing models describing such interactions in the upper atmosphere to select which ones best describe such events.

LHCf is the smallest of the six official LHC experiments. Each of the two detectors weighs only 40 kilograms and measures 30 cm long by 60 cm high and 10 cm wide. (Image: Lorenzo Bonechi/ CERN)

“Over previous runs, we’ve found significant discrepancies between our data and the most advanced hadronic interaction models, which are used to model how cosmic rays shower down onto the earth when they interact with our atmosphere,” Bonechi says. “LHCf is trying to find evidence that could help prove which of these models provide the most reliable description.

In the past, LCHf has only been able to run when the LHC is operating at low-luminosity — meaning that the number of collisions is at a minimum. This is because an excess of forward-projected, high-energy particles could cause damage to the detectors by over-heating them. But, with new upgrades, the LHCf is preparing to probe these interactions at higher collision energies than before in the next run of operations, kicking-off in 2021. Also, as a result of these ongoing improvements, the experiment should also increase the number and type of particles that can be detected and studied.

Researchers working with the LCHf also plan to measure forward-flung particles emitted from collisions of protons with light ions — most likely oxygen ions — in addition to those created in proton-proton collisions. This is a vital step in understanding cosmic ray collisions in the upper atmosphere as the first interactions that trigger extensive air showers involve mainly light atomic nuclei such as oxygen and nitrogen.

This means that during the next run of the LHC, the LCHf looks likely to cast new light on cosmic-ray interaction models at high energies.

Bonechi concludes: “Now, scientists working in this field are making an effort to integrate our results into their models, and we might see a revolution in them in the near future.”