Physicists Recreate Cosmic Ray Collisions in the Lab to Decode Air Showers from Deep Space

Every second of every day, the Earth’s atmosphere is bombarded by particles arriving from far beyond the Solar System. Some of them originate in the Sun and arrive with relatively modest energies, producing the familiar glow of the Northern Lights when they strike the gases of the upper atmosphere. Others are vastly more violent visitors: high-energy cosmic rays, packing more than one million times the energy of the solar particles, that are thought to be flung across the galaxy by exploding stars known as supernovae. When these energetic interlopers slam into atmospheric gases, they trigger cascades of secondary particles—so-called air showers—that rain down over the planet. For decades, scientists have had to rely almost entirely on computer simulations to understand what happens in those collisions, and those simulations have produced varied and sometimes contradictory answers. A new experiment has now replaced simulation with direct measurement, delivering the first laboratory-controlled portraits of these cosmic rainstorms.

The research, carried out by an international team of physicists and published in the journal Physical Review Letters, took an unprecedented approach to a long-standing problem. Rather than modeling how high-energy cosmic rays interact with the Earth’s atmosphere, the team recreated the process directly using CERN’s Large Hadron Collider, the world’s most powerful particle accelerator, located outside Geneva. The work was led by Jesse Liu, an assistant professor of physics at New York University, and included Cigdem Issever, a physicist at Humboldt-University of Berlin, along with colleagues from the DESY Research Center in Hamburg. All of the researchers are part of CERN’s ATLAS collaboration, the enormous international experiment devoted to exploring the fundamental nature of matter and the basic forces that shape the universe. Their goal was deceptively simple to state and extraordinarily difficult to achieve: make protons collide with oxygen under conditions that faithfully mimic a cosmic ray striking the sky, and then measure the debris with precision no simulation can match.

The choice of oxygen was not arbitrary. Oxygen is a foundational component of the Earth’s atmosphere, so any realistic account of how cosmic rays generate air showers must describe in detail what happens when a high-energy proton strikes an oxygen nucleus. In a landmark first for the Large Hadron Collider, the accelerator team introduced beams of oxygen nuclei into the machine for the first time and steered them into collisions with proton beams generated by the LHC itself. Those protons stand in for the cosmic rays that emanate from supernovae and other exotic astrophysical sources, arriving at the top of the atmosphere with energies far beyond anything the Sun can produce. By colliding protons and oxygen at an energy of 9.62 teraelectronvolts per nucleon pair, the experiment reproduced the collision conditions that occur when the most energetic cosmic rays plunge into the atmosphere, bringing a process that normally unfolds kilometers above our heads into a controlled laboratory setting.

Capturing what happens in those collisions demanded instrumentation of extraordinary speed and sensitivity. The collisions in the LHC occur up to 40 million times per second, and each one lasts a vanishingly small fraction of a moment before the resulting particles fly apart. To record the outcomes, the researchers used a sophisticated 100-million-pixel camera that is part of the ATLAS detector system. This device took millions of closeup photographs of the collision aftermath, registering the number of particles produced, their energies, and the angles at which they emerged from the proton-oxygen impacts. Those three quantities—the multiplicity, the energy spectrum, and the angular distribution of the secondary particles—are precisely the parameters that atmospheric air-shower models must get right. With millions of measured events in hand, the team assembled precise portraits of the birth of cosmic rainstorms, effectively turning the LHC into a machine for studying the sky rather than only the deepest structure of matter.

The measurements revealed something that the scientific community had long suspected but never before confirmed with direct data: the computer models that researchers have used for years to describe these interactions are, in many respects, significantly wrong. Issever emphasized the unusual value of the result, noting that the published data are unique and that the measurements were able to show that previous models for this type of interaction are actually very inaccurate. That finding matters far beyond the technical details of particle physics. Air-shower simulations underpin a wide range of astrophysical research, from interpreting the signals recorded by ground-based cosmic-ray observatories to estimating the energies and compositions of the original particles that struck the atmosphere. If the underlying collision physics is modeled inaccurately, every downstream inference inherits that error. By replacing modeled assumptions with measured reality, the new results give the entire field a firmer empirical foundation.

The significance of the work becomes clearer when one considers how different high-energy cosmic rays are from their solar counterparts. The particles streaming from the Sun carry energies low enough that their interactions with atmospheric gases, while still not fully understood, can be studied with a variety of direct and indirect methods, and their most famous consequence is the aurora that illuminates the Northern Hemisphere for millions of observers. High-energy cosmic rays, by contrast, are so energetic and so rare that no detector can simply sit and watch one collide with the atmosphere in a controlled way. Their collisions happen at altitudes and energies that are effectively inaccessible, which is why simulations became the standard tool. Those simulations extrapolate from particle-physics measurements made in other contexts, and different extrapolation schemes have yielded conflicting predictions about how many secondary particles an air shower should produce and how they should be distributed. The LHC experiment cuts through that uncertainty by measuring the relevant collisions directly.

Liu framed the achievement in terms of the bigger picture of cosmic understanding. We now have a better understanding of how particles from objects more exotic than the Sun interact with our atmosphere, Liu said, adding that by advancing knowledge of high-energy cosmic collisions in the Earth’s atmosphere, the team has taken another step toward unraveling the many mysteries of the universe. The phrase particles from objects more exotic than the Sun captures the astrophysical stakes of the measurement. Supernovae, and possibly other extreme cosmic accelerators, are capable of boosting protons and nuclei to energies millions of times higher than anything the Sun emits, and tracing those particles back to their sources is one of the great open challenges of high-energy astrophysics. Every improvement in the description of how they interact with air brings scientists closer to reading the sky as a record of the galaxy’s most violent events.

The technical details of the measurement, published under the title describing charged-particle production in proton-oxygen collisions at 9.62 teraelectronvolts as a probe of cosmic-ray air showers with the ATLAS detector, will now feed directly into the models used by cosmic-ray researchers worldwide. Air-shower simulations incorporate particle-production data as their physical input, so the new measurements can be inserted where previously only extrapolations existed. The effect is expected to sharpen predictions for the giant detector arrays that monitor extensive air showers across hundreds or thousands of square kilometers of the Earth’s surface, and to reduce the systematic uncertainties that have historically separated different experiments’ estimates of cosmic-ray composition and energy. In this sense, a particle-physics experiment designed to probe the fundamental nature of matter has delivered a gift to astrophysics, demonstrating once again how the boundaries between these disciplines blur at the highest energies.

The experiment also marks a milestone for the Large Hadron Collider itself. Oxygen beams had never before been circulated in the machine, and producing them required the accelerator complex to ionize, accelerate, and handle a new species of particle alongside its routine proton operations. That capability opens the door to future studies of other atmospheric and astrophysically relevant nuclei, potentially extending the laboratory reconstruction of cosmic collisions to an even broader range of elements that make up the air and the cosmic rays that strike it. For now, the team’s results stand as the world’s first measurements of cosmic rainstorms recreated in controlled laboratory conditions, a demonstration that some of the most energetic processes in the universe can be brought down to Earth, photographed millions of times over, and finally understood not through approximation but through direct observation. These new results, Liu concluded, significantly sharpen our knowledge of these subatomic interactions and will help scientists further explore the nature of cosmic particles raining from the sky.

Subject of Research: Laboratory recreation of high-energy cosmic ray collisions with atmospheric oxygen using proton-oxygen collisions at the Large Hadron Collider to improve air shower models.

Article Title: Physicists recreate particles from outer space to advance understanding of the universe’s mysteries

Article References: Physicists recreate particles from outer space to advance understanding of the universe’s mysteries. (n.d.). Original publication

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Keywords: cosmic rays, Large Hadron Collider, ATLAS, air showers, supernovae, oxygen beams, particle physics, CERN, Physical Review Letters, aurora, cosmic-ray simulations, astroparticle physics