Four ways for cosmic rays

The Alpha Magnetic Spectrometer (AMS) collaboration has now analysed the spectra of 20 cosmic-ray elements. These new results, published in Physical Review Letters, sort the cosmic radiation into four classes, two primary and two secondary. None behave quite as current models predict.

AMS has been taking data aboard the International Space Station (ISS) since May 2011, when it arrived on the space shuttle Endeavour (CERN Courier July/August 2011 p19). Since then, it has detected more than 260 billion events. Cosmic rays range in energy from about 1 GeV to as much as 10⁸ TeV, far beyond the reach of any collider, and analysing the elements found amongst them provides a direct way to study their origin and journey through space.

Cosmic rays are traditionally divided into primary and secondary components. Primary cosmic rays originate in astrophysical sources, producing secondary rays when they collide with matter in the interstellar medium. The two can be told apart by their energy spectra: although both fluxes decrease with energy, that of secondary rays falls much more steeply.

Precision improvement

The first major results from AMS, in 2013, showed the fraction of positrons among cosmic rays rising with energy rather than falling, as it would if they came only from secondary collisions (CERN Courier October 2013 p23). The picture has grown more complex ever since. “AMS provides an orders-of-magnitude improvement in precision over previous detectors,” explains spokesperson Samuel Ting (see “You have to go forward”). “And most of its results disagree with current theory, necessitating the development of new models of cosmic rays.”

Earlier AMS surveys of 15 elements had revealed that primary cosmic rays can be divided into two distinct classes, according to how their flux varies with rigidity – the momentum of a charged particle divided by its electric charge, which determines how strongly it is deflected by magnetic fields.

The new analysis of phosphorus, chlorine, argon, potassium and calcium draws on about one million events collected over 13.5 years. The fluxes reveal that secondary cosmic rays can also be divided into two classes based on their rigidity dependence. Across the 20 elements whose spectra AMS has measured so far, out of the 28 it has identified, the fluxes therefore fall into four classes. For the five new elements, the analysis also determines the primary and secondary contributions to each flux.

The upgrade will allow for the collection of as much data in five years as was recorded in the previous 15

“When measurements are inaccurate, even the most simplified theoretical models can adequately account for the data,” says Sunil Gupta, an astroparticle physicist and honorary fellow of the Tata Institute of Fundamental Research and president-designate of the International Union of Pure and Applied Physics. “But those explanations fail when precision improves.”

The even-numbered elements argon and calcium prove more abundant at the cosmic-ray source than their odd-numbered neighbours phosphorus, chlorine and potassium. Possible explanations for this, and for AMS’s other unexpected results, remain under debate. Gupta cautions that further analysis, and more refined propagation models, will be needed, but says the new data “offer extremely significant constraints on the theoretical models essential for progress in this field.”

Understanding what cosmic rays are made of and how they travel is also the first step in the search for new physics in space, since they form the background against which any dark-matter signal or trace of primordial antimatter would have to stand out. AMS is set to sharpen its measurements with an additional silicon tracker layer, due to be installed in 2027. “The upgrade will increase the acceptance of AMS by 300%,” says Ting, “allowing for the collection of as much data in five years as was recorded in the previous 15. This will greatly extend the capabilities of AMS in the study of dark matter, antimatter and high-Z cosmic rays.”