LHCb brings cosmic collisions down to Earth

13 April 2017

In an effort to improve our understanding of cosmic rays, the LHCb collaboration has generated high-energy collisions between protons and helium nuclei similar to those that take place when cosmic rays strike the interstellar medium. Such collisions are expected to produce a certain number of antiprotons, and are currently one of the possible explanations for the small fraction of antiprotons (about one per 10,000 protons) observed in cosmic rays outside of the Earth᾿s atmosphere. By measuring the antimatter component of cosmic rays, we can potentially unveil new high-energy phenomena, notably a possible contribution from the annihilation or decay of dark-matter particles.

In the last few years, space-borne detectors devoted to the study of cosmic rays have dramatically improved our knowledge of the antimatter component. Data from the Alpha Magnetic Spectrometer (AMS-02), which is attached to the International Space Station and operated from a control centre at CERN, published last year are currently the most precise and provide the antiproton over proton fraction up to an antiproton energy of 350 GeV (CERN Courier December 2016 p26). The interpretation of these data is currently limited by poor knowledge of the antiproton production cross-sections, however, and no data are available so far on antiproton production in proton–helium collisions.

LHCb physicists were able to mimic cosmic collisions between 6.5 TeV protons and at-rest helium nuclei

The LHCb’s recently installed internal gas target “SMOG” (System for Measuring Overlap with Gas) provides the unique possibility to study fixed-target proton collisions at the unprecedented energy offered by the LHC, with the forward geometry of the LHCb detector well suited for this configuration. The SMOG device allows a tiny amount of a noble gas to be injected inside the LHC beam pipe near the LHCb vertex detector region. The gas pressure is less than a billionth of atmospheric pressure so as not to perturb LHC operations, but this is sufficient to observe hundreds of millions of beam–gas collisions per hour. By operating SMOG with helium, LHCb physicists were able to mimic cosmic collisions between 6.5 TeV protons and at-rest helium nuclei – a configuration that closely matches the energy scale of the antiproton production observed by space-borne experiments. Data-taking was carried out during May 2016 and lasted just a few hours.

LHCb’s advanced particle-identification capabilities were used to determine the yields of antiprotons, among other charged particles, in the momentum range 12–110 GeV. A novel method has been developed to precisely determine the amount of gas in the target: events are counted where a single electron elastically scattered off the beam is projected inside the detector acceptance. Owing to their distinct signature, these events could be isolated from the much more abundant interactions with the helium nuclei. The cross-section for proton–electron elastic scattering is very well known and allows the density of atomic electrons to be computed.

The result for the antiproton production has been compared to the most popular cosmic-ray models describing soft hadronic collisions, revealing significant disagreements with their predictions. The accuracy of the LHCb measurement is below 10% for most of the accessible phase space, and is expected to contribute to the continuous progress in turning high-energy astroparticle physics into a high-precision science.

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