The Large Hadron Collider (LHC) could open the way to a deeper understanding of neutrinos by measuring a hitherto unobserved process involving them, reports an international group of researchers in a paper published in the current issue of the journal Physical Review D, with scientists from the National Centre for Nuclear Research leading the study.

Neutrinos are the least interacting elementary particles discovered so far, and at the same time they currently represent the surest window into the knowledge of phenomena beyond the Standard Model of fundamental interactions. For this reason, they are the subject of numerous experimental studies. In the past few years, the Large Hadron Collider has joined these efforts. A novel research programme was initiated there as part of the FASER experimental collaboration, one of the initiators of which was dr Sebastian Trojanowski from the NCBJ's Theoretical Physics Division.

The new research programme recently made its first neutrino observations at the LHC. One of the main goals of this work is to better understand the strong interactions, one of the four fundamental forces of nature, in a situation that eludes standard theoretical calculations (known as perturbations) and requires the use of approximations with significant uncertainties. Neutrinos produced in proton collisions at the LHC can take information about these far beyond the dense and hot centre of interaction. Among other things, this will allow the collisions of cosmic rays constantly bombarding the Earth's atmosphere to be explained more accurately and limit their impact on the observation of neutrinos from outside our galaxy in the growing field of neutrino astronomy.

Neutrino collisions in the detector, where attempts are being made to observe them, are also of great interest to scientists. "Weak neutrino interactions are both their curse and their blessing. Attempting to detect them can drive us crazy, for example, we expect that only one in a billion neutrinos flying through the detector will leave any trace in it" - says dr Sebastian Trojanowski. "On the other hand, once this is achieved, neutrinos provide insights into otherwise almost inaccessible phenomena. In our latest work, we studied even rarer processes involving them. It turns out that the LHC is a powerful enough source of neutrinos to finally observe them in the future."

The process discussed in the latest paper is so-called "trident" neutrino scattering. Typically, neutrino interactions produce one charged lepton, such as an electron or a muon. This process occurs through "charged currents". It is also possible that no charged leptons are produced at all (so-called "neutral currents"). However, in a trident process, the production of as many as two such particles occurs. "The contribution to this rarefied scattering is made by charged and neutral currents at the same time, which makes it possible to analyse their interference and provides a window into the study of so-called new physics" - notes dr Toni Mäkelä, who did a postdoctoral fellowship at NCBJ while working on the project and is now continuing his research at the University of California, Irvine, USA.

Trident neutrino scattering has so far not been observed, although attempts have been made to do so. Initial observations recorded more than 20 years ago have been challenged in later years by the inclusion of new background processes that can mimic trident scattering and disrupt the measurement. In the latest paper, the researchers presented the measurement strategy for the proposed improved neutrino detector at the LHC, FASER𝜈2, and again discussed the background processes in detail. The results show that the first observation with statistical significance sufficient to announce a discovery should be possible in the coming years, if the improved detector is constructed according to current proposals.

"The author of one of the first theoretical papers speculating on the scattering of trident-type neutrinos was Professor Wiesław Czyż of the IFJ PAN and the Jagiellonian University in Kraków. This was almost exactly 60 years ago" - adds dr Trojanowski. "We have been waiting for quite a long time. We invited current industry leaders from recognised centres in Europe and the USA to collaborate on this project. Their enthusiasm reassured us that finally this measurement could be successful."

The detailed results of the work are available in the publication Discovering neutrino tridents at the Large Hadron Collider, W. Altmannshofer, T. Mäkelä, S Sarkar, S Trojanowski, K. Xie, and B. Zhou, Phys. Rev. D 110, 072018, DOI: https://doi.org/10.1103/PhysRevD.110.072018

The research described in this article is co-financed by the grant ‘Study of light particles of the dark sector of the Universe’ No. 2021/42/E/ST2/00031 awarded by the National Science Centre within the SONATA BIS programme.