CERN Accelerating science

Exotic searches with NA62 experiment

by Panos Charitos

The NA62 experiment at CERN focuses in the study of kaon physics; a branch of physics that has been one of the major protagonists of the field in the second half of the twentieth century, as it played a key role in the development of the Standard Model and in exploring the reasons of the observed CP violation: a phenomenon that may be involved in the observed matter-antimatter asymmetry in the universe.

The NA62 facility at CERN (Image Credits: CERN). 

The experiment aims at the very challenging task of measuring with 10% relative error the branching ratio of the ultra-rare decay K+→ π+v vbar which is expected to occur only in about 8 out of 1011 kaon decays. This will be achieved by means of an intense hadron beam, an accurate kinematical reconstruction and a redundant veto system for identifying and suppressing all background events.

The most immediate possibility to search for new physics is through the core NA62 goal namely the precise measurement of K-> π v vbar which puts a very precise prediction of the Standard Model under scrutiny. Presently, theories of physics beyond the Standard Model have been proposed that would give sizable differences in both K+ → π+ v vbar  and K→ π0 ν νbar branching ratios. The tiny branching ratios present an experimental challenge as these decays are extremely difficult to measure. NA62 aims at measuring a number of about 100 events in about two years of data taking. For this purpose at least 1013 K+ decays are required assuming an acceptance of O (∼ 10%). To obtain a contribution of the background below 10%, a rejection factor of 1012 in rejecting the other kaon decay modes is needed and this is what drives to a large extent the design of the NA62 experiment.

Apart from high-precision measurement of rare processes, the NA62 collaboration is also looking into the possibility of the existence of exotic particles. One key insight is the fact that NA62 is actually dumping a large number of protons in the effort to produce the K+ beam. The interactions of these protons can be further studied as they may provide a hint for new physics. Out of the dumped protons, new weakly interacting particles with very long lifetimes could be produced. Another possibility is that such particles could be produced from upstream meson decays.

Babette Dobrich, a CERN fellow working in the NA62 experiment, explains: “Due to their very weak coupling, such particles would be able to traverse the material in front of the decay volume without interacting and reveal themselves by decaying in the sensitive volume of the experiment. Depending on the final states we are expecting, data can be taken in "dump mode" or in parallel with the data taking for kaons by applying appropriate triggers. In this way we look for scalar- and pseudo-scalar bosons, like axions, dark photons and heavy neutral leptons.”

Following, her PhD in particle phenomenology, specifically on new physics searches and QED tests with laser-based experiments, Babette applied to DESY for her first post-doc. Babette’s research activities are focused - but not limited - to axions and axion-like particles. Her interest stems from the fact that particularly QCD axions themselves are an excellent dark matter candidate. She explains: “in general pseudo-scalars have received lately much interest for example in the context of dark matter model-building, because they can be a mediator for the interactions between dark matter and SM particles” and adds: “More generally, until we have a very clear picture of where new physics is hiding, I am a fan of the idea of putting the search instruments that we have at hand to a broad use and NA62 provides an excellent opportunity in this respect.”.

The search for new physics in NA62 presents certain experimental challenges. Perhaps one of the main difficulties is to better understand the halo particle background; mainly muons from upstream meson decays that enter the decay volume in data taking with beam or in "dump mode".

View of the "TAXes" of NA62 (during installation without side- and roof-shielding) downstream of the target in which  NA62's Kaon beam is produced. The "TAXes" act as a dump for all SPS protons that pass the target without interaction. New, weakly interacting particles might be produced from the dumped protons and can be detected in the downstream decay volume. (Image Credit: Sylvain Girod/CERN).  

Searches with NA62 for long-lived new particles complement well LHC as well as lab-size new physics searches. Sticking to the example of axion-like particles, one will find a growing literature arguing for the existence of these particles over a wide mass range. Some regions of their parameter space can be excluded purely by astrophysical arguments. However, in the experimentally unexplored parameter space, LHC experiments could find axions roughly above the GeV scale while there is a number of running and proposed experiments to cover the mass region below keV.

“In the case of NA62, we expect to be mainly sensitive in the MeV up to GeV region, thus helping to fill a gap. Albeit NA62 was not built for this purpose I think we can do a good job in this area. After all, unless we have a very clear picture of where the new physics is, we should make a broad use of the opportunities we have.”

For Babette starting to work with NA62 involved a personal challenge since her previous experimental experience was limited to a lab-based laser experiment in DESY. She recalls: “there were many things I had to learn from the start like certain software tools and some high-energy detector concepts. Once someone told me that I should ‘think less like a theorist’.” and continues “However it turned out I was lucky to join a very welcoming collaboration. Since we are a comparably small experiment, it was easy to identify the right people who could help with a particular question. I am very much indebted to several people who took time to discuss and answer my questions”.

Today there is a growing number of people in the NA62 collaboration interested in this sort of physics. They are brought together under the “long-lived exotica” working group. It turns out that some of the analyses for exotics have many synergies with the core analysis, like the optimization of veto conditions. Regarding the “dump runs" Babette explains: “we realized that even very small samples of just a day  in some instances may have physics impact, whilst being, e.g., a useful tool to asses backgrounds that are also present in regular data taking”.

The NA62 collaboration has put forward a proposal to have some dedicated run-time for the physics searches explained above after the second long shut-down. This is one of the proposals submitted to the Physics Beyond Collider working group. Babette notes: “an easy improvement to get better parameter reach "on paper" would be, for example, to have a "dump" closer to the decay volume. This would make us sensitive to a sizeable un-probed parameter space of particles that -if they exist - now decay before reaching our decay volume. The feasibility of such beam-line modifications has yet to be assessed. But it is already exciting to study our impact on models that we might be able to test in the near future.”



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