Physicists from the NA62 Collaboration at CERN reported today that they had observed a candidate event of an extremely rare charged kaon decay (K+ → π+νν–).
NA62 experiment in CERN’s North Area. Image credit: NA62 / CERN.
What if the odds of an event occurring were about one in ten billion? This is the case for the decay of a positively charged particle known as a kaon into another positively charged particle called a pion and a neutrino-antineutrino pair.
Yet, such a rare event, which has never been observed with certainty, is something that particle physicists really want to get their hands on.
The reason? The Standard Model predicts such one-in-ten-billion odds with an uncertainty of less than 10%. A deviation from this prediction, revealed by a precise measurement of the decay, could therefore be a clear indicator of physics beyond the Standard Model.
In a seminar taking place today at CERN, the NA62 team reports a candidate event of this ultra-rare kaon decay found using a new ‘in-flight decay’ approach.
While this single event cannot be used to probe beyond-Standard-Model physics, it demonstrates that the approach works well and can be applied to catch more events in the next run of data-taking.
NA62’s candidate event of a ultra-rare kaon decay. Octagons show hits in the RICH detector. Circles show predicted ‘Cherenkov rings’ for positively charged pion (+), positively charged muon (+) and antielectron (e+) decay particles. Image credit: NA62 / CERN.
“The NA62 experiment is a particle physics experiment at CERN using a 400 GeV proton beam from the Super Proton Synchrotron (SPS) accelerator,” the physicists said.
“To look for kaon decays, we first make beams rich in kaons by firing high-energy protons from the SPS accelerator into a beryllium target. The collision creates a beam of nearly one billion particles per second, only about 6% of which are kaons,” they explained.
“Next, we send the beam through a Cherenkov detector, which positively identifies the kaons from the Cherenkov radiation that they produce.”
“A silicon-pixel detector then determines the momentum of the kaons with a time resolution of 100 picoseconds.”
“A device called a straw tracker, placed inside the vacuum tank, in turn measures the momentum of the charged daughter particles into which the kaons decay, and another Cherenkov detector called RICH determines the particles’ type. Other devices known as calorimeters reject unwanted background events with photons and muons.”
In the analysis of data taken over the course of 2016, the NA62 team identified a candidate event of the decay of a positively charged kaon into a positively charged pion and a neutrino–antineutrino pair that escapes undetected.
The result allowed the physicists to put an upper limit on the relative frequency, or ‘branching fraction,’ of the decay of 14 in 10 billion.
The result is compatible with the Standard-Model prediction, which is 8.4 in 100 billion (with an uncertainty of 1), but more data is needed to probe beyond-Standard-Model theories, which predict deviations from the Standard-Model value.
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Giuseppe Ruggiero et al. 2018. K+ → π+νν–: First Result from the NA62 Experiment. CERN EP Seminar;
