Hints of Rare Kaon Decay Seen at CERN

Today at a seminar at CERN, the NA62 collaboration reported the possible observation of an extremely rare kaon decay, an enormously rare event.

The odds of a positively charged particle known as a kaon into another positively charged particle called an antineutrino pair are one in ten billion.

However, using a new “in-flight decay” approach, these odds were beaten.

This Rare Kaon Decay Event has Never Been Seen Before

This event, which had never been seen before today, was sought after by particle physicists for decades.

This is partially due to the fact that the Standard Model predicts these staggering odds along with an uncertainty of less than ten percent.

The Standard Model explains how the basic building blocks of matter interact.

A precise measurement of decay that contradicts this prediction, could, therefore, be a clear indicator of physics beyond the Standard Model.

The Process of Finding Kaon Decays

In order to find kaon decays, the NA62 team created beams that were rich in kaons. This was achieved by firing high-energy protons from the Super Proton Synchrotron (SPS) accelerator into a beryllium target.

Upon colliding, a beam of up to one billion particles per second is created. Although a staggering number, only an estimated 6% of these particles are kaons.

After this, the beam passes through a Cherenkov detector. This positively identifies the kaons by analyzing the Cherenkov radiation produced in the process.

The momentum of the kaons with a resolution of 100 picoseconds is then determined using a silicon-pixel detector.

Next, a straw tracker is placed inside a vacuum tank. This measures the momentum of the charged daughter particles. The kaons decay into these particles and another Cherenkov detector called the RICH detector is used to evaluate the particles’ type.

While this process is taking place, calorimeters reject events with activity involving photons and muons in the background. This is done as they are not needed during the process.

Mapping the Rare Event of Kaon Decay. The 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 courtesy of CERN)

Discovering That the Decay of an Antineutrino Pair Escapes Undetected

The NA62 team analyzed data that had been recorded throughout 2016. This led them to discover a candidate event for the decay of a positively charged pion and neutrino (otherwise known as an antineutrino pair) escaping undetected.

This result meant that the researchers could put a limit of 14 in 10 billion on the relative frequency, or “branching fraction”, of the decay. The Standard-Model prediction for this process is 8.5 in 10 billion, with an uncertainty of one. This means the result the researchers got is compatible with this model. However, more data must be collected in order to probe beyond Standard Model theories.

Hints of this Kaon Decay Have Been Previously Observed

This is not the first time this event has been observed. The E040 experiment along with the E787 experiment that came before, both reported several candidate event reports before this. However, this was not performed with the particles were in motion, which may have an effect on their behavior.

Such candidate events have inferred a branching fraction of 17.3 in 100 billion, with an uncertainty of approximately 11. This is a result that is consistent with the Standard Model prediction.

Today’s event can’t be used to venture beyond Standard Model physics. However, it does demonstrate that we are headed in the right direction. The approach used at CERN has proven its worth and will be used to capture more events like this in the data-tracking runs scheduled to take place next month.