CERN experiment sees hints of a rare kaon decay

March 27, 2018 by Ana Lopes, CERN
NA62 experiment in CERN's North Area. 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 ten percent. 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 collaboration 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, which kicks off in mid-April. The result was also presented earlier this month at the Rencontres de Moriond conference in La Thuile, Italy.

To look for kaon decays, the NA62 team first makes beams rich in kaons by firing high-energy protons from the Super Proton Synchrotron (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. Next, the team sends 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.

NA62’s candidate event of a 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. Credit: NA62/CERN

In their 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 researchers 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.

This is not the first time that hints of this decay have been observed. Several candidate events have been previously reported by the E949 experiment and its predecessor E787 at Brookhaven National Laboratory in Long Island, New York. These candidate events have been used to infer a branching fraction of 17.3 in 100 billion (with an uncertainty of about 11), which is consistent, within large errors, with the Standard-Model prediction.

But there is a difference between the Brookhaven experiments and NA62: whereas the former observed the kaon decays with the particles at rest in a target, NA62 observes them while the are in flight within the vacuum tank. This new in-flight approach has advantages because it provides much more room for detection and background-event immunity.

The NA62 team expects to identify more events of the rare kaon decay in the ongoing analysis of a twenty-fold-larger dataset taken in 2017, and it will begin taking data again in mid-April for a record number of 218 days. If all goes to plan, the collaboration should be able to measure the branching fraction of the decay with a small enough uncertainty to make a precise test of the Standard Model.

Explore further: LHCb finds new hints of possible deviations from the Standard Model

Related Stories

LHCb observes an exceptionally large group of particles

March 17, 2017

The LHCb experiment at CERN is a hotbed of new and outstanding physics results. In just the last few months, the collaboration has announced the measurement of a very rare particle decay and evidence of a new manifestation ...

How universal is (lepton) universality?

June 4, 2014

Just as a picture can be worth a thousand words, so the rarest processes at the Large Hadron Collider (LHC) can sometimes have the most to tell us. By isolating and counting decays of B+ mesons to a kaon and two leptons, ...

The ATLAS Experiment's quest for the lost arc

March 28, 2017

Nature has surprised physicists many times in history and certainly will do so again. Therefore, physicists have to keep an open mind when searching for phenomena beyond the Standard Model. 

Recommended for you

A quantum magnet with a topological twist

February 22, 2019

Taking their name from an intricate Japanese basket pattern, kagome magnets are thought to have electronic properties that could be valuable for future quantum devices and applications. Theories predict that some electrons ...

Sculpting stable structures in pure liquids

February 21, 2019

Oscillating flow and light pulses can be used to create reconfigurable architecture in liquid crystals. Materials scientists can carefully engineer concerted microfluidic flows and localized optothermal fields to achieve ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (3) Mar 27, 2018
With those odds, it sounds like pure quantum randomness. Keep blasting and watching long enough and they will probably find a 'kitchen sink' particle.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.