NA62

Based at CERN, NA62 is making ultra-precise measurements of some of the rarest forms of particle decay in a search for beyond-standard model forces or particles. 

NA62 is one of TRIUMF’s major CERN-based international collaborations, which include the ALPHA anti-matter experiment and ATLAS detectors at the Large Hadron Collider. NA62, or North Area 62, refers to the location of the experiment at CERN. 

Particle physicists believe that exactly how very short-lived particles called kaons decay provides a unique and powerful window into possible beyond-standard model (SM) physics. Kaons have previously been responsible for establishing important aspects of the SM, including parity (left-right) symmetry violation, and the existence of the strange quark. 

Unlike protons and neutrons which consist of three quarks, kaons belong to the family of particles called mesons, made of a quark and an anti-quark.  (The term kaon is a contraction of k meson). Each charged kaon consists of a strange quark and an up quark and has a half-life of about 12 nanoseconds, or billionths-of-a-second. 

A positively charged kaon (K+) decays in several ways producing lighter particles, including pions, muons, electrons and neutrinos. The most common decay modes involve a muon and a neutrino (63%), and two pions (21%). But, in one-in-ten-billion instances, a K+ decays into a charged pion and a neutrino-antineutrino pair. 

The ratio of this rare K+ decay relative to all decays (the branching ratio) has been precisely calculated using theoretical values emerging from the SM. However, the known experimental value for the K+ branching ratio is far less precise. 

Thus, NA62 scientists are measuring the K+ decay ratio with the greatest precision ever in order to compare theoretical predictions with highly precise experimental results. Any deviation between the experimental results and the theoretical predictions would point to the existence of new particles and interactions possibly hypothesized in beyond-SM theories. These could include leptoquarks, supersymmetric particles, or even dark matter interactions. Alternately, if NA62’s results are consistent with theoretical predictions, they will provide important constraints on theorized new physics interactions. 

Notably, the NA62 experiment is sensitive to a very wide mass range of potential new particles, including ones a thousand times heavier (up to 1000 TeV) than can currently be produced in the world’s most powerful accelerator.  

If the Standard Model’s prediction for rare kaon+ decays is experimentally verified, during NA62’s three years of data taking scientists will detect just 50 of the rarer decay from amid trillions of particle detections. NA62 also studies a wide variety of other kaon and pion decays in other searches for beyond-SM physics effects. 

The NA62 experiment is an international collaboration with 200 scientists from ten countries.   

At 250-metres-long, NA62 is the world’s best particle sorting machine. Using a series of custom-built, highly sensitive detectors, the experiment is searching for ultra-rare kaon+ decays using a technique that’s the equivalent of removing nearly all the individual pieces of straw from a haystack, one-by-one, in order to find a few unique needles. The enormous challenge of accurately identifying one particular particle decay in 10-billion, and suppressing the background noise, means that NA62 has to eliminate background processes which appear similar to the sought-after mode with an efficiency of one-in-100 billion. 

To find the rare K+ decays, NA62 uses a multistep process: 

Making K+

To make K+, the NA62 team fires high-energy protons from CERN’s  400 GeV Super Proton Synchrotron (SPS) into a stationary target. This collision creates a shower of particles which are electromagnetically selected by momentum to create a high-intensity beam of almost one-billion- particles-per-second.  

Identifying (Tagging) K+

Only about six percent of the beam particles entering NA62 are K+, and the first two detectors are designed to identify, or tag, K+, from amid the bulk of beam particles, and log the K+‘s time and momentum.  

The first detector, KTAG, identifies particles from their Cherenkov radiation. The velocity of a particle determines the Cherenkov radiation produced, and thus K+ can be distinguished from other beam particles, such as pions and protons. Since all the beam particles have a common momentum, those with heavier masses travel at lower velocities. 

The second detector, the GigaTracker, is the unique centrepiece of the tracking system. This silicon pixel detector is used to measure the arrival time and momentum of the incoming K+, information that is correlated with later decay particles from the kaon decay.  

The GigaTracker is one of the highest intensity, highest accuracy silicon tracking devices ever built. While similar to the silicon pixel detectors used in the LHC experiments, NA62 involves a much higher number of particles-per-second. Thus, the GigaTracker uses a first-of-its-kind microchannel cooling system in order to keep each detector segments at about -20°C, and avoid its melting.   

Decay Chamber

Next, the particles pass through a 100-metre long vacuum chamber in which the K+, and other particles in the beam, decay. 

Decay Detectors

The back-end of NA62 consists of a suite of sophisticated detectors designed to identify rare K+ decay products and sort these from the other decays.  

First, all the particles pass through a series of powerful magnets, in which a particle’s path is bent based on its momentum. By measuring the radius of curvature of the particles in the magnetic field, NA62 scientists identify and determine a pion’s momentum.  

A ring imaging Cherenkov (RICH) detector similarly identifies the nature of each decay particle, distinguishing incoming pions from the flood of background muon decay particles. A 10,000-litre liquid krypton calorimeter measures the energy of each incoming pion and detects any accompanying photons or other particles which accompany the background processes. NA62 has the highest detection efficiency for charged particles and photons ever achieved using the liquid krypton calorimeter and other detectors. 

Muons are relatively weakly interacting particles and can penetrate the previous detectors, whereas pions cannot. Thus, the above collection of detectors involves others designed to detect muons and photons, and thus be able to reject these unwanted signals from the detections. 

TRIUMF and NA62 

Since joining the NA62 collaboration in 2016, TRIUMF scientists, engineers and technicians are contributing a combination of analytical and hardware components. 

Data Analysis 

TRIUMF affiliate scientist Douglas Bryman is leading TRIUMF’s contribution to NA62 and in so doing, continuing 30-years of pivotal TRIUMF research expertise and experience in rare kaon decay detection. In 1997, Bryman was a leader of the international team that first measured and experimentally confirmed the K+ to charged pion and a neutrino-antineutrino pair decay. TRIUMF was heavily involved in that experiment, based at Brookhaven National Laboratory, designing and building many of the beam line and detector components. After 25-years as a TRIUMF staff scientist, Bryman now holds the TRIUMF-funded Warren chair at the nearby University of British Columbia.  

Liquid Krypton Detector Purity 

Led by Bryman, TRIUMF’s detector and electronics groups have recently contributed a detector to ensure the purity of the krypton in NA62’s 10,000-litre liquid krypton detector (LKr). The LKr is crucial to NA62 because it measures the energy of pions produced by the rare K+ to charged pion and a neutrino-antineutrino pair decay and efficiently detects background photons. The precision of this measurement is critical to NA62. The krypton must be extremely pure for the LKr to function efficiently, with no more than one-part-in-a-billion contamination, such as from oxygen or water. The unique TRIUMF-designed and built krypton purity detector acts as a security monitor removing one key area of potential systemic uncertainty from the NA62 experiment.