PIENU

TRIUMF’s PIENU (Pion to Electron and NeUtrino) experiment is searching for beyond-standard model physics by measuring with the greatest level of accuracy ever achieved the rare decays of particles called pions. 

While protons and neutrons each consist of three quarks, pions belong to the family of particles called mesons, made of a quark and an anti-quark. (The term pion is a contraction of π meson;  π is the Greek letter pi.) Pions are extremely short-lived decaying with a mean lifetime of just 26 nanoseconds, or billionths-of-a-second.  

Particle physicists believe that exactly how pions decay provides a unique and powerful window into possible beyond-standard model (SM) physics.  

This is because pions decay in only a few different ways. Almost always, a positive pion decays into a muon and a muon neutrino, but in about one-in-12,000 instances, the positive pion decays into a positron and an electron neutrino. 

The ratio (or branching ratio) of these two pion decay routes is one of the most precisely calculated theoretical values involving quarks emerging from the standard model. The rarer form of decay is predicted to occur in only one-in-12,352 decays. However, the current experimental value for the pion decay branching ratio is known with about ten times less precision.  

Thus, PIENU is measuring the pion decay ratio with the greatest precision ever, aiming to improve the existing experimental precision five-fold. Any deviation between PIENU’s results and the theoretical predictions for the branching ratio would indicate that previously unknown forces or particle interactions were involved and point to the existence of new particles and interactions hypothesized in beyond-SM theories. This could include leptoquarks, supersymmetric particles, or even dark matter interactions. Similarly, a deviation from the theoretical expectation could mean that electrons and muons have fundamentally differing weak interactions, which would violate a SM hypothesis known as lepton universality.  

If PIENU’s results are consistent with theoretical predictions, they will provide important constraints on theorized new physics interactions. Notably, the PIENU experiment is sensitive to a very wide mass range of new particles, including ones a thousand times heavier (up to 1000 TeV) than can currently be produced in the world’s most powerful accelerator. 

PIENU is an international collaboration led by TRIUMF’s Toshio Numao and the University of British Columbia‘s Douglas Bryman. It is the third in a sequence of TRIUMF experiments spanning three decades producing ever more precise measurements of the pion branching ratio.  

PIENU involves 25 researchers from six countries.

Scientists ran the PIENU experiment from 2009 to 2012, detecting about 10 million of the rare pion decays amid ten trillion total decays. Since 2012, the international PIENU team has been analyzing the data and producing interim results, particularly focused on identifying and removing any systematic errors that would reduce overall precision.  

PIENU was located in TRIUMF’s main Meson Hall at the M13 beamline. The approximately metre-squared experimental facility was designed to minimize systematic errors and to maximize the detector energy resolution in order to differentiate and precisely measure the two types of pion decay. It’s notable that given a pion’s half-life is just 26 billionth’s-of-a-second, the entire process described below occurs in just a small multiple of that time. 

PIENU involved three key components: pion production; pion detection; and pion decay product detectors of electrons and photons. 

Pion Production

Pions for PIENU were produced by firing TRIUMF’s main cyclotron‘s intense 520 MeV proton beam at a carbon target. This produced a shower of reaction products, including a high percentage of positively-charged pions. These were electromagnetically sorted and focused by the M13 beamline and transported to the PIENU target. 

PIENU Target 

In PIENU, the low energy beam of pions hits an 8-millimeter thick polystyrene target which stops the pions. The target is a plastic scintillator – it’s doped with an organic material that emits light when charged particles interact in it. Thus, every time a pion hits the target, it emits a tiny, billionth-of-a-second flash of light that’s detected by sensitive photosensors. If the light produced is characteristic of the energy of a pion, PIENU scientists know when and where a pion has hit the target and stopped.  

Decay Energy Detection

Pions decay at rest in the target material, with a 26-nanosecond half-life. Almost all of the pions decay to a muon and a muon neutrino. The muon, in turn, is also stopped in the target material and decays in two microseconds into a positron and two neutrinos. Thus, the final particle count of this decay route is a positron and three neutrinos.  

In the rare decay path, the pion decays directly to a positron and a neutrino, each with a fixed energy. What’s important to note here is that in the common decay involving a muon, the positron energy is variable and lower than in the rare decay process allowing the two routes of decay to be distinguished.   

PIENU’s detectors are designed to be exquisitely sensitive to positron detection and to measuring positron energy along with any accompanying photons. The main detector, directly downstream from the target, is a special sodium iodide crystal calorimeter. At 48 cm in diameter and about 48 cm long, the crystal is the largest single-crystal sodium iodide ever grown and the purest. Like the target, it’s a scintillator – an inorganic crystal scintillator doped with thallium to increase the light output – and produces a flash of light characteristic of the energy of each incoming positron. The crystal is viewed by 19 photomultiplier tubes which record the positron-induced light bursts. 

Finally, the sodium iodide crystal is surrounded by a ring of two layers of pure cesium iodide crystal detectors. This second layer of detectors is used to observe electromagnetic shower leakage. Positrons hitting the first detector produce a shower of electrons and photons which radiate out through the crystal and their energy is measured. However, in some cases, this energetic shower is more than the crystal can contain and some of this energy leaks out.  

The surrounding layer of cesium iodide crystals captures and records this energy, thereby greatly increasing the overall energy collection efficiency of the experiment. The detections in the sodium iodide and surrounding cesium iodide crystals are timed and thus detections collected in coincidence can be tracked back to the same event – a single positron. 

The neutrino emissions in these decays are inferred; neutrinos rarely interact with matter and are not detected in the experiment. 

Thus, by comparing the energies and times of emission of the detected positrons (see illustration below), PIENU scientists can identify the number of each type of pion decay that takes place and obtain the sought-after branching ratio with high precision.  

Since PIENU measures positron energies from direct pion decay so precisely, it is also sensitive to the presence of heavy neutral particles which might substitute for the nearly massless neutrinos which usually accompany the decays.  PIENU has obtained very tight limits on the existence of these heavy neutrinos which are hypothesized in many beyond-Standard Model theories.