The TRIUMF/UBC Rare Decay Group is presently focused on studies of pion decays with the TRIUMF PIENU experiment and kaon decays at CERN NA62.
The branching ratio for pion decays to electrons (e) compared to those to muons (µ), Re/µ= Γ(π+→e+ν+e+νγ)/Γ(π+→µ+ν+µ+νγ) (“PIENU”), provides the best test of the universality of the weak interaction couplings, an important hypothesis in the Standard Model of particle physics. Re/µ has been calculated with extraordinary precision at the 10-4 level: Re/µ= (1.2352 ± 0.0002) × 10-4, perhaps the most precisely calculated weak interaction observable involving quarks. Since the uncertainty of the SM calculation for Re/µ is very small and the decay π+→e+ν is suppressed by the structure of charged currents, Re/µ is extremely sensitive to the presence of pseudo-scalar (and scalar) couplings absent from the SM; a measurement in disagreement with the theoretical expectation would imply the existence of new physics beyond the SM; at of 0.01 % experimental precision, new physics beyond the SM up to the mass scale of 3000 TeV may be accessed by a deviation from the precise SM expectation. Possible sources of deviation include new interactions involving scalar particles like Majorons, charged Higgs particles, and leptoquarks among many others. Another example of new physics which could modify Re/µ involves sterile massive neutrinos which are also sought in the PIENU experiments. Currently, the most accurate measurement reported by PIENU, Re/µ=(1.2344 ±0.0023(stat)±0.0019(syst))×10−4, which is at the 0.2% level of precision, resulted in the test of e − µ universality ge/gµ = 0.9996 ± 0.0012, expressed as the ratio of potentially distinct weak couplings for the electron and muon; this is in excellent agreement with the SM expectation. The goals of the present TRIUMF PIENU (and PSI PEN) experiments are to improve the measurement precision by another factor of 2 or more to a level of <0.1%. However, this still leaves room for experimental improvement by more than an order of magnitude in uncertainty to confront the SM prediction and to search for BSM effects. NA62 is aimed at a high precision measurement of the ultra-rare K^+→π^+ νν ̄ decay, one of the most incisive probes of new physics involving flavor. The SM prediction for the branching fraction is B(K+→π +νν ̄) = (8.22 ± 0.69 ± 0.29) ×10−11. Only a precious few experimentally accessible highrder quark processes can be predicted with this level of certainty. Suppression of the K→πνν decays in the SM framework along with the high precision of the prediction allows physics beyond the SM to contribute dramatically to the branching ratio. Numerous new physics contributions to K^+→π^+ νν ̄" and " K_L^0→π^0 νν ̄ have been considered including Supersymmetry, minimal flavor violation, little Higgs models, among many others, probing a mass scale of O(3000 TeV) well above that accessible at colliders. If new physics is discovered at the LHC, K^+→π^+ νν ̄ and a few other cleanly interpretable processes will be essential for deciphering the flavor structure of the new effects; if no new physics is evident, high precision measurements of K^+→π^+ νν will extend the indirect search to higher mass scales. The experimental challenge of identifying and suppressing backgrounds to enable measurement of K^+→π^+ νν ̄ at the 1 in 10 billion SM rate has been met successfully. Several events of K^+→π^+ νν ̄ decay were observed in prior work at Brookhaven National Laboratory resulting in the measured branching ratio B(K^+→ π^+ νν ̄)=1.73_(-1.05)^(+1.15) x10^(-10). Now, CERN experiment NA62 is in the process of extending the precision by more than an order of magnitude to determine if new physics contributes. Using slow extraction, 75 GeV kaons are produced at the CERN SPS for NA62 which is aiming at a precision of 10% for the K^+→π^+ νν branching ratio measurement at the SM level. NA62 recently presented results from the 2016-2018 running periods B(K^+→ π^+ νν ̄)=11〖.0〗_(-3.5)^(+4.0) x10^(-11) compatible with previous measurements and the SM prediction. Further studies are planned to reach 10% precision to stringently test the SM.
For more information about PIENU, please click here or visit the experiment website. For more information about NA62, please click here.