Abstract
PIONEER is a next-generation experiment to measure the charged pion branching ratios to electrons vs. muons Re/μ=Γπ+→e+ν(γ)Γπ+→μ+ν(γ) and pion beta decay (Pib) π+→π0eν. The pion to muon decay (π→μ→e) has four orders of magnitude higher probability than the pion to electron decay (π→eν). To achieve the necessary branching-ratio precision it is crucial to suppress the π→μ→e energy spectrum that overlaps with the low energy tail of π→eν. A high granularity active target (ATAR) is being designed to suppress the muon decay background sufficiently so that this tail can be directly measured. In addition, ATAR will provide detailed 4D tracking information to separate the energy deposits of the pion decay products in both position and time. This will suppress other significant systematic uncertainties (pulse pile-up, decay in flight of slow pions) to <0.01%, allowing the overall uncertainty in to be reduced to O (0.01%). The chosen technology for the ATAR is Low Gain Avalanche Detector (LGAD). These are thin silicon detectors (down to 50 μm in thickness or less) with moderate internal signal amplification and great time resolution. To achieve a 100% active region several emerging technologies are being evaluated, such as AC-LGADs and TI-LGADs. A dynamic range from MiP (positron) to several MeV (pion/muon) of deposited charge is expected, the detection and separation of close-by hits in such a wide dynamic range will be a main challenge. Furthermore, the compactness and the requirement of low inactive material of the ATAR present challenges for the readout system, forcing the amplifier chip and digitizer to be positioned away from the active region.
Highlights
The branching ratio Re/μ Γ(π+→e+ν(γ)) Γ(π+→μ+ν(γ))for pion decays to electrons over muons provides the best test of electron–muon universality in charged-current weak interactions.it is extremely sensitive to new physics at high mass scales
Because the uncertainty of the Standard Model (SM) calculation for Re/μ is very small and the decay π+ → e+ν is helicity-suppressed by the V − A structure of charged currents, a measurement of Re/μ is extremely sensitive to the presence of pseudo-scalar couplings absent from the SM; a disagreement with the theoretical expectation would unambiguously imply the existence of new physics beyond the SM
With 0.01% of experimental precision, new physics beyond the SM (BSM) up to the mass scale of 3000 TeV may be revealed by a deviation from the precise SM expectation [3]
Summary
For pion decays to electrons over muons provides the best test of electron–muon universality in charged-current weak interactions. TRIUMF PIENU [6,7] and PSI PEN [8,9,10] experiments expect to improve the measurement precision by another factor of 2 or more to a level of ≤0.1% Even when these goals are realized, this still leaves room for experimental improvement by more than an order of magnitude in uncertainty to compare with the SM prediction. Π+ → π0e+ν(γ), provides the theoretically cleanest determination of the CKM matrix element Vud. With current input one obtains Vud = 0.9739(28)exp(1)th, where the experimental uncertainty comes almost entirely from the π+ → π0e+ν(γ) branching ratio (BRPB) [16] (the pion lifetime contributes δVud = 0.0001), and the theory uncertainty has been reduced from (δVud)th = 0.0005 [19,20,21] to (δVud)th = 0.0001 via a lattice QCD calculation of the radiative corrections [22]. The current precision of 0.3% on Vud makes π+ → π0e+ν(γ) not presently relevant for the CKM unitarity tests because super-allowed nuclear beta decays provide a nominal precision of 0.03%
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