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Rare Pion Decays and Tests of the Standard Model Rare pion decays and tests of the Standard Model Dinko Poˇcani´c University of Virginia 7 November 2019 Fundamental Symmetries Research with Beta Decay Institute for Nuclear Theory University of Washington, Seattle 4 { 8 November 2019 Known and measured pion and muon decays decay B:R: physics interest + + π ! µ ν 0:9998770 (4) (πµ2) + −4 µ νγ 2:00 (25) × 10 (πµ2γ) + −4 e ν 1:230 (4) × 10 (πe2) ( `U, BSM (T ,. ) + −7 (π) (π) e νγ 7:39(5) × 10 (πe2γ) ( BSM (T ,. ), FA ; FV ,. 0 + −8 π e ν 1:036 (6) × 10 (πe3) ( q`U(Vud ), BSM loops + + − −9 e νe e 3:2 (5) × 10 (πe2ee) π0 ! γγ 0:98798 (32) ( χ anomaly e+e−γ 1:198 (32) × 10−2 (Dalitz) e+e−e+e− 3:14 (30) × 10−5 e+e− 6:2 (5) × 10−8 µ+ ! e+νν¯ ∼ 1:0 (Michel) ( BSM weak int. terms e+ννγ¯ 0:014 (4) (RMD) e+νν¯e+e− 3:4 (4) × 10−5 D. Poˇcani´c(UVa) Rare π decays: Study subjects 7 Nov '19/INT 2 / 34 The electronic (πe2) decay: π+ ! e+ν(γ) BR ∼ 10−4 D. Poˇcani´c(UVa) Rare π decays: The πe2 decay 7 Nov '19/INT 3 / 34 ∼ 2:5 × 10−5 I Strong SM helicity suppression amplifies sensitivity to PS terms (\door" for New Physics) by factor2 mπ=me (mu + md ) ≈ 8000. Rπ I e/µ tests lepton universality: in SM e, µ, τ differ by Higgs couplings only; there could also be newS or PS bosons with non-universal couplings (New Physics); repercussions also in the neutrino sector, SUSY, ALPS... I Experimental world average is 23× less accurate than SM calculations! [1:2327(23) × 10−4] πe2 decay: SM calculations, lepton universality I Early evidence for V − A nature of weak interaction. 2 2 2 2 2 π Γ(π ! eν¯(γ)) ge me (1 − me =mµ) Re/µ = = 2 2 2 2 2 1 + δRe/µ Γ(π ! µν¯(γ)) gµ mµ (1 − mµ=mπ) π Γ(π ! eν¯(γ)) I Modern SM calculations: R = = e/µ Γ(π ! µν¯(γ)) 8 CALC 1:2352(5) × 10−4 Marciano and Sirlin, [PRL 71 (1993) 3629] <> 1:2354(2) × 10−4 Finkemeier, [PL B 387 (1996) 391] :>1:2352(1) × 10−4 Cirigliano and Rosell, [PRL 99 (2007) 231801] D. Poˇcani´c(UVa) Rare π decays: Motivation 7 Nov '19/INT 4 / 34 Rπ I e/µ tests lepton universality: in SM e, µ, τ differ by Higgs couplings only; there could also be newS or PS bosons with non-universal couplings (New Physics); repercussions also in the neutrino sector, SUSY, ALPS... I Experimental world average is 23× less accurate than SM calculations! [1:2327(23) × 10−4] πe2 decay: SM calculations, lepton universality I Early evidence for V − A nature of weak interaction. ∼ 2:5 × 10−5 2 2 2 2 2 π Γ(π ! eν¯(γ)) ge me (1 − me =mµ) Re/µ = = 2 2 2 2 2 1 + δRe/µ Γ(π ! µν¯(γ)) gµ mµ (1 − mµ=mπ) π Γ(π ! eν¯(γ)) I Modern SM calculations: R = = e/µ Γ(π ! µν¯(γ)) 8 CALC 1:2352(5) × 10−4 Marciano and Sirlin, [PRL 71 (1993) 3629] <> 1:2354(2) × 10−4 Finkemeier, [PL B 387 (1996) 391] :>1:2352(1) × 10−4 Cirigliano and Rosell, [PRL 99 (2007) 231801] I Strong SM helicity suppression amplifies sensitivity to PS terms (\door" for New Physics) by factor2 mπ=me (mu + md ) ≈ 8000. D. Poˇcani´c(UVa) Rare π decays: Motivation 7 Nov '19/INT 4 / 34 I Experimental world average is 23× less accurate than SM calculations! [1:2327(23) × 10−4] πe2 decay: SM calculations, lepton universality I Early evidence for V − A nature of weak interaction. ∼ 2:5 × 10−5 2 2 2 2 2 π Γ(π ! eν¯(γ)) ge me (1 − me =mµ) Re/µ = = 2 2 2 2 2 1 + δRe/µ Γ(π ! µν¯(γ)) gµ mµ (1 − mµ=mπ) π Γ(π ! eν¯(γ)) I Modern SM calculations: R = = e/µ Γ(π ! µν¯(γ)) 8 CALC 1:2352(5) × 10−4 Marciano and Sirlin, [PRL 71 (1993) 3629] <> 1:2354(2) × 10−4 Finkemeier, [PL B 387 (1996) 391] :>1:2352(1) × 10−4 Cirigliano and Rosell, [PRL 99 (2007) 231801] I Strong SM helicity suppression amplifies sensitivity to PS terms (\door" for New Physics) by factor2 mπ=me (mu + md ) ≈ 8000. Rπ I e/µ tests lepton universality: in SM e, µ, τ differ by Higgs couplings only; there could also be newS or PS bosons with non-universal couplings (New Physics); repercussions also in the neutrino sector, SUSY, ALPS... D. Poˇcani´c(UVa) Rare π decays: Motivation 7 Nov '19/INT 4 / 34 πe2 decay: SM calculations, lepton universality I Early evidence for V − A nature of weak interaction. ∼ 2:5 × 10−5 2 2 2 2 2 π Γ(π ! eν¯(γ)) ge me (1 − me =mµ) Re/µ = = 2 2 2 2 2 1 + δRe/µ Γ(π ! µν¯(γ)) gµ mµ (1 − mµ=mπ) π Γ(π ! eν¯(γ)) I Modern SM calculations: R = = e/µ Γ(π ! µν¯(γ)) 8 CALC 1:2352(5) × 10−4 Marciano and Sirlin, [PRL 71 (1993) 3629] <> 1:2354(2) × 10−4 Finkemeier, [PL B 387 (1996) 391] :>1:2352(1) × 10−4 Cirigliano and Rosell, [PRL 99 (2007) 231801] I Strong SM helicity suppression amplifies sensitivity to PS terms (\door" for New Physics) by factor2 mπ=me (mu + md ) ≈ 8000. Rπ I e/µ tests lepton universality: in SM e, µ, τ differ by Higgs couplings only; there could also be newS or PS bosons with non-universal couplings (New Physics); repercussions also in the neutrino sector, SUSY, ALPS... I Experimental world average is 23× less accurate than SM calculations! [1:2327(23) × 10−4] D. Poˇcani´c(UVa) Rare π decays: Motivation 7 Nov '19/INT 4 / 34 The PEN/PIBETA apparatus • πE1 beamline at PSI • stopped π+ beam PEN detector • active target counter Runs 2-3 • 240 module spherical pure pure CsI calorimeter CsI • central tracking • beam tracking PH • digitized waveforms MWPC1 π+ mTPC VACUUM PMT beam AD AT BC MWPC2 ~3 m flightpath 10 cm BC: Beam Counter PH: Plastic Hodoscope (20 stave cylindrical) AD: Active Degrader MWPC: Multi-Wire Proportional Chamber (cylindrical) T C i i T m r j c i n C a b rT Active Target mTPC: mini-Time Projection ChamberAT: D. Poˇcani´c(UVa) Rare π decays: Experimental method 7 Nov '19/INT 5 / 34 π-exp Experimental branching ratio (Re/µ ) Recognizing that: I timing gates affect the analyzed number of πe2 and π ! µ ! e events; I MWPC efficiency depends on energy, peak π-exp Nπ!eν (1 + tail) fπ!µ!e(Te) (Eµ!eνν¯)MWPC Aπ!µ!e we have: Re/µ = Nπ!µν fπ!eν (Te) (Eπ!eν )MWPC Aπ!eν rf r rA 16000 14000 Ec Ec = cutoff energy 12000 N = number of events 10000 Counts A = acceptance 8000 6000 tail(Ec ) = tail to peak ratio 4000 π ! µ ! e π ! eν (E)MWPC = efficiency of MWPC 2000 −3 ×10 f (Te) = decay probability during 0 10 20 30 40 50 60 70 80 90 observation time window E CsI (MeV) D. Poˇcani´c(UVa) Rare π decays: Method 7 Nov '19/INT 6 / 34 Branching ratio/uncertainties peak π Nπ!eν fπ!µ!e(Te) (Eµ!eνν¯)MWPC Aπ!µ!e Re/µ = (1 + tail) Nπ!µν fπ!eν(Te) (Eπ!eν)MWPC Aπ!eν | {z } | {z } | {z } | {z } rN rf r rA | {z } blinded Uncertainties: s δR δr 2 δ 2 δr 2 δr 2 δr 2 = N + tail + f + + A R rN 1 + tail rf r rA MWPC(E): chamber efficiencies. rA: acceptances PEN goal: δR=R ' 5 × 10−4 D. Poˇcani´c(UVa) Rare π decays: Method 7 Nov '19/INT 7 / 34 Discriminating πe2 and πµ2 in TGT + e+ t(e ) in AT predicted from t(PH) mTPC mTPC ) (x; y) of πstop in AT ToF from B0 π+ PMT E(π+) AD AT Edep(AD) ) Edep; z of πstop in AT t(π+) in AT predicted from t(AD) Predicted π+ and e+ energies agree VERY well with observations: 11 7 + 10.5 e 6 10 9.5 5 9 4 (MeV) (MeV) 8.5 tgt obs e tgt obs π 8 E E 3 7.5 2 7 + 6.5 π 1 6 1 2 3 4 5 6 7 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 pred pred E (MeV) Eπ tgt (MeV) e tgt ) E and t predictions are used for πe2/πµ2 discrimination. D. Poˇcani´c(UVa) Rare π decays: Method 7 Nov '19/INT 8 / 34 Target waveforms and event type discrimination Measurement jrawj 0.07 20000 π ! eν simulation filtered π ! µν simulation predicted 0.06 15000 µ found 0.05 0.04 10000 target waveform yield 0.03 (2GS/s) 5000 0.02 0.01 0 0 400 450 500 550 600 650 700 750 800 -2 -1.5 -1 -0.5 0 0.5 1 time (0.5 ns/bin) ∆χ2 0.030 Measurement 2 π ! eν simulation ∆χ uses predicted and observed 0.025 π ! µν simulation timings and energies. Steps: 0.020 2 1. evaluate 2 peak fit ) χ2, 0.015 2 2. evaluate 3 peak fit ) χ3, yield 0.010 2 2 2 3. find ∆χ = χ2 − χ3 (normalized). 0.005 Best of ∼ dozen similar observables at 0.000 discriminating π / π event classes.
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