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Muon Decay Parameters from TWIST Muon Decay Parameters From TWIST Art Olin, TRIUMF/UVic for the TWIST Collaboration Outline •Muon decay physics •TWIST Experiment •Systematics •Results Muon decay spectrum The energy and angle distributions of positrons following polarized muon decay obey: 2 d Γ 2 x0 ∝3−3x ρ 4x−33η 1−x x2 dxdcos θ 3 x 2 Pμ ξ cos θ 1−x δ 4x−3 [ 3 ] (+ rad. corr.) Ee where x= Ee,max Lorentz invariant, local weak interaction. SM postulates maximally parity violating V-A: ρ=δ=3/4; ξ=1;η=0. Mediated by W boson. Empirically based – tested in present work. 2 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Muon decay matrix element Most general Lorentz-invariant, local, lepton-number conserving muon decay matrix element: = 4GF γ Γγ ν ν Γ µ M ∑ gεµ eε | | ( e )n ( µ )m | γ | µ 2 γ =S,V ,T ε ,µ=R,L γ The muon decay parameters are bi-linear combinations of the g εμ V In the Standard Model, g LL = 1, all others are zero Pre-TWIST global fit results (all 90% c.l.): 3 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Goal of TWIST Search for new physics that can be revealed by order-of-magnitude improvements in our knowledge of ρ, δ, and Pμξ Two examples Model-independent limit on muon handedness µ = 1 + 1 ξ − 16 ξδ QR 1 2 3 9 Left-right symmetric model: SU(2)L x SU(2)R x U(1) 3 3 W = W cosζ +W sinζ − ρ = ζ 2 L 1 2 ⇒ 4 2 2 4 = iω ( − ζ + ζ ) M M WR e W1 sin W2 cos − ξ = ζ 2 + ζ 1 + 1 1 Pµ 4 M2 M2 4 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test The TWIST Experiment TRIUMF Weak Interaction Symmetry Test 5 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Muon production and transport m 500 MeV u d o proton e n t c a a beam r y g e t TECs fringe field region ν π+ μ+ ⇒ ⇒ 6 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Positron tracking Variable density gas degrader 7 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Detector Array -4 Al and Precision (<10 ) select variable Ag In z Wire position dE/dx density targets gas for μ+ degrader in target trigger scintilla tor R. Henderson et al., Nucl. Instr. and Meth. A548 (2005) 306-335 8 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Muon Decay Spectrum Acceptance Pμ 9 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Spectrum Fit Procedure •Decay spectrum linear in ∂ N ρ, P ξ, P ξδ. N Data = N MC + μ μ ∂ • Derivative spectra are simulated. •Offset added to parameters to blind them. •Consistency is tested and systematics determined before unblinding. ∂ N ∂ N •Spectrum edge is fit in angle slices to the P + P ∂ P ∂ P simulation. A momentum – calibrated spectrum is regenerated and fit. α = {ρ, 10 Artδ, Olin: ξ} PANIC2011 TRIUMF Weak Interaction Symmetry Test Spectrum fit quality All data sets: 11x109 events, 0.55x109 in (p,cosµ) fiducial Simulation sets: 2.7 times data statistics 11 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Fringe field, solenoid entrance Position Angle 2 m 12 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Fringe field, solenoid entrance Position Angle 2 m 13 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Measured average muon positions nominal beam The average muon beam trajectory inside the detector is sensitive to the muon transverse momentum . Identify changes in muon beam properties between TEC measurements Comparisons between nominal and mis-steered beams mis-steered beam . Observed muon beam trajectories within the detector . Difference in the decay asymmetry in data vs that predicted by the simulation. Resulting uncertainties are asymmetric. 14 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Improved drift chamber calibration Equal-time contours Chamber position resolution Direct determination of the effective distance vs time relation. Accounts for small plane-to-plane fabrication differences. Improved momentum resolution (near the endpoint) . Was ~ 69 keV/sin(θ) in simulation and ~ 74 keV/sin(θ) in data. Now ~ 58 keV/sin(θ) in both. A. Grossheim et al., Nucl. Instrum. Methods A 623, 954 (2010).15 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Bremsstrahlung Normal muon stopping target Leading systematic for ρ and δ. Separately determined from upstream stops and broken decay tracks. Larger in Ag target. Both consistent with GEANT3 simulation at 2.5% level. 16 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Consistency of data sets ) Ag Al 4 110 - 0 100 Key: 1 ( 90 68 – zstop shifted Δρ m s 80 70 – B = 1.96 T o r r e 70 71 – B = 2.04 T f t e 80 72 – TEC in a m t a 70 74 – Nominal a r 60 d a 75 – Nominal p Δδ 50 f 40 76 – Mis-steered o n o 30 83 – External e i 20 c t material n a e l 120 84 – Nominal r u e m 100 86 – Mis-steered f i f s 80 87 – Nominal i ΔP ξ μ D n 60 91 – Low momentum e d 40 92 – Low momentum d i 68 70 71 72 74 75 76 83 84 86 87 91 92 93 93 – Low momentum h 14 data sets for ρ and δ, χ2 of 14.0 and 17.7 respectively 2 9 data sets used for Pμξ, χ = 9.7 statistical uncertainties only, after corrections 17 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Decay parameter results ρ = 0.74977 ± 0.00012 (stat) ± 0.00023 (syst) (<1σ from SM, -1.4x10-4 from blind) δ = 0.75049 ± 0.00021 (stat) ± 0.00027 (syst) (+1.4σ from SM, -2.3x10-4 from blind) +0.00165 π P μξ = 1.00084 ± 0.00029 (stat)-0.00063 (syst) (+1.2σ from SM, same as blind) Pπ ξδ/ρ > 0.99909 (90%CL) μ from global analysis R. Bayes et al., Phys. Rev. Lett. 106, 041804 (2011). J. Bueno et al., Phys. Rev.D (August). Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test 18 Decay parameter results ρ = 0.74977 ± 0.00012 (stat) ± 0.00023 (syst) -4 Implies(<1σ from a negative SM, -1.4 decayx10 fromrate! blind) Triggered a full analysis review. Identifiedδ = 0.75049 sensitivity ± 0.00021 to (stat) stop ± position,0.00027 (syst)target-dependent systematics.(+1.4σ from SM, -2.3x10-4 from blind) Final results are slightly modified. +0.00165 π P μξ = 1.00084 ± 0.00029 (stat)-0.00063 (syst) (+1.2σ from SM, same as blind) Pπ ξδ/ρ > 0.99909 (90%CL) μ from global analysis Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test 19 Implications for the Weak Couplings The final TWIST results have been included in a new muon decay global analysis together with all previous muon decay parameter measurements Find significantly tighter 90% c.l. upper limits on the coupling of right-handed muons to right- or left-handed electrons: . S | g RR| < 0.031 Factor of ~2 smaller than V . | g RR| < 0.015 pre-TWIST values . | gS | < 0.041 LR Factor of ~3 smaller than . | gV | < 0.018 LR pre-TWIST values T . | g LR| < 0.012 μ -4 New limit on right-handed muon couplings: Q R < 5.8x10 (90% c.l.) – Factor of ~9 smaller than pre-TWIST value Uncertainty for η reduced by 1/3 compared to 2005 global analysis – η = -0.0033 ± 0.0046 . Important for the determination of GF 20 Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test Left-Right Symmetric limit comparison “manifest” LRS, 90%CL generalized or non-manifest LRS, 90%CL 2 2 m2 > 592 GeV/c (gL/gR)m2 > 578 GeV/c -0.020 < ζ < +0.017 -0.020 < (gR/gL)ζ < +0.020 Other W’ direct search mass limits . ATLAS: >1.49 TeV/c2, 95%CL (LLWI11) Other limits on mixing angle ζ . CMS: >1.58 TeV/c2, 95%CL (LLWI11) . Hardy and Towner: <0.0005 (MLRS), <0.04 . CMS: >1.36 TeV/c2, 95%CL (2011) (generalized) 2 . CDF: >1.12 TeV/c , 95%CL (2011) . K decay: <0.004 (MLRS) . D0: >1.0 TeV/c2, 95%CL (2008) Art Olin: PANIC2011 TRIUMF Weak Interaction Symmetry Test 21 Inclusive Limits on μ→e X0 Limits when X0 is undetectable. Massive X0 Decays First search for two-body decays with possible asymmetries. Order of magnitude improvement for massive isotropic decays. Peak structures at the endpoint are produced by small momentum scale mismatches, so this sets the systematic limit. Decay Massless Jodidio more sensitive to Anisotropy B 90% CL isotropic signal but completely -6 insensitive to asymmetric decay. A= -1 52x10 -6 Limits on 3-body X0 decays are A=0 20x10 obtained from decay parameter A=+1 10x10-6 values. -6 Jodidio A=0 2.6x1022 Art Olin: PANIC2011 TRIUMF Weak Interaction SymmetryPRD34,1967(1986) Test Limits for heavy sterile neutrinos Muon decay spectrum shape places limits on heavy neutrino mass and mixing in a mass region inaccessible with π or K decays. R.R. SchrocK, Phys. Rev. D 24, 1275 (1981). P. KalyniaK and J.N. Ng, Phys. Rev. D 25, 1305 (1982). M.S. Dixit et al., Phys. Rev. D 27, 2216 (1983). Heavy sterile neutrino model Art Olin: PANIC2011 TRIUMF Weak InteractionS.N. Symmetry GninenKo, Test arXiv:1009.5536v3, Jan 2011 LLWI2011, Feb 20−26 2011 23 G.M. Marshall, Muon Decay Parameters fromTWIST Summary and Outlook TWIST has significantly improved the measurements of ρ, δ, and Pπ ξ. These results μ are in agreement with the Standard Model.
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