Exploring Muon Decay with TWIST by wulinqing

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									                       Exploring Muon Decay
                                with
                               TWIST
                                    Carl A. Gagliardi
                                   Texas A&M University
                          for the TWIST Collaboration
                                         Outline
                                • Physics of muon decay
                                • TWIST experiment
                                • TWIST results to date
                                • How will we do better?

Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06   1
                       Muon decay matrix element
     • Most general local, derivative-free, lepton-number conserving muon
       decay matrix element:
                    4GF
                 M
                      2
                                    
                                    g e |  | ( e ) n (  ) m |  | 
                                  S ,V ,T
                                 ,  R ,L
     • In the Standard Model, gVLL = 1, all others are zero
     • Pre-TWIST global fit results (all 90% c.l.):




     • Theoretical constraints have recently been derived on the LR and
       RL terms from neutrino mass limits (hep-ph/0608163)
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                        2
                             Muon decay spectrum
     • The energy and angle distributions of positrons following
       polarized muon decay obey the Michel spectrum:
                      d 2                    2                 x
                                  (3  3x )   ( 4 x  3)  3 0 (1  x )
                x 2 dxd (cos  )              3                  x
                                                 2            
                             P cos (1  x )   ( 4 x  3)
                                                 3            
                                   Ee
                      where  x
                                 E e ,m ax
     • Pre-TWIST accepted values for the Michel parameters:
                                                                 SM
                           = 0.7518 ± 0.0026                    3/4
                           = -0.007 ± 0.013                     0
                          P = 1.0027 ± 0.0079 ± 0.0030         1
                           = 0.7486 ± 0.0026 ± 0.0028           3/4
                          P(/) > 0.99682 (90% c.l.)          1

Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                      3
                                     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
                        1 1     16 
                     QR  1     
                         2 3      9  
     • Left-right symmetric models
           3     3 2                                     ML   ML  
                                                               2      4

                                  1  P  4    
                                                    2
                                                               
                                                         M  M    
           4     2                                       R  R 
                                                 
     • …..

Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                   4
                                  What is required?




          Must:
             • Understand sources of muon depolarization
                     -- Pμ and ξ come as a product
               • Determine spectrum shape
                     -- All three parameters
               • Measure forward-backward asymmetry
                     -- For Pμξ and δ
          to within a few parts in 104
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06   5
                               Surface muon beam

                              TRIUMF
                             M13 beam line




Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06   6
                          TWIST spectrometer




Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06   7
            Typical events

     • Use pattern recognition (in
       position and time) to sort hits
       into tracks, then fit to helix




     • Must also recognize beam
       positrons, delta tracks,
       backscattering tracks



Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06   8
                    2-d momentum-angle spectrum
                                           In angular
                                              fiducial




                                       In momentum
                                            fiducial


                      Acceptance of the TWIST spectrometer
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06     9
                                                           αMC hidden
      Fitting the data distributions                        blind analysis




  • Fit data to sum of a MC
    base spectrum plus MC-
    generated derivative
    distributions.

  • Decay distribution is
    linear in the Michel
    parameters, so this is
    exact, no matter what
    values (αMC) are used in
    the MC base spectrum.



Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                      10
                                  Physics data sets
     • Fall 2002
           – Test data-taking procedures and develop analysis techniques
           – First physics results – ρ and δ
           – Graphite-coated Mylar target not suitable for Pμξ


     • Fall 2004
           – Al target and Time Expansion Chamber enabled first Pμξ
             measurement
           – Improved determinations of ρ and δ are underway


     • 2006-07
           – Achieve ultimate TWIST precision for ρ, δ, and Pμξ



Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                   11
         Fitting the 2002 data to determine ρ and δ




  Normalized residuals
  [(Data-Fit)/sigma] of the 2-d momentum-angle fit
  Fit describes the data well, even when
  extrapolated far outside the fiducial region



                                    Angle-integrated results

Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06       12
             Fitting the 2004 data to determine Pμξ




                                                           Separating the asymmetry
                                                           into components




Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                              13
                                     Results to date
     • From Fall, 2002 run:
           – ρ = 0.75080 ± 0.00032 (stat) ± 0.00097 (syst) ± 0.00023 (η)
                                      PRL 94, 101805
           – δ = 0.74964 ± 0.00066 (stat) ± 0.00112 (syst)
                                      PRD 71, 071101
     • New global analysis (PRD 72, 073002) using the ρ and δ
       results, together with previous measurements and recent
       e+ transverse polarization measurements (PRL 94, 021802):
           – Significant improvements in the limits for gS,V,TLR
           – η = -0.0036 ± 0.0069
     • From Fall, 2004 run (so far):
           – Pμξ = 1.0003 ± 0.0006 (stat) ± 0.0038 (syst)
                                      PRD 74, 072007
     • Factors of 2-3 improvements on pre-TWIST precisions

Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                   14
          New limits in left-right symmetric models
         Restricted (“manifest”) LRS model                 General LRS model




         Initial TWIST measurements already provide significant new limits


Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                       15
                                     Results to date
     • From Fall, 2002 run:
           – ρ = 0.75080 ± 0.00032 (stat) ± 0.00097 (syst) ± 0.00023 (η)
                                      PRL 94, 101805
           – δ = 0.74964 ± 0.00066 (stat) ± 0.00112 (syst)
                                      PRD 71, 071101
     • New global analysis (PRD 72, 073002) using the ρ and δ
       results, together with previous measurements and recent
       e+ transverse polarization measurements (PRL 94, 021802):
           – Significant improvements in the limits for gS,V,TLR
           – η = -0.0036 ± 0.0069
     • From Fall, 2004 run (so far):
           – Pμξ = 1.0003 ± 0.0006 (stat) ± 0.0038 (syst)
                                      PRD 74, 072007
     • Factors of 2-3 improvements on pre-TWIST precisions

Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                   16
       Systematics in the previous measurements




                                                       The same effects tend to dominate
                                                       the systematic uncertainties for all
                                                       three parameters.


Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                                      17
                 Reducing the leading systematics
     • Issues that were unique to 2002 data
          – Stopping target thickness uncertainty
          – Chamber orientation uncertainty with respect to magnetic field


     • For all three Michel parameters
          – Chamber response
             • Improved gas system regulation and monitoring
             • Improved determination of foil geometry
             • Improved treatment of drift chamber behavior
          – Positron interactions


     • Specific to Pμξ
          – Muon depolarization when crossing fringe field
          – Muon depolarization in the stopping target

Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                     18
          Controlling positron interaction uncertainties –
                        “upstream stops”
     • Our Monte Carlo must simulate positron interactions properly
     • Need a well-understood test beam to validate the Monte Carlo

     • We use the Michel spectrum!




Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06              19
       Validating the Monte Carlo with “upstream stops”
                       (Downstream fit result) – (upstream fit result)




             Bremsstrahlung                                    Scattering

     • The TWIST Monte Carlo provides an excellent description of the
       hard interaction physics
     • Took ~50 times more upstream stop events in 2004 than in 2002
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                    20
        Muon depolarization across the fringe field
                                              Use Time Expansion Chamber (TEC) to
                                              measure and optimize the muon beam




                                               •   First installed for 2004 run
                                               •   Found vertical beam offset – now corrected
                                               •   Now take frequent beam characterizations
                                               •   Have techniques to identify when the beam
       2004 muon beam spot                         changes between TEC measurements
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                                        21
                Muon depolarization after stopping




       • Observed significant depolarization in 2004 data
       • New techniques allow us to veto muons that stop outside the
         metal stopping target
       • More sensitive analysis procedures to determine the residual
         depolarization rate
       • Now taking data with a Ag target to explore material dependence
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                   22
                                        Conclusions
     • The initial TWIST measurements have improved our
       knowledge of the Michel parameters ρ, δ, and Pμξ by
       factors of 2-3

     • Improvements by additional factors of 3-5 are anticipated

     • Stay tuned!




Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06           23
                                 TWIST Participants
              TRIUMF                             Alberta                        Kurchatov Institute
           Ryan Bayes ¤y                    Andrei Gaponenko ¤¤
           Yuri Davydov                                                      Vladimir Selivanov
                                            Peter Kitching                   Vladimir Torokhov
           Jaap Doornbos                    Robert MacDonald ¤
           Wayne Faszer                     Maher Quraan
           Makoto Fujiwara                  Nate Rodning x                      Texas A&M
           David Gill                       John Schaapman
           Alex Grossheim                                                    Carl Gagliardi
                                            Glen Stinson                     Jim Musser ¤¤
           Peter Gumplinger
           Anthony Hillairet ¤y                                              Bob Tribble
                                               British Columbia
           Robert Henderson                                                  Maxim Vasiliev
           Jingliang Hu                     James Bueno ¤
           John A. Macdonald x              Mike Hasinoff                       Valparaiso
           Glen Marshall                    Blair Jamieson ¤¤
           Dick Mischke                                                      Don Koetke
           Mina Nozar                            Montréal                    Paul Nord
           Konstantin Olchanski             Pierre Depommier                 Shirvel Stanislaus
           Art Olin y
           Robert Openshaw                       Regina                      ¤ Graduate student
           Tracy Porcelli z                                                  ¤¤ Graduated
           Jean-Michel Poutissou            Ted Mathie
                                            Roman Tacik                      y also U Vic
           Renée Poutissou
                                                                             z also Manitoba
           Grant Sheffer
           Bill Shin zz                                                      zz also Saskatchewan
                                                                             x deceased

                Supported under grants from NSERC (Canada) and DOE (USA).
                Additional support from TRIUMF, NRC (Canada), and the Russian Ministry of Science.
                Computing facilities of WestGrid are gratefully acknowledged.

Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                                              24
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06   25
        Coupling constants and Michel parameters
     • The Michel parameters are bilinear combinations of the
       coupling constants:




Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06        26
                                      Detector array




    •   56 low-mass high-precision planar chambers symmetrically placed around thin
        target foil
    •   Measurement initiated by single thin scintillation counter at entrance to detector
    •   Beam stop position controlled by variable He/CO2 gas degrader

Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                                     27
                                   Analysis method

         • Extract energy and angle distributions for data:
              – Apply (unbiased) cuts on muon variables.
              – Reject fast decays and backgrounds.
              – Calibrate e+ energy to kinematic end point at 52.83 MeV.

         • Fit to identically derived distributions from simulation:
              –   GEANT3 geometry contains virtually all detector components.
              –   Simulate chamber response in detail.
              –   Realistic, measured beam profile and divergence.
              –   Extra muon and beam positron contamination included.
              –   Output in digitized format, identical to real data.




Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                        28
                 Data set by data set 2-d fit results
                    2002
                     ρ
                                                           2004
                                                            Pμξ




                        δ




Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06          29
               Recent muon decay global analysis
                                                                PRD 72, 073002



                                                           These improvements arise
                                                           from the TWIST ρ and δ
                                                           measurements.




     • Fit also finds η = -0.0036 ± 0.0069, a factor of ~2 more precise than
       the previously accepted value, -0.007 ± 0.013.
     • Significant improvement in η comes from new measurements of the
       transverse polarization of the e+ in muon decay (PRL 94, 021802), but
       the TWIST ρ and δ measurements also play an important part.
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                              30
                   Validating the soft physics simulation
                           with “upstream stops”




                            dE/dx                          Multiple Scattering

            • Multiple scattering is well reproduced
            • Small differences in dE/dx are seen between data and MC
               – Exploring differences between GEANT3 and GEANT4

Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                         31
                             Tracking (in)efficiency




      • Measured in upstream stop events
      • Reconstruct track upstream, then ask if downstream also reconstructs
      • Overestimates
  Difference between true inefficiency – not all tracks reach the downstream half
  inefficiencies in data and
  Monte Carlo events

Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06                            32

								
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