Weak probe of the nucleon in electron scattering by gegeshandong

VIEWS: 1 PAGES: 20

									                     Weak probe of the nucleon
                      in electron scattering

                                 •Weak Current far from
                                  the Z pole

                                 •History

                                 •Elastic Scattering and strange
                                  content of the nucleon

                                 •Perspectives in charged
                                  current and DIS kinematics

                                 •Conclusion


David Lhuillier , CEA Saclay
     Principle                                                               2
                                             e’                         e’
                                    e                      e

                                            g(Q2)
         e-   N          e- X   =                      +           Z0


                                        N         X            N         X
                                                       Nice perturbative probe
     Q2 << MZ2 at typical “hadronic scale”

     But weak interaction violates P

       Build a pseudo-scalar observable, prop. to MZ:

                                                    ALR~GFQ2/4pa
                                             ALR~10-4 at Q2=1 (GeV/c)2


David Lhuillier , CEA Saclay
                A Bit of History: E122 @ SLAC

                                                       e’
             DIS off deuteron target:       e
                                                                 x=Q2/2Mn
                                                   g   ,Z0
                                                                   y=n/E

                                            2H
                                                             X




  •Isoscalar target: fu(x)=fd(x)        x dependence factorizes out
  •Neglect sea distributions: x > 0.2


David Lhuillier , CEA Saclay
                A bit of History: E122 @ SLAC

                                                                        C.Y.Prescot et al,
                                                                   Phys. Lett. 84B, 524 (1979)




      First constraint establishing
      the Weinberg-Salam Model:




                                                          Hybrid
                                                              sin2qW=
                                                          0.224+/-0.020

           Extensions:
           •Low x: probe sea contribution with sin2qW as input
           •Larger x: precise test of sin2qW
            (Jlab LOI, complementary to E158 and Qweak)


David Lhuillier , CEA Saclay
                       Parity Violation in e- Scattering


                             Test SM     Elastic
     Factor 1000 !



                                          e--N




                     Mutual enrichment between test of SM and hadronic structure
                     Impressive performances of polarized beam

David Lhuillier , CEA Saclay
                           Experimental Techniques
                                                     Injector          Beam line       Hall
Key elements:                    Source

 •High Pe and I                            l/2

 •Rapid helicity flip
 •Feedbacks
                                                                “Table-Top” Experiments




        Half-wave plate reversals

                                     IN


                                     OUT                            Charge asymmetry
                                                                        at 1 GeV



                                                 Final: AI=-8.4+/-7.8 ppb (10-9) !

  David Lhuillier , CEA Saclay
                               Elastic Scattering

                                         e’                             e’     2
                                e                       e
                                        g(Q2)
  e-   N           e- N   =                       +                Z0


                                    N         N             N           N




             Electromagnetic FF
                                                                         Weak FF
                      GFQ2                                      few ppm
           ALR=                 f(F1g,F2g,F1Z,F2Z,GA)       at Q2<1 (GeV/c)2
                     2pa 2

David Lhuillier , CEA Saclay
  Strange content of the Nucleon

    Sum over 3 quarks flavours (u,d,s)
    weighted by elec. and weak charges


                                                     Charge symmetry




     Extraction of the strange quarks contribution

                                                                Same relations
   3x3 equations                                               stand for GE/GM…




   Motivation triggered by DIS results on Ds but the measured matrix elements
   are independent: vector <N|sgms|N> instead of pseudo-vector <N|sgmg5s|N>


David Lhuillier , CEA Saclay
                     Leading Non Zero Moments
                               Measurements at low Q2

   Strange Charge Radius:




                                            <rs2>Sachs (fm2)
               Scale: <r2n>Sachs=0.12 fm2


   Strange Magnetic Moment:




                        Scale: mN


David Lhuillier , CEA Saclay
                               Elastic e-p Asymmetry




                                     ms= -0.3

                                                         Forward angle:
                                  <rs2>Sachs= 0.33 fm2      GsE,GsM


                                                         Backward angle:
                                                             GsM,GA



David Lhuillier , CEA Saclay
                          Experimental Strategy

Complete Separation GE/GM/GA
                                              Rosenbluth Method
   over Q2=0.11 (GeV/c)2
        Forward and backward measurements with the same detector setup




              G0 @ Jlab                        PVA4 @ MAMI
          Q2=0.3-0.8 (Gev/c)2              Q2=0.1 and 0.25 (Gev/c)2
               but not enough lever arm to extract the 3 contributions


David Lhuillier , CEA Saclay
                          Experimental Strategy

Complete Separation GE/GM/GA
                                                     Different targets
   over Q2=0.11 (GeV/c)2

     Quasi-elastic e-Deuteron                 Elastic e-4He:   Isoscalaire Transition pure GE




                          SAMPLE
                         MIT-Bates
                                                                    HAPPEx2
                                                                     JLab
                                     Q2=0.1 GeV/c2




David Lhuillier , CEA Saclay
                           Experimental Program

Q2 dependence
   G0, PVA4, HAPPEx1

    0.2 < Q2 < 0.8 (GeV/c)2


First moments
   SAMPLE, HAPPEx2, PVA4

         Q2 = 0.1 (GeV/c)2

   •First Constraint on ms:
       ms=0.37+/-0.20stat+/-0.26syst+/-0.15theo

   •Expected combined accuracy:
       dms ~ 0.16 mN           d<rs2>Sachs ~ 0.020 fm2 ~ <rn2>Sachs/6

David Lhuillier , CEA Saclay
                    Weak Charged Current
                      in e-p scattering                             0
                                                                W- Z W+


                         The NEPTUNE experiment


                               n
       e-
                     W-            Letter Of Intent at JLab (A. Deur)

                                         •High luminosity
                                     +   •Direct measurement
            p                  n
                V                    -   •Only detected particle = n
            GE,M GA GP



David Lhuillier , CEA Saclay
                    NEPTUNE Experimental Setup


                                       •Background reduction:
                                            Backward kinematic
                                            Sweepnig magnet
                                            Recoil detector
                                            Pulsed beam

                                             S/B~0.1
 e-


                                           •APV=10%
                                             Accurate
                                             normalization
Symmetric w.r.t. beam axis                   despite bad S/B.
    (not represented)




   David Lhuillier , CEA Saclay
                    NEPTUNE Projected Results

           n                   p electroproduction         e-pnn
                                                     GA   E=1-2 GeV
                                                          qe=120deg
                                                          Q2=0.8-2.7




                                                                       Q2
                                                                    (GeV/c)2

        Accurate data in an unexplored kinematical domain
         of a fundamental observable of the nucleon


David Lhuillier , CEA Saclay
          Parity Violating Asymmetry in e-p DIS

                                                                  e’
                                                   e+,e-

                                                             g ,Z0

                                                                       X
                                                      p




          Extract G3  separate valence and sea contributions …

David Lhuillier , CEA Saclay
         Parity Violating Asymmetry in e-p DIS

 Single spin asymmetry:

                                      •G3 term is suppressed
                                      •but ratio G2/F2 depends on d/u
                                      •ALR ~ 10-5 Q2, allows accurate
                                       measurements


  Charge asymmetry:


          [se+-se-] a xG3      •First data from HERA,
                                Stat. Limited
                               •Intense neutrino beams


David Lhuillier , CEA Saclay
                               Conclusion


      Weak Neutral Current in e- scattering:

             •New probe of the nucleon structure
             •Important experimental program ongoing to extract
              the contribution of strange quarks to the vector
              matrix element <N|sgms|N>.


       Development of experimental techniques

               •Perspectives in charged current (GA)
                and DIS neutral current (d/u).
               •Precise test of the interaction itself


David Lhuillier , CEA Saclay
                               Back to the Future




 •New results from ALR in Moller scattering from E158 at SLAC.
 •Parity violating asymmetries allow precise test of SM at low energy

David Lhuillier , CEA Saclay

								
To top