Fatiha Benmokhtar - Thomas Jefferson National Accelerator ... 1 Strange Quark Contribution to the Electromagnetic

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  • 1

    Strange Quark Contribution to the

    Electromagnetic Properties of the

    Nucleon,

    G0 Results

    Fatiha Benmokhtar

    Carnegie Mellon University

    Hall C Summer workshop, Aug 6-7th 2009

    fatiha@ernest.phys.cmu.edu

    F. Benmokhtar

  • 2

    • Physics Motivation

    • G0 Backward angle Setup

    • Some Analysis

    • From Physics Asymmetry to Strange FFs

    • Results

    • Summary

    Outline

    F. Benmokhtar

  • 3

    Nucleon Constituents

    - The sea contains all flavors, but

    • the u and d sea are hard to distinguish from the valence

    • the heavier quarks (c,b,t) are too heavy to contribute much

    • Strange quark is the natural candidate to study the sea.

    With how much do virtual strange pairs contribute

    to the structure of the nucleon ?

    .....+ gssdduuuudp 

    « sea= virtual pairs »valence

    F. Benmokhtar

  • 4

    Strange Quark Contribution to the Nucleon Properties

    NssNσ s  Hyp ->

    p-N ->  Up to 30 % with big theoretical

    uncertainties.

    DI -Nucleon scattering (NuTeV)

    %~dx))x(s)x(s(x 42 1

    0 

    • Mass:

    • Longitudinal Momentum:

    F. Benmokhtar

  • 5

    09.015.0 s

    N|ss|N 5 

    . -p elastic scattering (E734 BNL)

    • Polarized Semi-Inclusive DIS (HERMES)

    • PQCD predicts -0.1

    ** Accepted JLab HallB 12GeV proposal ( E09-007 )

    (H. Avagian, F. Benmokhtar, K. Hafidi, A. El-Alaoui, M. Marazita )

    F. Benmokhtar

    0.02 0.006 0.029 0.007s   

    • Spin:

  • 6

    Strangeness Contribution to the Nucleon Electromagnetic

    Properties

    ??NssN  s

    M

    s

    E GG ,

    Goal: Determine the contributions of the strange quark sea ( )

    to the charge and current distributions in the nucleon :

    “strange form factors” GsE and G s M

    ss

    How do we measure them? F. Benmokhtar

  • 7

    Strange Form Factors

    Z

    sin2qW = 0.2312 ± 0.00015

    Charge symmetry ->

    measured

    ?

    (see G. A. Miller PRC 57 (98) 1492.)

    F. Benmokhtar

    s

    MEW

    d

    MEW

    u

    MEW

    Z

    ME GGGG

    ,

    2

    ,

    2

    ,

    2

    , sin 3

    4 1sin

    3

    4 1sin

    3

    8 1 

      

     q

      

     q

      

     q

    s

    ME

    d

    ME

    u

    ME

    p

    ME GGGG ,,, ,

    , 3

    1

    3

    1

    3

    2 

    nspsnupdndpu GGGGGG ,,,,,, ;; 

      pZnpWs MEMEMEME GGGG ,,,2 ,,,, sin41  q

  • 8

    Parity Violation Asymmetry

     2Z

    2

    EM

    NC

    PV

    LR

    LR PV

    M

    Q ~

    M

    M ~

    σσ

    σσ A

     

    • Scatter polarized electrons off unpolarized target,

    • Asymmetries (R,L) of the order of ppm

    Electric Magnetic Axial

    5

    Z

    10 M

    M , 

    • Proton target

    • Deuteron target

    (static case)

    • Helium target  

      

     

    )(2 sin

    2

    2 2

    n

    E

    p

    E

    s

    E W

    F PV

    GG

    GQG A q

    p

    -Complete calculation by R. Schiavilla et al.,

    is available ( priv. communication)

    - Enhanced sensitivity to Axial form factor.

    p

    AME

    2

    F PV

    σ

    AAA

    24π

    QG A

      

      

     

    s

    EG

    np

    nnpp

    PV

    AA A

    ss

    ss

     

    e

    AM

    e

    AWA

    s

    MM

    Z

    MM

    s

    EE

    Z

    EE

    GGG)sin(A

    GGGA

    GGG)(A

    

    

    

    q

    t

    q

    241

    Left

    Handed

    spi

    n Right

    Handed Direction of motion

    Left

    Handed

    spin Right

    Handed Direction of motion

    F. Benmokhtar

  • 9

    PV. Experiments

    H, DHH, 4HeHH,DTarget

    F/BF/BFFBAngle

    Ge s, GM

    s , GA (p+n)GE

    s, GM sGE

    s , GM sGE

    s + 0.4 GM

    s GM

    s, GA

    (p+n) Separation

    0.12 - 1.0

    (0.23, 0.62)

    0.1, 0.23

    (0.23,0.6) 0.10.480.04, 0.1Q2 (GeV/c)2

    G0

    (JLab)

    2003-2007

    PVA4

    (MAMI)

    2002-2008

    HAPPEX II

    (JLab)

    2004-2005

    HAPPEX I

    (Jlab)

    1998-2002

    SAMPLE

    (MIT-Bates)

    1998-2002

    Beam

    LH2Target

    Superconducting Coils ParticleDetectors

    • HAPPEX-III in preparation: 2009 at 0.6 GeV2

    e

    AM

    e

    AWA

    s

    MM

    Z

    MM

    s

    EE

    Z

    EE

    GGG)sin(A

    GGGA

    GGG)(A

    

    

    

    q

    t

    q

    241

    Forward

    Backward

    F. Benmokhtar

      

    )1()1(

    tan)1(21

    4

    2

    12

    2

    2

    2

    tt

    t

    t

    q

    

    

    M

    Q

  • 10

    Results from the Forward Angle

    Compared to ANVS (“No VectorStrange”)

    EM form factors : Kelly PRC 70 (2004) 068202

    D.S. Armstrong, et al., PRL 95, 092001 (2005)

    G0 Backward

       NVSphys

    V

    p

    E

    p

    M

    p

    E

    F

    s

    M

    s

    E AA RG

    GG

    QG GG 

     

    )0(

    22

    2 1

    24

    tp 

    Examining full data set,

    probability that

    GE s+GM

    s ≠ 0 is 89%F. Benmokhtar

  • 11

    G0 Backward Angle Experiment

    (06-07)

    F. Benmokhtar

  • 12

    12

    Experimental Setup

    • Electron Beam: 362 and 687 MeV -> Q2: 0.23 and 0.62 GeV2

    • Electron off 20 cm LH2 or LD2 target, electron detection : Θ =108°, W ~ 0.5 sr.

    •Turn-around of the magnet, change polarity

    • Add Cryostat Exit Detectors (9 CEDs per Octant)-> separate elastic

    and inelastic electrons in the CED*FPD space.

    • Aerogel Cerenkov detector per octant for p/e separation. (pp < 500 MeV/c)

    Cerenkov

    CED

    FPD

    electrons

    pions

    FPD C

    E D

    C E

    D

    Elastic

    Inelastic

  • 13

    G0 beam monitoring

    Spokesman

    Target service module

    Detectors(Mini- Ferris wheel)

    CED+Cherenkov

    Sup. Cond. Magnet (SMS)

    Detectors (Ferris Wheel)

    FPD

  • 14

    Beam Specifications

    Acor  Ameas - 1

    2Y i

     Y

    Pi Pi

    • Helicity correlated beam properties

    -> false asymmetry.

    Correction : linear regression:

    • 2ns beam structure

    • 86 % longitudinal polarization

    • Helicity changed every 1/30 sec (MPS).

    • Form Asym. from a pseudo-random

    quartet structure in helicity (+--+ or -++-).

    • Slow helicity reversal: 1/2 wave plate IN and OUT

    ~0.1ppm

    F. Benmokhtar 10 -6

    < 0.3 x 10 -6

    Beam halo

    34 eV2.5 +/- 0.5Energy difference

    4 nrad0.0 +/- 0.1y angle difference

    4 nrad-0.8 +/- 0.2x angle difference

    40 nm-17 +/- 2y position difference

    40 nm-19 +/- 3x position difference

    2 ppm0.09 +/- 0.08Charge asymmetry

    “Specs”Achieved (IN-OUT)/2Beam Parameter

  • 1515

    Recorded Data (Hz/μA/Oct)

    45 C

    LH2, 687 MeV

    LD2, 687 MeV

    LH2, 362 MeV

    LD2, 362 MeV

    90 C 120 C

    70 C 45 C

  • 1616

    Raw Blinded Asymmetries

    per Kinematics

    octant # (azimuthal distribution)

    F. Benmokhtar

    LH2 362 MeV

    LD2 687 MeVLH2 687 MeV

    LD2 362 MeVLH2 362 MeV

  • 1717

    Blinding Factors

    X0.75-1.25

    H, D Raw Asymmetries, Ameas

    Corrections Scaler counting correction

    Rate corrections from electronics

    Helicity-correlated corrections

    Background asymmetry

    Beam polarization

    Nucleon EM form factors

    Q2 Determination

    Analysis Strategy

    H, D Physics Asymmetries