Where%is%the%Proton%Spin?% - Stony Brook drees/PHY599/ %R.%Wood%%%%%@%%%%%Proton%Spin%

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Text of Where%is%the%Proton%Spin?% - Stony Brook drees/PHY599/ %R.%Wood%%%%%@%%%%%Proton%Spin%

  • Where%is%the%Proton%Spin?%

    Kevin%R.%Wood%Abhay%Deshpande,%Stacy%Karthas%

    September%21,%2015%

    1%Kevin%R.%Wood%%%%%@%%%%%Proton%Spin%

  • 2%Kevin%R.%Wood%%%%%@%%%%%Proton%Spin%

    Where%is%the%Proton%Spin?%

    Quark&Polariza,on&

    %@%%SLAC@Yale%&%EMC%polarized%DIS%measurements%

    Gluon&Polariza,on&

    %@%%RHIC%polarized%proton@proton%collisions%

    Parton&Angular&Momentum&

    %@%%Plans%for%the%EIC%

    Summary%

  • Spin%from%Quarks?%

    Qp = +1= qq

    ?%

    The%charge%of%the%proton%is%completely%accounted%for%by%the%sum%of%that%of%its%consQtuent%quarks.%

    s = 12

    s = 12

    s = + 12

    u%q = 2

    3

    u%q = 2

    3

    d%q = 1

    3

    Sp =12= Sq

    12?%

    Is%the%situaQon%for%the%spin%of%the%proton%analogous%to%that%of%its%charge?%

    3%Kevin%R.%Wood%%%%%@%%%%%Proton%Spin%

  • %%Use%deep%inelasQc%scaUering%of%polarized%(&%electrically%charged)%lepton%beam%off%%%%%%protons%polarized%parallel%and%anQ@parallel%with%respect%to%the%beam%

    4%

    longitudinally%polarized%lepton%beam%

    fixed%proton%targets%polarized%(anQ@)parallel%to%beam%

    %%Measure%the%asymmetry%in%differenQal%cross%secQon%[1]:%

    %%%%where%E%is%the%energy%of%the%incoming%lepton%and%E%that%of%the%scaUered%lepton%Kevin%R.%Wood%%%%%@%%%%%Proton%Spin%

    A =d ( ) d ( )d ( ) + d ( ) d

    d 2 E,E ',( )ddE '

    Measuring%Quark%PolarizaQon%Inside%the%Proton%%

  • 5%Kevin%R.%Wood%%%%%@%%%%%Proton%Spin%

    %%Experimentally,%one%counts%the%number%of%scaUered%leptons%(per%energy%and%solid%angle)%and%calculates%the%asymmetry%[2]:%

    %%If%the%nave%model%menQoned%previously%holds%true,%we%would%expect%this%asymmetry%%%%%to%be%%%=%1/3%%

    *%

    s = !

    2 s = !

    2

    s = + !

    2

    =N ( ) N ( )N ( ) + N ( )

    %%Leptons%interacts%with%a%quark%electro@%%%%magneQcally%via%a%virtual%photon%(spin%1)%

    %%Thus%the%lepton%switches%its%spin%state%from%+%%%%%%to%@%by%emiing%a%+1%spin%photon%

    %%Absorbing%this%photon%changes%the%spin%state%%%%of%the%quark%(spin%%parQcles);%only%spin%@%%%%%quarks%can%do%this%

    %%The%anQ@aligned%protons%has%twice%as%many%%%%%%spin%@%quarks%as%the%aligned%protons%

    Measuring%Quark%PolarizaQon%Inside%the%Proton%%

  • Kevin%R.%Wood%%%%%@%%%%%Proton%Spin% 6%

    (1978)%SLAC@Yale:%DIS%with%polarized%~25%GeV%electrons%[1]%

    (1989)%EMC:%DIS%with%polarized%~200%GeV%muons%[3]%

    The&quarks&spins&only&account&for&a&small&por,on&(~30%)&of&the&Protons&spin!&

    Measuring%Quark%PolarizaQon%Inside%the%Proton%%

  • Kevin%R.%Wood%%%%%@%%%%%Proton%Spin% 7%

    More%to%the%Proton%

    Sp =12 12

    But%the%proton%also%consists%of%gluons!%

    Gluons%are%spin%1%parQcles..%

    Could%there%be%a%net%gluon%polarizaQon%within%the%proton%to%account%for%the%spin?%

    Sp =12= 12 + g?%

    [5]%

  • Kevin%R.%Wood%%%%%@%%%%%Proton%Spin% 8%

    Measuring%Gluon%PolarizaQon%Inside%the%Proton%%

    %%The%RelaQvisQc%Heavy%Ion%Collider%(RHIC)%then%measured%the%contribuQon%from%the%%%%%%gluon%spin%(spin%1%parQcle)%

    %%Collide%longitudinally%polarized%protons%with%(anQ)aligned%polarizaQons%

    %%Measure%cross%secQon%asymmetry%for%neutral%pion%producQon%

    %%%%tags%a%strong%interacQon%(mediated%by%%%%%gluons)%

    %%An%emiUed%gluon%will%only%be%absorbed%by%%%%%a%parton%with%the%proper%spin%

  • Kevin%R.%Wood%%%%%@%%%%%Proton%Spin% 9%

    Measuring%Gluon%PolarizaQon%Inside%the%Proton%%

    There%is%a%net%gluon%polarizaQon%within%the%proton!%

    %But%its%sQll%not%enough%

    Gluons&only&contribute&an&addi,onal&~30%&of&the&protons&total&spin.&

    Where%is%the%addiQonal%~40%%of%the%protons%spin?%

    In%the%orbital%moQon%of%the%partons?&

    Sp =12 12 + g

    Sp =12= 12 + g + Lq + Lg

    ?%

    Phys.%Rev.%LeU.%115,%092002%(2015)%

  • Kevin%R.%Wood%%%%%@%%%%%Proton%Spin% 10%

    Measuring%the%Parton%Angular%Momentum%

    %%The%proposed%Electron%Ion%Collider%(EIC)%plans%to%take%advantage%of%the%resoluQon%from%%%%polarized%DIS%experiments%and%the%highly%relaQvisQc%protons%from%collider%experiments%

    *%Will%also%drasQcally%improve%the%precision%of%%%%%and%g%

    [5]%

  • Summary%

    11%Kevin%R.%Wood%%%%%@%%%%%Proton%Spin%

    The%proton%gets%its%spin%from%a%variety%of%its%consQtuents,%not%just%the%valence%quarks.%

    We%can%measure%the%various%contribuQons%from%polarized%DIS%and%collision%experiments.%

    There%is%sQll%spin%that%is%unaccounted%for,%but%the%next%generaQon%experiment,%EIC,%hopes%to%seUle%the%maUer.%

    Understanding%of%the%proton%spin%can%be%applied%to%other%to%other%hadrons.%

  • References%

    [1]%%Phys.%Rev.%LeU.%41&(2)%70@73%(1978)%

    [2]%%Gerhard%K.%Mallot,%Spin*of*the*nucleon*in*deep*inelas2c*lepton*sca3ering*experiments,*Conf.:9201264*(1992)*

    [3]%%Nucl.%Phys.%B%328&1@35%(1989)%

    [4]%%Phys.%Rev.%LeU.%115,%092002%(2015)%[5]%%arXiv:1212.1701%Electron%Ion%Collider:%The%Next%QCD%FronQer%

    Cover%art:%hUp://www.physastro.iastate.edu/research/highlights/unlocking@mystery@proton@spin%

    12%Kevin%R.%Wood%%%%%@%%%%%Proton%Spin%