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Proton structure at high Q 2 at HERA

Proton structure at high Q 2 at HERA. Yujin Ning. Introduction Experiment setup High Q 2 PDF from HERA I New PDF from HERA II High x study in ZEUS Summary & outlook. Introduction. Deep inelastic scattering (DIS). an important tool to study the proton structure. Deep: small scale

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Proton structure at high Q 2 at HERA

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  1. Proton structure at high Q2 at HERA Yujin Ning Introduction Experiment setup High Q2 PDF from HERA I New PDF from HERA II High x study in ZEUS Summary & outlook

  2. Introduction

  3. Deep inelastic scattering (DIS) an important tool to study the proton structure Deep: small scale Inelastic: proton is broken Lorentz-invariant variables: Q2=-q2=-(k-k’)2Virtuality: scale x=-q2/2pq Parton momentum fraction y=pq/pk Inelasticity: transferred energy fraction W2=(p+q)2 Square of the invariant mass of hadronic final state s Square of center-of -mass  or Z0:neutral current (NC) ep->eX W:charged current (CC) e-p->X and e+p->X

  4. Neutral current cross sections and structure functions At Born level (Lowest order in QED) Longitudinal structure function  valence + sea partons dominant  valence quarks important at high Q2 Parton density function(PDF) probability density finding parton q or q carry the momentum fraction x with Q

  5. Charged current cross sections sensitive to d quark sensitive to u quark MW=80.4GeVCross sections are suppressed CC good to disentangle flavor content of the proton (u or d by using e+/e-)

  6. Experiment setup

  7. HERA ep collider Ee=27.6GeV Ep=920(820 before year 98)GeV ep collisions x z Study the inner structure of proton and fundamental forces

  8. ZEUS NC and CC events Scattered neutrino • NC: • Well isolated electron with high transverse momentum • Net transverse momentumis zero • CC: • Large missing transverse momentum from the undetected neutrino

  9. HERA I PDF

  10. ZEUS NC and CC cross sections NC CC ZEUS-JETS (a new QCD fit using ZEUS inclusive cross section and jet production) describes data well

  11. H1 structure functions F2 contribution from  exchange xF3 FL from  exchange Q2<890GeV2 Good agreement with H1 PDF 2000

  12. ZEUS-JETS PDF • Only ZEUS data used in ZEUS-JETS, well understood systematic uncertainty • High Q2 NC/CC (94-00), constrain at high x • DIS inclusive jet and dijet in photo production (PHP) (96-97), jet cross section is sensitive to gluon density • First time HERA jet used in QCD fit ZEUS-JETS constrain the uncertainty on gluon density

  13. HERA II PDF

  14. HERA update HERA II Ee=27.6 GeV Ep=920 GeV sqrt(s)=318 GeV e- 205 pb-1 • Longitudinally polarized lepton beam: sensitive to weak interaction • Increase specific luminosity: increase statistics at high Q2 e- 62 pb-1 e+ 85 pb-1

  15. ZEUS NC cross section and xF3 xF3  valence quark More precise measurement of xF3 with 120 pb-1 e- data, 6 times more than HERA I Unpolarized reduced cross section for ep

  16. ZEUS CC e-p cross sections Higher statistics, good agreement

  17. ZEUS-pol QCD fit with HERA II polarized lepton beam ZEUS-JET+polarized e-p All EM parameters fixed to SM value Center value does not change compared to ZEUS-JETS Uncertainties reduced, especially on u valence quark

  18. High x study in ZEUS

  19. Motivation for high x study • DGLAP does not predict x-dependence • PDFs decrease very quickly at high x, hard to measure because of low statistics and large migration • Highest measured point x=0.75, BCDMS, data is available at higher x but at low W, need huge correction. • HERA covers much larger phase space to high Q2 and low x, but not to high x • ZEUS published x=0.65 • The uncertainties on PDF grow with x, might be infinite at x=1 New reconstruction methods are needed to reach the highest x

  20. ZEUS published method • Double angle method (DA) • All the energy deposits in CAL: scattered electron and all others->hadron • Electron polar angle • Spatial distribution hadronic deposits->hadronic polar angle • Insensitive to CAL overall energy scale • Sensitive to the accurate simulation of all effects in CAL • Hadronic energy lost in beam hole at high x, no accurate reconstruction on x • Highest published point, x=0.65

  21. New method • Electron + Jet • Electron is well reconstructed for Q2>450GeV2, very high acceptance in whole x range • Define Q2 bins from Ee and e: • In each Q2 bin, define x bins: • If leading jet is NOT near the beam hole • low x, jet is well reconstructed, x from Ejet and jet, good resolution • If jet IS near beam hole • high x, jet is not well reconstructed  no jet, count events without jet xedge<x<1 integral of cross section

  22. Compare with ZEUS published In the ZUES published kinematic region, new method agrees well with published result New method measures cross section to x=1

  23. Measured cross section low x x jet in FCAL high x Agree well at measured low x region More information on high x jet lost in beam hole

  24. Cross section ratio to theory Theories describe data well at low x. Expectation is lower than data in the highest x bin, unmeasured before With new method, the cross section at high x was measured as precisely as other x range.

  25. Summary • A lot of work done on PDF: ZEUS-JETS and H1 PDF 2000. • A new method measure cross section to x=1. • The luminosity delivered by HERA II is more than 350 pb-1. • NC and CC DIS cross section for e+p and e-p are measured with HERA II and agree with the SM predictions. • New fit including HERA II polarized data, the uncertainty is reduced • HERA II will totally delivery 1fb-1, more precise measurement.

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