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Exploring Proton Structure at High Q²: Insights from HERA Experiments

This study investigates the inner structure of protons utilizing deep inelastic scattering (DIS) measured at the HERA collider, focusing on high Q² and high x values. The experimental setup featuring an ep collider allows for precise measurements of neutral and charged current cross sections, leading to advancements in parton distribution functions (PDFs). The analysis includes new methods for measuring jets in high x regions, enhancing our understanding of valence quarks and gluon density. Key findings and methodologies illustrate the evolution and constraints of PDFs from HERA I to II.

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Exploring Proton Structure at High Q²: Insights from HERA Experiments

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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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